Quality control in the

knitting process

Prof.Dr.S.Kathirvelu

EiTEX, BDU.

1

• Knitted fabric is gaining popularity in the

textile and clothing industry. The demand

for knitted fabrics is increasing:

consumers today are looking for comfort,

fashion and style, which results in ever-

changing demands on the apparel

market.

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The advantage of knitted fabrics is

that they are able to meet consumer

demand for such properties as a

softer feel, good draping quality

and wrinkle recovery.

3AKFM

• Knitted fabric is therefore an ideal material

for manufacturing sportswear, intimate

garments and casual wear as it allows for

stretch and free body movement.

4

• Major Contribution Quality control

• Saving time and money. When a defect occurs,

the knitting machine has to be stopped to correct

the fault, resulting in lost time, which is

uneconomical in the overall production process.

In addition, it may increase the subsequent

remedial costs of the fabric finishing and garment

manufacturing process as a result of defective

fabric.

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• Reducing customer discontent. The knitted

fabric may be rejected by customers if quality

requirements are not met, which will have an

adverse effect on the company’s reputation.

• knitted fabric quality was maintained and

achieved by manual inspection.

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• As a matter of fact, an effective monitoring of

the knitting process is required. Its function is

to avoid or detect the fabric faults as well as

to locate the defect and its causes as soon as

possible in attempts to cut down the

undesirable return of goods and avoiding

productivity and quality losses.

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• Weight per unit area. The mass per unit

area of fabric is measured to determine

the consistency of the fabric weight of

the sampled knitted fabric. The weight

deviations of circular knitted fabric

should not exceed ± 5% from the stated

weight.8

• Courses per centimetre and wale spacing.

Courses and wales per centimetre are

measured by placing a centimetre glass on the

fabric, and counting the number of courses

and wales contained within the area.

• Handles. The feel of the knitted fabric to the

hand, including the softness or stiffness of the

fabric.

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• Elasticity. Each knitted fabric has its own

unique elastic property, which is measured by

bursting strength testing methods.

• knitted fabric quality is usually defined as a

number of loops in a square of prescribed

dimensions (known as loop density) and

represented by the number of loops per

square inch or loops per square centimetre.10

• Loop density is the most important element in

defining knitted fabric quality and is directly

related to

• fabric appearance, weight per unit area,

dimensional stability, fabric weight,

tightness factor, drape and many other

factors.

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• Quality control in the knitting process

• Two main causes of inconsistency that lead to

fabric defects are raw material management

before the knitting process and variation in

parameters during the knitting process.

Therefore, the best way to improve the quality

of knitted fabric is to monitor the knitting

parameters and the knitting conditions.

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• Yarn appearance. The appearance of the yarn directly

affects the appearance of the fabric after the knitting

process. Several factors influence the appearance of the

yarn, including cleanliness, fluffy texture and colour. A

yarn can be labelled as good in appearance when it is

free from impurities, contains a reasonable amount of

projecting strands and has the minimum level of

spinning defects such as short or long yarn slubs.

13

• Yarn count. Yarn count or linear density is used to

express the mass per unit length or length per unit

mass of a yarn. It has a direct influence on the

weight and dimensional stability of the knitted

fabric. The selection of yarn with a proper yarn

count is essential in determining the knitted fabric

quality, since only the correct yarn count gives

optimal knitting performance for a specific machine

gauge and structure.14

• Yarn evenness. This refers to the yarn

irregularity and non-conformity, which directly

affects the knitted fabric quality and the

knitting performance. Yarn evenness is

expressed by mean deviations (U %) or

commonly by the coefficients of variation (CV

%).

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• Yarn elasticity. An elastic yarn is easier to

knit and results in fewer knitting faults

such as drop stitches, holes and bad

selvedges. In a general case, wool yarns

perform better than cotton yarns in

knitting due to the higher elastic

property of wool.16

• Yarn twist. The direction of yarn twist plays a

decisive role in knitted fabric quality. The yarn

twist should be in the same direction, either S

or Z in knitting the same fabric roll. In

addition, the amount of twist has a significant

influence on yarn torque. Excessive or

improper yarn twist causes distortion of the

finished knitted fabric, i.e. skewed fabric.

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• Yarn friction. The coefficient of yarn friction

should be set as low as possible in the knitting

process. The higher the yarn friction, the higher

the knitting tension will be. When the knitting

tension is greater than the yarn strength, the yarn

will break or cause a fabric fault. In practice, the

yarn friction can be reduced by adding lubricants.

