Striping of Corrugated Board in Flexo Post-Printing - tappi

Striping of Corrugated Board in Flexo Post-Printing M. Holmvall and T. Uesaka Fibre Science and Communication Network (FSCN), and Department of Natural Sciences Mid Sweden University, SE-851 70 Sundsvall, Sweden ABSTRACT Striping is a print defect that is common in flexo post-printing on corrugated board. Striping is print density and/or print gloss variations that appears as stripes on the printed liner parallel to the flutes of the corrugated medium. The periodic print variations are often thought to originate from a non-uniform printing pressure caused by the corrugated board structure. Experiments and numerical simulations have been performed to study how print density and print gloss variations can be related to local contact pressure during flexo post-printing on corrugated board. Print gloss and print density have been measured for prints with known printing pressure. The local printing pressure in an industrial flexo post-printing press has been simulated with a finite element model. The results showed that the print density variations are caused by the printing pressure variations, depending on the ink transfer characteristics of the liner board, while the print gloss variations are caused by surface distortion of the corrugated board, such as “washboarding”. INTRODUCTION When corrugated boards are printed directly in a flexo press (called flexo “post-“printing), the problem is very often striping. Striping is print density and/or print gloss variations parallel to the flutes of the corrugated medium, as shown in Figure 1. Periodic print quality defects as striping are generally rated lower than random noise prints by observers [1]. Almost all corrugated boards that are manufactured today are printed, and the majority of them are printed by flexo post-print. Hence, striping is a great concern for the flexo post-printing industry. Striping is often thought to be caused by washboarding [2 - 6]. Washboarding is, in this work, defined as a purely geometrical defect that is a wavy structure that partially follows the flutes of the corrugated medium, as can be seen in Figure 2. The magnitudes of washboarding reported in the literature are mostly between 10-100 microns [5, 7]. Washboarding is a common structural defect of the corrugated board liner, often considered to be an effect of the adhesive used to glue the liner and medium together [8]. The reason for washboarding to be blamed for the striping is that it is assumed to cause impression variations (printing pressure variations) over the printing area [4, 6]. These contact pressure variations are then supposed to yield print density variations. The correlation between the contact pressure variations and print density variations has been shown in experiments by Hallberg et al. [9].

In our previous paper [10], a finite element model has been developed to determine the flexo post-printing nip parameters that have the most significant effect on pressure variations. It was found that the thickness and stiffness of the photopolymer printing plate as well as those of the top liner board had the greatest effect on the pressure

with washboarding

Corrugated board

without washboarding

Corrugated board

Figure 2. Corrugated board seen from the side in CD showing corrugated board without washboarding (top) and with washboarding (bottom).

Figure 1. Photograph of a corrugated board with printed striping.

variations. Compared to the effects of the photopolymer printing plate and the top liner board, the washboarding had a rather minor effect. To avoid the periodic print density variations in flexo post-printing, printers often try to increase the printing pressure. However Lu and Chen [10] showed that the corrugated board medium for B-flute boards was damaged for pressures about 0.23 MPa. Hence, there is a limit on how much the pressure can be raised without damaging the corrugated board structures. Although a number of important insights have been presented in the literature, it has not been shown, quantitatively, how the periodic print density/print gloss variations are produced by the printing pressure variations. It has also not been shown what really constitutes “striping” seen in the actual prints, i.e., print density variations, print gloss variations, or both. The objectives of this paper are to establish quantitative relationships between pressure variations and print density/gloss variations, and then to clarify the root causes of striping in flexo post printing. METHODS Measurements of Ink Transfer Characteristics of Single Liner Board In order to relate printing pressure to print density and print gloss quantitatively, ink transfer characteristics of single liner board must be established. Samples of an uncoated white top kraft liner with a grammage of 140 g/mm2 have been printed in an IGT F1 Printability Tester where the applied printing force can be controlled. The 50 mm wide samples were printed with a solid tone strip of 40 mm width for printing forces between 30 and 500 N. Five samples were printed for each applied force. The anilox cylinder, which places ink on the photopolymer, has a screen angle of 60o, a 120 lines/cm screen ruling and a volume of 12 ml/m2. The ink used was an ink from Sun Chemical AB called Turbocryl with a viscosity of 20 s, measured with a Zahn cup #2 [12]. The solid tone printing plate photopolymer that was used had a thickness of 1.7 mm and a width of 49 mm. The print density of each sample was measured with a spectrophotometer called SpectroEye from GretagMacbeth. It was set for print density measurements with a 2o observer angle, a C illuminant and DIN density standard. Each sample was measured twenty times and the average was calculated. Then an average over the five samples for each printing force was calculated and used as the print density corresponding to the printing force. The gloss of each sample was also measured. The gloss was measured with a ZLR 1050 M laboratory glossmeter at a 75o measuring angle. Each sample was measured five times and the average was calculated. Then an average over the five samples for each printing force was calculated and used as the print gloss corresponding to the printing force. Measurements of Printing Pressure The printing pressure in the IGT F1 printing press was determined with a 5051 model pressure sensor from Tekscan (Tekscan, Inc., South Boston, USA). The pressure sensor is built as a two dimensional rectangular mesh of metal threads creating 1936 censoring cells with a size of 1.27 mm x 1.27 mm. The metal threads are placed and laminated between two plastic films. The thickness of the entire sensor is 0.1 mm. The censoring parts of the sensor are where the metal threads cross. The sensor consists of 44 censoring points in each direction. When the printing pressure is measured and the photopolymer is pressed against the metal thread mesh, it will expand into the parts on the sensor that is between the thread crossings. This makes some of the applied pressure not to be measured by the sensor since the sensor only measures the pressure that is supported by the metal thread crossings. Since the applied force is known from the IGT F1 press it is possible to calibrate the system. For example, assuming that the real pressure, P , is a function of the measured pressure, mP for each sensor cell:

