Comparative study of printability analysis on pvc cast and pet g films

International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN

0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 4, May – June (2013), © IAEME

208

COMPARATIVE STUDY OF PRINTABILITY ANALYSIS ON PVC

CAST AND PET-G FILMS

Akshay V. Joshi 1

1 Department of Printing Engineering, P. V. G’s College of Engineering and Technology, 44,

Vidyanagari, Shivdarshan, Pune, Maharashtra-411009, India

Swati Bandyopadhyay 2

2 Department of Printing Engineering, Jadavpur University, LB Block, Plot No. 8, Salt Lake Bypass, Sector III, Salt Lake City, Kolkata, West Bengal 700098, India

ABSTRACT

The effect of process parameters on printability indicators such as density, dot area,

tone curve and dot structure and defects such as dot skips were investigated on shrink films such as PVC cast and PET-G (Polyethylene terephthalate-Glycol). These flexible substrates were printed on a pilot scale gravure machine with varying process variables like viscosity, speed, hardness and line screen. Since even a slight change in printability indicators can cause a sense of ambivalence in customers and subsequently lead to product rejection, hence the need for optimization is so important. A general full factorial method was designed with above-mentioned process variables and analyzed by ANOVA (Analysis of Variance), Main and Interaction effects. Line screen ruling and hardness were the most influential factors in minimizing the dot skips. A reduction of defects by 79% and 87% was observed in PVC and PET-G respectively. The two substrates exhibited similar trends for printability indicators. KEYWORDS: Shrink Films, Process Variables, Printability, Dot Skips, ANOVA 1. INTRODUCTION

Gravure printing is a process of transferring ink onto a substrate via an engraved

cylinder. PVC Cast and PET-G films that find wide applications in various products such as cosmetics, retail products etc. are majorly printed using this process. The global consumption of these films in the form of shrink sleeves is increasing at an unprecedented rate. These films not only differ in terms of cost but also exhibit significant differences in printability. The

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ISSN 0976 - 6480 (Print) ISSN 0976 - 6499 (Online) Volume 4, Issue 4, May – June 2013, pp. 208-221 © IAEME: www.iaeme.com/ijaret.asp Journal Impact Factor (2013): 5.8376 (Calculated by GISI) www.jifactor.com

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growing aesthetic conscious market has brought with itself a grueling challenge for the printers to reproduce immaculate print quality on these films with minimum dot skips or missing dots. Numerous experiments have been performed to investigate the relationship between printability and missing dots with process parameters on porous substrates. However, the presence of these defects on plastic films viz. PVC cast and PET-G has not been explored much hitherto. The aim of this research work is to fill up the gap and determine an optimal combination of parameters that minimize dot skips and render good printability to shrink films. The occurrence of missing dots is due to the size of depression on the substrate or its roughness that obstructs the ink flow from the diameter of the cell. An increase in viscosity can also result in missing dots by reducing the amount of ink transferred [1, 2, 3]. A need to quantify this defect was felt in order to control it. A machine vision algorithm that gives accurate missing dot count by adapting to the printing parameters and lighting conditions was developed. [1]. A reliable condition that could quantitatively predict the severity of missing dot counts on a commercial image analysis system was also proposed. Missing dots were selected by adjusting the gray level threshold for feature detection [4]. A GRI press with soft rubber roller was also found to simulate this count very well [5]. A very good inter-relationship also exists between surface topographical parameters like depth, volume, slope and concentration of crater on the substrate and the missing dot count. However, some non-topographical features such as electric properties of the substrate, fibre and filler distribution, ink absorbency, compressibility and paper independent features like ink or machine can also influence missing dots [6]. Moreover, printing impression, press speed and doctor blade pressure affects speckle frequency. A wear in the doctor blade system can further aggravate missing dots [7]. Missing dots are observed in direct gravure with enclosed doctor blade (EDB), though direct gravure retains the circularity of dots. Direct gravure shows the best printability on PET film [8]. Super Calendered paper shows minimum number of missing dots despite its high roughness value. Coated paper with starch sizing has more missing dots than the samples without starch sizing. The roughness of a substrate alone cannot predict missing dots. However, a good correlation can be achieved when compressibility is also included as a factor [9]. Different cell structures have a strong effect to ink transfer and the characteristics of missing dots. E.S.A (full form) shows significant improvements, in terms of ink release for both electromechanical and laser engraved cylinders [8]. A smooth substrate having low bulk is more suitable for a smoother cylinder whereas a higher bulk substrate gives lesser missing dots under rough cylinder [10]. Printability is largely dependent on the type of substrate used. Furthermore, depressions on unprinted paper surface cause significantly increased probability of abnormal printability [11]. This study investigates the effects of gravure process parameters on dot skips and will help in determining an optimal combination of these parameters that minimizes dot skips on shrink films. 2. MATERIALS AND EXPERIMENTAL PROCESS