A good waxed yarn can reduce the coefficient of

friction by nearly 50%.

18

• Quality control during the knitting process

• In order to produce faultless knitted fabric,

precision in the settings of the knitting

machine is essential during the whole knitting

process. The optimal setting of a knitting

machine should be balanced in the following

parameters:

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• Yarn tension before and after the yarn

feeder. The yarn tension should be set at

the minimum prior to the yarn feeder or

with direct feeding (without yarn feeder).

• Fabric take-up tension should be set as

low as possible.

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• Drawing-in of yarn at the cylinder and the

dial. In knitting, a larger distance between

cylinder and dial gives a greater chance of

obtaining a loosely knitted fabric.

• Height of the dial. The tightest setting should

be set between cylinder and dial to ensure the

fabric can freely pass through without being

torn.

21

• Faults resulting from poor cleaning can be due to

the following:

• lint and/or yarn fragments in the camming system,

tricks, needles, or sinkers;

• variation in oil content for machine and needle

track lubrication;

• worn needles, which generally produce length-

direction streaks;

• worn cylinder and/or dial.

22

• Parameters of knitting control

• Loop length

• Loop length is defined as the amount

of yarn used to form one unit loop.

• The loop length is the absolute

quantity of any knitted fabric and is

directly related to loop density.23

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• Stitch density, tightness factor, weight and

thickness are all inversely proportional to the loop

length.

• As a result, it is vital to keep the variations of loop

length to a minimum so that loop length is

maintained uniformly and consistently throughout

the knitting process. The only effective and reliable

way to ensure the consistent loop length is by

means of a positive yarn feeding system.

25

• Positive yarn feeding

• Positive yarn feeding is a system often fitted on circular

knitting machines to positively drive the yarn at a fixed rate

relative to the surface speed of the needle cylinder. It is

currently being considered as a standard quality control

installation in all modern circular knitting machines. The main

function of this system is to regulate the yarn knitting tension

to a desired value, by enabling a predetermined length of

yarn to be fed positively and consistently to all the needles for

each revolution of the machine cylinder.

26

• The predetermined length of yarn is commonly

referred to as course length; that is the length of

yarn per needle or stitch multiple by the

number of needles knitting per revolution in the

cylinder or cylinder and dial. The positive

feeders aim to control the fabric quality by

making the course length align with the desired

yarn delivery speed.

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• Knitting tension

• The loop length on knitted fabric is inversely

proportional to the knitting tension. Therefore, a

uniform loop length can only be produced on a knitting

machine with regular knitting tension. By carefully

controlling the knitting tension, the variations in loop

length can be minimized and the quality of knitted

fabric improved. Some parameters affecting knitting

tension include yarn-unwinding tension, package

diameter and package density:

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Yarn unwinding method. The yarn knitting

tension changes during the yarn unwinding

process as the yarn is pulled out from the

top package layer by layer. This creates a

higher unwinding tension at the bottom of

the cone, while a lower tension occurs at

the shoulder level.

• Package size refers to the package diameter.

The package diameter changes constantly from

full size to empty when unwinding the yarn.

The unwinding tension changes upon the

package diameter. The unwinding tension on a

full size package is much lower than that on

those with a small package size or on empty

ones.

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• Package density. Package density has a

proportional relationship to yarn unwinding

tension as well as to the knitting tension.

When unwinding yarns from a high-density

package, higher yarn frictions between coils

are produced, resulting in higher unwinding

tension.

31

• Tightness factor indicates the relative tension

of a plain-weft knitted structure. It is defined

as the ratio of the area covered by the yarn in

one loop to the area occupied by that loop. In

simplified formula expression:

• tightness factor (K) in SI units,

• where Tex is the unit of yarn count and L is the

stitch length in millimetres.

32

• Yarn input tension

• Yarn input tension (YIT) is used to tune

the feeding of the yam into the knitting

zone. The optimal YIT ranges from 2 to 4

grams. An excessive value in YIT results in

yarn breaks and machine downtime,

both of which are uneconomical.

33

• YIT can be used as a means of process

control, so that defects can be prevented or

quickly detected. The variation of YIT is an

ideal indicator to reflect the formation of a

loop. An exceedingly high yarn tension can

arise from improper threading up, dirt and

fluff in the yarn path, tilted cones, poorly

wound cones or incorrectly set tensioners.

