Warp knitting of metal fibre cloths for use as separation...
Transcript of Warp knitting of metal fibre cloths for use as separation...
Warp knitting of metal fibre cloths for use as separation material in automotive glass production
Daan De Keyzer
Promotors: prof. dr. ir. Lieva Van Langenhove, dr. ir. Filip Lanckmans Thesis submitted to obtain the degree of Master of Science in Textile Engineering
Department of Textiles Chairman: prof. dr. Paul Kiekens Faculty of Engineering Academic Year 2011-2012
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Acknowledgement
Hereby I would like to thank everyone who contributed to the accomplishment of my thesis.
First I would like to thank my external promotor at Bekaert, dr. ir. Filip Lanckmans, for the
extensive guidance during my research and the time spent on reviewing my writings. Thank you
also, Frank De Ridder, for your valuable input and advice.
I would like to thank my internal promotor, prof. dr. ir. Lieva Van Langenhove, for providing me
the opportunity to choose this topic, and for the useful advice throughout the research.
Furthermore, I would like to thank prof. dr. Yordan Kyosev, for providing me the license to his
program TexMind. It proved to be very useful for pattern selection of warp knits.
I would like to express my appreciation to all the people at Bekintex that helped me during my
practical tests. Thanks to the people from the R&D team, for all the amusing distractions and
the pleasant atmosphere.
Thanks to all my E-team classmates for two years of unforgettable moments. It was truly an
incredible experience. Special thanks go to Sam, for reviewing this work.
Thank you Charlotte, my girlfriend, for being at my side in Istanbul and motivating me whenever
needed. Also thank you Linde, my sister, for your practical and moral support.
Finally I would like to thank my parents, for making it possible to follow E-team and supporting
me during all the years: “nen grote merci”.
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Copyright notice
In so far as allowed by the non-disclosure agreement of 18 January 2012 between Bekintex NV,
Universiteit Gent and Daan De Keyzer; the author gives permission to make this master
dissertation available for consultation and to copy parts of this master dissertation for personal
use. In the case of any other use, the limitations of the copyright have to be respected, in
particular with regard to the obligation to state explicitly the source when quoting results from
this master dissertation.
Daan De Keyzer
May 31, 2012
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Warp knitting of metal fibre cloths for use as
separation material in automotive glass production
By
Daan DE KEYZER
Promoters: prof. dr. ir. Lieva Van Langenhove, dr. ir. Filip Lanckmans
Thesis submitted to obtain the academic degree of
Master of Science in Textile Engineering
Department of Textiles Chairman: prof. dr. Paul Kiekens
Faculty of Engineering Academic Year 2011-2012
Summary
In this thesis, warp knitted metal fibre cloths for use as separation material in automotive glass
production are investigated. The separation materials currently used are made by circular weft
knitting technology. Warp knitted fabrics are structurally very different and this results in a
completely different deformability, air permeability and thickness, all important for the
application as separation material. The aim of this thesis is to investigate the effect of structural
warp knitting parameters on the fabric properties. Various types of warp knitted structures are
selected and knitted.
Two structural parameters are found to influence the fabric deformability, the machine gauge
and take-off speed, which determine the loop density in respectively wale and course direction
of the fabric. Another parameter is the movement variation of the guide bars, which influences
the length of the underlap and adds stability to the fabric when increased. Next, the presence of
inlay threads is found to influence the deformability and air permeability. Finally, double needle
bed structures are found to be the least promising method to tailor the fabric properties.
Two warp knitted structures show promising properties when compared to the reference weft
knitted sample, and resulted in a patent application. These two fabrics can be further tested,
and improved by altering the investigated parameters.
Keywords: warp knitting, metal fibre, deformability, air permeability
Warp Knitting of Metal Fibre Cloths for use as
Separation Material in Automotive Glass Production
Daan De Keyzer
Supervisors: prof. dr. ir. Lieva Van Langenhove, dr. ir. Filip Lanckmans
Abstract - This research describes the properties of warp
knitted metal fibre fabrics that can be used as heat resistant
separation material in automotive glass production. Various
types of warp knitted structures are selected and knitted. The
fabrics are analysed to assess the potential of warp knitted
structures for automotive glass production.
Keywords - warp knitting, metal fibre, deformability, air
permeability
I. INTRODUCTION
abrics made out of stainless steel (SS) fibres are used in
the production process of press bended automotive
glass. The process consists of pressing flat glass around a
mould at high temperatures (500-700 °C). Direct contact
between the glass and the mould would lead to optical
distortions and defects such as scratches. To avoid this,
knitted stainless steel fibre fabrics are used as a heat resistant
separation material (HRSM) to cover the mould and protect
the glass. Due to the direct contact between the fabric and the
glass, the structural fabric characteristics strongly influence
the quality of the formed glass. The increasing complexity of
the glass shapes and higher optical requirements for windows
have led to a demand for a new generation of HRSM fabrics.
The currently used technology, circular weft knitting, does not
provide enough patterning possibilities to meet the demand
for improved fabrics. An alternative technology with more
patterning possibilities is warp knitting, but it also has an
inherently different structure, as shown in Figure 1.
Figure 1: Comparison of a) warp and b) weft knitted structure [1].
This structural difference results in a completely different
deformability, air permeability and thickness, all important for
the application as HRSM. The aim of this thesis is to
investigate the effect of structural warp-knitting parameters on
the fabric properties.
II. IMPORTANT HRSM FABRIC PROPERTIES
A. Fabric deformability
The deformability is determined by the tensile force at
break and elongation of the fabric. Knitted fabrics have
anisotropic properties. Generally the fabrics are measured in
course and wale direction. The leading car glass
manufacturers have set a standard of 49 N for the tensile force
at break in each direction.
The elongation at 49 N is ideally between 20 to 40 % in
both fabric directions. Higher elongations can possibly lead to
overstretching the fabric when draping it on the mould. This
deforms the structure and results in a lower optical quality.
The ratio of wale over course elongation at break (w/c) is
preferably close to one, this enables good drape ability
without structural deformations of the fabric over the mould.
B. Air permeability
The air permeability of the fabric determines the necessary
vacuum pressure in the process to form the bended glass.
When the air permeability is too low, the process complexity
increases and the window may not be shaped correctly. The
preferred value is considered to be 800 l/(dm2.min).
C. Fabric thickness
The fabric thickness influences the necessary mould and
glass temperature to form the bended glass. Thicker fabrics
will slow down the heat transfer from the mould to the glass.
III. EFFECT OF WARP KNITTING PARAMETERS
A. Sample specifications
All samples were made on a Karl Mayer Raschel warp
knitting machine with gauge twelve. The yarn used is a
stainless steel (AISI 316L) fibre with Nm 15/2 yarn count.
B. Effect of the underlap length
A warp knitted loop consists of an overlap and underlap.
The underlap length is determined by the sideways shogging
movement of the guide bar.
As can be found in the literature, single guide bar (GB)
structures are dimensionally to unstable and split easily when
damaged [2]. Warp knitted structures knitted with two guide
bars are more stable and already provide many patterning
possibilities. Different patterns are made by changing the
sideways movement of both guide bars. The movement is
defined by the number of needles lapped during the shog. The
set of samples consists of patterns with a different value for
both guide bars. Two statistical design of experiments (DOE)
were performed on the samples to determine the effect of
varying GB 1 and/or 2.
The results show that the underlap length has a statistical
significant influence on the elongation at 49 N in course
direction. The course elongation at 49 N for satin-pillar was
significantly lower compared to cord-pillar, respectively 88%
and 112%. In wale direction there is a slight decrease when
changing from pillar to tricot on GB 2, while keeping a tricot
stitch on GB 1.
F
The tensile force at break in wale direction is mainly
influenced by the machine gauge: it will increase for a higher
gauge. In course direction it increases for a longer underlap.
When comparing tricot-pillar with satin-pillar the course
tensile force at break is respectively 19 N versus 170 N.
Longer underlaps result in a closer and tighter structure,
which decrease the air permeability. However, all structures
are well above the preferred value of 800 l/(dm2.min).
The presence of an underlap between more than two wales
adds an extra thread on the fabric surface, which increases the
thickness. An even longer underlap however, will not further
increase the thickness.
C. Effect of inlay threads
The insertion of inlay threads is the second structural
parameter used to obtain a dimensionally stable fabric. The
properties of the satin-pillar structure were compared with a
pattern consisting of a pillar and inlay thread over four
needles (Figure 2).
Figure 2: Pillar stitch combined with (a) knitted satin loop and (b)
inlay threads over four needles [3]
The results show a significant difference in fabric properties
between both structures. The air permeability is significantly
higher for the inlay pattern because the inlay threads are not
knit into loops like the satin stitch. The thickness is
significantly lower, with 1,18 mm versus 1,95 mm for the
knitted loop pattern. Furthermore, the inlay pattern has
improved elongations at 49 N for both wale and course
directions, with 20 w% and 45 c% compared to 27 w% and 88
c% for the satin-pillar sample. This results in an increased
ratio w/c, as shown in Figure 3.
Figure 3: Difference in ratio w/c between satin-pillar and pillar-
(4)inlay with 95% confidence levels.
D. Effect of single versus double needle bed
Warp knitted fabrics can be knitted on either a single needle
bed or double needle bed, both methods result in a different
structure with different properties. Different types of double
needle bed structures with two guide bars, such as double
tricot and cord, were knitted and compared to the promising
single needle bed samples (satin-pillar and pillar-inlay).
The results show that these structures do not give any
improved properties compared to the single needle bed
samples. The thickness and elongations at break are almost
twice as high, which makes them not so interesting for HRSM
fabrics.
IV. COMPARISON WEFT-WARP KNITTED HRSM
The two most promising patterns for HRSM fabrics are the
satin-pillar and pillar-inlay patterns. Although the pillar-inlay
pattern has better deformation properties, the satin-pillar is
interesting for its relative high air permeability for a thick
fabric, which could result in good wear properties. The course
elongation at 49 N could be lowered even more with a longer
underlap e.g. velvet stitch instead of satin. When both samples
are compared to the weft knitted ½ pattern (Table 1), it is
clear that the pillar-inlay has comparable properties for air
permeability and thickness, but a significantly improved
elongation at 49 N.
Table 1: Comparison between data weft and warp knitted HRSM
Pattern Weft Knit 1/2 Satin-pillar Pillar-(4)inlay
AP (l/(dm2.min)) 1862 1138 1750
Thickness (mm) 1,24 1,95 1,18
WaleBF (N) 150 304 277
CourseBF (N) 255 169 175
WaleE49 (%) 120 27 20
CourseE49 (%) 68 88 45
Ratio w/c 1,78 0,30 0,44
Legend: BF = tensile force at break / E49 = elongation at 49 N
V. HIGH TEMPERATURE CHARACTERISATION
Additional investigation was done on the behaviour of SS
fibre fabrics at high temperature.
The first test consisted of testing the effect of high
temperature oxidation on the tensile properties at fibre, yarn
and fabric level. At 780 °C, SS will be oxidised at the surface.
The loss of weight results in a decrease of strength and
elongation at all three levels.
The second test, named sagging test, is meant to test the
cyclic loading on the HRSM fabrics in a high temperature
environment. The result is expressed in an amount of mm that
the fabric “sags” after cyclic loading. The results showed that
warp knitted fabrics had slightly higher values: 25 mm versus
21 mm for weft knitted fabrics.
VI. CONCLUSION
Warp knitting is a promising alternative to weft knitting for
HRSM fabrics. Two types of warp knitted structures have
been found to have interesting properties. Specifically the
pillar-inlay type has significant improvements over the weft-
knitted sample in terms of elongation at break.
REFERENCES
[1] ROZANT, O., P.E. BOURBAN, and J.-A.E. MANSON,
Drapability of dry textile fabrics for stampable thermoplastic
preforms. Composites: Part A, 2000. 31: p. 1167–1177. [2] RAZ, S., Warp knitting production.1987, Heidelberg: Melliand
Textilberichte. ISBN 3875290224
[3] KYOSEV, Y. and W. RENKENS. TexMind 2011; Available from: www.texmind.com.
a b
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Contents
ACKNOWLEDGEMENT ................................................................................................................................ i
COPYRIGHT NOTICE .................................................................................................................................. ii
SUMMARY .............................................................................................................................................. iii
EXTENDED ABSTRACT .............................................................................................................................. iv
CONTENTS ............................................................................................................................................... vi
LIST OF FIGURES ..................................................................................................................................... viii
LIST OF TABLES ......................................................................................................................................... x
UTILISED ABBREVIATIONS ........................................................................................................................ xi
CHAPTER 1 INTRODUCTION ....................................................................................................................... 1
CHAPTER 2 LITERATURE REVIEW ............................................................................................................... 2
2.1 AUTOMOTIVE GLASS PRODUCTION TECHNOLOGY .................................................................................................... 2 2.1.1 Tempered glass ............................................................................................................................... 3 2.1.2 Laminated glass .............................................................................................................................. 6 2.1.3 Production technology .................................................................................................................... 7
2.2 MOULD COVERING FABRICS .............................................................................................................................. 10 2.2.1 Warp knitting versus weft knitting ............................................................................................... 10 2.2.2 Warp knitting machines ................................................................................................................ 13 2.2.3 Warp knitted structures ................................................................................................................ 17
2.3 MODELLING OF WARP KNITTED STRUCTURES ....................................................................................................... 23 2.3.1 Overview of existing models ......................................................................................................... 23 2.3.2 TexMind model ............................................................................................................................. 26
2.4 PROPERTIES OF WARP-KNITTED FABRICS ............................................................................................................. 31
CHAPTER 3 METHODOLOGY .................................................................................................................... 33
3.1 INTRODUCTION .............................................................................................................................................. 33 3.2 TEXTILE PARAMETERS ...................................................................................................................................... 34
3.2.1 Yarn parameters ........................................................................................................................... 34 3.2.2 Structural fabric parameters ......................................................................................................... 35
3.3 MATERIALS ................................................................................................................................................... 35 3.4 DESIGN OF EXPERIMENT .................................................................................................................................. 36 3.5 CHARACTERISATION OF THE TEXTILE STRUCTURE ................................................................................................... 39
3.5.1 At room temperature .................................................................................................................... 39 3.5.2 At process temperature ................................................................................................................ 40
CHAPTER 4 TEST RESULTS ........................................................................................................................ 44
4.1 EFFECT OF STRUCTURAL TEXTILE PARAMETERS ON FABRIC PROPERTIES ...................................................................... 44 4.1.1 Effect of gauge and underlap movement GB 1 ............................................................................. 44 4.1.2 Effect of gauge and underlap movement GB 2 ............................................................................. 50
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4.1.3 Effect of inlay threads and take-off speed .................................................................................... 54 4.1.4 Effect of the amount of needle beds ............................................................................................. 56
4.2 HIGH TEMPERATURE CHARACTERISATION ............................................................................................................ 57 4.2.1 Effect on the fibre properties ........................................................................................................ 57 4.2.2 Effect on the yarn properties ........................................................................................................ 59 4.2.3 Effect on the fabric deformability ................................................................................................. 62
4.3 COMPARISON OF WEFT AND WARP KNITTED SAMPLES ........................................................................................... 65 4.4 SUMMARY .................................................................................................................................................... 67
CHAPTER 5 CONCLUSION ........................................................................................................................ 68
APPENDIX A: WARP KNITTED STRUCTURES .............................................................................................. 70
APPENDIX B: FABRIC TEST DATA .............................................................................................................. 83
LITERATURE LIST ..................................................................................................................................... 89
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List of Figures