2.1.1. Substrate and Inks

The flexible substrates chosen were Polyethylene terephthalate Glycol (PETG) and Poly Vinyl Chloride Cast (PVC) both 40µm thickness. Solvent based Process Black Vinyl ink was acquired and diluted with a mixture of Ethyl Acetate and Toluene, used in the ratio 1:1.



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2.1.2. Gravure Press

A pilot scale gravure machine with a maximum printing speed of 120 m/min., equipped with real-time embedded modules to control web tension, an E.S.A system, and a pneumatic impression system was employed for the purpose of printing. The gravure cylinder was electronically engraved with 175 and 200 LPI line screen with a cell depth of 42 µm and 36 µm respectively.

Fig. 1: Gravure Press (left) and Cells at 175 LPI (mid) and 200 LPI (right)

2.1.3. Measurements and Characterization The viscosity of the ink for the entire run was continuously monitored and measured

using a B-4 Ford cup. The ink behavior was studied using a viscometer (Brookfield Engineering Labs, INC., LVDV, spindle No.61) and a ULA adapter. A Gretag-Macbeth Spectro-Eye was used to measure density and dot area. The density of the print was measured to check the ink lay-down on the substrate. The ink spread on the substrate was evaluated by dot area. The quality of print in halftone area was quantified by dot area measurement. The tone curve provided information on linear relation between dot and density. Fibro D.P.M software was used to facilitate accurate measurements of dot skip count in the 30 % patch. The dot structure was also analyzed at 30% patch of the step wedge and processed through Dexel Imaging software V 2.4.4.

Fig. 2: Dot Skips: Un-processed (left) and processed (right)



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2.2 Experimental Process

A monotone layout was designed that comprised of elements such as step wedge, solids, skin tone, logo, surface and reverse text at 175 and 200 LPI line screen. Production runs were conducted for a couple of days with defined set of process parameters to define the baseline for the defects and a target was set to minimize it by 50%. The evaluation was carried out for 25 sheets being considered as sample size. The sheets were cut and marked for dot skips. The number of dot skips was counted on a 30% patch of the step wedge. A general full factorial with two replicates (54 runs per replicate) was designed with three levels for viscosity, speed and hardness and two levels for line screen (Table 1 and Table 2). The experiments were conducted to correlate the above mentioned parameters to each of the output response i.e. dot skip count. The design of experiment (DOE) was analyzed by analysis of variance (ANOVA) to figure out the significant factors affecting the defect. Fisher’s F- test was used in the ANOVA. The best settings minimizing the dot skips were identified through main and interaction plots. The best settings were verified by conducting a press and then checked for its consistency. Furthermore, printability on shrink films was also gauged by indicators such as density, dot area, tone curve and dot structure.

Fig. 3: Elements in the Layout

Table 1: Process Variables

S. No. Variables Notation Unit Levels

Low Mid High

1 Line Screen A LPI 175 - 200

2 Viscosity B sec. 17 19 21

3 Speed C m/min. 80 100 120

4 Hardness D Shore A 60 70 80



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Table 2: General Full Factorial Design for Dot Skips/Sheet