34

• Yarn length per stitch:

• The length of yarn in one stitch is another

important factor which permanently affects

the quality of a knitted fabric. The yarn length

per stitch determines the dimensions and

stitch density of the fabric. It is therefore

essential to keep the variations of the loop

dimension to a minimum.35

• Take-down tension : It is a well-known

fact that the take-down tension can materially

affect the dimensions of knitted fabric and so the

fabric quality will deteriorate. An excessively high

take-down tension would induce undesirable

stretching in the length direction of fabric,

resulting in a narrower fabric with a lower value

of courses per unit length.

36

• Machine gauge thus plays an influential role in

the choice of yarn count and can have an effect

on fabric properties such as weight and

appearance. Therefore, it is important to obtain

an optimal balance of yarn count and machine

gauge in order to ensure the best knitting

performance for a specific machine gauge and

structure, with high machine efficiency and

minimum fabric fault rate.

37

• STARFISH – Engineered knitted program for cotton

circular knits. The name STARFISH is contracted

from the phrase ‘START as you mean to FINISH’.

STARFISH is a computer program that resembles a

simulator. It models the influence of the major

variables in the production and processing of

circular knitted cotton fabrics and calculates their

effects on the final properties of the finished

fabric.

38

• Using STARFISH, the most appropriate

combination of yarn count, stitch length and

type of knitting machine necessary to deliver

the desired combination of properties in the

finished fabric can be calculated in an efficient

• and accurate way, without using production

time or materials to excess.

39

• STARFISH helps knitting manufacturers to

rapidly develop new fabric qualities or

optimize existing qualities in an effective

way without recourse to expensive trial

and error sampling.

40

• In addition, it also helps to optimize the

development process and make direct

savings in development time and costs. It

further helps to optimize the process

management and production control

procedures, in order to improve product

quality and consistency.41

• Mayer and Cie MCTmatic Quality MonitoringSystem

• The MCTmatic system uses computer-

controlled adjustment and a processor

controlled braking system installed on the

knitting machine. It is a monitoring system for

setting and altering the yarn delivery and

tensioning.

42

• The MCTmatic system allows the motors to be

set for feed wheel, central stitch adjustment

and fabric take-down. The MCTmatic system is

very useful in ensuring knitted fabric quality

throughout the production process. When non-

conformity is detected, the knitting machine

will stop and the knitted faults will be

indicated on the MCTmatic display panel.

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• Knitting faults can be categorized into horizontal

and vertical components. The first category is

mainly due to yarn inconsistencies and

inappropriate raw material management. The

second category usually results from

inappropriate knitting conditions, especially

incorrect machine settings and maintenance,

poor monitoring of the machine performance

and improper yarn delivery.

44

• In order to eliminate or reduce knitting faults,

manufacturers endeavour to set up a standard

quality control method with the aid of fault

detecting devices. Examples include a needle

detector to find closed latches for rising

needles, and a yarn breakage detector to

show up broken yarn during production.

45

• The online monitoring system is based on the

principle of measuring the important knitting

parameters in real-time process and

comparing these data with predetermined set

values for a particular product quality.

46

• The monitoring system offers the following

parameters during operations:

• yarn input tension (YIT),

• speed of knitting machine (m/s),

• yarn delivery speed (m/min),

• yarn consumption per course,

• fabric production in kg,

• tightness factor (K) and loop length.

47

• YIT can be measured by a set of sensors. The

measurement system is composed of a force

sensor, which is installed close to the feeding

zone and encoders and an optical sensor.

Next, the monitoring software is connected

and primarily used for analysing the

waveforms of the YIT, such as the

MonitorKnit.48

• The resulting waveform is analysed by a signal

processing technique, which produces a signal

when a knitting fault is formed. Therefore, by

inspecting and comparing waveforms resulting

from normal and abnormal knitting, fabric faults

and malfunctioning of the knitting machines can

be quantified and identified in an accurate way,

which constitutes a major step in reducing repair

time.

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• The other online monitoring system is that of

fabric image acquisition. Samples of different

knitted defects are acquired by image-capture

equipment. In image processing, the sensed

image (e.g. by a video camera) is translated into

a digital image (i.e. a two-dimensional array of

numbers or grey levels) by an analogue-to digital

• converter.