FIGURE 2.1: HRSM PRODUCTS USED IN AUTOMOTIVE GLASS PRODUCTION [2] ..................................................................................... 2 FIGURE 2.2: STRESS DISPERSION IN GLASS THICKNESS [3] ................................................................................................................. 3 FIGURE 2.3: GLASS TEMPERING PROCESS [3] ................................................................................................................................. 4 FIGURE 2.4: EFFECT OF UNEVEN HEATING ON GLASS SHAPE [3].......................................................................................................... 5 FIGURE 2.5: TEMPERATURE DIFFERENTIAL DURING QUENCHING [3] .................................................................................................... 5 FIGURE 2.6: LAMINATED GLASS [2] .............................................................................................................................................. 6 FIGURE 2.7: GRAVITY SAGGING FOR LAMINATED GLASS [2] ............................................................................................................... 6 FIGURE 2.8: OVERVIEW OF BENDED GLASS TECHNOLOGIES................................................................................................................ 7 FIGURE 2.9: IN-FURNACE GRAVITY SAG BENDING [2] ....................................................................................................................... 8 FIGURE 2.10: IN-FURNACE PRESS BENDING [2]............................................................................................................................... 8 FIGURE 2.11: OUT-OF-FURNACE PRESS BENDING [2] ....................................................................................................................... 9 FIGURE 2.12: COMPARISON BETWEEN PLAIN WOVEN (A), WARP-KNITTED (B) AND WEFT-KNITTED (C) STRUCTURE [9] ............................... 11 FIGURE 2.13: DIFFERENCE BETWEEN WEFT (LEFT) AND WARP (RIGHT) KNITTING [10] .......................................................................... 11 FIGURE 2.14: TECHNICAL FACE OF PLAIN KNITTED STRUCTURE [10] .................................................................................................. 12 FIGURE 2.15: FLOAT (LEFT) AND TUCK (RIGHT) STITCHES [10] ......................................................................................................... 12 FIGURE 2.16: DIFFERENCE BETWEEN TRICOT (LEFT) AND RASCHEL (RIGHT) WARP KNITTING ................................................................... 13 FIGURE 2.17: GUIDE BAR LAPPING MOVEMENT ............................................................................................................................ 13 FIGURE 2.18: BASIC OVERLAP/UNDERLAP VARIATIONS [11] ........................................................................................................... 14 FIGURE 2.19: DIFFERENCE BETWEEN OPEN (A) AND CLOSED (B) LOOP [11] ........................................................................................ 15 FIGURE 2.20: PROPERTIES OF OPEN LOOPS VERSUS CLOSED LOOPS [12] ............................................................................................ 15 FIGURE 2.21: LOOP FORMATION ON SINGLE NEEDLE BED RASCHEL MACHINE [10] ............................................................................... 16 FIGURE 2.22: LOOP FORMATION ON DOUBLE NEEDLE BED RASCHEL MACHINE [10] ............................................................................. 17 FIGURE 2.23: PLAITING OF THREADS [10] ................................................................................................................................... 18 FIGURE 2.24: PLAITING PRINCIPLE DURING FRONT GB OVERLAP [10] ............................................................................................... 18 FIGURE 2.25: TECHNICAL BACK OF SINGLE GUIDE BAR WARP KNITTED FABRIC [10] .............................................................................. 19 FIGURE 2.26: TECHNICAL FACE OF BALANCED DOUBLE TRICOT STRUCTURE ......................................................................................... 19 FIGURE 2.27: COMMON PATTERNS WITH TWO GUIDE BARS [11] ..................................................................................................... 20 FIGURE 2.28: PRINCIPLE OF INLAY [11] ...................................................................................................................................... 21 FIGURE 2.29: DOUBLE NEEDLE OPEN PILLAR STITCH [11] ............................................................................................................... 21 FIGURE 2.30: PRODUCTION OF DOUBLE FACED FABRIC [11] ............................................................................................................ 22 FIGURE 2.31: THE LOOP MODEL BY G.L. ALLISON [20] .................................................................................................................. 23 FIGURE 2.32: THE MACHINE STATE LOOP MODEL [11] ................................................................................................................... 24 FIGURE 2.33: MODELLING HIERARCHY OF KNITTED STRUCTURES [31] ............................................................................................... 25 FIGURE 2.34: INTRA AND INTER-LOOP INTERACTIONS [33] ............................................................................................................. 25 FIGURE 2.35: 2D LOOP TOPOLOGY WITH (A) MAIN DIMENSIONS AND (B) ANCHOR POINTS [31] ............................................................. 26 FIGURE 2.36: KEY POINTS IN 3D LOOP TOPOLOGY ........................................................................................................................ 27 FIGURE 2.37: GENERATED DOUBLE NEEDLE BAR STRUCTURE [32] .................................................................................................... 28 FIGURE 2.38: TENSILE DIAGRAM OF A KNITTED STRUCTURE IN WALE DIRECTION [36] ........................................................................... 29 FIGURE 2.39: EFFECT OF FRICTION ON THE TENSILE PROPERTIES IN WALE DIRECTION [36] ..................................................................... 30 FIGURE 2.40: JAMMING MECHANISMS DURING BENDING OF A TWO-BAR WARP KNITTED FABRIC [40] ..................................................... 31 FIGURE 2.41: INFLUENCE OF UNDERLAP LENGTH ON THE BREAKING STRESS IN COURSE DIRECTION [44] ................................................... 32 FIGURE 3.1: PROJECT FLOW ...................................................................................................................................................... 34 FIGURE 3.2: EXAMPLE OF PATTERN CARD .................................................................................................................................... 38 FIGURE 3.3: PREPARATION OF FABRIC SAMPLE FOR OXIDATION TEST ................................................................................................. 40
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FIGURE 3.4: SAGGING SAMPLE PREPARATION ............................................................................................................................... 41 FIGURE 3.5: SCHEMATIC PROCEDURE OF THE SAGGING TEST ............................................................................................................ 42 FIGURE 3.6: TEST SET-UP IN OVEN WITH SAMPLE .......................................................................................................................... 42 FIGURE 3.7: TYPICAL PROCESS DIAGRAM OF SAGGING TEST ............................................................................................................. 43 FIGURE 4.1: EFFECT OF GB 1 AND GAUGE ON THE BREAKING STRENGTH FOR WALE (TOP) AND COURSE (BOTTOM) DIRECTION IN A MEANS PLOT
WITH 95 % CONFIDENCE LEVELS (LEFT) AND A PARETO CHART OF STANDARDISED EFFECTS (RIGHT) ................................................ 46 FIGURE 4.2: EFFECT OF GB 1 AND GAUGE ON THE ELONGATION AT 49 N FOR WALE (TOP) AND COURSE (BOTTOM) DIRECTION IN A MEANS PLOT
WITH 95 % CONFIDENCE LEVELS (LEFT) AND A PARETO CHART OF STANDARDISED EFFECTS (RIGHT) ................................................ 47 FIGURE 4.3: EFFECT OF GB 1 AND GAUGE ON THE AP IN A MEANS PLOT WITH 95 % CONFIDENCE LEVELS (LEFT) AND A PARETO CHART OF
STANDARDISED EFFECTS (RIGHT) ....................................................................................................................................... 48 FIGURE 4.4: EFFECT OF GB 1 AND GAUGE ON THE THICKNESS IN A MEANS PLOT WITH 95 % CONFIDENCE LEVELS (LEFT) AND A PARETO CHART OF
STANDARDISED EFFECTS (RIGHT) ....................................................................................................................................... 48 FIGURE 4.5: EFFECT OF GB 1 ON TENSILE FORCE AT BREAK AND ELONGATION AT 49 N IN A MEANS PLOT WITH 95 % CONFIDENCE LEVELS ..... 49 FIGURE 4.6: EFFECT OF GB 2 AND GAUGE ON THE BREAKING STRENGTH FOR WALE (TOP) AND COURSE (BOTTOM) DIRECTION IN A MEANS PLOT
WITH 95 % CONFIDENCE LEVELS (LEFT) AND A PARETO CHART OF STANDARDISED EFFECTS (RIGHT) ................................................ 51 FIGURE 4.7: EFFECT OF GB 2 AND GAUGE ON THE ELONGATION AT 49 N IN WALE DIRECTION IN A MEANS PLOT WITH 95 % CONFIDENCE LEVELS
(LEFT) AND A PARETO CHART OF STANDARDISED EFFECTS (RIGHT) ............................................................................................ 52 FIGURE 4.8: EFFECT OF GB 2 ON THE ELONGATION AT 49 N IN COURSE DIRECTION (LEFT) AND ON THE RATIO W/C (RIGHT) IN A MEANS PLOT
WITH 95 % CONFIDENCE LEVELS ....................................................................................................................................... 52 FIGURE 4.9: EFFECT OF GB 2 ON THE AIR PERMEABILITY IN A MEANS PLOT WITH 95 % CONFIDENCE LEVELS (LEFT) AND A PARETO CHART OF
STANDARDISED EFFECTS (RIGHT) ....................................................................................................................................... 53 FIGURE 4.10: EFFECT OF GB 2 ON THE FABRIC THICKNESS IN A MEANS PLOT WITH 95 % CONFIDENCE LEVELS (LEFT) AND A PARETO CHART OF
STANDARDISED EFFECTS (RIGHT) ....................................................................................................................................... 53 FIGURE 4.11: PILLAR STITCH COMBINED WITH KNITTED LOOP (LEFT) AND INLAY (RIGHT) OVER FOUR NEEDLES ........................................... 54 FIGURE 4.12: COMPARISON OF COURSE ELONGATION AT BREAK (LEFT) AND RATIO W/C (RIGHT) BETWEEN SATIN-PILLAR AND PILLAR-INLAY
STRUCTURE IN A MEANS PLOT WITH 95 % CONFIDENCE LEVELS ............................................................................................... 55 FIGURE 4.13: COMPARISON OF COURSE ELONGATION AT 49 N (LEFT) AND RATIO W/C (RIGHT) BETWEEN TWO PILLAR-INLAY SAMPLES WITH
DIFFERENT COURSE DENSITIES IN A MEANS PLOT WITH 95 % CONFIDENCE LEVELS ....................................................................... 55 FIGURE 4.14: TENSILE CURVES OF OXIDISED (- -) AND NON-OXIDISED (− −) 12 µM STAINLESS STEEL FIBRES .............................................. 57 FIGURE 4.15: EFFECT OF HTO ON FIBRE TENSILE FORCE (LEFT) AND ELONGATION (RIGHT) AT BREAK IN A MEANS PLOT WITH 95 % CONFIDENCE
LEVELS ......................................................................................................................................................................... 58 FIGURE 4.16: SEM IMAGES OF OXIDISED 12 µM FIBRES AT 1000X (LEFT) AND 5000X (RIGHT) ............................................................. 59 FIGURE 4.17: TENSILE PROPERTIES OF OXIDISED (- -) AND NON-OXIDISED (− −) NM 11/2 YARN ............................................................ 59 FIGURE 4.18: TENSILE PROPERTIES OF OXIDISED (- -) AND NON-OXIDISED (− −) 15/2 NM YARN ............................................................ 60 FIGURE 4.19: EFFECT OF HTO ON THE TENSILE FORCE AND ELONGATION AT BREAK OF NM 11/2 AND 15/2 YARNS IN A MEANS PLOT WITH 95
% CONFIDENCE LEVELS (LEFT) AND PARETO CHART OF STANDARDISED EFFECTS (RIGHT) ............................................................... 61 FIGURE 4.20: RATIO OF TENSILE FORCE AT BREAK PRE- AND POST-OXIDATION FOR FIBRES, YARNS AND FABRICS ......................................... 62 FIGURE 4.21: RATIO ELONGATION AT BREAK PRE- AND POST-OXIDATION FOR FIBRES, YARNS AND FABRICS ............................................... 63 FIGURE 4.22: CORRELATION BETWEEN FABRIC WEIGHT AND SAGGING WITH R² = 0,7402 .................................................................... 64 FIGURE 4.23: CORRELATION BETWEEN ELONGATION AT BREAK AND SAGGING FOR SAMPLES 4A, 5A_1 AND 5A_2 ................................... 65
x
List of Tables
TABLE 3.1: YARN PROPERTIES ................................................................................................................................................... 35 TABLE 3.2: THEORETICAL COMPOSITION RANGE OF EN 1.4404 ALLOY IN % MASS [45] ....................................................................... 35 TABLE 3.3: SAMPLES IN THE MAIN DOE ...................................................................................................................................... 36 TABLE 3.4: SAMPLES FOR THE INVESTIGATION OF OTHER STRUCTURAL PARAMETERS ............................................................................ 37 TABLE 3.5: OVERVIEW OF UTILISED ISO NORMS FOR TESTS AT ROOM TEMPERATURE ........................................................................... 39 TABLE 3.6: TENSILE TEST SPECIFICATIONS FOR FIBRES AND YARNS ..................................................................................................... 41 TABLE 3.7: SAGGING TEST SETTINGS ........................................................................................................................................... 42 TABLE 4.1: SUB-DOE A .......................................................................................................................................................... 45 TABLE 4.2: SUMMARY OF AVERAGE TEST RESULTS OF SUB-DOE A ................................................................................................... 45 TABLE 4.3: SUB-DOE B ........................................................................................................................................................... 50 TABLE 4.4: SUMMARY OF AVERAGE TEST RESULTS SUB-DOE B ........................................................................................................ 50 TABLE 4.5: AVERAGE TEST DATA OF SATIN-PILLAR AND PILLAR-INLAY STRUCTURE ................................................................................ 54 TABLE 4.6: PATTERN DETAILS OF DOUBLE NEEDLE BED SAMPLES ....................................................................................................... 56 TABLE 4.7: AVERAGE TEST DATA OF DOUBLE NEEDLE BED SAMPLES ................................................................................................... 56 TABLE 4.8: FIBRE TEST DATA ..................................................................................................................................................... 58 TABLE 4.9: YARN TEST DATA ..................................................................................................................................................... 60 TABLE 4.10: SAGGING TESTING VALUES ...................................................................................................................................... 63 TABLE 4.11: SUMMARY OF TEST RESULTS FOR COMPARISON WITH WEFT KNITTED REFERENCE SAMPLE .................................................... 65 TABLE 4.12: SAGGING COMPARISON BETWEEN WEFT AND WARP KNITTING ........................................................................................ 66 TABLE 4.13: IMPORTANT DRIVERS FOR WARP KNITTED HRSM ........................................................................................................ 67
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Utilised abbreviations
AP Air permeability
BF Tensile force at break
BE Elongation at break
DOE Design of experiment
E49 Elongation at 49 N
GB Guide bar
GF Glass fibre
HRSM Heat resistant separation material
HTO High temperature oxidation
Nm Number metric, unit for yarn count, expressed in m/g
SS Stainless steel
1
Chapter 1
Introduction
Fabrics made out of stainless steel (SS) fibres are used in the production process of press bended
automotive glass. The process consists of pressing flat glass around a mould at high temperature
(up to 700 °C). Direct contact between the glass and the mould would lead to optical distortions
and defects such as scratches. To avoid this, knitted SS fibre fabrics are used as a heat resistant
separation material (HRSM) to cover the mould and protect the glass. Due to the direct contact
between the fabric and the glass, the structural fabric characteristics strongly influence the quality
of the formed glass. Mould covering fabrics made out of SS fibres have a high maximum working
temperature and wear resistance. The increasing complexity of the glass shapes (with a higher
radius of curvature) and the recently developed quantitative method by ISRA Vision [1] to measure
optical distortion have led to a demand for improved HRSM fabrics. Furthermore, a new method to
produce windshields, which have higher optical requirements than side and back windows, by
press bending instead of gravity sagging, puts even more pressure on the development of a new
generation of HRSM fabrics.
The currently used technology, circular weft knitting, does not provide enough patterning
possibilities to meet the demand for improved fabrics. They also have certain disadvantages such
as non-isotropic deformation characteristics and sagging of the fabric at high temperature (700 °C).
An alternative technology is warp knitting, which has more structural parameters that can be
adjusted and therefore more patterning possibilities. It has an inherently different structure and a
higher flexibility regarding the orientation, length and shape of the loop. However, this structural
difference results in a completely different deformability, air permeability and thickness, all
important for the application as HRSM. The aim of this thesis is to investigate the effect of
structural warp knitting parameters on the fabric properties, which is the first step in developing a
new generation of HRSM fabrics. The goal is to obtain a fabric with improved deformability,
sagging and draping behaviour, in the end to improve the overall optical quality of the glass.
The focus of this work is on the properties of warp knitted fabrics at room temperature, followed
by an indication of the behaviour at process temperature.
In the next chapter a literature review is given, from the production of automotive glass to the
characteristics of warp knitted fabrics. The third chapter will describe the material specifications
and test methods used. In the next chapter, the test results of the different samples will be given
and compared. In the last chapter a conclusion is made, indicating which warp knitted structures
are promising to use as a mould covering fabric.
2
Chapter 2
Literature review
This chapter will give a literature review of all topics regarding the production of automotive glass
and warp knitted SS fibre fabrics. In the first part an overview of the glass production technologies
will be given and in which parts of the process the fabrics are being used. In the second section the
technology of warp knitting and the differences with weft knitting will be discussed. Furthermore,
several warp knitted structures will be explained in detail. In the third section an overview of the
existing models and visualization methods of warp knitted structures will be presented. In the
future these models could allow an accurate prediction of the fabric properties and simulate its
deformation behaviour. The fourth section gives a review of all the research regarding the
properties of warp knitted fabrics.
2.1 Automotive glass production technology
Figure 2.1 gives an overview of the heat resistant separation material (HRSM) products used in
automotive glass production [2]. Three types of applications for HRSM exist and only the mould
covering fabrics are discussed in this paragraph. The other two HRSM applications, for example the
roller covering sleeves in the furnace, are not included the scope of this study.
Figure 2.1: HRSM products used in automotive glass production [2]
3
Automotive glass can be subdivided in the front (windshield), back, side, quarter and sunroof
windows. The industrial term given to these windows is “lite”, for example sidelites. Quarterlites
are the smaller shaped windows in the side flanks of a car.
Two types of safety glass are used in automobile windows: tempered and laminated glass. Front
windows or windshields are made from laminated glass and the other windows are made of
tempered glass to increase the strength of the window.
2.1.1 Tempered glass
Tempering is a process which makes glass stronger by creating a protective compressive stress on
the glass surface with a thermal toughening process [3]. The definition of fully tempered glass by
the American Society for Testing and Materials (ASTM-C 1048) is “thermally treated glass having a
final surface compression of 10,000 psi (69 MPa) or more, or an edge compression of 9,700 psi (67
MPa) or more”[4]. For tempered glass to qualify as safety glass it must also meet the requirements
by ANSI Z97.1, which limits the size of the broken glass particles in a crash [5].
The stress dispersion throughout the thickness of tempered glass is shown in Figure 2.2. The
tempering process creates a differential stress between the outside and inner glass surface. The
outer layers are under a compressive stress, which gives the tempered glass its high strength and
protects the weaker inner surface. In order for tempered glass to be broken, the magnitude of the
impact (bending) force must overcome the built-in compressive surface stresses.
Figure 2.2: Stress dispersion in glass thickness [3]
When the surface of glass is penetrated by a deep scratch or impact, the stress gradients will make
the crack propagate in a curved shape. This leads to the formation of small glass pieces called
cullet. Cullet is safer than the sharp edged pieces formed by broken annealed glass.
The higher the compressive stress level, the more cracks that will form, the faster they will
propagate and the smaller the cullet particles will be.
4
The tempering process is possible due to the viscoelastic properties of glass at high temperature
and the thermal expansion characteristics. Glass will become less viscous and expand with an
increasing temperature.
For glass tempering two conditions are required: a uniform heating of the glass followed by a rapid
uniform cooling of the whole surface. The process of tempering glass is subdivided into three
phases: heating, quenching and cooling. The temperature change during the three phases is shown
in Figure 2.3.
Figure 2.3: Glass tempering process [3]
The first phase is a uniform heating of the glass in the furnace. The heating time depends on the
glass thickness; the general rule is 40 seconds per mm glass. As the glass is heated up, it expands.
During the heating in the furnace the surface of the glass gets hot faster than the inner section.
When the glass reaches a temperature above 540 °C (annealing point), all residual stresses in the
glass from previous processing are removed. The glass is heated up further to a temperature
between 620-640 °C. Below 620 °C, glass is too cold to develop the correct amount of compressive
stress required for tempered glass. Above 640 °C, problems will arise with optical distortion and
shape stability. At the end of the heating phase, the temperature should be the same at the top
and bottom surface of the glass. The uniform heating is very important, since a temperature
differential between both sides leads to a non-uniform bending, as shown in Figure 2.4. This non-
uniform bending is caused by a larger contraction of the hotter surface compared to the colder
surface during cooling.