Dot Skip/Sheet -

PVC

Dot Skip/Sheet - PETG

Run

Order A B C D

Replicate

1 Replicate 2

Replicate

1 Replicate 2

1 175 17 80 60 7.4 7.52 1.88 1.6 2 175 19 80 60 3.24 2.12 2.44 2.2 3 175 21 80 60 2.64 1.36 4.44 4.04 4 175 17 80 70 3.92 4.48 1.52 1.16 5 175 19 80 70 4.56 3.24 2.36 1.84 6 175 21 80 70 2.12 0.96 2.64 2.48 7 175 17 80 80 6.04 5.68 0.56 0.32 8 175 19 80 80 3.88 1.92 0.68 0.28 9 175 21 80 80 0.88 0.52 0.92 1.04 10 175 17 100 60 9.52 8 1.96 1.44 11 175 19 100 60 3.96 2.16 2.04 1.84 12 175 21 100 60 2.92 1.32 2.36 2.64 13 175 17 100 70 10.88 7.36 1.6 1.28 14 175 19 100 70 4.08 6.48 1.92 1.52 15 175 21 100 70 2.68 0.96 2.2 1.08 16 175 17 100 80 6.88 7.8 0.4 0.2 17 175 19 100 80 4.28 4.44 0.68 0.2 18 175 21 100 80 1.56 0.8 0.84 0.44 19 175 17 120 60 13.36 13.12 0.84 0.76 20 175 19 120 60 4.76 8.48 1.6 1.44 21 175 21 120 60 4.92 1.44 1.92 1.64 22 175 17 120 70 10.48 15.4 0.76 0.48 23 175 19 120 70 4.52 7.68 1.36 0.96 24 175 21 120 70 4.28 2.16 1.68 0.8 25 175 17 120 80 8.48 7.68 0.36 0.16 26 175 19 120 80 4.64 6.8 0.48 0 27 175 21 120 80 2.32 1.08 0.64 0.2 28 200 17 80 60 16.64 16.64 14.32 13.72 29 200 19 80 60 7.64 6.36 16.2 15.72 30 200 21 80 60 2.64 3.52 18.4 17.64 31 200 17 80 70 11.68 10.2 10.68 8.88 32 200 19 80 70 7.96 7.48 11.72 9 33 200 21 80 70 5.52 5 12.36 9.92 34 200 17 80 80 14.4 13.36 1.52 1.12 35 200 19 80 80 8.68 6.12 1.76 1.64 36 200 21 80 80 2 2.2 3.6 2.96 37 200 17 100 60 24.08 22.68 8.4 7.28 38 200 19 100 60 11.4 11.84 9.96 9.08 39 200 21 100 60 4.48 4 10.36 10.52



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40 200 17 100 70 17.88 22.12 5.72 6.24 41 200 19 100 70 12.76 10.84 7.6 7.16 42 200 21 100 70 7.8 7.36 8.92 8.44 43 200 17 100 80 15.64 15.92 0.76 1 44 200 19 100 80 10.48 8.04 0.88 0.56 45 200 21 100 80 4.6 4.16 1.36 0.92 46 200 17 120 60 28.44 30.36 3.44 2.68 47 200 19 120 60 20.96 20.52 4.88 5.36 48 200 21 120 60 5.28 4.72 6.56 7.44 49 200 17 120 70 17.36 24.8 2.08 1.48 50 200 19 120 70 12.52 11.52 2.72 2.88 51 200 21 120 70 12.28 9.4 5.44 5.12 52 200 17 120 80 22.6 18.16 0.56 0.52 53 200 19 120 80 14.76 8.12 0.64 0.44 54 200 21 120 80 5.28 6.04 0.76 0.52

3. Results and Discussions

A production run on a pilot gravure machine was conducted for five days with defined settings i.e. 175 LPI line screen, 19 sec viscosity, 100 m/min press speed, 70 shore A hardness and 3.5 kg/cm2 pressure. The baseline of 4.544 dot skips/sheet for PVC and 1.528 dot skips/sheet for PETG was defined through the production runs and the target was set to minimize by 50% i.e. 2.272 dot skips/sheet and 0.764 dot skips/sheet for PVC and PET-G respectively.