)( mPfP = . (1)

then, the force F is:

∫∫= PdAF , (2)

where dA is the area element. Expressing in a discrete form, we have:

∑∑= =

Δ=N

i

N

jij APF

1 1. (3)

N is the total number of cells in each direction and AΔ is the cell area. Row and column number for a censoring cell is denoted by the indices i and j . Assuming a linear function )( mPf gave a reasonable result when the calibrated force is within 15 percent of the applied force. Assuming a second order function only improved the mean square error by approximately 1 percent. Using the calibrated values of the pressure measurement, it is now possible to calculate the nip peak pressure for each applied printing force. The nip peak pressure is the highest pressure over the nip width in the printing direction. Measurements of Reflection at 0º and 45º for Printed Corrugated Boards In order to obtain information on diffused reflectance and specular reflectance components of printed corrugated board, as represented by print density and print gloss, respectively, reflectances detected at 0º and 45º were determined by using a mottle measurement system developed at Pulp and Paper Research Institute of Canada. Two samples, one coated and one uncoated, of B-flute corrugated board have been illuminated with a 45º He-Ne laser. Detectors at 0º and 45º register the reflected light intensity. The 0º detector is intended to measure diffuse reflectance components, as represented by print density and the one at 45º to measure specular components, as represented by print gloss. The uncoated sample had a clear striping pattern printed on it. The coated sample had a barely visible striping pattern. The samples were both printed with a cyan ink. Finite Element Analyses of Printing Pressure Variations of Corrugated Board The printing pressure for a flexo post-printing press has been simulated with a finite element model, which was reported earlier [10] (see also Figure 3). It models a printing nip corresponding to an industrial flexo post-printing press and a corrugated board of so called “B-flute” type. The model included the impression cylinder, the corrugated board and the printing form. The printing cylinder was regarded as rigid and the printing form was modeled as a layered composite of plate cushion, mounting plastic and photopolymer. The corrugated board was modeled as a composite of top and bottom liner boards and a corrugated medium. The plate cushion and photopolymer materials properties were assumed to be hyperelastic Mooney-Rivlin materials and the material constants were determined from tensile tests. The liner boards and the medium were modeled as orthotropic linear elastic materials and the nine elastic constants were determined by an ultra sonic testing method [13]. The mounting plastic and the impression cylinder materials were treated as isotropic linear elastic materials. The model simulates a unit structure of one flute of the corrugated board. The side of the model that cuts through the center of the nip and the two sides that cut through the adhesion region between the top liner and the medium had symmetrical boundary conditions. The impression cylinder was fixed and printing was simulated by translating the printing cylinder core towards the corrugated board in the thickness direction. The magnitude of the translation, the print squeeze, was 200 microns for the control case. All other surfaces were considered as traction free. The effects of boundary conditions and mesh density were investigated in the earlier paper [10].

RESULTS Ink Transfer Characteristics and Printing Pressure The print density results from the printing experiment are shown in Figure 4. The curve shows a typical print density development curves reported in the literature [14]. The print density increased rapidly with increasing printing forces upto125 N and then the print density tended to reach a plateau at around 250 N. The calibration result from the pressure measurement on the IGT F1 printability tester is shown in Figure 5. It is seen that the relation between nip peak pressure and applied printing force is approximately linear.