620°C
MIDPLANE
SURFACE
SURFACE
MIDPLANE
580°C
410°C
100
200
300
400
500
600
700 TEMPERATUREDEGREESC
HEATING TIME QUENCH TIME
COOLING TIME
690°C FURNACE TEMPERATURE
TIME
540°C STRESS RELIEF TEMP
5
Figure 2.4: Effect of uneven heating on glass shape [3]
The second phase is quenching, this is a rapid (± 5 s) and uniform cooling of the glass surface. Due
to quenching, the outside surface of the glass rapidly loses its heat and starts to shrink. The best
quenching is obtained with quench settings that lead to a temperature difference of 170 °C
between the centre and the surface of the glass, as shown in Figure 2.5. The goal is to hold this
temperature difference until the centre is colder than 540 °C. By cooling the glass further, the
inside zone starts to shrink thermally, but the already solid outer surfaces cannot shrink any
further. This stress differential puts the surface into compression and the centre in tension, which
is the goal of the tempering process. The surface compression strengthens the glass and protects
the more vulnerable central tension layer. Thinner glass needs a more rapid quenching to obtain
the necessary temperature differential.
The third phase consists of cooling the glass to reach a comfortable temperature for further
processing.
Figure 2.5: Temperature differential during quenching [3]
410oC
170oC
170oC
410oC
580o
C
Q u e n c h A i r
Q u e n c h A i r
615°C Exit temperature of hot glass
= 20 °C Differential
610°C
630°C
Glass shape after quenching and cooling
6
2.1.2 Laminated glass
Laminated glass consists of two layers of glass bonded together under heat and pressure with a
tough plastic polyvinylbutyral (PVB) layer. This glass/PVB ‘sandwich’ behaves as a single unit and
looks like normal glass. Laminated glass breaks rather easily on impact, but the plastic interlayer
prevents the fragments from falling out of the window and causing injuries. Furthermore the PVB
layer blocks UV-radiation. In Figure 2.6 the principle of laminated glass is shown.
Figure 2.6: Laminated glass [2]
Laminated automobile glass is used mainly for windshields, but also for sidelites with a darker or
customized colour. It is traditionally produced by a gravity sagging technique, where the glass is
transported on a carousel skeleton through a heating zone (Figure 2.7). In the heating zone, two
glass layers are simultaneously bent by their own weight in order to guarantee exactly the same
shape. The shape of the windshield is determined by the skeleton ring. After gravity sagging the
two glass pieces are separated and an intermediate PVB layer is inserted. Since the glass is shaped
purely by its own weight and not by a pressing mould, HRSM fabrics are not used in this carousel
technology. However, due to the increasing shape complexity of automotive glass, there is a trend
to use press-bending for the complex shapes, in which HRSM fabrics are used. Specifically for
windshields, which have higher optical requirements than side or backlites, there is a need for
improved HRSM fabrics to fulfil these requirements.
Figure 2.7: Gravity sagging for laminated glass [2]
7
2.1.3 Production technology
Depending on the type of safety glass and application area there are different technologies to
produce automobile glass, as shown in Figure 2.8. The two main technologies differ in the way the
glass is bent: for in-furnace technology the glass is bent inside the hot furnace, in out-of-furnace
technology this happens directly after the furnace. The two technologies can be further subdivided
into gravity sagging and press bending techniques.
Figure 2.8: Overview of bended glass technologies
2.1.3.1 In-furnace
The in-furnace bending technology was mainly developed by the companies Glasstech and Sekurit
Saint-Gobain. Two methods for shaping the glass in-furnace exist: press bending and gravity sag
bending. The complexity of the glass is the main parameter for determining which of these
methods is used. Windows with a low complexity will be made by gravity sagging, while for more
complex shapes a press bending process is used.
The first step of the process is heating the flat glass part, which is heated from room temperature
until approximately 620°C in a furnace with temperature of 690°C. The furnace is normally heated
by a combination of hot air convection and infrared radiation. In the second step either a gravity
sagging or press bending technique is used to shape the window at the end of the furnace. In the
gravity sagging process the heated glass is bent by its own weight, the shape depends on the shape
of the shuttle ring. In Figure 2.9 the gravity sag bending system is shown. The pick-up is done by a
8
mould under vacuum that is covered with HRSM material. The mould can be flat or slightly bent to
pre-bend the shape before sagging. The mould drops the hot window on the shuttle ring which
results in the sagging of the window. Finally, the window is rapidly quenched in order to get the
required temperature differential.
Figure 2.9: In-furnace gravity sag bending [2]
In the press bending process, shown in Figure 2.10, the window is bent by a hot mould with extra
vacuum that presses the glass in the shuttle ring. Again, the mould is covered with the knitted
HRSM material. There is only a small difference between the two technologies: the action of the
mould in the pick-up step. In the gravity sagging process the mould only has a vacuum suction
function, while in the press bending process the mould has both a vacuum and pressing function.
Figure 2.10: In-furnace press bending [2]
In some cases a separate bend and quench ring are used in the press bending process. This enables
the production of more complex shapes, for example the highly curved backlites in modern cars.
9
When only one ring is used for both bending and quenching, it is covered with a quenching fabric
strip [6] [7], as previously shown in Figure 2.1. The function of this warp-knitted HRSM fabric strip
is completely different compared to the mould covering fabrics. The fabric not only has to prevent
the glass from breaking when the window falls from the mould on the ring, but it also has to have a
very open structure to permit air flow during quenching. However, this limits the pressing force
that can be used, since a high pressing force would leave an imprint of the knit structure on the
edges of the glass window. When a separate bending and quench ring is used, the bending ring is
not covered with HRSM fabric and is also narrower, which allows a higher pressing force and thus
more complex shapes. Additionally, highly complex shapes require special bend rings with side
wings to bend the sides of the window.
2.1.3.2 Out-of-furnace
Out-of-furnace bending is the process of bending glass outside of the furnace. Due to the fact that
the window is shaped outside of the hot furnace, the temperature loss is the reason that only
tempered windows with a lower complexity, such as small side and quarter windows, can be made
using out-of-furnace bending. The advantage of this technology is that it can be used to produce
both tempered and laminated glass, depending on the quench settings. In case of tempered glass,
quenching will cool the window rapidly, for laminated glass it will be slowly annealed. Traditionally,
laminated windshields are produced by a carousel gravity sagging method, which does not use
knitted HRSM since there is no mould. However, the increasing complexity of windshields requires
the use of press bending techniques. Windshields also have higher optical requirements, which
demands for improved knitted HRSM fabrics. Again different variations of the press bending
technology exist. In Figure 2.11 the technology with the segmented bending ring is shown. The
window is heated in the hot furnace. Upon its exit, it is pressed by the mould on the segmented
bend ring that has moved up from under the rollers. After pressing, the bending ring moves down
and the rollers transport the window to the quenching zone.
Press bending of laminated windows presents new challenges, for instance to get two windows
with exactly the same shape for lamination since the two windows are bent separately. In the
traditional skeleton method this was no issue since the two windows were bent simultaneously.
Figure 2.11: Out-of-furnace press bending [2]
10
2.2 Mould covering fabrics
Mould covering fabrics are all fabrics used to cover the moulds in the automotive glass production,
as discussed in the previous paragraph. The fabrics currently used are made by a circular weft
knitting technology of 100% SS or mixed GF/SS yarns.
The requirements, based on experiences with manufacturers, for a mould covering fabric are:
High temperature resistance (> 680 °C). Fabrics need to be able to withstand the high
temperature developed in the furnace. Although metal oxides are formed on the surface of
the fibres, SS fibres are able to withstand these conditions.
Sufficient air permeability (> 800 l/(dm2.min)). The fabric needs to allow sufficient air flow
to enable the vacuum between the mould and glass. A low permeability would result in an
incomplete bent glass.
Thickness (< 2 mm). A higher thickness will require the furnace to be set at a higher
temperature for sufficient heat transfer, which increases the energy consumption.
High softness.
Breaking strength in all directions should be higher than 49 N. This is a standard set by the
automobile glass manufacturers. It is assumed to be the average force subjected to the
fabric by the stretching process over the mould, the vacuum pressure and the releasing
force when a glass is pressed.
Sufficient drape-ability, which is determined by the tensile elongation (%) of the fabric. Not
only the absolute value of the elongation in wale and course direction is important, but also
the ratio between both influences the drape-ability. Ideally the ratio wale over course
elongation is approximately one.
Woven fabrics cannot be applied as mould covering materials due to their very limited drape-
ability. They are characterised by a low tensile elongation of the fabric, due to the straight
orientation of the threads. Also the air permeability is in general too low to permit sufficient air
flow. Figure 2.12 shows a comparison of plain woven, warp-knitted and weft-knitted structures.
Clear differences in thread orientation and permeability of the structure can be noted.
2.2.1 Warp knitting versus weft knitting
Knitted fabrics are defined as fabrics in which at least one system of threads is formed into knitted
loops, that are intermeshed into stitches [8]. The knitted stitch is formed when the knitting needle
receives a new loop and knocks over the old loop from the previous knitting cycle.
Knitted fabrics can be subdivided into two main groups: weft and warp knits, depending on the
way the stitches are formed. In weft knitted fabrics the stitches are made from the weft thread
across the width of the fabric. In warp knitted fabrics the stitches are made from each warp thread
along the length of the fabric. Similar to the warp and weft terms for weaving, the terms wale and
11
course exist for knitting. The difference with woven structures can be seen in Figure 2.12. The rows
of knitted loops across the width of the fabric are called courses and the columns along the length
of the fabric are called wales.
Figure 2.12: Comparison between plain woven (a), warp-knitted (b) and weft-knitted (c) structure [9]
The methods used to make these fabrics are substantially different and are shown in
Figure 2.13. The needles (A-B-C-D) move up and down to intermesh the formed loops into stitches.
In a weft knitting machine, the yarn feeding and loop formation occur at successive needles across
the needle bed in one knitting cycle. One thread at a time is fed to form a course of loops from this
single thread. In a warp knitting machine there is a simultaneous yarn feeding and loop forming
action, which occurs at every needle across the needle bar during the same knitting cycle. The
warp yarns are fed simultaneously from a warp beam to the needles by the guides (E-F-G-H) to
form one loop per needle per cycle. The wales are interconnected by the sideway movements of
the guides.
Figure 2.13: Difference between weft (left) and warp (right) knitting [10]
12
The possible stitch types that can be produced with weft knitting machines are more limited
compared to warp knitting. In weft knitting there are four basic structures: plain, rib, interlock and
purl. Rib and interlock are two types of double faced structures produced on double needle bed
knitting machines. Purl is a special single face structure that shows the same left loops on both
sides of the fabric.
Figure 2.14: Technical face of plain knitted structure [10]
A plain single needle bed structure is also called single jersey. Such weft knitted fabric types are
currently used to cover the moulds in automotive glass bending. Single jersey fabrics are made on
circular weft knitting machines, which are more productive and efficient than flat weft knitting
machines. Besides the knitted loop stitch, shown in Figure 2.14, there are two more stitches
commonly used in single jersey knits: the float stitch and tuck stitch. Both stitches, shown in Figure
2.15, are made with a held loop, which is an old loop that the needle retained in the previous
knitting cycle. Tuck stitches are used to increase the stability of the fabrics in the course direction.
Other stitch variations include elongated loops and transferred loops. However, these stitches are
not so interesting to tailor the mechanical properties of the HRSM fabrics and are more used for
aesthetical purposes.
Figure 2.15: Float (left) and tuck (right) stitches [10]
13
2.2.2 Warp knitting machines
Two types of warp knitting machines exist: tricot and raschel machines, both shown in Figure 2.16.
The machines differ only in the design and function of the sinkers during loop formation. In tricot
machines the sinker has two functions: the first one is to prevent the knitted loops from coming
upwards with the needles and the second one is to clear the formed loops from the needle head
and knock-over the previous loop. The sinker in raschel machines is only used to keep the fabric
down, the loop clearing and knock-over is done by the trick-plate and a high take-up tension. Latch
needles can be used on both tricot and raschel machines, but bearded needles can only be used on
tricot machines. Raschel machines are more flexible in what types of fabric that can be made.
Open net structures for example, cannot be held easily by the sinkers on tricot machines. This is
not a problem on raschel machines since the sinker does not have a fabric supporting function.
Figure 2.16: Difference between tricot (left) and raschel (right) warp knitting
For both types the movement of the guide bars for the loop formation is the same, as shown in
Figure 2.17. The loop formation is characterised by an overlap and underlap, which is realised by a
shog and swing movement of the guide bar.
Figure 2.17: Guide bar lapping movement
14
Every knitted loop is made by one of the five variations of overlap and underlap, which are shown
in Figure 2.18. Every black dot represents a needle. The letters O and U stand for overlap and
underlap. A row of numbers represents the movement of the warp guide bar. Every number
represents a position between two needles. In (a) the guide bar moves from position one to zero
(overlap) and then from position zero to one (underlap) to form one closed knitted loop. In the
chain link notation only the overlap movements are written out, the underlap is represented by a
‘slash’.
(a) Closed lap: overlap followed by an underlap in the opposite direction
(b) Open lap: overlap followed by an underlap in the same direction
(c) Only overlaps
(d) Laying-in: only underlaps and no overlaps
(e) Miss-lapping: no overlap or underlap
Figure 2.18: Basic overlap/underlap variations [11]
The difference between an open and closed loop can be seen in Figure 2.19. An open loop is
formed when the overlap and next underlap are made in the same direction. When done in
opposite direction, a closed loop is formed.
15
Figure 2.19: Difference between open (a) and closed (b) loop [11]
Regarding the choice of open loops versus closed loops for the application as HRSM material for
mould covering, the following preferred properties need to be looked at:
High elastic strain recovery: when the fabric is pulled over the mould the elastic strain on
the loops keeps the fabric close to the mould and could have an influence on sagging
Low and equal extensibility in course and wale direction: this enables the fabric to be
applied similarly each time, regardless of the operator.
Reduced edge curling: edge curling decreases the handling ability of the fabric when
applying it to the mould
High lustre: a smooth, shiny fabric will improve the optical quality of the glass
The properties of the knitted fabric are different for open and closed loops, as shown in
Figure 2.20. As can be seen in the figure, open loops have all the properties desired in the fabric,
except for the elastic strain recovery. As the fabric extensibility is considered to be an important
factor for the draping procedure of the fabric over the mould, open loops are preferred. However,
open loop movements are more difficult to form, except for the open pillar stitch. The successful
formation of open loops strongly depends on the tension of the warp yarns. If the applied tension
is incorrect, the loop can slip off the needle during the next knitting cycle.
Fabric property Open loops Closed loops Desired in fabric
Elastic strain recovery Lower Higher High
Extensibility Lower Higher Low
Edge curling Lower Higher Low
Lustre Higher Lower High
Wear on knitting elements Low High Low
Figure 2.20: Properties of open loops versus closed loops [12]
16
2.2.2.1 Single needle bed raschel machines
The loop formation on a single needle bed raschel machine is shown in Figure 2.21, which can be
divided into six steps:
(a) The sinker holds down the fabric while the guide bars move into position for the next loop
(underlap).
(b) The needles rise, the loop in the needle head opens the latch and clears the needle
(c) The two guide bars swing from the front to the back of the needle bar, every thread of
every guide forms an overlap on the corresponding needle on the needle bar.
(d) Both guide bars make the return swing from the back to the front to finish the overlap
(e) The needle bar moves down, the previous loop closes the latch which holds the new loop in
the needle head
(f) The needle bar moves further down and the previous loop is knocked-over
Figure 2.21: Loop formation on single needle bed raschel machine [10]
17
2.2.2.2 Double needle bed machines
Double needle bed raschel machines have a second needle bed that is opposite to the one in single
needle bed machines. The loop formation, shown in Figure 2.22, is similar, but the loops are
formed alternating on the front and back needle bed.
(a) The front needle bar rises to clear the previous course from the needle heads and latches
(b) During the overlap the guide bar swings around the needles
(c) The needle bar descends to knock-over the stitches and the guide bar does the underlap
shog
(d) The back needle bar starts its knitting cycle
Figure 2.22: Loop formation on double needle bed raschel machine [10]
2.2.3 Warp knitted structures
Two main structures can be distinguished in warp knitting: single needle and double needle bed
structures. Single needle bed structures always have two structurally different sides called the
technical face and back. The technical back, the visible top side during knitting, is characterised by
the underlaps. The technical front side is characterised by the knitted loops. Double needle bed
structures have either two different or two identical sides and are called double face structures.
Besides the amount of needle beds, the amount of guide bars is another important parameter that
greatly determines the structure. This amount can vary from minimum one guide bar to six or
more. More complex structures will require more guide bars. Every guide bar is programmed with
a certain pattern and the combination of these patterns leads to the warp knitted structure. Two
guide bars are commonly used and already provide many patterning possibilities.
18
When two guide bars with loop-forming function are used, there is a plaiting phenomenon. Only
the threads of a certain guide bar will be visible on the top or bottom side of the knit, as shown in
Figure 2.23. Plaiting is inherent to the set-up of the machine. When the two guide bars make the
overlap movement, shown in Figure 2.24, the threads will cross and the threads of the front guide
bar will be on top of the technical back. When the guide bars shift and form the underlap, the
threads of the front guide bar will be on top and visible on the technical back. For double needle
bed structures the phenomenon is similar and allows the production of an identical double face
structure with two different threads on each face. For HRSM fabrics this can be advantageous,
since a fabric with on the glass side 100% glass fibre (GF) and on the mould side 100% SS fibres is
possible. Research has shown that mixed GF/SS fabrics result in better optical quality of the
window compared to 100% SS fibre fabrics [13].
Figure 2.23: Plaiting of threads [10]
Figure 2.24: Plaiting principle during front GB overlap [10]
19
2.2.3.1 Single needle bed structures
The most basic structures that can be made with a single needle bar machine are those with one
guide bar. These structures are dimensionally unstable and split easily when damaged [11]. An
example of a single guide bar fabric with an unbalanced loop structure is shown in Figure 2.25. The
non-linear configuration of the wale loops is also called loop inclination.
Figure 2.25: Technical back of single guide bar warp knitted fabric [10]
Double guide bar structures are more stable due to the opposite loop orientation of the two guide
bars. When the yarn tension in both guide bars is balanced the loops will be erect, as can be seen
in Figure 2.26.
Figure 2.26: Technical face of balanced double tricot structure
In Figure 2.27 the common double guide bar patterns are shown. The left pattern represents the
pattern of the front guide bar, the right pattern the back guide bar. When the patterns are
switched, as in b-e / c-f / d-g, this will give a different structure with different appearance and
handle due to the plaiting property of the machine. For example, the difference between locknit
(b) and a reverse locknit (e) is that the locknit will give a softer touch and higher elasticity due to
the free-floating underlaps. In the reverse locknit the longer back guide bar underlaps are locked
under the shorter front guide bar underlaps, which restrict the movement of the structure. There
20
are similar effects between satin and sharkskin structures, with the satin having a smooth technical
back and the sharkskin a rough one. Another important parameter is the length of the underlap.