200175

6

4

2

211917

12010080

6

4

2

807060

Line Screen

Mean

Viscosity

Speed Hardness

Main Effects Plot for Dot Skip/SheetData Means

Fig. 4: Main effect plot for Dot Skip/Sheet – PVC (left) and PET-G (right)

The main effect plot shows similar behavior for PVC and PETG with line screen and hardness as major influential factors while viscosity and speed as important factors in minimizing the dot skip count. A lower number of dot skips occur at 175 LPI, 17 sec viscosity, 120 m/min and 80 shore A hardness while more dot skips occur at 200 LPI, 21 sec, 80 m/min and 60 shore A hardness. At lower line screen, ink volume of a cell is more resulting in higher ink transfer. The lower ink viscosity allows the ink to flow out easily from a cell. The presence of residual ink is more at higher line screen and viscosity leading to cell clogging. These results in uneven ink transfer and increased dot skip counts. At lower speed, the travel time of cylinder dipping back into the ink trough is more. The solvents in the ink

200175

15.0

12.5

10.0

7.5

5.0

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15.0

12.5

10.0

7.5

5.0

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Line Screen

Mean

Viscosity

Speed Hardness

Main Effects Plot for Dot Skip/Sheet_PVCData Means



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may begin to evaporate which increases the viscosity leading to cell clogging. This condition is referred to as drying-in and it causes the ink to remain in the cells rather than flowing together to form a continuous ink film on the web resulting in higher dot skip count. The chances of ink drying in cells are less at higher speeds. The centrifugal force is more at lower viscosity and higher speed leading to more ink transfer from the cell. Also the hard roller generates more nip force at a given pressure allowing more ink to evacuate from the cell.

Table 4: ANOVA for Dot Skips/Sheet – PVC

Source DF Seq SS Adj SS Adj MS F P

A 1 1234.11 1272.64 1272.64 346.66 0.000 B 2 189.98 168.85 84.42 23.00 0.000 C 2 94.68 90.71 45.36 12.35 0.000 D 2 2185.05 2229.46 1114.73 303.64 0.000

A*D 2 296.71 300.59 150.30 40.94 0.000 B*D 4 161.77 157.10 39.28 10.70 0.000 C*D 4 60.36 60.36 15.09 4.11 0.004 Error 85 312.05 312.05 3.67 Total 102 4534.72

S = 1.91603 R-Sq = 93.12% R-Sq(adj) = 91.74%

Table 5: ANOVA for Dot Skip/Sheet – PETG

Source DF Seq SS Adj SS Adj MS F P

A 1 546.986 592.118 592.118 647.88 0.000 B 2 26.212 38.674 19.337 21.16 0.000 C 2 209.904 236.732 118.366 129.51 0.000 D 2 472.190 495.283 247.641 270.96 0.000

A*B 2 4.808 7.701 3.851 4.21 0.018 A*C 2 124.460 134.214 67.107 73.43 0.000 A*D 2 231.856 237.452 118.726 129.91 0.000 B*D 4 8.153 9.110 2.278 2.49 0.049 C*D 4 78.749 78.749 19.687 21.54 0.000 Error 85 77.684 77.684 0.914 Total 106 1781.001

S = 0.955995 R-Sq = 95.64% R-Sq(adj) = 94.56%

The ANOVA table for dot skips/sheet on PVC and PET-G indicates that all the main

factors and their interactions with each other were significant as the p-value are below α value of 0.05. Line screen and hardness were seen to influence dot skips significantly as is evident from a high F- statistic values on both the ANOVA tables. The larger F-statistics with p < 0.05 from the ANOVA table confirms the significance of all the main effects and interaction between line screen, viscosity, speed and hardness on dot skips at 95% confidence level. The high percentage of coefficient of determination (R²) indicates that 93.12% and 95.64% of the variability could be explained by the model for PVC and PET-G respectively.