By utilizing the results from the pressure measurements done with the pressure sensor in Figure 5, it is possible to convert the printing force to printing pressure, as seen in Figure 6. In the same figure, the peak and valley pressures for the finite element model are marked with vertical lines for reference. It can then be clearly seen that the saturated print density is not reached at these pressures.

0 100 200 300 400 5000

0.5

1

Force [N]

Pre

ssur

e [M

Pa]

Figure 5. Nip peak pressure as a function of the set printing force on the IGT F1.

100 200 300 400 5000.85

0.95

1.05

1.15

Force [N]

Prin

t Den

sity

[−]

Figure 4. Print density as a function of app-lied printing force.

Figure 3. Finite element model of an industrial flexo printing press.

In Figure 7 and Figure 8 the results from the gloss measurements are presented on the same samples for which the print density was measured. The gloss was measured in both the machine direction (MD) and the cross machine direction (CD) of the liner. The MD gloss is slightly higher than the CD gloss. Clearly the printed gloss has very little dependence on printing force (and thus printing pressure).

Figure 9 shows the printing pressure distribution result for the control case as reported in [10]. The local printing pressure is typically higher at ridges of flutes than at valleys (see also Figure 3). Using the result presented in Figure 6, we can convert the printing pressure distribution curve into a print density variation curve, as shown in Figure 10. The shape of the pressure curve was changed dramatically, because of the non-linearity of print density-printing pressure relation. Performing the same conversion to the print gloss results yielded the result in Figure 11. As expected, print gloss, on the other hand, showed no variation across the flute width. This means that the printing pressure variations causes only the print density variations, and print gloss is very little affected. In this regard, the ink transfer characteristics curve, i.e., print density vs. printing pressure relations, has a significant effect on the print density variations. Those papers requiring higher pressure to reach the plateau of print density, such as higher surface roughness, less compressible boards, may suffer more the striping problem. Figure 12 shows the effect of increased print squeeze on the print density. An increase in print squeeze decreases the print density variations drastically. It is, therefore, understandable that the industry often tries to avoid striping by increasing the print squeeze. However, as discussed earlier, this practice entails the substantial risk of damaging the board permanently, and should be avoided.

100 200 300 400 5000

2

4

6

8

10Print gloss Liner gloss

Force [N]

Glo

ss [−

]

Figure 8. Print gloss and liner gloss in CD as a function of the applied printing force.

0 0.2 0.4 0.6 0.8 10.85

0.95

1.05

1.15

Print density

Peak pressure

Valley pressure

Pressure [MPa]

Prin

t Den

sity

[−]

Figure 6. Print density as a function of printing pressure. Vertical lines represent the peak and valley pressure from the finite element model.

100 200 300 400 5000

2

4

6

8


Force [N]

Glo

ss [−

]

Figure 7. Print gloss and liner gloss in MD as a function of the applied printing force.

Reflectance Variations at 0º and 45º of Printed Corrugated Board Samples The data from the reflectance measurements were analyzed by using a fast Fourier transform (FFT). The FFT produces a spectral power distribution showing the power of a signal distributed over the frequencies or periods. The spectral distributions presented were averaged over three scanning lines and are presented only in the interval of interest between 0-11 mm containing the medium wavelength. The results for the uncoated sample are shown in Figure 13 and Figure 14. The result for the 0º detector clearly showed a distinct peak at about 6.4 mm, corresponding to the wavelength of the corrugated medium (Figure 13). This also corresponds to the striping pattern clearly visible in this uncoated board sample. The 45º detector results in Figure 14, however, showed no obvious dominating peak. First, the intensity of the variations (power) is one order of magnitude lower than that of the 0º detection. Second the 6.4 mm peak does exist but is buried in other peaks. Therefore, in the case of the uncoated board sample, the human perception of this visible strip pattern is clearly originated from the print density variations. Figure 15 and Figure 16 show the results for the coated sample. Interestingly, the power of the peaks for the 0º detector measurements (Figure 15) was only 1/20 of the power for the corresponding uncoated board measurements (Figure 13). Again, the peak at 6.4 mm wave length does exist but buried among other peaks. Therefore, for the coated board sample, the striping pattern has very weak intensity and will be very difficult to detect by human eyes among other noisy variations. The peaks at 45º detection, on the other hand, have higher power, but again the 6.4 mm peak was buried in other peaks. These results suggest that print gloss variations for coated board consist of various wave lengths of surface distortion, including washboarding.

0 1 2 3 4 5 60.85

0.95

1.05

1.15

1.25 Control 300 μm 400 μm500 μm

Position [mm]

Prin

t den

sity

[−]

Figure 12. Print density over one flute width for different print squeezes.