Longer reciprocating lapping movements are used to increase course wise stability, weight and
density of the fabric. The longer the floating underlaps on the technical back are, the brighter and
smoother the fabric will be [11] [14-16].
Figure 2.27: Common patterns with two guide bars [11]
Another technique to increase the width wise stability is by laying-in course threads. These threads
are not knitted into loops, but are laid in the loops of the pillar stitches. Figure 2.28 shows the
principle of inlay with one thread. The front guide bar is fully threaded and is responsible for the
formation of the pillar stitches, while the back guide bar only has one thread to lay into the pillar
loops. Different laid-in patterns can be made depending on the threading of the guide bar (partial
or full) and the inlay length. Yarns that are technically difficult to knit can be used and inserted as
laid-in threads in the knitted structure, which is the main advantage of this technique. An example
21
of such materials are yarns with a bending stiffness that is too high to form loops but low enough
to form curves when inserted in the structure.
Figure 2.28: Principle of inlay [11]
Two-needle overlap stitches, shown in Figure 2.29, are a special type of stitches used to add body
and stability to a single guide bar fabric. Each yarn is wrapped around two needles during the
overlap cycle and both these needles draw the loops simultaneously. However, the fact that two
loops are drawn from the same relatively small amount of yarn causes a large amount of stress on
the yarn and needles. Since the metal fibre yarn is difficult to knit and requires a large amount of
oil to remove the friction, this additional stress can have a potential negative influence on the knit
ability.
Figure 2.29: Double needle open pillar stitch [11]
2.2.3.2 Double needle bed structures
Double needle bed machines can be used to make a large variety of structures. The possibilities of
these structures in HRSM materials is that thicker structures can be made, which could have a
22
softer and more dampening effect on the windows. Some disadvantages for using these structures
as HRSM fabrics is their lower air permeability and higher thickness, which restricts the heat
transfer from the hot mould to the glass. Designing this type of structures is also more complex
since an additional parameter, the amount of needle beds, must be taken into account. Pattern
drawings for a double needle bed machine are made by a different procedure compared to single
needle bed structures, as can be seen in Figure 2.30. Two rows of dots now represent one cycle of
the machine: the front needle bar cycle is always represented by the first row, while the second
row represents the back needle bar cycle.
Figure 2.30: Production of double faced fabric [11]
Three basic types of double needle bed structures can be distinguished. The first type are the
double faced fabrics, in which both guide bars overlap on both needle bars. Each lapping
movement is doubled on both needle beds before an underlap is done. An example of this type is
shown in Figure 2.30. Double needle bed machines can also be used to produce two separate
single faced fabrics, this is the second type. If the front guide bar only overlaps on the front needle
bed and miss laps on the back bed, and the back guide bar only overlaps the back bed and miss
laps on the front bed, the two fabric sides will be separate. A third type is when the two fabrics are
connected by the underlaps to form one fabric. This occurs when the back bar only overlaps the
front needle bed and the front bar only overlaps the back needle bed.
Warp knitting makes it possible to combine two or more different sets of loops in the same
structure. This provides warp knitting with more possibilities, compared to weft knitting, to tailor a
fabric with the desired properties. However, it also becomes more complex to predict the outcome
and behaviour of a certain structure. This is why the use of a modelling program can be very useful
to speed up the design process and reduce the amount of time spent in practical trials.
23
2.3 Modelling of warp knitted structures
It is of interest in the development of new knitted structures that one has the ability to model
certain structures prior to experimentally producing them. Modelling structures allow a
preselection and optimization in order to reduce the (often costly) development time.
In this paragraph an overview of the existing models regarding warp knitted structures is given,
together with a description of the model specifically used to visualize the warp knitted mould
covering fabrics.
2.3.1 Overview of existing models
In the past decades numerous studies have been done on the geometry of knitted structures, most
of which have been done of weft knits. Not only was the industrial significance of warp knitted
fabrics lower, the warp-knitted structure is also more complex due to its dependency on the
threading and the lapping movement. Early attempts consisted of experimental studies on the
dimensional properties of warp knitted structures [17, 18] [19]. The first attempt to relate stitch
length to fabric geometry was the geometrical model of Allison, in which the warp knitted cell was
split into four sections: the loop’s head as a semi-circle, two straight lines for the loop’s legs and a
third straight line for the underlap. The model is shown in Figure 2.31.
Figure 2.31: The loop model by G.L. Allison [20]
Although this model was more based on geometrical shapes than on the physical reality, it
provided accurate results regarding the amount of yarn run-in. A few years later, Grosberg
developed the first model based on the physical yarn configuration in the knitted cell [21]. The
model was based on the assumption that the yarn is an elastic unit and that the shape of the loop
is created by forces at the base of the loop. The shape created under these conditions is called an
“elastica”, which exhibits a constant relationship between loop height and loop length.
Experiments showed that the underlaps are straight in the fabric on the machine, but part of a
circle in the relaxed fabric. The loops however, do not change noticeably after relaxation of the
24
fabric. This led to a simplification of the model by assuming that the loop and underlap are isolated
from each other by friction at the cross-over point, so the unit loop can be considered as two
separate parts. Later, Grosberg experimentally proved this assumption by experimentally
comparing the properties of relaxed tricot fabrics with his model. The resulting formula is very
complex, but the Grosberg model made it possible to calculate very accurate run-in values and to
check fabric analyses [22].
Another approach was by Raz with the machine state loop model, in which he states that the loop
shape of the fabric in the machine is more likely to be determined by the physical pull of the take-
up mechanism, than by the bending forces suggested in Grosberg’s model [11]. The geometry of
this model is shown in Figure 2.32.
Figure 2.32: The machine state loop model [11]
The geometrical models previously discussed have one thing in common: they do not take the real
three-dimensional geometry of the knitted loop into account. Since these two-dimensional models
do not provide a complete definition of the actual structure, they cannot be used in modern
computer-modelling to predict the mechanics of the warp-knitted structure [23]. An overview of
the different modelling theories and methods, applied on textiles, has been given by Sherburn [24].
Different methods have been reported to develop a real three-dimensional model, most of which
use complex mathematics to define the topology of the loop structure [25],[26] ,[27, 28],[29, 30].
These models can only be applied to a limited amount of knitted structures and cannot be used at
an industrial level.
A step-by-step approach to model and predict the properties of a general warp knitted fabric, as
shown in Figure 2.33, was given by Kyosev and Renkens [31]. There are two different approaches:
on a structural level or on a material level. Defining an accurate and simple topology is the first
step in making a general model for warp knitted structures on a structural level. From this
topology, a geometrical model can be defined. The third and most complex step is to take the yarn
25
mechanics into account to predict the fabric properties. On a material level the different effects
occurring in and between the yarns are considered. In Figure 2.34 the intra and inter-yarn
interactions in a plain weft knitted loop are given, similar effects will occur in the warp knitted
structure. The model of Kyosev and Renkens has been successfully developed into an industrial
tool under the brand name of TexMind [32]. The program, although still under development,
provides a useful tool in the daily visualization of all warp knitted structures.
Figure 2.33: Modelling hierarchy of knitted structures [31]
Figure 2.34: Intra and inter-loop interactions [33]
26
2.3.2 TexMind model
The TexMind model distinguishes itself from the other models by its simple topology definition and
the ability to automatically generate the mathematical model for warp knitted structures. The
topology is similar to the one defined for weft-knitted fabrics by Moesen et al [34]. The control
parameters of warp knitting machines, such as lapping movement, threading, take-up speed and
machine gauge, are used as input data of the model. The modelling process is divided into three
steps: pre-processing, solution and post-processing. The pre-processing step is basically checking
the knitability of the input data, for example to check the diameter of the yarn with the space
between the needles (gauge). In the solution step the basic structural elements are created
(topology), from which the yarn path in every element is calculated (loop form calculation). The
post-processing of the data comprises of visualizing the structure and exporting it to other
programs, for example to finite element modelling programs. In this section the solution step of
the model will be discussed to give a clear view on how the model is built up and is based on the
information found in the following articles [31, 35, 36].
2.3.2.1 Topology
The first step in a structural model is defining the topology elements of which the knitted structure
exist. The 2D topology can be defined using the contact points between the loops in the X-Y plane.
The yarn thread of each loop is described by a curve through six contact points, as shown in Figure
2.35. The position of the contact points are defined by the loop height B, loop width L, the distance
between two wales A, the distance between the feet K and the height of the feet yB.
Figure 2.35: 2D loop topology with (a) main dimensions and (b) anchor points [31]
The coordinates of these points are defined as (for 0 < i < n):
27
The loop height B, which is the distance between two courses, is defined by the take-up speed of
the warp knitting machine. The distance between the wales A can be defined as
with E the machine gauge in needle per inch. The distance may not be bigger than the distance
between the needles, but in reality the fabric relaxes which decreases the distance A. The
parameters K and yB are determined directly by the yarn radius r:
The points 1, 2, 2’, 1’ define the position of a loop head, all coordinates of other loops in the
knitted structure can be calculated using a simple translation of these points in X and Y directions.
In order to make the structure more visual, smoothing curves such as splines can be drawn through
the key points.
The 3D topological representation can be derived from the two-dimensional one by considering
the z-axis, as shown in Figure 2.36.
Figure 2.36: Key points in 3D loop topology
28
Every point 1i,j and 2i,j is now associated with two points +z and –z, with Δz > R (yarn radius):
The key-points selected for the visual representation depend on the warp knitted structure. For
instance the loops in a double tricot warp knit consist of two yarns, which means that four z-
positions are required for every point (-z, -2z, z, 2z).
The key points are located around the local X-Y plane for a single needle bed machine. For a double
needle bed machine two such local planes will be required to visualize the structure.
2.3.2.2 Loop form calculation
Once the key points of single loops are known, the next step is building the geometrical and
mechanical model. The geometric modelling consists of adjusting the positions of the key points
according to the yarn geometrical parameters and the calculation of the yarn axis form. Basically,
all the distances between the key points need to be checked and adjusted according to cross-
section of the used yarns. An example of an image generated by TexMind is shown in Figure 2.37. It
shows a double needle bar fabric with double tricot pattern.
Figure 2.37: Generated double needle bar structure [32]
The geometrical model can generate an accurate image of the warp knitted structure, but it
represents an idealized image. This image does not take into account the internal and external
forces in the geometry which occur for example during fabric relaxation. These forces are
considered in the mechanical models, which can be calculated by continuum models (force and
energy), or with a discrete model, which reduces the yarns to mass-spring systems. A different
approach is the use of FEM software, which can be used to calculate the deformations (small or
big) of the knitted structure. In the TexMind model, three different FEM tools are used and
implemented to consider the mechanical influences. The first one is based on truss, which
represents the loop as a frame of trusses to calculate uni- and biaxial deformations. At low
29
deformations, the knitted structure can be seen as a truss framework in order to model the
deformation behaviour [37]. The second tool is based on beam elements, since they transmit
bending moments, which can be used to calculate the compressibility. The third one is an explicit
FEM to simulate the knitting process for the contact calculations between yarns (friction effects).
These calculations are important for the mechanical modelling, but the main difficulty is to obtain
a stable algorithm with an acceptable calculation time.
The effect of fabric relaxation or tensile forces on the structure is described in mechanical models.
Kyosev and Renkens have described a theoretical model for the tensile properties of warp knitted
structures [36]. Different effects occur within the structure when stretched. Depending on the
tensile direction (wale or course) the structural deformation will be different.
In wale direction the tensile diagram is divided into two phases. In each phase a different
parameter determines the shape of the curve. The first phase in the tensile diagram is determined
by the bending stiffness of the yarn. In Figure 2.38 it is shown how the wale loops are bent and
increase in length when strained. Stiffer yarns will require a higher force to be strained. The second
phase in the diagram starts when the loop is completely stretched (structural elongation is
maximum) and the yarns themselves are being stretched. This phase is mainly determined by the
yarn tensile properties.
Figure 2.38: Tensile diagram of a knitted structure in wale direction [36]
For a more accurate and realistic prediction, a third parameter, can be taken into account. This
parameter describes the effect of friction and sliding between the loops and underlaps. As
discussed in the Grosberg model in the previous paragraph, the friction between the loops and the
underlaps plays an important role in the deformation of the structure under tensile stress. The
yarn sliding and friction effects concerning single guide bar structures under tensile stress was
previously investigated by Stumpf et al. [38]. The effect of yarn slippage on the friction force
between the yarns was determined experimentally. The results showed that from a certain force
30
level, the yarn slippage stops and further fabric deformation is based on elastic elongation of the
yarns. Furthermore, it was determined that the friction is independent of the sliding velocity. It
was also proposed to model the deformation of the structure during yarn slippage by a truss
framework, since there is relatively little change in the loop structure in this phase.
The effect of friction and sliding on the tensile diagram is shown in Figure 2.39. The loops will start
sliding when put under strain. The effect of friction and sliding becomes greater at higher
elongation values. It is only when the loops are completely stretched that the material will be
strained, which is in the second phase in the diagram. However, the effect of this parameter is
difficult to define and is therefore sometimes neglected to simplify the model. Similar effects will
occur during the deformation in course direction, but the tensile diagram will be completely
different. Straightening of the loops through sliding will contribute greatly to the, in general, higher
elongation in course direction. It can be expected that structures with a longer underlap will
stabilize the structure in course direction, due to the relative smaller length of yarn in the loop
compared to the underlap length. Therefor smaller loops and longer underlaps will decrease the
tensile elongation in course direction. Smaller loops can be achieved by decreasing the machine
take-off to increase the amount of courses per centimetre.
Figure 2.39: Effect of friction on the tensile properties in wale direction [36]
Mechanical models could be very useful to predict the properties of warp knitted fabrics. For
HRSM fabrics, it would allow a prediction of the tensile and draping properties of the fabric when
stretched over the mould. Up until now however, little research has been conducted on the
deformability of warp knitted fabrics. The main reason is that the deformability properties are very
structure and material specific, and cannot be generalized easily for all warp knitted structures,
especially for SS yarns.
31
2.4 Properties of warp-knitted fabrics
In this paragraph, a review of the past and more recent research on the properties of warp knitted
fabrics is given. Although little research can be directly applied to the HRSM fabrics made out of SS,
the research gives an indication of the effects occurring within the structure that can possibly
influence the fabric properties. The review is focused on the two-guide bar structures, since single
guide bar structures have a too open and unstable structure to be applied as HRSM fabric.
In the past some research has been done on the influence of two-guide bar warp knitted structures
on the different fabric properties. Yanagawa and Kawabata (1972) determined the biaxial tensile
properties of two-bar warp-knit fabrics [39]. The bending properties of warp-knitted outerwear
fabrics were experimentally investigated by Gibson et al [40] for a wide range of materials and
structures. Different two-bar fabrics with a different underlap length were tested and the influence
of the underlaps on the bending rigidity was described. Certain jamming mechanisms in the knitted
structure were proposed as cause for the different bending rigidity in course or wale direction, but
also for positive (technical face on outside of curve) or negative (technical face on inside of curve)
curvature. In Figure 2.40 two different jamming mechanisms are described, in (a) the length
jamming is shown, when the bending moment is applied parallel to the courses for a positive
curvature, in (b) the width jamming is shown, when the bending moment is applied parallel to the
wales for a negative curvature.
Figure 2.40: Jamming mechanisms during bending of a two-bar warp knitted fabric [40]
The results show that the bending rigidity has the strongest correlation with the fabric weight per
unit area and too lesser extent with the fabric thickness. Since the weight per unit area is greatly
influenced by the length of the underlap, it can be concluded that a longer underlap will provide a
greater resistance to bending. This is due to the fact that for a relatively long underlap, more yarns
are located between each course at any needle space, which provides a greater resistance to the
bending of these underlaps.
Similarly, the shear properties were also investigated [41] to determine important properties such
as the drape and handle of warp-knitted outerwear fabrics. This led to the conclusion that fabrics
32
with longer underlaps have a higher resistance to shear deformation. Again, this can be explained
by the fact that longer underlaps result in a higher amount of yarns between each course at any
needle space.
More recent studies on the bending rigidity and shear friction of warp knitted structures have been
done by Jeddi et al [42, 43], which have given similar results regarding the influence of the
underlap length as stated by Gibson et al. Moreover, it was also shown that the density of the
structure influences the bending rigidity: tighter knits with higher density will have a higher
bending rigidity. For the surface friction, Jeddi et al reported a decrease in surface friction for an
increased fabric density. This can be easily explained by the fact that more densely knitted
structures will have a tighter and smoother surface.
In another recent study [44], the influence of the knitted structure on the tensile properties and
fatigue behaviour was determined. Two structural parameters were found to have an important
effect on the fabric elasticity: the space available for yarn movement and the length of the
underlap. The space available for yarn movement is determined by the space between the
overlaps and the front guide bar underlap. For example a locknit structure will have more space
than a reverse locknit structure. Tests were carried out on tricot, locknit, reverse locknit, three-
needle satin, four-needle satin, three-needle sharkskin and four-needle sharkskin structures with
different knit densities. The results show that for an increasing underlap length, the breaking strain
will decrease and the breaking stress will increase, which is shown in Figure 2.41. The breaking
stress is given in cN/course, to eliminate the difference in fabric course density of the tested
samples.
Figure 2.41: Influence of underlap length on the breaking stress in course direction [44]
33
Chapter 3
Methodology
3.1 Introduction
In this chapter the methodology of the thesis is discussed. The materials used, the investigated
textile parameters and test methods are described in detail. In Figure 3.1 the project methodology
is schematically shown in a flow chart.
The aim of this thesis is to investigate the effect of structural warp knitting parameters on the
properties of HRSM fabrics. The first step in this investigation is to determine the deformability
characteristics at room temperature, and to determine which structural fabric parameters
influence it. Based on the literature review, a design of experiment is made to investigate the
effect of these parameters. The TexMind program is used to make 3D images of the structures,
which helps visualization and selection of the most interesting structures for this study.
In the next step, the samples are produced. One part of the samples is made in-house at Bekaert
on a single needle bed crochet warp knitting machine. Another part of the samples were
outsourced to a partner and were made on a single needle bed raschel warp knitting machine.