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211917 12010080 807060

20

10

020

10

020

10

0

Line Screen

Viscosity

Speed

Hardness

175

200

Screen

Line

17

19

21

Viscosity

80

100

120

Speed

Interaction Plot for Dot Skip/SheetData Means

Fig. 5: Interaction plot for Dot Skip/Sheet – PVC

211917 12010080 807060

10

5

010

5

010

5

0

Line Screen

Viscosity

Speed

Hardness

175

200

Screen

Line

17

19

21

Viscosity

80

100

120

Speed

Interaction Plot for Dot Skip/SheetData Means

Fig. 6: Interaction plot for Dot Skip/Sheet – PETG

From the above plot, it is observed that hardness has an interaction with line screen,

viscosity and speed at 95% confidence level for both PVC and PETG films. The interaction plot shows minimum dot skip count per sheet at 175 LPI, 17 sec, 120 m/min and 80 shore A hardness, hence considered as best settings. The best settings were confirmed by conducting a press run and then checked for its consistency by re-running for 5 days (Table 6).



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Table 6: Verification and Consistency Runs for PVC and PET-G

Trials A B C D

Dot Skip/Sheet-

PVC

Dot Skip/Sheet-

PETG

Verification 175 17 120 80 1.200 0.2

Consistency 175 17 120 80 0.952 0.192 The above table shows negligible variation in dot skips/sheet between Verification and Consistency trials for both PVC and PETG films. These trials confirmed that the results were in close agreement with each other for both the films.

Table 7: Production and Verification Run for PVC and PET-G

Trials Dot Skip/Sheet-

PVC

Dot Skip/Sheet-

PET-G

Production Run 4.544 1.528 Verification Run 0.952 0.192

A significant reduction in dot skips/sheet from Production Run to Verification Run

was observed for both PVC and PET-G films. The dot skip count per sheet was reduced to 79% in PVC while 87% in PETG, which surpassed the set target of 50% reduction.

In addition, the effect of gravure process variables on print attributes such as density, dot area, tone reproduction curve and dot structure was investigated. The density of the print was measured to check the ink lay-down on the substrate. The ink spread on the substrate was evaluated by dot area. The quality of print in halftone area was quantified by dot area measurement. The tone reproduction curve provided information on linear relation between dot and density.

Effect of Process Variables on Density

200175

1.76

1.75

1.74

1.73

1.72

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1.76

1.75

1.74

1.73

1.72

807060

Line Screen (LPI)

Mean

Viscosity (sec.)

Speed (m/min.) Hardness (Shore A)

Process Variables Vs. Density - PVC

200175

1.74

1.72

1.70

1.68

1.66211917

12010080

1.74

1.72

1.70

1.68

1.66807060

Line Screen (LPI)

Mean

Viscosity (sec.)


Process Varaibles Vs. Density - PETG

Fig. 7: Effect of Process Variables on Density-PVC (left) and PETG (right)

For both PVC and PETG, higher density was achieved at 175 LPI line screen, 21 sec. viscosity, 80 m/min speed, and 60 Shore A hardness. This is because the lower line screen has a larger cell opening leading to more ink transfer. Furthermore, the presence of high solid content at higher viscosity leads to more ink film thickness. Also the lower speed allows more contact time at the nip and the soft roller creates a wider nip width. This combined effect results in higher ink film thickness. Next, the higher line screen comprises of finer



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cells have a lower ink volume, leading to lesser ink transfer resulting in lower ink density. Also, the presence of higher solvent content at lower viscosity causes the ink to spread more; thereby reducing the ink film thickness. The higher centrifugal forces and shortened dwell time in the nip at higher speed and less nip width of hard roller results in more ink spread on the substrate and lower density.

Effect of Process Variables on Dot Area

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60.0

57.5

55.0

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Mean

Viscosity (sec.)


Process Variables Vs. Dot Area - PVC

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62

60

58

56

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60

58

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Line Screen (LPI)

Mean

Viscosity (sec.)


Process Variables Vs. Dot Area - PETG

Fig. 8: Effect of Process Variables on Dot area-PVC (left) and PETG (right)

The dot area was measured at 50% patch of the step wedge. The dot area was maximum at 175 LPI, 17 sec. viscosity, 80 m/min speed, and 60 Shore A hardness. The lower line screen has a larger cell volume as compared to higher line screen. The ink at lower viscosity contains more solvent content, thus evacuates easily from the cell and spreads on the substrate. The maximum dwell time in the nip at slower speed and the wider nip width of the soft roller results in more spread of the ink onto the substrate. The finer cells at higher line screen has a smaller cell opening and lower ink carrying capacity that results in lower dot area. Also less amount of ink is evacuated from the cell at higher viscosity that reduces the dot area and may result in dot skips. At higher speed, the web remains in the nip contact for a short duration of time leading to less ink transfer, thus resulting in lower dot area. However the above figure indicated that speed has a lesser impact as compared to other process variables.