0 1 2 3 4 5 60.85

0.95

1.05

1.15

Position [mm]

Prin

t den

sity

[−]

Figure 10. Print density over one flute width for control case.

0 1 2 3 4 5 60

0.1

0.2

0.3

Position [mm]

Pre

ssur

e [M

Pa]

Figure 9. Pressure distribution over one flute width for control case.

0 1 2 3 4 5 60

2

4

6

8

10

Position [mm]

Prin

t glo

ss [−

]

Figure 11. Print gloss in MD over one flute width for control case.

CONCLUSIONS The results from the print density and print gloss experiments showed that striping perceived by human eyes is a result of the print density variations, and not of the print gloss variations. The print density variations, in turn, are caused by the local printing pressure variations in the flexo printing nips. The pressure variations will have a great influence if the printing pressure is below the plateau region of the print density curve. Therefore, ink transfer characteristics, i.e., print density vs. printing pressure relationships, of liner boards are one of the important factors affecting the print defect. Those liners which have higher roughness, for example, may suffer the striping problem more, because they tend to require more printing pressure to achieve a higher print density. The reflectance variation experiments done in this work indicates that striping is print density variations and not print gloss variations. However, there might be gloss effects other than print gloss variations. Corrugated boards stacked together might for instance affect the roughness of the liner board above medium corrugation ridges by mechanical polishing and hence induce gloss variations. Increasing the printing pressure might work to avoid striping since both the ridge and valley pressures reach the saturated print density. This, however, may cause damages in the corrugated boards. The pressure required to reach the saturated print density (~ 0.2-0.3 MPa) is, unfortunately, also close to the pressure that will damage a B-flute corrugated board medium (~0.23 MPa) [11].

0 5 100

2

4

6

8

x 107

Period [mm]

Pow

er

Figure 16. Spectral power distribution for the 45º measurement data of the coated sample.

0 5 100

1

2

3

4

x 105

Period [mm]

Pow

er

Figure 14. Spectral power distribution for the 45º measurement data of the uncoated sample.

0 5 100

1

2

3x 10

6

Period [mm]

Pow

er

Figure 13. Spectral power distribution for the 0º measurement data of the uncoated sample.

0 5 100

5

10

15

x 104

Period [mm]

Pow

er

Figure 15. Spectral power distribution for the 0º measurement data of the coated sample.

ACKNOWLEDGEMENTS The authors would like to acknowledge financial support from SCA Packaging Research for this project. They acknowledge Anders From, Ragnhild Dölling, and Folke Österberg of SCA Packaging Research in Sundsvall for valuable discussions and continuous encouragement. The authors would also want to thank Roger Nilsson at CA Mätsystem for assisting with the pressure measurements. References 1. Lindberg, S. and Fahlcrantz, C.-M., “Perceptual Assessment of Simulated Print Noise with Random and

Periodic Structure”, J. Vis. Commun. Image R., 16(3):271-287(2005). 2. Zang, Y.H. and Aspler, J.S., “Factors that Affect the Flexographic Printability of Linerboards”, TAPPI

Journal, 78(240): 23 -33(1995). 3. Haglund, N., “Improving the Printability of Kraftliner”, Paper Technology Industry, Aug. 1988, 265 –

267(1988). 4. Snyder, P.A., “Controlling Adhesive Application”, FLEXO, Sept. 1995, 24 – 27(1995). 5. Wendler, S.D., “The Measurement of Corrugated Liner Washboard and its Effect on Full Tone Print

Quality”, In: Proceedings of 55th Appita Annual Conference, 30 April – 2 May, Hobart, Australia, 495 – 500(2001).

6. Jensen, K.W., “Flexo Printability of Coated White-Top Liner with Consideration of Convertability”, Graphic Arts in Finland, 18(1):14 – 20(1989).

7. Hallberg, E., Odeberg-Glasenapp, A. and Lestelius, M., ”Quantification of Banding on Printed Corrugated Board Using Spatial Frequency Analysis”, Packag. Technol. Sci., 18:89-95(2005).

8. DiDominicis, A.J. and Klein, G.H., “Corrugating”, Pulp and Paper, Chemistry and Chemical Technology, 3rd edition, vol. IV, 26:2373 – 2398(1983), Wiley, New York.

9. Hallberg, E., Rättö, P., Lestelius, M., Thuvander, F. and Odeberg-Glasenapp, A., ”Flexo Print of Corrugated Board: Mechanical Aspects of the Plate and Plate Mounting Materials”, TAGA Journal, 2:16-28(2005).

10. Holmvall, M., and Uesaka, T., “Nip Mechanics of Flexo Post-Printing on Corrugated Board”, submitted to Journal of Composite Materials.