After the samples are knitted, the mechanical properties of both the gauge eight and twelve
samples are characterised. In the next chapter the results are statistically analysed to determine
the effect and interactions of the structural parameters, both material and machine wise.
Additionally, high temperature tests are performed to give an indication of the fabric properties at
the process temperature.
Parallel to the evaluation of the warp knit samples, a currently used weft knitted HRSM is tested.
The results of the weft and warp knitted HRSM are compared to determine if warp knitted fabrics
are a viable alternative.
From the parametrical analysis of warp knitted fabrics and the comparison between weft and warp
knitted fabrics, a conclusion will be made regarding the possibilities of warp knitting for HRSM
fabrics.
34
Figure 3.1: Project flow
3.2 Textile parameters
The yarn and structural fabric parameters need to be taken into account when designing warp
knitted textile structures based on metal fibres.
3.2.1 Yarn parameters
The following yarn parameters influence the properties and handle of the warp knitted fabric
structure:
Type: Spun yarn or continuous filament yarn
Material: type of alloy (AISI or EN)
Fibre diameter
35
Fibre length
Twist [twist/meter]
Tensile force at break [N]
Elongation at break [%]
3.2.2 Structural fabric parameters
In the warp knitting process SS yarns are knitted into a HRSM fabric, as discussed in Chapter 2. The
type of machine and the parameters set determine the structure, density and handle of the fabric.
The following structural parameters determine the knitting pattern:
Gauge [needles per inch]: fixed value, depends on the machine
Amount of needle beds: single or double
Amount of guide bars
Amount of warp threads: has an influence on the fabric width
Guide bar threading: full or partial threading influences the pattern
Underlap length
Open or closed loops
Presence of inlay threads
3.3 Materials
In this paragraph the specifications of the materials used in the knitting tests are given. Two types
of spun yarns were available for the knitting tests. For the gauge eight tests, a SS Nm 11/2 yarn is
used. For the gauge twelve tests, a SS Nm 15/2 yarn is used, the Nm 11/2 type is too coarse for the
knitting machine. In Table 3.1 the details of the used yarns are given. As can be seen, the yarns
have practically no tensile elongation which makes the warp knitting more difficult. Both yarns
have been oiled to improve the knit ability.
In Table 3.2 the theoretical composition range of the used EN 1.4404 alloy is given, this alloy is also
known as AISI 316L according to the AISI standards.
Table 3.1: Yarn properties
Yarn Count [Nm] Alloy Twist [tpm] Tensile strength [N] Elongation at break [%]
11/2 EN 1.4404 140 14,37 1,15
15/2 EN 1.4404 140 11,98 1,10
Table 3.2: Theoretical composition range of EN 1.4404 alloy in % mass [45]
C Si Mn P max. S N Cr Cu Mo Nb Ni
≤ 0,030 ≤ 1,00 ≤2,00 0,045 ≤0,015 ≤0,11 16,5-18,5 - 2,00-2,50 - 10,0-13,0
36
3.4 Design of experiment
The goal of this thesis is to explore the use of warp knitted SS fibre fabrics as an HRSM for mould
covering, by investigating the effect of structural warp-knitting parameters on the fabric
properties.
The first step in this investigation is to determine the deformability characteristics at room
temperature, and to determine which structural parameters influence it.
In the literature review it is found that single guide bar (GB) warp knitted structures are
dimensionally too unstable and split easily when damaged [11]. Warp knitted structures knitted
with two guide bars are more stable and already provide numerous patterning possibilities.
Different patterns can be made by changing the sideways movement of both guide bars. The
movement is defined by the number of needles lapped during the sideways shog. However, these
two guide bar fabrics are generally more stable in warp direction, and more stretchable in course
direction.
The underlap length, which is determined by the guide bar movements, is claimed to have a
significant influence on the fabric stability and deformability in the course direction. The main
design of experiment (DOE) is made to determine the effect of gauge and the movement variation
of GB 1 and/or 2 on the deformability of warp knitted stainless steel fibre fabrics. The set of
samples consists of six different structures at two machine gauges, based on six different
combinations of guide bar movements. The DOE contains a total of twelve samples, as shown in
Table 3.3. The second (GB1) and third (GB2) column represent the movements of the guide bars,
expressed in an amount of needles lapped. This amount expresses between how many needles the
guide bar shifts during the underlap. For example, when two needles are lapped the guide bar
shifts between two needles, which results in a tricot stitch. The minimum lap is over one needle,
which results in a pillar stitch.
Table 3.3: Samples in the main DOE
Pattern GB 1
[# needles lapped]
GB 2
[# needles lapped]
Gauge 12
[with 15/2 Nm]
Gauge 8
[with 11/2 Nm]
Tricot-pillar 2 1 X X
Cord-pillar 3 1 X O
Satin-pillar 4 1 X X
Tricot-tricot 2 2 X X
Cord-tricot 3 2 O O
Satin-tricot 4 2 ? ?
Legend: X = sample made; O = limited material availability; ? = limited by machine
37
The choice is made to utilize closed loops as much as possible, except for the pillar stitch. Although
open loops could have certain advantages for the deformability of HRSM fabrics (Chapter 1), it is
technically more difficult to form open loops with SS yarn (other than the pillar stitch) compared to
closed loops.
First, the gauge eight samples were made to test the knit-ability of SS yarn, when this proved
successful the gauge twelve samples were made. Two samples were not made in gauge eight, with
the main reason being the limited amount of Nm 11/2 yarn available. Keeping in mind however,
that the currently used weft knitted fabrics are produced on machines with gauge twelve or higher,
the decision is made to focus more on the gauge twelve samples. The production of the satin-tricot
sample was not successful on both gauges, due to the technical difficulties in knitting this structure
faultless with SS fibre yarn.
For each structure a pattern card with chain notation and lapping diagram is made, together with
3D images of the structure to aid visualization. An example of such a pattern card can be seen in
Figure 3.2. The other pattern cards can be found in Appendix A.
Besides the machine gauge and guide bar movements, three other structural parameters are
expected to influence the deformability of warp knitted structures. The first one is the use of inlay
threads, which can reduce the elongation at break in course direction. The second one is the use of
double needle overlaps, in which the overlap is done over two needles. As previously discussed in
Chapter 1, double needle overlaps put a large amount of stress on both needles and threads,
which is why it is technically too difficult to knit this structure with SS fibre yarns.
However, single needle bed structures with inlay threads can be produced. Two variations of one
structure with inlay threads are produced, differing in their course density. Additionally, the
influence of the third structural parameter, the amount of needle beds, on the overall fabric
properties is investigated. Four variations of double needle bed structures are produced. It is not
possible to do a full scale DOE for these parameters, only a limited amount of variants are be
made. The structures are summarized in Table 3.4. Similar to the main DOE samples, all detailed
patterns can be found in Appendix A.
Table 3.4: Samples for the investigation of other structural parameters
Name Gauge Needle bed Yarn
[Nm]
Structure [# needles lapped]
GB 1 GB 2
Pillar-inlay low dens. 12 Single 15/2 1 Inlay over 4
Pillar-inlay high dens. 12 Single 15/2 1 Inlay over 4
Double face 12 Double 15/2 1-2 1-2
Double tricot 12 Double 15/2 2 2
Double cord 12 Double 15/2 2-2 2-2
Double pillar with inlay 12 Double 15/2 1 Inlay over 4
38
Figure 3.2: Example of pattern card
Pillar-cord
Lapping diagram and chain link notation 3D Images
Technical face
Technical back
Selvedge
Guide bar threading
39
3.5 Characterisation of the textile structure
The warp knitted fabrics are characterised by tests at room and process temperature. The
characterisation of the fabrics at room temperature is the core of the thesis, but the high
temperature tests are done to give a first indication of the high temperature behaviour. As
discussed in Chapter 1, the mould has a maximum process temperature of 680 °C.
3.5.1 At room temperature
The following parameters are measured at room temperature:
Wale density [number of wales per 10 cm]
Course density [number of courses per 10 cm]
Fabric weight [g/m2]
Fabric thickness [mm]
Air permeability [l/(dm2.min)]
Tensile force at break in wale and course direction [N]
Elongation at break in wale and course direction [%]
Elongation at 49 N in wale and course direction [%]
These parameters are measured by a variety of tests. Some of the tests are done according to ISO
norms, others according to own specification. The utilised ISO norms are shown in Table 3.5.
Table 3.5: Overview of utilised ISO norms for tests at room temperature
Test parameter [unit] Norm
Air permeability [l/dm2/min] ISO 9237:1995
Fabric weight [g/m2] In a conditioned atmosphere (ISO 139)
Fabric thickness [mm] ISO 5084:1996
Tensile force at break [N] ISO 13934-1:1999
Elongation at break [%] ISO 13934-1:1999
The apparatus used for the tensile tests is a Zwicki 1120 by Zwick Roell Instruments.
The wale and course densities are measured by visually counting the amount of respectively wales
and courses in 10 cm of fabric.
The fabric thickness [mm] is measured with an Interapid thickness gauge with deep throat, reading
to 0,001 mm. The presser-feet have a diameter of 30 mm and the pressure is 2 N. All samples are
measured at least five times in the fabric areas specified by ISO 5084:1996. The fabric is measured
in a relaxed state.
40
3.5.2 At process temperature
In the press bending process, HRSM fabric is subjected to a maximum surface temperature of 680
°C. This affects the fabric properties at both yarn and fibre level due to high temperature oxidation
(HTO) effects. Two tests are performed to measure the influence of HTO on the deformability: an
oxidation test and a sagging test. These tests only give an indication of the fabric tensile properties
at process temperature and are not used for a detailed analysis. Additionally, scanning electron
microscopy (SEM) is used to visualise the effects occurring at fibre surface by HTO.
3.5.2.1 Oxidation test
The goal of an oxidation test is to simulate the high temperature effects within the furnace. The
test consists of heating test material (fabric, yarn or fibres) in an oven at 680 °C for 1 hour. After
this, the samples are subjected to the same mechanical tests at room temperature. The results are
then analysed to determine the effect of high temperature on the material properties. The test is
performed at fabric, yarn and fibre level.
The samples are prepared by clipping the material without tension on a metal frame, which goes in
the oven. The preparation of a fabric sample is shown in Figure 3.3.
Figure 3.3: Preparation of fabric sample for oxidation test
The following parameters are then measured to indicate the effects of this degradation:
Fabric tensile strength [N] in wale and course direction after 1 h at 680 °C
Fabric elongation at break [%] in wale and course direction after 1 h at 680 °C
Yarn tensile strength [N]
Yarn elongation at break [%]
Fibre tensile strength [N]
Fibre elongation at break [%]
The fabric tensile strength and elongation at break are measured according to ISO 13934-1:1999
on the same apparatus as at room temperature. The tensile properties for yarns and fibres are
41
measured according to own specification. For these tests, a DMA Q800 of TA Instruments is used in
the DMA Strain Rate mode. The test specifications are shown in Table 3.6.
Table 3.6: Tensile test specifications for fibres and yarns
Material Gauge [mm] Preload [N] Strain rate [%/min]
Fibre 10 0,02 0,2
Yarn 20 0,5 0,5
3.5.2.2 Sagging test
HRSM fabrics are subjected to mechanical loads during their application as mould covering fabrics
in automotive glass production. The goal of the sagging test is to simulate the forces on the fabric
generated by the vacuum suction and sticking force on the fabric after bending. It measures the
resistance of the fabric to cyclic loading at a temperature of 680 °C.
The sample preparation is done by stretching the fabric in a metal ring with a pre-load of 10 kg.
This load stands for the amount of stretching done on the fabric to pull it over the mould. The
preparation procedure for the samples is shown in Figure 3.4.
Figure 3.4: Sagging sample preparation
One cycle of the test consists of loading the fabric with the plunger to 4 N and then returning to
the initial position. The pressing distance necessary to reach the pre-set load of 4 N is measured. A
summary of the settings is shown in Table 3.7.
42
Table 3.7: Sagging test settings
Number of cycles 100
Force [N] 4
Speed of plunger [mm/s] 5
Temperature furnace [°C] 680
The test procedure, schematically shown in Figure 3.5, is done as followed:
1) One cycle at room temperature
2) 100 cycles at 680 °C. The distance measurement of the last cycle is used to determine the
value of sagging, defined by Y-X [mm].
3) Cooling of the sample to room temperature
Figure 3.5: Schematic procedure of the sagging test
Figure 3.6 shows the test set-up in the oven. After the test is performed, the sagging of the fabric
can be seen (marked by the red arrow).
Figure 3.6: Test set-up in oven with sample
The online process diagram of the sagging test is shown in Figure 3.7. The red curve represents the
vertical position of the plunger and the blue curve the force of the load cell. A higher value of
43
sagging means that the fabric is less resistant to cyclic loading at high temperature. This means
that the plunger needs to press deeper (higher value for vertical position) to obtain the same load
of 4 N. Similarly, the plunger will only start registering at a deeper distance since the fabric is not
stretched anymore in the ring. This will be seen in the diagram, with the red curve shifting up
towards the end of the test cycles.
Figure 3.7: Typical process diagram of sagging test
3.5.2.3 Scanning electron microscopy (SEM)
SEM analysis is used to investigate the changes at the surface of the fibres due to high
temperatures. A protective oxide layer will be formed at the surface of the fibres, which will
influence the friction properties at both fibre and yarn level. With SEM the change in surface
friction of the fibres and yarns due to high temperature oxidation can be visualized. The images
were made at 15 kV and at different magnifications.
However, a detailed analysis of the inter-yarn and fibre friction at process temperature is not
investigated, this falls outside of the scope of this study.
44
Chapter 4
Test results
In this chapter the test results of the samples described in Chapter 3 are given. From these results
the effect of structural fabric parameters on the properties, important for an HRSM fabric, are
determined. As discussed in Chapter 2 those properties are:
Fabric deformability, determined by the tensile force at break (> 49 N), the elongation at 49
N and a ratio w/c approaching one. Preferably the elongation at 49 N is minimal to have a
fabric with sufficient stiffness. The ratio w/c is defined by the ratio of elongation at 49N in
wale and course direction.
Sufficient air permeability, preferably above 800 l/(dm2.min).
Thickness, preferably lower than 2 mm to enable adequate heat transfer between mould
and glass.
The deformability properties are the most critical parameters for the drape ability of the fabric
over the mould, and determine the quality of the HRSM fabric. Therefore these parameters are
investigated and compared for the different types of fabrics, in order to relate the structural fabric
parameters to the fabric properties.
4.1 Effect of structural textile parameters on fabric properties
In this paragraph the influence of machine gauge, underlap movement GB 1 and 2, inlay threads
and the effect of single or double needle bed on the fabric properties are investigated.
4.1.1 Effect of gauge and underlap movement GB 1
The fact that due to technical complications not all samples in the main DOE can be made brings
along difficulties to do a full factorial analysis. This is why the main DOE is split up into two smaller
sub-DOE’s to analyse the effects of both guide bars and the machine gauge.
Sub-DOE A investigates the effect of two independent parameters: the machine gauge and the
underlap movement of GB1. In this set of samples the underlap movement of GB 2 is kept constant
at one needle lap (pillar stitch). Two variants of two factors result in a DOE with four samples. A
summary of the samples with their structural details is shown in Table 4.1. The cord-pillar sample
3A technically does not fit in the DOE, but it is added to analyse the gauge twelve samples further
in a one-way ANOVA.
45
Table 4.1: Sub-DOE A
Name GB 1
(# needles lapped)
GB 2
(# needles lapped)
Gauge 12
(sample nr.)
Gauge 8
(sample nr.)
Tricot-pillar 2 1 2A 2B
(Cord-pillar) 3 1 3A O
Satin-pillar 4 1 4A 3B
Legend: O = not made due to material availability
Five measurements were done of each sample. In Table 4.2 the average values for the dependent
variables of each sample are shown. The detailed test results, which are used in the statistical
analysis, can be found in Appendix B. In theory, the wale and course density should be equal for all
samples. The wale density is mainly determined by the gauge, which is fixed at gauge eight for the
“B” samples and at twelve for the “A” samples. Even so, there is a difference between the gauge
eight or twelve samples. This is due to the structural contraction of the fabric after knitting and the
washing process to remove the knitting oil. The contraction is different for the different knitting
patterns. The patterns with the highest underlap length will have the highest contraction and
therefor the highest wale density. The course density is less influenced by the knitting pattern and
is mainly determined by the machine take-off setting. However, due to technical reasons it is not
always possible to obtain a similar value for each knitting pattern.
Table 4.2: Summary of average test results of sub-DOE A
Nr. Wales/ 10 cm
Courses/ 10 cm
Wale_BF (N)
Course_BF (N)
Wale_E49 (%)
Course_E49 (%)
Ratio w/c
AP (l/(dm
2.min))
Thickness (mm)
2B 32 32 214 52 39 155 0,25 2660 1,54
3B 35 41 216 250 32 49 0,67 1446 1,69
2A 44 41 285 19 35 (239) - 1886 1,53
3A 51 42 284 132 40 112 0,35 1322 1,77
4A 55 42 304 169 27 88 0,30 1138 1,95
Legend: AP = air permeability ; BF = tensile force at break ; E49 = elongation at 49 N; ( ) = elongation at break
Figure 4.1 shows the effect on the tensile force at break in both wale and course directions. It can
be expected that the underlap movement of GB 1 will not have a significant influence on the
breaking force in the wale direction (top chart), since the underlap is oriented in course direction.
This is confirmed in the pareto chart of standardised effects (top-right) which is lower than p = 0,05
for GB 1. However, the machine gauge does have a significant influence in wale direction. Although
the yarn used in the gauge 12 samples is less strong (Nm 15/2 versus Nm 11/2) than in the gauge 8
samples, the tensile force at break is higher for the gauge 12 samples. This indicates that the gauge
itself has a significant effect on the tensile force at break. The higher wale density, resulting from
the higher gauge causes the increased tensile force at break. The effect can be clearly seen in the
top-left means plot.
46
In course direction, both the machine gauge and underlap movement of GB 1 have an effect on the
tensile force at break. When the movement of GB 1 increases, then the tensile force at break will
increase because the underlaps are longer and straighter in the course direction. This effect is
illustrated in the pareto chart of standardised effects in Figure 4.1. From this chart it can also be
seen that a higher gauge results in a lower tensile force at break. However, this can attributed to
the weaker yarn used in the gauge 12 tests. In theory, the machine gauge should not have a
significant effect on the tensile force in course direction. The small interaction seen in the “1 by 2”
bar is due to the higher course density of sample 3B.
