Docstoc

COMPARATIVE STUDY OF PRINTABILITY ANALYSIS ON PVC CAST AND PET-G FILMS

Document Sample
COMPARATIVE STUDY OF PRINTABILITY ANALYSIS ON PVC CAST AND PET-G FILMS Powered By Docstoc
					  International Journal of Advanced Research OF ADVANCED RESEARCH IN
  INTERNATIONAL JOURNAL in Engineering and Technology (IJARET), ISSN
  0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 4, May – June (2013), © IAEME
             ENGINEERING AND TECHNOLOGY (IJARET)

ISSN 0976 - 6480 (Print)
ISSN 0976 - 6499 (Online)
                                                                        IJARET
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)
                                                                        ©IAEME
www.jifactor.com




        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

                                                208
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

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.



                                               209
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

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)



                                            210
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

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



                                             211
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

               Table 2: General Full Factorial Design for Dot Skips/Sheet
                                     Dot Skip/Sheet -         Dot Skip/Sheet - PETG
                                          PVC
   Run                           Replicate                    Replicate
             A   B     C    D                Replicate 2                  Replicate 2
  Order                               1                           1
    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

                                           212
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

               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.

                          Main Effects Plot for Dot Skip/Sheet_PVC                                                  Main Effects Plot for Dot Skip/Sheet
                                               Data Means                                                                              Data Means

                                                                                                                   Line Screen                           Viscosity
                               Line Screen                       Viscosity                        6
        15.0
        12.5
                                                                                                  4
        10.0
         7.5
                                                                                                  2
         5.0
                                                                                           Mean
 Mean




                         175                 200            17      19       21                              175                 200                17      19          21
                                                                                                                     Speed                               Hardness
                                 Speed                           Hardness                         6
        15.0
        12.5
                                                                                                  4
        10.0
         7.5
                                                                                                  2
         5.0

                    80            100          120          60      70       80                         80            100              120          60     70           80




                    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

                                                                                     213
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

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.

                                              214
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



                                      Interaction Plot for Dot Skip/Sheet
                                                        Data Means
                                 17       19       21   80    100    120   60      70      80
                                                                                                20
                                                                                                        Line
                                                                                                     Screen
                   Line Screen                                                                  10       175
                                                                                                         200
                                                                                                0
                                                                                                20
                                                                                                     Viscosity
                                                                                                           17
                                       Viscosity                                                10         19
                                                                                                           21
                                                                                                0
                                                                                                20
                                                                                                     Speed
                                                                                                        80
                                                             Speed                              10     100
                                                                                                       120
                                                                                                0




                                                                                Hardness




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



                                      Interaction Plot for Dot Skip/Sheet
                                                        Data Means
                                 17       19       21   80    100    120   60      70      80
                                                                                                10
                                                                                                        Line
                                                                                                     Screen
                                                                                                5        175
                   Line Screen
                                                                                                         200
                                                                                                0
                                                                                                10
                                                                                                     Viscosity
                                                                                                           17
                                                                                                5          19
                                       Viscosity
                                                                                                           21
                                                                                                0
                                                                                                10
                                                                                                     Speed
                                                                                                        80
                                                                                                5      100
                                                             Speed
                                                                                                       120
                                                                                                0




                                                                                Hardness




                    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).




                                                             215
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

                 Table 6: Verification and Consistency Runs for PVC and PET-G
                                                     Dot Skip/Sheet-   Dot Skip/Sheet-
              Trials       A      B      C     D          PVC              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
                                               Dot Skip/Sheet-     Dot Skip/Sheet-
                                Trials
                                                     PVC               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
                   Process Variables Vs. Density - PVC                                               Process Varaibles Vs. Density - PETG
                       Line Screen (LPI)                Viscosity (sec.)                                 Line Screen (LPI)                Viscosity (sec.)
           1.76                                                                              1.74
           1.75                                                                              1.72
           1.74
                                                                                             1.70
           1.73
                                                                                             1.68
           1.72
    Mean




                                                                                             1.66
                                                                                      Mean




                   175              200          17          19          21                          175                 200       17           19           21
                      Speed (m/min.)                  Hardness (Shore A)                                 Speed (m/min.)                 Hardness (Shore A)
           1.76                                                                              1.74
           1.75                                                                              1.72
           1.74
                                                                                             1.70
           1.73
                                                                                             1.68
           1.72
                                                                                             1.66
                  80         100           120   60           70           80                       80         100           120   60           70           80


           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

                                                                                216
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

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
                    Process Variables Vs. Dot Area - PVC                                                 Process Variables Vs. Dot Area - PETG
                         Line Screen (LPI)                 Viscosity (sec.)                                   Line Screen (LPI)                   Viscosity (sec.)
           62.5                                                                                 62