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• The digital image can then be analysed

by using image processing techniques to

reflect the knitting faults and defects.

The defects can be analysed and

identified by the image processing

algorithms and filters.

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• The imperfections of knitted fabric may be

due to faulty yarns, malfunctioning knitting

machine parts or poor finishing. The defects

in knitting construction are considered here in

terms of appearance and nature. The major

defects in knitted fabric can be divided into (1)

bands and streaks and (2) stitch defects.

52

• Barré effect: A barré effect has the

appearance of a stripe with shaded edges. It

is a continuous visual barred or striped

pattern parallel to the yarn direction. Barré is

caused by inconsistencies in fibre properties,

yarn characteristics, knitting parameters and

processing.

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Knitting fault: barré effect.

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• Skewing: Skew can be defined as a fabric

condition occurring when the knitted wales

and courses are angularly displaced from the

ideal perpendicular angle. The skewing effect

is seen as a line or design running at a slight

angle across the cloth. It is an inherent defect

mainly caused by yarn twist parameters.

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• Firstly, the high yarn twist levels result in

yarns that exhibit high inherent torsion

energy as a result of their great tendency

to untwist. The yarn exhibits significant

snarling effect, high liveliness and

consequently, poor fabric dimensional

stability.

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• Second, the effect of fabric skewness is caused

by yarn twist direction. Yarn twist direction

depends on the direction of machine rotation

• For machines rotating in a counterclockwise

• direction, yarns made using Z twist direction

yielded fabric of lower spirality than those

made using S direction.

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• Bowing: A bow effect is observed when the

course line form an arc across the width of

knitted fabric. It is defined as an excessive

curvature of the courses in a knitted fabric that

may or may not extend over the full width.

Bowing is the distortion caused by a faulty take-

up mechanism on the knitting machine. It can

also be caused by incorrect feeding during the

finishing process.

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• Streak or stop mark: A straight

horizontal streak or stop mark in the

knitted fabric is due to a difference in

tension in the yarns, caused by the

machine being stopped and then

restarted.

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• Needle line: Needle line is a vertical creak that

is different from the adjacent normal wales

(Fig. 9.5). This is caused by needle movement

due to a tight fit in its slot or a defective

sinker. It can also be caused by a misaligned or

broken needle, which will produce distorted

stitches.

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• Dropped stitch: This is a lost stitch caused

either by the yarn carrier not having been set

properly or the stitch having been knitted too

loosely. This may be attributed to improper

setting of the yarn feeder or insufficient yarn

tension. To solve the dropped stitch problem,

re-adjust the yarn feeder or increase the yarn

tension.64

• Cloth press off: This defect results from broken

yarn coming away from the knitting needles

during knitting. A serious press off can be a big

section or the entire tube of circular knitted

fabric coming off the knitting needles. Press

off often occurs accidentally with yarn

breakage(s).

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• Cockled or puckered: The knitted fabric

appears wavy when spread flat. This is

difficult to detect during visual inspection on

an inspection machine with fabric under roller

tension. It is usually due to uneven stitches,

stitch distortion, uneven yarn relaxation or

shrinkage.

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• Crack or hole: Large holes could be

caused by weak places in the yarn,

resulting in the yarn breaking during

loop formation. Small holes are often the

result of a broken yarn before (or after) a

knot or splice, since the yarn end with

the knot sits tightly in the last stitch.

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• Tucking (bird’s eye): This appears as a small

aperture occurring occasionally in a wale (Fig.

9.8). It is generally caused by unintentional

tucking from a malfunctioning needle, with two

small, distorted stitches, side by side. Another

reason is incorrect dial settings. If the dial is set

too high, the dial needles do not support the

fabric, which then pulls the fabric up. Tucking is

also caused by incorrect feeding during finishing.

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• Float: A float is defined as a length of yarn

extending unbound over a number of wales

with which it should be intermeshed. This is

caused by a missed stitch, which can be due

either to the failure of one or more needles to

rise to catch the yarn, or to the yarn

prematurely casting off from the knitting

elements.

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• The important basis of good quality control in knitting

rests with loop consistency, i.e. in achieving equal

loop length and producing regular knitted structures.

Modern technology and developments have been able

to facilitate the installation of positive and storage

feeding devices to achieve this purpose. In summary, a

good quality control system should start with the

selection of good quality yarn materials and the

proper maintenance of machine parts.

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