Figure 4.1: Effect of GB 1 and gauge on the breaking strength for wale (top) and course (bottom) direction in a
means plot with 95 % confidence levels (left) and a pareto chart of standardised effects (right)
When considering the preferred value for tensile force at break, all samples but 2A (2B barely)
have values higher than 49 N. Because sample 2A has a tensile force at break lower than 49 N in
course direction, the elongation at 49 N can only be determined in the wale direction, as shown in
Figure 4.2. The value for elongation in course direction in Table 4.2 is substituted by the elongation
at break. The results suggest that the wale elongation at 49 N decreases for a higher underlap
length and gauge. From a theoretical point of view, this effect is difficult to explain, especially since
samples 2A and 4A have the same course density. In wale direction the elongation is mainly
determined by the elongation of the open pillar stitch. An increased density of pillar stitches would
47
in theory not affect this elongation, unless there is a significant interaction between the pillars. In
these samples however, the connection by the tricot or satin loops is the interaction between the
pillars. It can be expected that this interaction is higher for a longer underlap, for example a satin
loop, since more than two adjacent pillars (as in the tricot loop) are connected.
In course direction, no conclusions can be drawn between both machine gauges since for sample
2A break was reached at 19 N. However, when comparing the values in Table 4.2 for samples 2B
and 3B, respectively 155 % versus 49 %, it can be concluded that an increased movement of GB 1
significantly reduces the elongation at 49 N in course direction. A lower elongation at 49 N means
that the fabric is more stiff and provides a better response to the applied force. The reduction in
course elongation also results in an increase from 0,25 to 0,67 for the ratio w/c, which means that
the properties in both directions become more equal.
Figure 4.2: Effect of GB 1 and gauge on the elongation at 49 N for wale (top) and course (bottom) direction in a
means plot with 95 % confidence levels (left) and a pareto chart of standardised effects (right)
In Figure 4.3, the variation of the air permeability is shown. A higher underlap length decreases the
air permeability because the structure becomes more closed and has less voids. Increasing the
gauge from eight to twelve increases the wale density, which also makes the fabric less air
permeable. However, even for the highest underlap length (over four needles) the air permeability
is still well above the preferred value of 800 l/(dm2.min) for HRSM fabrics. In the pareto chart
(right) correlation can be found between gauge and movement GB 1 in the “1 by 2” bar. However,
this can be considered as distortion since sample 3B (Gauge 8; GB 1 = 4) has an increased number
of courses per cm compared to sample 2B. This increased course density results in less air
permeability.
Figure 4.4 shows the variation in fabric thickness. The underlap length of GB 1 clearly affects the
thickness, as can be seen in the pareto chart. The difference between a tricot-pillar and a satin-
pillar structure is that the underlap in a tricot-pillar is between two adjacent wales, while in a satin-
pillar it laps across two wales. When the underlap is laid upon a pillar lap this adds thickness to the
fabric. The machine gauge should in theory not have a significant influence, though the plot shows
48
correlation between machine gauge and fabric thickness when GB 1 laps over four needles. This
could be explained by the fact that a higher gauge results in a denser structure and therefor the
longer underlaps will lie closer, and perhaps even cross-over.
Figure 4.3: Effect of GB 1 and gauge on the AP in a means plot with 95 % confidence levels (left) and a pareto chart
of standardised effects (right)
Figure 4.4: Effect of GB 1 and gauge on the thickness in a means plot with 95 % confidence levels (left) and a pareto
chart of standardised effects (right)
The effects of the movement of GB 1 on the dependent variables can be checked with a one-way
ANOVA, in which sample 3A (movement GB 1 = 3) is included. The requirements for ANOVA are
confirmed with two tests. First, the Levene test is performed to test the homogeneity of variance
between the different groups of data. Second, the Shapiro-Wilk test is done to test the normality
of the data in one group. The data passed both tests and therefor ANOVA is applicable.
The average test values of sample 3A for air permeability and thickness are situated as expected
between sample 2A and 4A, which can be seen in Table 4.2. A higher underlap length results in a
fabric with lower air permeability and a slightly higher thickness. The effect of GB 1 on the absolute
49
values for the tensile force at break and elongation at 49 N is shown in Figure 4.5. For the tensile
force at break (top plots) the effects found in the DOE analysis are confirmed. The underlap length
has no significant effect on the wale tensile force at break, but a longer underlap increases the
tensile force at break in course direction. It should be mentioned that the tensile force at break of
the tricot-pillar (2A) is insufficient for use as an HRSM fabric (> 49 N). For the wale elongation at 49
N (bottom plot), the trend found in the DOE analysis cannot be confirmed since the value for the
cord-pillar sample is significantly higher than the tricot-pillar sample. The interaction between the
different wales, as previously suggested, does not seem to have an influence other than in the
satin-pillar sample 4A. In course direction the elongation trend, previously found in the gauge 8
samples, is confirmed for gauge 12. A higher underlap length will decrease the course breaking
elongation significantly. The data suggest that the satin-pillar sample is the best option for
application as HRSM fabric, due to its lowest course elongation at 49 N of 88%. When looking at
the w/c ratio, this sample does not have the best ratio (0,3), because the wale elongation at 49 N
does not decrease accordingly. However, the first step is to obtain low elongations at 49 N before
finding the sample with the best ratio w/c. Fabrics with ratio w/c equal to one but elongations
above 200 % are not interesting for an HRSM.
Figure 4.5: Effect of GB 1 on tensile force at break and elongation at 49 N in a means plot with 95 % confidence
levels
50
4.1.2 Effect of gauge and underlap movement GB 2
Sub-DOE B investigates the effect of the machine gauge and the underlap movement of GB 2 on
the fabric properties. Similar to sub-DOE A, this also results in a DOE with four samples. The
underlap movement of GB 1 is kept constant at a two needle lap (tricot stitch). It is investigated
whether the substitution of the pillar by a tricot on GB 2 improves the overall fabric properties for
use as HRSM. A summary of the samples with their structural details is shown in Table 4.3.
Similar to sub-DOE A, five measurements were done of each sample. In Table 4.4 the average
values for the dependent variables of each sample are shown. The detailed test results that were
used for the statistical analysis can be found in Appendix B. The tensile force at break in course
direction of sample 1A could not be measured since these samples have an elongation at break
that exceeds the maximum extension of the testing apparatus (250 %). The test was stopped
before break was reached. The obtained tensile force is lower than 49 N, which means that the
elongation at 49 N cannot be determined. Table 4.3: Sub-DOE B
Name GB 1
(# needles lapped)
GB 2
(# needles lapped)
Gauge 12
(sample nr.)
Gauge 8
(sample nr.)
Tricot-pillar 2 1 2A 2B
Tricot-tricot 2 2 1A 1B
Table 4.4: Summary of average test results sub-DOE B
Nr. Wales/ 10 cm
Courses/ 10 cm
Wale_BF (N)
Course_BF (N)
Wale_E49 (%)
Course_E49 (%)
Ratio w/c
AP (l/(dm
2.min))
Thickness (mm)
1B 42 38 341 108 24 171 0,14 1476 1,54
2B 32 32 214 52 39 155 0,25 2660 1,54
1A 53 42 385 ((18)) 32 ((248)) - 1434 1,62
2A 44 41 260 19 35 (239) - 1886 1,53
Legend: AP = air permeability ; BF = tensile force at break ; E49 = elongation at 49 N;
(( )) = not measured to break ; ( ) = elongation at break
First the deformability of the samples is investigated. Figure 4.6 shows a higher tensile force at
break in wale direction for the tricot-tricot structure compared to the tricot-pillar structure. The
additional connection between the wales strengthens the structure by stabilizing it and dispersing
the tensile forces in an additional direction. For the same reason the tensile force at break in
course direction should also be slightly higher for the tricot-tricot structure. When looking at the
gauge eight samples, this difference is significantly higher due to the increased course density for
the tricot-tricot sample. No difference can be seen between the gauge twelve samples (bottom-left
plot) but this is because the maximum value for sample 1A could not be determined. The gauge 12
data for tensile force in course direction is thereby inconclusive. It can be concluded though from
the gauge 8 data that the tensile force at break for small GB movements depends more on the
51
course and wale densities, other than on the presence of an additional small underlap like in the
tricot-tricot pattern.
Figure 4.6: Effect of GB 2 and gauge on the breaking strength for wale (top) and course (bottom) direction in a
means plot with 95 % confidence levels (left) and a pareto chart of standardised effects (right)
The wale elongation at 49 N in the top-left plot of Figure 4.7 appears to be lower for the tricot-
tricot structure than for the pillar-tricot structure. The difference between the gauge eight samples
is higher due to a higher course density of the tricot-tricot sample (GB 2 = 2). A higher course
density decreases the wale breaking elongation due to shorter loop lengths. The gauge twelve
samples have the same course density and therefor can be compared. The gauge 12 data show
that the effect of pillar versus tricot on the wale breaking elongation is minimal, but that there
might be a slight decrease in favour of tricot loops. The extra connection for the tricot-tricot
between the wales could cause this effect.
The data for course elongation at 49 N (and thus ratio w/c) is only available for the gauge 8
samples (Figure 4.8). The plot for course elongation (left) shows an increase for the tricot-tricot
structure compared to the pillar-tricot structure, which is opposite to the effect found in wale
direction. The effect is difficult to explain theoretically since the underlap length between the
52
wales stays constant in both patterns. A possible reason could be that the pillar loops limit the
extensibility in course direction by keeping the tricot loops vertically together.
The ratio w/c decreases when the underlap movement by GB 2 is increased, as shown in the plot
(right). The extra connection between the wales in a tricot-tricot pattern decreases the wale
elongation, but increases it in course direction. This leads to an overall decrease of the w/c ratio,
which is less interesting for the application as HRSM fabric. It can therefore be concluded that the
use of a tricot loop instead of pillar loop does not improve the deformation characteristics.
Figure 4.7: Effect of GB 2 and gauge on the elongation at 49 N in wale direction in a means plot with 95 % confidence
levels (left) and a pareto chart of standardised effects (right)
Figure 4.8: Effect of GB 2 on the elongation at 49 N in course direction (left) and on the ratio w/c (right) in a means
plot with 95 % confidence levels
Figure 4.9 shows the variation of the air permeability for the different structures. A higher gauge
and tricot-tricot structure results in a lower air permeability due to the higher loop density. The
distortion seen in the plot and the high “1 by 2” interaction in the pareto chart can be attributed
53
again to the higher course density of sample 1B (Gauge 8; GB 2 = 2). However, all samples still have
air permeability above the preferred value of 800 l/(dm2.min).
Theoretically it can be expected that there will be not a significant difference in fabric thickness
between a tricot-pillar and tricot-tricot structure, since both have the same underlap. This is
confirmed in Figure 4.10.
It can be concluded that increasing the movement of GB 2 does not improve the overall
characteristics of the fabric for use as an HRSM.
Figure 4.9: Effect of GB 2 on the air permeability in a means plot with 95 % confidence levels (left) and a pareto chart
of standardised effects (right)
Figure 4.10: Effect of GB 2 on the fabric thickness in a means plot with 95 % confidence levels (left) and a pareto chart of standardised effects (right)
54
4.1.3 Effect of inlay threads and take-off speed
The third method to tailor the deformability of knitted fabrics is to insert inlay threads in the
structure. The effect of an inlay thread in a warp knitted structure is investigated by comparing the
satin-pillar and the pillar-inlay (over 4 needles) pattern. Figure 4.11 shows the structural
differences between both patterns. In the pillar-inlay pattern the threads of the second guide bar
are not knitted in the structure but laid into the pillar loops. The consequence is that the fabric
weight and thickness is significantly lower, while the air permeability is almost twice as high, as can
be seen in Table 4.5. This means that a pillar-inlay structure shows certain advantages as an HRSM
fabric in comparison with the satin-pillar pattern. Examples of such advantages are the higher air
permeability and lower thickness, which improves the suction force and heat transfer between
mould and glass, while also reducing the energy consumption.
Figure 4.11: Pillar stitch combined with knitted loop (left) and inlay (right) over four needles
Table 4.5: Average test data of satin-pillar and pillar-inlay structure
Nr. Wales/ 10 cm
Courses/ 10 cm
Wale_BF (N)
Course_BF (N)
Wale_E49 (%)
Course_E49 (%)
Ratio w/c
AP (l/(dm
2.min))
Thickness (mm)
4A 55 42 304 169 27 88 0,30 1138 1,95
5A1 43 41 211 136 25 69 0,37 1966 1,40
5A2 43 51 277 175 20 45 0,44 1750 1,19
Legend: AP = air permeability ; BF = tensile force at break ; E49 = elongation at 49 N
Samples 4A and 5A_1 are compared to determine the effect of inlay threads versus knitted loops.
Both samples are made at the same take-off speed, so their course density is near to equal. In
Table 4.5 it can be seen that the elongation at 49 N in wale direction does not differ much. The
values of 4A and 5A_1 are very close, with respectively 27 % versus 25 %. In course direction the
55
presence of inlay threads compared to knitted loops does have a significant effect, as shown in
Figure 4.12. The strong decrease in elongation at 49 N in course direction (left) also results in an
increase of the ratio w/c (right) from 0,30 to 0,37.
Figure 4.12: Comparison of course elongation at break (left) and ratio w/c (right) between satin-pillar and pillar-inlay
structure in a means plot with 95 % confidence levels
The fourth structural parameter that influences the fabric properties is the take-off speed. A lower
take-off speed will result in an increased course density, characterised by shorter loops.
To determine this effect, a second variant 5A_2 of the pillar-inlay sample is produced with higher
course density, with respectively 51 courses per 10 cm versus 41. The fabric properties are also
shown in Table 4.5. The most important parameters, when comparing these two structures, are
their mechanical deformation properties. The elongation at 49 N in wale direction is lower due to
the shorter loops, with 20 % versus 25 %.
The change in elongation at 49 N in course direction is even higher, with 45 % versus 69 %. This
increases the ratio w/c ratio significantly to a value closer to 1 (Figure 4.13).
Figure 4.13: Comparison of course elongation at 49 N (left) and ratio w/c (right) between two pillar-inlay samples
with different course densities in a means plot with 95 % confidence levels
56
4.1.4 Effect of the amount of needle beds
The fifth structural parameter that influences the properties of warp knitted fabrics is the amount
of needle beds. In order to determine this influence, some basic double bed structures were made
to compare with the single bed ones. Due to high structural differences between single and double
needle bed patterns the fabrics cannot easily be compared with one another. Four different types
of double needle bed structures were produced and tested according to the same procedures as
for the single needle bed samples. The pattern details are shown in Table 4.6 and the detailed
pattern visualisations can be found in Appendix A.
Table 4.6: Pattern details of double needle bed samples
Name Gauge Number Yarn
(Nm)
Structure (# needles lapped)
GB 1 GB 2
Double face 12 6A 15/2 1-2 1-2
Double tricot 12 7A 15/2 2 2
Double cord 12 8A 15/2 2-2 2-2
Double pillar with inlay 12 9A 15/2 1 Inlay over 4
The properties of these fabrics are compared with the two most promising warp-knitted structures
from the previous paragraphs: samples 4A and 5A. The data of all samples are shown in Table 4.7.
When comparing the tensile properties of the samples, it is found that sample 8A fulfils the
minimum tensile force at break of 49 N in both directions. Samples 6A and 7A have an elongation
that is too high to be measured and thus the force at break could not be determined. Sample 9A
fails to fulfil the requirement in course direction. When the course elongation at 49 N is compared
with the single needle bed structures it is seen that the value is more than twice as high, even for
the double needle bed inlay variant. Additionally, the thickness is almost two times higher when
compared to sample 4A, which would require a higher furnace temperature. From these results it
can be concluded that double needle bed warp knitted fabrics are not promising to use as HRSM
fabrics, mainly due to their higher elongations at 49 N in course direction.
Table 4.7: Average test data of double needle bed samples
Nr. Wales/ 10 cm
Courses/ 10 cm
Wale_BF (N)
Course_BF (N)
Wale_E49 (%)
Course_E49 (%)
Ratio w/c
AP (l/(dm
2.min))
Thickness (mm)
4A 55 42 304 169 27 88 0,30 1138 1,95
5A1 43 41 211 136 25 69 0,37 1966 1,40
5A2 43 51 277 175 20 45 0,44 1750 1,19
6A 36 35 421 ((33)) 58 ((249)) - 1238 3,24
7A 44 36 437 ((24)) 66 ((250)) - 1066 4,26
8A 35 36 368 138 67 188 0,36 1312 3,51
9A 74 36 223 24 13 (193) - 999 4,27
Legend: BF = tensile force at break ; E49 = elongation at 49 N; ( ) = elong. at break ; (( )) = not measured to break
57
4.2 High temperature characterisation
At high temperatures, the metal fibres will be subjected to high temperature oxidation (HTO), and
will go through metallurgical transformations. This will affect the deformability and stability of the
knitted fabric once it is stretched over the mould. The focus of these experiments is to indicate the
change in fabric properties by investigating the effect of HTO on the fibre, yarn and fabric tensile
properties. First, the effect on the fibres will be investigated, then on the yarns and finally in the
fabrics to determine the correlation between the three levels. Additional measurements, for
example of the inter yarn friction, are not examined in this thesis but are necessary to fully
characterise the fabric behaviour.
4.2.1 Effect on the fibre properties
The effect on the yarn properties is determined by comparing the tensile tests of oxidised fibres
and non-oxidised fibres. The two types of yarns used, Nm 11/2 and 15/2, are both spun from
12 µm fibres. The tensile force of these fibres was determined before and after oxidation. The
oxidation process consisted of heating the fibres for one hour at 780 °C. Ten samples of each type
were tested. Figure 4.14 shows the tensile curves of the tested fibres. The curves suggest that
oxidised fibres have a significantly lower elongation at break and as well as a lower tensile force at
break. This can be explained by the formation of metal oxides on the surface of the fibre that
makes the fibres thinner and less strong.
Figure 4.14: Tensile curves of oxidised (- -) and non-oxidised (− −) 12 µm stainless steel fibres
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Ten
sile
fo
rce
(N
)
Elongation (%)
58
Table 4.8 shows the maximum values of the ten samples before and after oxidation. The data are
used for an ANOVA analysis to determine if the difference pre and post oxidation is significant.