11. Lu, T.J. and Chen, C., “Compressive Behaviour of Corrugated Board Panels”, J. Compos. Mater., 35(23):2098 – 2126(2001).

12. Cusdin, G., Ink on Press, In G. Cusdin (Ed.) “Flexography: Principles & Practices”, 5th edition, vol. 5, 67-113(1999).

13. Mann, R.W., Baum, G.A. and Habeger, C.C., “Determination of All Nine Orthotropic Elastic Constants for Machine-made Paper”, TAPPI, vol. 63, (2):163-166.

14. Havlínova, B., Cicák, V., Brezová, V., “Water-reducible Flexographic Printing Inks – Rheological Behaviour and Interaction with Paper Substrates”, Journal of Materials Science 34:2081-2088(1999).

2006-11-16Institutionen för naturvetenskap

Striping of Corrugated Board in Flexo Post-Printing

Martin Holmvall and Tetsu UesakaFSCN, Mid Sweden University

International Printing and Graphic Arts ConferenceSeptember 20 - 22, 2006, Cincinnati, Ohio, USA


Striping - A Print Quality Challenge for Flexo Post-Printing

• Main print defect in Flexo post-printing


The Striping Issue

Striping

Print density variations Print gloss variations

Pressure variations


Analysis of Pressure Variations

• Developed a non-linear finite element model

• Performed parametric studies

(M. Holmvall and T. Uesaka, EUROMECH Colloquium 2006)


Summary: Pressure Variations

• Most influential parameters: printing plate and top liner board

• Increasing printing pressure can damage the corrugated board

• Proposed an optimised design of printing plate that reduces the pressure variation drastically

(M. Holmvall and T. Uesaka, EUROMECH Colloquium 2006)


Questions:

• What is “striping”? : Print density variations, print gloss variations, or both ?

• How does print density/gloss relate to printing pressure?


Approaches

Measure diffuse and specular light reflectance

What is striping?

Correlate pressure and print density/gloss

Pressure vs. print density/gloss


Print Density and Print GlossDete

ctorHeNe−laser

Det

ecto

r

• HeNe-laser• Reflectance

detectors at 0 and 45 degrees

• Diffuse and specular reflectance

(Measured at PAPRICAN, Canada)


Pressure Dependence• Used an

IGT F1

• Measured print density and print gloss as a function of printing “force”

Doctor blade

Imprint cylinder

Liner board

Plastic sheet

Fibre glass carrier

PhotopolymerAdhesive tape

Printing cylinder

Anilox cylinder

Adhesive tape

F


Pressure Measurements

• Used an I-Scan• Measured

pressureP

F


Pressure - Force Relationship

0 100 200 300 400 5000

0.5

1

Force [N]

Pre

ssur

e [M

Pa]


Results


0° Reflectance – Uncoated Board

0 5 100

1

2

3x 10

6

Period [mm]

Pow

er


0° Reflectance – Coated Board

0 5 100

5

10

15

x 104

Period [mm]

Pow

er


45° Reflectance – Uncoated Board

0 5 100

1

2

3

4

x 105

Period [mm]

Pow

er


45° Reflectance – Coated Board

0 5 100

2

4

6

8

x 107

Period [mm]

Pow

er


Reflectance and Striping

• Print density variations clearly show up as striping

• Print gloss variations are also detectable• Are these variations in print density and

print gloss induced by the pressure variations?


Pressure Dependence of Print Density

0 0.2 0.4 0.6 0.8 10.85

0.95

1.05

1.15

Pressure [MPa]

Prin

t Den

sity

[−]


Pressure Dependence of Print Gloss (MD)

0 0.2 0.4 0.6 0.8 10

2

4

6

8


Pressure [MPa]

Glo

ss [−

]


Predicted Print Density

← M

D →

Ext

ensi

on [c

m]

← CD →Extension [cm]

01.

32.

63.

95.

26.

50

1.3

2.6

3.9

5.2

6.5

0 1 2 3 4 5 60.85

0.95

1.05

1.15

Position [mm]

Prin

t den

sity

[−]


Predicted Print Gloss

0 1 2 3 4 5 60

2

4

6

8

10

Position [mm]

Prin

t glo

ss [−

]• No pressure dependence


Conclusions• Striping consists of print density variations

with possible influence of print gloss variations

• Print density variations can be explained mostly by printing pressure variations

• Print gloss variations do exist, but might be caused by other reasons


Acknowledgements

• SCA Packaging Research• Paprican, L. Cormier


Thank you for your attention!

Striping of Corrugated Board in Flexo Post-Printing - tappi

Documents

Transcript of Striping of Corrugated Board in Flexo Post-Printing - tappi