           60.0                                                                                 60
                                                                                                58
           57.5
                                                                                                56
           55.0
    Mean




                                                                                         Mean
                                                                                                54
                    175              200            17          19          21                           175              200              17          19          21
                       Speed (m/min.)                    Hardness (Shore A)                                 Speed (m/min.)                      Hardness (Shore A)
           62.5                                                                                 62

           60.0                                                                                 60
                                                                                                58
           57.5
                                                                                                56
           55.0                                                                                 54
                   80          100            120   60           70           80                        80          100             120    60           70           80


           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
                   Process Variables Vs. Tone Curve - PVC                                               Process Variables Vs. Tone Curve - PETG
                          Line Screen (LPI)                Viscosity (sec.)                                     Line Screen (LPI)                 Viscosity (sec.)
           0.930                                                                                0.925
           0.915
                                                                                                0.900
           0.900
                                                                                                0.875
           0.885
           0.870                                                                                0.850
    Mean




                                                                                         Mean




                        175              200        17           19           21                             175                  200      17            19          21
                          Speed (m/min.)                 Hardness (Shore A)                                     Speed (m/min.)                  Hardness (Shore A)
           0.930                                                                                0.925
           0.915
                                                                                                0.900
           0.900
                                                                                                0.875
           0.885
           0.870                                                                                0.850
                   80           100           120   60           70           80                         80           100            120   60            70          80


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

                                                                                   217
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

        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-        Mean               Mean               Mean
              able          Area            Circularity        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.




                                              218
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

                     Table 9: Effect of Viscosity (B) on Dot Structure
            Vari-           Mean                 Mean               Mean
            able            Area              Circularity         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-         Mean               Mean                Mean
             able          Area             Circularity         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-          Mean                Mean                Mean
            able            Area             Circularity         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


                                             219
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

        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.




                                            220
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

REFERENCES

1. Heeschen. W. A and Smith D. A (2000), “Robust Digital Image Analysis Method for
    Counting Missing Dots in Gravure Printing”, Proceedings of the International Printing
    and Graphics Arts Conference: TAPPI 2000, Atlanta, GA, USA, pp. 29-35.
2. Alexandra. Et. al. (2009), “Gravure printed features for printed electronics”, GravurEzine,
    Vol.4, No.2, pp. 14-23.
3. Heintze. H (2005), “The economical measurement of rotogravure cell skipping.” Pulp
    And Paper Canada, Vol. 106, No.1, pp. 33-36.
4. Heintze. H. U and Gordon R. W (1980), “Reliability of video-scanner measurement of
    gravure speckle on coated board”, Journal of the Technical Association of the Pulp and
    Paper Industry, Vol. 63, No. 9, pp. 125-128.
5. Heintze. H. U and Gordon R. W (1979), “Tuning of the GRI proof presses a predictor of
    rotonews print quality in pressroom”, Journal of the Technical Association of the Pulp
    and Paper Industry, Vol. 62, No. 11, pp. 97-101.
6. MARTORANA. E. et. Al. (2006), “Causes of missing dots in rotogravure prinitng”,
    Wochenblatt für Papierfabrikation, Vol. 134, No. 11-12, pp. 690-698.
7. Heintze. H. U (1982), “Press operation and gravure print quality”, Journal of the
    Technical Association of the Pulp and Paper Industry, Vol. 65, No. 6, pp. 109-112.
8. Rong. X and Pekarovicova. A (2007), “The study of missing dots of electromechanical
    and laser engraved cylinders”, Proceedings of Technical Association of the Graphic Arts:
    TAGA 2007, pp. 596-604.
9. Kai. H, Bousfield. D. W and Unerti. W. N (2004), “The influence of roughness and
    elastic modulus on gravure print quality: A new test method and model”, Proceedings of
    Coating Conference: Technical Association of the Pulp and Paper Industry 2004.
10. Sjöblom. Richard (2003), “Rotogravure one colour ink transfer investigations on LWC”,
    Thesis (MS), Åbo Akademi University, Faculty of Chemical Engineering.
11. Mettanen. M (2010), “Methods for measuring and predicting the printability of paper”,
    Nordic Pulp and Paper Research Journal, Vol. 25, No. 3, pp. 391-404.
12. M. M. Kodabagi and S. R. Karjol, “Script Identification from Printed Document Images
    using Statistical Features”, International Journal of Computer Engineering & Technology
    (IJCET), Volume 4, Issue 2, 2013, pp. 607 - 622, ISSN Print: 0976 – 6367, ISSN Online:
    0976 – 6375.




                                             221

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:0
posted:7/4/2013
language:English
pages:14