Figure 4.15 clearly shows that HTO will lower the fibre tensile force and elongation at break.
Table 4.8: Fibre test data
SS Fibre ø 12 µm
Non-oxidised Oxidised
Test nr. Tensile force at break (N)
Elongation at break (%)
Tensile force at break (N)
Elongation at break (%)
1 0,1695 1,790 0,131 1,158
2 0,1579 1,737 0,1254 1,052
3 0,1572 1,504 0,1166 0,938
4 0,1669 1,844 0,1185 1,155
5 0,1324 1,385 0,1333 1,151
6 0,1692 1,738 0,1276 1,065
7 0,1462 1,451 0,1351 1,398
8 0,1551 1,505 0,1272 1,076
Average 0,1568 1,619 0,1268 1,124
Stdev 0,0119 0,165 0,0062 0,124
Figure 4.15: Effect of HTO on fibre tensile force (left) and elongation (right) at break in a means plot with 95 %
confidence levels
The presence of metal oxide fragments on the fibre surface can be seen by optical analysis with a
scanning electron microscope (SEM). Figure 4.16 shows the presence of the metal oxide fragments
on the fibre surface. The metal oxide fragments are brittle and make the fibre surface less strong.
59
Figure 4.16: SEM images of oxidised 12 µm fibres at 1000x (left) and 5000x (right)
4.2.2 Effect on the yarn properties
The effect on the yarn properties is determined by comparing the tensile test results of oxidised
and non-oxidised yarns. Figure 4.17 and Figure 4.18 show the tensile curves for Nm 11/2 and 15/2
yarns.
A clear effect of oxidation on the tensile properties of Nm 11/2 yarn can be seen in the plot. The
oxidised yarns have a lower tensile force and elongation at break compared to the non-oxidised
samples. With an average fibre length of 80 mm and a test gauge length of 15 mm, the fibre tensile
properties can be related to these results. The lower tensile force and elongation of the oxidised
fibres will contribute to the effect seen in the figure. The inter-fibre friction undoubtedly also has
an effect, due to the presence of metal oxide particles, but has not been investigated in this thesis.
Figure 4.17: Tensile properties of oxidised (- -) and non-oxidised (− −) Nm 11/2 yarn
0
2
4
6
8
10
12
14
16
18
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Ten
sile
Fo
rce
(N
)
Elongation (%)
60
The effects for the Nm 15/2 yarn are not as clear as for the Nm 11/2 yarn. While all samples have a
lower maximum force and strain, the slope of the curves are different. This variation could be
explained by an unequal oxidation in the different sections of the yarn, possibly due to a higher
amount of spinning oil.
Figure 4.18: Tensile properties of oxidised (- -) and non-oxidised (− −) 15/2 Nm yarn
The effects on both yarns are statistically analysed in a DOE with two factors, oxidised/non-
oxidised and yarn count. The test values are shown in Table 4.9.
Table 4.9: Yarn test data
SS 11/2 Nm SS 15/2 Nm
Non-oxidised Oxidised Non-oxidised Oxidised
Test nr. Tensile force at
break (N)
Elongation at break
(%)
Tensile force at
break (N)
Elongation at break
(%)
Tensile force at
break (N)
Elongation at break
(%)
Tensile force at
break (N)
Elongation at break
(%)
1 15,12 1,142 11,73 0,901 12,22 1,081 11,34 1,001
2 14,16 1,137 12,74 0,983 13,01 1,152 9,35 0,985
3 15,54 1,163 12,19 0,882 12,68 1,147 8,99 0,968
4 13,75 1,165 12,91 0,982 11,92 1,230 9,07 1,083
5 13,97 1,181 13,15 1,003 12,18 1,081 7,74 0,979
6 15,36 1,170 12,66 0,942 12,38 1,064 11,00 1,001
7 13,53 1,133 12,69 0,874 11,03 1,064 10,35 0,921
8 15,20 1,136 13,81 1,039 12,01 1,115 9,81 0,867
9 11,94 1,032 12,43 0,956 10,08 1,031 10,90 0,818
10 15,15 1,203 13,35 0,919 12,24 1,079 11,29 0,966
Average 14,37 1,146 12,77 0,948 11,98 1,104 9,99 0,959
Stdev 1,07 0,044 0,56 0,052 0,80 0,055 1,13 0,071
0
2
4
6
8
10
12
14
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Ten
sile
fo
rce
(N
)
Elongation (%)
61
The results of the analysis are shown in Figure 4.19. The tensile force at break (top) is significantly
lower for Nm 15/2 yarn than for Nm 11/2 yarn, due to the higher fineness. Oxidation clearly lowers
the tensile force at break, for both the Nm 11/2 and 15/2 yarn. There is no “1 by 2” interaction
effect between the yarn count and oxidation treatment for tensile force.
The elongation at break is not influenced by the yarn count: a thinner yarn does not necessarily
result in a lower strain. Oxidation does have a clear effect: for both yarn counts it lowers the
elongation at break significantly.
Figure 4.19: Effect of HTO on the tensile force and elongation at break of Nm 11/2 and 15/2 yarns in a means plot
with 95 % confidence levels (left) and pareto chart of standardised effects (right)
62
4.2.3 Effect on the fabric deformability
The effect on the deformability is determined by the sagging and oxidation test. The oxidation test
measures the influence of oxidation on the fabric tensile properties, while the sagging test is used
to determine the stretch stability of the fabric under a cyclic force at high temperature.
4.2.3.1 Oxidation test
The oxidation test consists of the tensile testing of oxidised fabric strips. Figure 4.20 shows the
effect of oxidation on the tensile force at break at fibre, yarn and fabric level. Samples 6A, 7A and
9A are not included in the graph because their elongation exceeds the maximum values of the
testing apparatus (> 250 %).
There is a clear loss in the tensile force at break at all levels. The loss in tensile force at fibre level
does not correspond with the loss at yarn level. This is because the tensile force at break of a yarn
is mainly determined by the inter-fibre friction due to the yarn twist. At fabric level there is also a
significant loss, which can be related to the strength loss of the yarns. Once the knitted structure
has completely deformed under tension, the breaking load will be determined by the yarn
properties. For the different fabric samples, the loss is not equal in both directions, but no trend
can found between the samples.
Figure 4.20: Ratio of tensile force at break pre- and post-oxidation for fibres, yarns and fabrics
Figure 4.21 shows the effect of oxidation on the elongation at break at fibre, yarn and fabric level.
There is a loss in elongation at break for all samples. Similar to the tensile force loss, the loss in
elongation at fibre level does not correspond with the loss at yarn level. It can be expected that the
lowered elongation at break of the yarns will results in a lowered value at fabric level.
0
10
20
30
40
50
60
70
80
90
100
FibreØ12 µm
11/2Yarn
15/2Yarn
REF 1B 2B 3B 3A 4A 5A_1 5A_2 8ARat
io t
en
sile
fo
rce
at
bre
ak p
re/p
ost
ox.
(%
)
Sample
Fibre Yarn Fabric wale direction Fabric course direction
63
Figure 4.21: Ratio elongation at break pre- and post-oxidation for fibres, yarns and fabrics
4.2.3.2 Sagging test
The sagging test represents the cyclic loading of the HRSM fabric. It measures the resistance of the
fabric at high temperature to the vacuum force between mould and glass, and the sticking force
when the glass is released from the mould. Due to the large amount of fabric required for this test,
not all samples could be tested. The two most promising single needle bed patterns were tested,
as were also three double needle bed samples. The sagging values for each sample can be found in
Table 4.10. Table 4.10: Sagging testing values
Sample Sagging (mm) Weight (g/m2)
WaleE49 (%)
CourseE49 (%)
Ratio w/c
4A 25,36 735 27 88 0,30
5A_1 27,99 461 25 69 0,37
5A_2 25,07 534 20 45 0,44
6A 32,03 805 58 (249) -
7A 41,54 1063 66 (250) -
8A 38,32 902 67 188 0,36
Legend: AP = air permeability ; BF = tensile force at break ; E49 = elongation at 49 N;
( ) = maximum elongation measured
The five parameters previously discussed, that influence the fabric pattern, can be expected to
have an influence on fabric sagging. The properties determined by the pattern such as weight,
elongation at 49 N are investigated to determine which parameters influence the sagging.
0
10
20
30
40
50
60
70
80
90
100
FibreØ12 µm
11/2Yarn
15/2Yarn
REF 1B 2B 3B 3A 4A 5A_1 5A_2 8A
Rat
io e
lon
gati
on
at
bre
ak p
re/p
ost
ox.
(%
)
Sample
Fibre Yarn Fabric wale direction Fabric course direction
64
The sagging is plotted in function of fabric weight in Figure 4.22. Double needle bed fabrics with a
higher weight appear to have higher sagging values, but this can be mainly explained by the
different fabric structures (single versus double) which also have different elongation values. The
higher weight is a direct consequence of the different structures. When the double needle bed
structures are compared, those with higher weight have higher sagging values. However when the
single needle bed structures are compared, the lightest fabric has the highest value for sagging
(sample 5A_1). From this it can be concluded that the fabric weight does not necessarily have an
influence on sagging.
Figure 4.22: Correlation between fabric weight and sagging with R² = 0,7402
Sample 5A_2, which has exactly the same pattern as 5A_1 except with a higher course density, has
less sagging, with 25,07 versus 27,99 mm. This indicates that the course density has an effect on
sagging, because it affects the elongation properties and ratio w/c.
The influence of the elongation at 49 N on sagging is determined in Figure 4.23 for the single
needle bed samples. Fabrics with a higher elongation at 49 N will be more easily strained than
those with a lower elongation. This could result in a higher susceptibility to sagging. However,
when looking at the curve, the sample (4A) with the highest elongations at 49 N does not have the
highest value for sagging. Both sample 4A and 5A_2 have similar values for sagging but a
completely different structure and elongation values. This indicates that sagging is not determined
by one structural parameter, but by a combination of them. Further research is necessary to
determine the exact influence of the structural parameters.
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
45.00
0.00 200.00 400.00 600.00 800.00 1000.00 1200.00
Sagg
ing
(mm
)
Fabric weight (g/m2)
65
Figure 4.23: Correlation between elongation at break and sagging for samples 4A, 5A_1 and 5A_2
4.3 Comparison of weft and warp knitted samples
In this paragraph the most promising warp-knitted samples, derived from the test results, are
compared with the reference weft-knitted sample. The samples, both weft and warp knitted, are
all made on a gauge twelve machine with a Nm 15/2 yarn. The two most promising warp knitted
patterns are the satin-pillar and pillar-inlay types. A summary of the values is shown in Table 4.11.
Table 4.11: Summary of test results for comparison with weft knitted reference sample
Nr Wales /10 cm
Courses /10 cm
Weight (g/m
2)
AP (l/dm
2.min)
Thickness (mm)
WaleBF (N)
CourseBF (N)
WaleE49 (%)
CourseE49 (%)
Ratio w/c
REF 68 90 576 1862 1,24 150 255 120 68 0,56
4A 55 42 735 1138 1,95 304 169 27 88 0,30
5A 2 43 51 533 1750 1,18 277 175 20 45 0,44
Legend: AP = air permeability ; BF = tensile force at break ; E49 = elongation at 49 N
When the wale and course densities are compared, it is found that the densities for the weft
knitted sample are both higher in wale and course direction. Still, the weight of the warp knitted
samples, especially for the pillar-inlay sample, is not so much different. This is because two sets of
threads are knitted into one warp knitted structure. For the pillar-inlay sample there is one guide
bar and one inlay bar, which results in a weight comparable to the weft knitted reference. For the
satin-pillar sample, the weight is higher because of the two guide bar structure, which knits two
sets of threads on one set of needles.
24.50
25.00
25.50
26.00
26.50
27.00
27.50
28.00
28.50
0.00 20.00 40.00 60.00 80.00 100.00
Sagg
ing
(mm
)
Elongation at 49 N (%)
WaleE49 (%) CourseE49 (%)
66
The reference sample has about the same air permeability as the pillar-inlay sample, but a higher
value than the satin-pillar sample. The long underlap is responsible for the lower air permeability,
but is still well above the requirement (> 800 l/(dm2.min)). This underlap is also responsible for the
higher thickness, which is still lower than the preferred value (2 mm). The higher thickness could
possibly be an advantage for the application in automobile glass production. It could have a higher
compressibility and result in a window with improved optical quality. However, mould tests needs
to be done to investigate this effect.
When the tensile forces at break are compared, the difference between weft and warp knitting can
be clearly seen. Warp knitted fabrics are stronger in warp direction, due to the pillar stitch. Weft
knitted fabrics are stronger in course direction. When comparing the elongations at 49 N, it can be
seen that the wale elongation at 49 N is significantly lower for the warp knitted samples. The weft
knitted sample has a wale elongation at 49 N of 120 % versus 20 % for sample 5A_2. The same
warp knitted sample also has a lower course elongation at 49 N: 45 % versus 68 %. Sample 4A has a
slightly higher value: 88 %. However, the lower values for the pillar-inlay sample do not result in a
ratio w/c that is closer to 1 compared to the weft knitted sample: 0,44 versus 0,56 for the weft
knitted sample. This means that the weft knitted fabric has more equal properties in both wale and
course direction.
Finally, the sagging, which is an important factor for the lifetime of the fabric, can also be
compared. The values are shown in Table 4.12. The values for the warp knitted fabrics are almost
equal with a value around 25 mm, while the weft knitted sample has a lower value of
21 mm. This means that the produced warp knitted samples do not yet improve the sagging
compared to weft knits.
Table 4.12: Sagging comparison between weft and warp knitting
Sample Structure Sagging (mm)
Reference weft knit Weft knit 21,18
4A Satin-pillar 25,36
5A_2 Pillar-inlay 25,07
The overall conclusion is that elongation properties have been improved compared to weft knitted
fabrics but other important properties such as sagging and ratio w/c are still better for weft-knitted
fabrics.
67
4.4 Summary
This investigation has identified the most important drivers influencing the properties of warp
knitted heat resistant separation fabrics. These drivers can be adjusted to tailor the properties of a
warp knitted HRSM. In Table 4.13 the drivers and their effects on the fabric properties are shown.
The effect of gauge on tensile force at break and elongation at 49 N in course direction is
inconclusive, because they could either not be measured or reached break before 49 N as
previously mentioned. The effect on the ratio w/c is not displayed since this depends on the exact
increase or decrease of the elongation.
Table 4.13: Important drivers for warp knitted HRSM
Drivers AP Thickness WaleBF CourseBF WaleE49 CourseE49
Gauge ↑ ↓ =/↑ ↑ ? =/↓ ?
Course density ↑ ↓ =/↑ ↑ ↑ ↓ =/↓
Underlap length ↑ ↓ =/↑ = ↑ = ↓↓
Knitted -> Inlay thread ↑ ↓ =/↓ ↓ = ↓
Single -> double bed =/↓ ↑ ↑ =/↓ ↑ ↑↑
Legend: AP = air permeability / E49 = elongation at 49 N / BF = tensile force at break
=/ ↑ : small influence ? = inconclusive
By adjusting the wale and course densities in structures with a high underlap length or with inlay
threads, a stable warp knitted fabric can be formed that would fulfil the requirements for HRSM
fabrics.
68
Chapter 5
Conclusion
Warp knitted fabrics have been investigated as an alternative to weft knitted heat resistant
separation materials (HRSM). The applicability of warp knitted fabrics as HRSM is determined by
five fabric properties: tensile force at break, elongation at 49 N, ratio wale/course elongation, air
permeability and the fabric thickness. The influence of five structural parameters, machine gauge,
course density, guide bar (GB) underlap movement, presence of inlay threads and single or double
needle bed, on these fabric properties is determined in this study.
A series of two guide bar warp knitted structures are selected with the aid of a visual 3D modelling
program to investigate these parameters. Not all samples could be successfully made due to
technical difficulties when knitting SS fibre yarn. The samples were subjected to a sequence of
tests to determine the fabric properties.
First, the parameters that influence the fabric thickness are looked into. It is found that the fabric
thickness is little influenced by the structural parameters, except for single or double bed
structures. The GB movement has little influence. There is a difference between pillar-tricot and
pillar-cord fabrics but for an even longer underlap there is not much difference. Inlay threads do
not influence the fabric thickness since they do not lie on top of the pillar stitches but are inserted
inside the pillar loops. Single needle bed or double needle bed patterns have a strong influence on
the thickness, with the double needle bed fabrics almost being twice as thick.
Second, the parameters that influence the air permeability are determined. All parameters have a
significant influence on the air permeability. A higher machine gauge, course density and a longer
underlap result in lower air permeability. Structures with inlay threads are generally more open,
which results in higher air permeability. Double needle bed fabrics are less air permeable due to
their higher thickness.
Third, the parameters that influence the tensile force at break are investigated. The tensile force at
break is subdivided in the force in wale and course direction. In wale direction, the tensile force at
break will increase for a higher gauge and for double needle bed structures instead of single bed.
The other parameters do not have a significant influence. In course direction, the force will
increase with an increased GB movement and increased course density. Switching from a knitted
loop to an inlay thread, and from single to double bed will decrease the tensile force at break.
69
Fourth, the parameters that influence the elongation at 49 N are characterised. Again, the
elongation is subdivided in wale and course direction. In wale direction, the elongation will
increase with the course density and when switching from single to double bed. In course direction
the elongation is strongly determined by the underlap length and the presence of inlay threads.
Increasing the gauge and switching from single to double bed will increase the course elongation.
Fifth and last, the parameters that influence the ratio wale/course elongation are looked into.
Since this property is fully dependent on the value for wale and course elongation, the same
effects found for the elongation at 49 N will affect the ratio w/c.
Next, the parameters that influence the fabric properties at high temperature are investigated. The
results show that the breaking load of the fabrics is clearly influenced by the high temperature. The
oxidation of the fibres leads to a decrease in tensile force at break on fibre, yarn and fabric level.
Sagging tests show that single bed structures better resist the cyclic loading at high temperature,
compared to double bed structures. It was found that a combination of structural fabric
parameters influences the sagging properties.