Effect of Process Variables on Tone Curve

200175

0.930

0.915

0.900

0.885

0.870

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12010080

0.930

0.915

0.900

0.885

0.870

807060

Line Screen (LPI)

Mean

Viscosity (sec.)


Process Variables Vs. Tone Curve - PVC

200175

0.925

0.900

0.875

0.850211917

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0.925

0.900

0.875

0.850

807060

Line Screen (LPI)

Mean

Viscosity (sec.)


Process Variables Vs. Tone Curve - PETG

Fig. 9: Effect of Process Variables on Tone Curve-PVC (left) and PETG(right)



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An Ideal tone curve value is considered as 1. The closer the value to 1, better is the tone reproduction curve, which implies an enhanced overall dot reproduction. From the above graphs, it is visible, that line screen ruling has the most significant impact on the tone curve. This is due to the difference in the cell structures which further cause variation in the dot reproduction and amount of ink transfer. PVC and PETG both have similar results for dot reproduction with the tone curve values being comparable with respect to the process variables. The best tone reproduction curve results were achieved at 175 LPI, 17 sec. viscosity, 80 m/min speed, and 60 Shore A hardness. Effect of Process Variables on Dot Structure

Fig. 10: Raw Image (left) and Processed Image (right)

To analyze the dot structure, 30% patch of the step wedge was captured using a

microscope at 200X magnification. The captured images were processed through Dexel Imaging V 2.4.4 software to calculate critical aspects of the dot like area, circularity and perimeter. The effect of process variables on dot fidelity for both PVC and PET-G were analyzed.

Table 8: Effect of Line Screen (A) on Dot Structure

Vari-

able

Mean

Area

Mean

Circularity

Mean

Perimeter

A PVC PETG PVC PETG PVC PETG

175 4728.1 5073 0.8275 0.8312 262.37 271.347

200 2853.9 3091.1 0.8361 0.8373 201.36 213.433

As can be seen in Table 8, line screen has a major impact on the mean dot area. This

is because as the line screen increases, the number of dots in a given area increases. Due to the need to accommodate more dots in a given area, the dot size is reduced, causing a reduction in the overall dot area. Further, line screen does not seem to have a significant effect on circularity, with both PVC and PET-G maintaining their overall roundness. However, perimeter shows significant variation, which indicates that as the dot becomes smaller with finer ruling, the dot perimeter will decrease. Also, it is evident that for both the line screens, PET-G has a larger mean dot area, mean dot circularity and mean dot perimeter as compared to PVC.



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Table 9: Effect of Viscosity (B) on Dot Structure

Vari-

able

Mean

Area

Mean

Circularity

Mean

Perimeter

B PVC PETG PVC PETG PVC PETG

17 4024.3 4279.2 0.8333 0.8321 241.04 247.608 19 3741.4 4049.8 0.8328 0.8367 232.2 241.435 21 3607.1 3917.2 0.8292 0.8339 222.37 238.126

Viscosity has a major impact on the dot area. The dot area increases as the viscosity

reduces. This is due to the fact that inks at lower viscosity are more fluid and spread easily thereby increasing the size of the dot. At higher viscosity i.e. 21 sec there is less fluidity hence lower spreading. The viscosity also has an effect on dot circularity. The circularity decreases with an increase in viscosity for PVC film. However for PETG the circularity increases with an increase in viscosity and after reaching to a point the circularity drops down. This is because the ink release from a cell is less at higher viscosity. The perimeter also decreases with an increase in viscosity for both PVC and PETG. This is due to the increased difficulty in evacuating the ink from the engraved cells. In comparison to PVC, the dot reproduced on PET-G has comparatively larger values of area, circularity and perimeters at all the viscosities.