By investigating the five structural parameters, two specific warp knits are found interesting for
further research: the combination of a pillar stitch and a stitch with long underlap (e.g. satin), or
the combination of a pillar stitch and inlay threads over at least four needles. The main parameter
influencing these structures is the underlap or inlay length, determined by the guide bar
movement. The most important properties for the application as an HRSM fabric are the
elongation at 49 N and the ratio between wale and course elongation. The sample with inlay
threads has the most promising values for elongation, ratio w/c and sagging. Following the results
of this thesis, a patent application was filed regarding the use of the satin-pillar and pillar-inlay
pattern for HRSM applications.
The unidirectional tensile tests performed in this study cannot account for the interaction from the
other fabric directions. Therefore, biaxial tensile testing of the fabrics is recommended for further
research.
General conclusion
From the results it can be concluded that it is possible to knit a fabric from stainless steel fibre
yarns by warp knitting technology. Two patterns have been identified as promising for use as an
HRSM fabric. When compared to weft knitted fabrics, certain properties are improved but others
remain better for weft knitted fabrics. Further research into these structures, with additional
mould testing, will show if the warp knitted fabrics are a feasible alternative to weft knitting
technology.
70
Appendix A: Warp knitted structures
In this appendix the detailed schematics of the structures defined in the DOE are given. The first
section includes an overview of the DOE samples in a table. The second section gives a detailed
description of every structure, including the chain link notation with drawn structure, as well as 3D
images generated by the TexMind program.
Overview Single needle bed structures
Name GB 1
[# needles lapped]
GB 2
[# needles lapped]
Tricot-pillar 2 1
Cord-pillar 3 1
Satin-pillar 4 1
Tricot-tricot 2 2
Cord-tricot 3 2
Satin-pillar 4 2
Pillar-inlay 1 Inlay over 4
Double needle tricot-tricot 2+1 2+1
Double needle bed structures
Name GB 1
[# needles lapped]
GB 2
[# needles lapped]
Double face 1-2 1-2
Double tricot 2 2
Double cord 2-2 2-2
Double pillar with inlay 1-1 Inlay over 4
71
Patterns
Single needle bed patterns
Tricot-pillar (TRPI)
Lapping diagram and chain link notation 3D Images
Technical face
Technical back
72
Cord-pillar (COPI)
Lapping diagram and chain link notation 3D Images
Technical face
Technical back
73
Satin-pillar (SAPI)
Lapping diagram and chain link notation 3D Images
Technical face
Technical back
74
Tricot-tricot (TRTR)
Lapping diagram and chain link notation 3D Images
Technical face
Technical back
75
Cord-tricot
Lapping diagram and chain link notation 3D Images
Technical face
Technical back
76
Satin-tricot
Lapping diagram and chain link notation 3D Images
Technical face
Technical back
77
Pillar-inlay (PINL)
Lapping diagram and chain link notation 3D Images
Technical face
Technical back
78
Double needle tricot-tricot
Lapping diagram and chain link notation 3D Images
Technical face
Technical back
79
Double needle bed patterns Double face (DNDF)
Lapping diagram and chain link notation 3D Images
Technical face
Technical back
Cross-section in course direction
80
Double tricot (DNDT)
Lapping diagram and chain link notation 3D Images
Technical face
Technical back
Cross-section in course direction
81
Double cord (DNDC)
Lapping diagram and chain link notation 3D Images
Technical face
Technical back
Cross-section in course direction
82
Double pillar with inlay (DNPI)
Lapping diagram and chain link notation 3D Images
Not possible with TexMind program.
83
Appendix B: Fabric test data
General properties
Single needle bed samples
Nr. Test Gauge (npi)
Structure GB 1
(# needles lapped)
GB 2 (# needles
lapped)
# Wales per 10 cm
# Courses per 10 cm
Weight
(g/m2)
AP
(l/dm2/min)
Thickness
(mm)
REF 1 12 Weft ½ - - 68 90 560,20 1890,00 1,23
REF 2 12 Weft ½
68 90 576,88 1850,00 1,24
REF 3 12 Weft ½
68 90 573,28 1870,00 1,24
REF 4 12 Weft ½
68 90 593,87 1850,00 1,24
REF 5 12 Weft ½
68 90 574,90 1850,00 1,25
1B 1 8 TRTR 2 2 42 38 670,69 1530,00 1,55
1B 2 8 TRTR 2 2 42 38 664,97 1460,00 1,51
1B 3 8 TRTR 2 2 42 38 659,45 1370,00 1,55
1B 4 8 TRTR 2 2 42 38 672,58 1440,00 1,58
1B 5 8 TRTR 2 2 42 38 671,45 1580,00 1,49
2B 1 8 TRPI 2 1 32 32 407,60 2680,00 1,56
2B 2 8 TRPI 2 1 32 32 400,47 2650,00 1,51
2B 3 8 TRPI 2 1 32 32 406,92 2600,00 1,55
2B 4 8 TRPI 2 1 32 32 405,75 2640,00 1,58
2B 5 8 TRPI 2 1 32 32 408,74 2730,00 1,52
3B 1 8 SAPI 4 1 35 41 741,52 1460,00 1,64
3B 2 8 SAPI 4 1 35 41 731,54 1450,00 1,68
3B 3 8 SAPI 4 1 35 41 748,11 1480,00 1,75
3B 4 8 SAPI 4 1 35 41 737,66 1410,00 1,68
3B 5 8 SAPI 4 1 35 41 742,81 1430,00 1,69
1A 1 12 TRTR 2 2 53 42 601,55 1490,00 1,61
1A 2 12 TRTR 2 2 53 42 598,75 1400,00 1,66
1A 3 12 TRTR 2 2 53 42 608,02 1450,00 1,58
1A 4 12 TRTR 2 2 53 42 604,28 1390,00 1,64
1A 5 12 TRTR 2 2 53 42 603,63 1440,00 1,59
2A 1 12 TRPI 2 1 44 41 508,63 1720,00 1,41
2A 2 12 TRPI 2 1 44 41 511,68 1970,00 1,57
2A 3 12 TRPI 2 1 44 41 507,42 1830,00 1,58
2A 4 12 TRPI 2 1 44 41 505,34 2060,00 1,60
2A 5 12 TRPI 2 1 44 41 502,05 1850,00 1,49
3A 1 12 COPI 3 1 51 42 664,17 1390,00 1,64
3A 2 12 COPI 3 1 51 42 655,35 1360,00 1,83
3A 3 12 COPI 3 1 51 42 669,04 1390,00 1,79
84
3A 4 12 COPI 3 1 51 42 670,16 1200,00 1,83
3A 5 12 COPI 3 1 51 42 669,12 1270,00 1,74
4A 1 12 SAPI 4 1 55 42 734,81 1120,00 1,96
4A 2 12 SAPI 4 1 55 42 731,56 1140,00 1,99
4A 3 12 SAPI 4 1 55 42 738,86 1160,00 1,91
4A 4 12 SAPI 4 1 55 42 729,63 1170,00 1,90
4A 5 12 SAPI 4 1 55 42 737,68 1100,00 1,98
5A_1 1 12 PINL 1 inlay 4 43 41 460,38 1940,00 1,33
5A_1 2 12 PINL 1 inlay 4 43 41 452,54 1920,00 1,41
5A_1 3 12 PINL 1 inlay 4 43 41 471,18 1930,00 1,40
5A_1 4 12 PINL 1 inlay 4 43 41 467,22 2090,00 1,41
5A_1 5 12 PINL 1 inlay 4 43 41 451,78 1950,00 1,44
5A_2 1 12 PINL 1 inlay 4 43 51 532,55 1750,00 1,18
5A_2 2 12 PINL 1 inlay 4 43 51 542,40 1630,00 1,22
5A_2 3 12 PINL 1 inlay 4 43 51 526,63 1640,00 1,15
5A_2 4 12 PINL 1 inlay 4 43 51 541,22 1680,00 1,22
5A_2 5 12 PINL 1 inlay 4 43 51 525,46 1600,00 1,16
Double needle bed samples
Nr. Test Gauge
(npi) Structure
GB 1: 1
st-2
nd cycle
(# needles lapped)
GB 2: 1
st-2
nd cycle
(# needles lapped)
# Wales per 10 cm
# Courses per 10 cm
Weight (g/m
2)
AP (l/dm
2/min)
Thickness (mm)
6A 1 12 DNDF 1-2 1-2 36 35 770,80 1230,00 3,22
6A 2 12 DNDF 1-2 1-2 36 35 802,25 1210,00 3,10
6A 3 12 DNDF 1-2 1-2 36 35 792,46 1290,00 3,24
6A 4 12 DNDF 1-2 1-2 36 35 831,59 1230,00 3,32
6A 5 12 DNDF 1-2 1-2 36 35 829,86 1230,00 3,31
7A 1 12 DNDT 2-2 2-2 44 36 1077,64 1100,00 4,41
7A 2 12 DNDT 2-2 2-2 44 36 1065,45 1060,00 4,15
7A 3 12 DNDT 2-2 2-2 44 36 1046,38 1020,00 4,25
7A 4 12 DNDT 2-2 2-2 44 36 1059,75 1060,00 4,24
7A 5 12 DNDT 2-2 2-2 44 36 1066,56 1090,00 4,25
8A 1 12 DNDC 2-2 2-2 35 36 919,65 1300,00 3,59
8A 2 12 DNDC 2-2 2-2 35 36 927,78 1310,00 3,49
8A 3 12 DNDC 2-2 2-2 35 36 861,38 1350,00 3,57
8A 4 12 DNDC 2-2 2-2 35 36 886,30 1270,00 3,43
8A 5 12 DNDC 2-2 2-2 35 36 914,27 1330,00 3,49
9A 1 12 DNPI 1-1 inlay 4 74 36 1030,26 1010,00 4,33
9A 2 12 DNPI 1-1 inlay 4 74 36 941,37 966,00 4,32
9A 3 12 DNPI 1-1 inlay 4 74 36 1080,40 1000,00 4,25
9A 4 12 DNPI 1-1 inlay 4 74 36 980,47 1000,00 4,24
9A 5 12 DNPI 1-1 inlay 4 74 36 1055,10 1020,00 4,23
85
Tensile characteristics
Single needle bed samples
Nr. Test Wale_BF
(N)
Course_BF
(N)
Wale_EB
(%)
Course_EB
(%)
Wale_E49
(%)
Course_E49
(%) Ratio w/c
REF 1 188,91 277,34 133,12 81,34 118,82 66,72 1,78
REF 2 174,08 240,01 132,99 79,24 121,24 68,29 1,78
REF 3 146,19 237,07 130,77 80,93 119,89 69,94 1,71
REF 4 121,98 241,69 130,79 83,09 120,34 65,87 1,83
REF 5 119,85 279,79 131,56 82,82 120,76 67,26 1,80
1B 1 390,32 115,51 31,28 184,93 24,38 168,29 0,14
1B 2 325,55 101,95 31,03 182,78 23,95 170,81 0,14
1B 3 381,64 116,44 33,52 185,78 22,72 169,42 0,13
1B 4 263,45 109,85 30,70 186,70 23,41 172,25 0,14
1B 5 344,99 94,01 28,43 186,53 24,01 172,64 0,14
2B 1 272,94 51,55 46,59 161,08 39,63 154,83 0,26
2B 2 224,13 51,51 45,17 163,31 38,35 153,34 0,25
2B 3 192,96 51,33 46,37 154,38 38,82 156,62 0,25
2B 4 176,58 51,25 44,02 158,64 37,56 155,11 0,24
2B 5 201,58 56,47 45,66 159,88 38,58 155,89 0,25
3B 1 204,59 246,55 37,76 58,59 32,82 48,48 0,68
3B 2 228,77 260,92 38,40 60,61 32,93 49,81 0,66
3B 3 216,26 240,10 39,76 61,98 31,45 48,32 0,65
3B 4 202,91 243,73 39,03 62,67 31,33 49,02 0,64
3B 5 228,66 260,66 39,65 60,23 33,79 48,28 0,70
1A 1 401,48 16,18 40,47 248,58 32,74 - -
1A 2 474,66 19,39 40,37 247,69 32,39 - -
1A 3 442,97 16,05 39,95 247,02 33,16 - -
1A 4 375,55 17,86 40,23 246,90 31,37 - -
1A 5 229,55 18,41 37,00 250,02 31,92 - -
2A 1 294,26 13,66 42,72 236,47 35,97 - -
2A 2 331,90 15,67 42,73 233,84 34,52 - -
2A 3 287,11 16,30 41,82 234,27 33,69 - -
2A 4 260,34 18,96 42,44 238,91 36,03 - -
2A 5 252,32 31,90 43,63 250,04 35,43 - -
3A 1 274,59 133,64 43,89 130,88 38,46 113,57 0,34
3A 2 290,52 125,00 45,55 127,34 39,21 113,11 0,35
3A 3 322,18 123,72 46,10 126,69 39,85 110,38 0,36
3A 4 280,04 141,10 47,24 124,41 40,37 112,08 0,36
3A 5 252,08 135,93 47,93 125,45 40,06 111,68 0,36
86
4A 1 322,65 156,05 30,92 101,75 27,39 89,22 0,31
4A 2 319,41 169,05 31,36 99,60 26,95 87,78 0,31
4A 3 299,35 176,69 30,54 99,64 26,58 88,62 0,30
4A 4 315,22 171,61 31,02 97,75 27,43 88,34 0,31
4A 5 264,43 171,70 32,21 101,91 25,81 87,92 0,29
5A_1 1 200,00 114,69 31,04 74,54 24,49 68,03 0,36
5A_1 2 189,96 163,07 30,47 76,00 25,57 69,92 0,37
5A_1 3 217,73 122,02 29,59 75,50 25,04 70,45 0,36
5A_1 4 228,68 123,69 29,85 73,19 26,16 69,31 0,38
5A_1 5 219,66 154,10 27,84 75,23 25,89 68,83 0,38
5A_2 1 276,67 174,74 24,24 50,61 19,27 43,15 0,45
5A_2 2 233,36 232,07 24,25 54,73 19,03 46,06 0,41
5A_2 3 242,98 212,29 23,76 54,27 20,19 45,62 0,44
5A_2 4 231,33 232,49 24,10 54,54 20,68 45,28 0,46
5A_2 5 216,05 220,83 23,14 53,92 19,82 44,72 0,44
Double needle bed samples
Nr. Test Wale_BF
(N)
Course_BF
(N)
Wale_EB
(%)
Course_EB
(%)
Wale_E49
(%)
Course_E49
(%) Ratio w/c
6A 1 394,81 28,66 67,07 246,73 57,81 - -
6A 2 357,07 34,98 67,90 250,13 58,29 - -
6A 3 413,98 33,08 68,43 248,73 56,56 - -
6A 4 464,56 31,80 68,87 249,94 57,13 - -
6A 5 475,71 36,34 71,06 250,03 57,94 - -
7A 1 442,30 17,95 83,22 250,03 66,41 - -
7A 2 434,57 17,53 83,32 250,03 67,51 - -
7A 3 439,95 17,48 83,60 250,00 65,04 - -
7A 4 438,17 20,64 81,76 250,03 66,24 - -
7A 5 432,04 46,38 84,52 250,03 66,67 - -
8A 1 370,03 140,60 80,32 220,58 66,82 185,68 0,36
8A 2 375,60 126,99 79,77 220,73 66,27 187,35 0,35
8A 3 388,09 140,53 81,03 225,75 65,59 189,37 0,35
8A 4 349,29 143,81 81,51 225,98 66,46 188,22 0,35
8A 5 357,94 138,50 81,13 229,42 67,91 187,12 0,36
9A 1 224,99 17,18 14,50 182,77 12,39 - -
9A 2 233,46 25,94 14,09 191,60 13,11 - -
9A 3 246,69 22,48 11,76 196,39 10,92 - -
9A 4 207,70 21,59 15,92 188,95 14,23 - -
9A 5 201,11 33,62 15,00 205,59 13,88 - -
87
High temperature properties
Single needle bed samples
Nr. Structure Test OX_Wale_BF
(N) OX_Course_BF
(N) OX_Wale_EB
(%) OX_Course_EB
(%) Sagging (mm)
REF Weft ½ 1 108,62 158,16 89,53 62,80 21,18
REF Weft ½ 2
REF Weft ½ 3
REF Weft ½ 4
REF Weft ½ 5
1B TRTR 1 257,51 50,76 23,09 104,93 -
1B TRTR 2
1B TRTR 3
1B TRTR 4
1B TRTR 5
2B TRPI 1 142,58 33,20 29,63 66,89 -
2B TRPI 2
2B TRPI 3
2B TRPI 4
2B TRPI 5
3B SAPI 1 161,14 201,95 26,30 26,14 -
3B SAPI 2
3B SAPI 3
3B SAPI 4
3B SAPI 5
1A TRTR 1 305,27 46,03 28,87 186,61 -
1A TRTR 2
1A TRTR 3
1A TRTR 4
1A TRTR 5
2A TRPI 1 188,00 35,28 27,49 40,66 -
2A TRPI 2
2A TRPI 3
2A TRPI 4
2A TRPI 5
3A COPI 1 201,86 60,88 34,81 68,24 -
3A COPI 2
3A COPI 3
3A COPI 4
3A COPI 5
4A SAPI 1 215,32 120,76 23,95 62,57 25,36
4A SAPI 2
4A SAPI 3
88
4A SAPI 4
4A SAPI 5
5A_1 PINL 1 121,09 108,83 16,19 47,34 27,99
5A_1 PINL 2
5A_1 PINL 3
5A_1 PINL 4
5A_1 PINL 5
5A_2 PINL 1 138,34 117,07 16,34 33,49 25,07
5A_2 PINL 2
5A_2 PINL 3
5A_2 PINL 4
5A_2 PINL 5
Double needle bed samples
Nr. Structure Test OX_Wale_BF
(N) OX_Course_BF
(N) OX_Wale_EB
(%) OX_Course_EB
(%) Sagging (mm)
6A DNDF 1 258,68 79,40 47,78 163,14 32,03
6A DNDF 2
6A DNDF 3
6A DNDF 4
6A DNDF 5
7A DNDT 1 328,74 91,54 56,60 220,57 41,54
7A DNDT 2
7A DNDT 3
7A DNDT 4
7A DNDT 5
8A DNDC 1 260,67 83,85 65,44 144,23 38,32
8A DNDC 2
8A DNDC 3
8A DNDC 4
8A DNDC 5
9A DNPI 1 152,46 78,72 39,28 75,49 27,89
9A DNPI 2
9A DNPI 3
9A DNPI 4
9A DNPI 5
89
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