Table 10: Effect of Speed (C) on Dot Structure

Vari-

able

Mean

Area

Mean

Circularity

Mean

Perimeter

C PVC PETG PVC PETG PVC PETG

80 3808.52 4038.9 0.8302 0.8328 228.68 242.414

100 3788.44 4117.6 0.8339 0.832 233.5 242.421

120 3775.98 4089.7 0.8313 0.838 233.41 242.336

Speed did not have a major impact on dot area, circularity and perimeter. PETG however has a larger dot area compared to PVC. This is due to the higher surface energy which causes the ink to spread easily. Circularity for both PVC and PETG films is similar indicating a consistent dot reproduction across the speed range. The dot perimeter on both the films increases with increase in speed but reduces thereafter, with PETG showing a larger variation.

Table 11: Effect of hardness (D) on Dot Structure

Vari-

able

Mean

Area

Mean

Circularity

Mean

Perimeter

D PVC PETG PVC PETG PVC PETG

60 3835.57 4239.7 0.8351 0.8301 235.17 247.19

70 3819.67 4106.6 0.8324 0.8364 231.72 242.33

80 3717.7 3899.9 0.8279 0.8363 228.71 237.64



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Hardness can be seen to have a major impact on the dot structure. Increasing hardness tends to reduce the dot area and perimeter for both the films. This is due to the smaller nip width which reduces the contact area between the cylinder and substrate. The circularity of both the films across all the hardness remains relatively constant showing that hardness of impression roller does not affect the roundness of the dot. The dots reproduced on PET-G displays higher mean area, mean circularity and mean perimeter at all hardness values, as compared to PVC Cast.

4. CONCLUSION

A general full factorial was designed and ANOVA was used to study and compare the

effect of line screen, viscosity, speed and hardness on dot skips for PVC and PET-G substrate. The results suggest that the effects of the main parameters and the interaction between them are significant with line screen and hardness as the influential factors minimizing the dot skips. The high percentage of coefficient of determination (R²) indicates that 93.12% and 95.64% of the variability could be explained by the model for PVC and PET-G films respectively. This study also found the optimal parameters minimizing the dot skips as 175 LPI, 17 sec, 120 m/min and 80 shore A. The dot skips count for PETG was minimized by a massive 87% as compared to 79% for PVC.

Furthermore, the two films were compared against their printability aspects. It was observed that a higher density was achieved at 175 LPI line screen, 21 sec. viscosity, 120 m/min speed and 60 Shore A hardness for both PVC cast and PET-G. However, since 60 Shore A compromises the quality in terms of dot skips, a roller with 80 Shore A hardness is more appropriate. A viscosity of 17 sec exhibits a comparable behavior as that of 21 sec viscosity with a minor density difference of 0.0392 and 0.0584 between the two for PVC and PET-G respectively. Furthermore, the best tone curve and higher dot area was achieved at 175 LPI, 17 sec. viscosity, 80 m/min speed, and 60 Shore A hardness. On similar lines, a speed of 120 m/min and hardness of 80 Shore A are also seen to perform optimally here. A negligible difference in tone curve values of 0.0107 and 0.012 were observed on PVC and PET-G respectively. Line screen and viscosity were seen to have a major impact on dot area. The circularity is unaffected by any of the studied parameters with both the films retaining their dot roundness. The dot perimeter is affected by all the parameters for PET-G while a consistent perimeter is observed for PVC at higher hardness. Furthermore, PET-G exhibited a higher mean dot area, mean dot circularity and mean dot perimeter for all the process parameters.

Thus, a configuration of 175 LPI, 17 sec, 120 m/min and 80 shore A hardness is seen to perform optimally in terms of lesser defects and good dot fidelity. 5. ACKNOWLEDGEMENT

The authors are grateful to Manish Packaging (Surat), Royal Inks (Surat), Acuprint

Systems Pvt. Ltd. (Navi Mumbai) and K. D. Joshi (Pune) for donating substrate, ink, cylinder and impression rollers for the project. The authors are thankful to Mr. Anirudha Khaddakar, Q.C Services for his timely support.



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Comparative study of printability analysis on pvc cast and pet g films

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