A Comparison of Conventional and New Technology Gravure Cylinders in Terms of Density, Lightness and Chroma, Color Gamut, and Quality of Dot, Gradients and ISO Images
Chandramohan Seetharamiah Srinivasaraju
M.S. Candidate, Rochester Institute of Technology
Gravure is losing package printing market share to Flexo in North America. Closing the cost gap between flexo and gravure is a prerequisite to regaining market share. This could be accomplished by adopting new cylinder technologies that reduce the cost of cylinder preparation and the size of cylinder inventories. This study examined one such technology, RotoHybrid Cylinders. RotoHybrid is new to the gravure industry and has just entered beta testing. Gravure printers would like to understand how RotoHybrid and Conventional Chrome cylinders compare in terms of ability to print images. This research assessed image quality differences between conventional Chrome cylinders and RotoHybrid cylinders (HCR Type 1) when cylinders are new. For this research, image quality was assessed on the basis of density, dot surface area, lightness and chroma, gamut volume and the visual quality of gradients and ISO images.
The print trial used Chrome and HCR Type 1 cylinders (electromechanically engraved copper on a steel base protected by a Hybrid Chrome Replacement coating). The result of the trial demonstrated that RotoHybrid cylinders outperform conventional Chrome cylinders in terms of density, dot quality, chroma and gamut volume. Based on these results, RotoHybrid cylinders could create an opportunity for gravure printers to regain market share in package printing.
The Gravure industry in North America has been declining and losing market share to Flexo in package printing for several decades (Gravure & Its Markets, 2018). It is understood from Karl Bardin’s statement (Kozak, 2004), a significant factor affecting this decline is the cost difference between preparing and engraving cylinders versus making and mounting flexo plates. Today, leading flexo printers prepare and mount flexo plates in their printing plants where cost is reduced by using shared resources. In contrast, gravure cylinder preparation and engraving is commonly outsourced to a third-party supplier. Hence, gravure has to bear the added burden of the supplier’s infrastructure cost, transport cost, and profit margin (Luman, 2017; Ellis, 2017).
Current technology gravure cylinders have two layers of metal on a steel base: soft engravable copper and hard protective chrome. The protective functionality of chrome is required to achieve an economically acceptable print life and high quality reproduction of images (Gravure Association of America, & Gravure Education Foundation, 2003, pp. 204-205). However, the chrome bath used in electroplating is toxic and environmentally hazardous to earth soil, air and water (Health Effects Notebook for Hazardous Air Pollutants, 2017; National Institute for Occupational Safety and Health, 1975; Chromic Acid, n.d.). The chrome electroplating process is not banned by the government, but it is heavily regulated (Environmental Protection Agency, 2012; Chromium Electroplating: National Emission Standards for Hazardous Air Pollutants, 2016; United States Department of Labor, n.d.). Printers are reluctant to accept this regulatory burden because it makes it difficult to run their plants efficiently. This regulatory burden is heavy enough to convince most printers not to bring current technology cylinder preparation and engraving in-house (Friedman, 1998).
Closing the cost gap between flexo and gravure depends on adopting in-plant cylinder production. Eliminating the need for chrome in gravure cylinder preparation requires new technology. Such technology would reduce the burden of environmental regulation and clears the path for printers to bring the cylinder production
into the plant.
Today, chromeless gravure cylinders are being developed by RotoHybrid (a research company developing new gravure cylinder technology). RotoHybrid’s chromeless cylinders are new to the gravure industry and have just entered beta testing. Gravure printers would like to understand difference between the RotoHybrid Cylinders and Conventional gravure cylinders in terms of image quality.
In this research, printed ink density and dot surface area are used as quantitative measure of image quality. For precise density measurement of ink alone, the density of the substrate has to be subtracted from the combined density of the ink and paper. This can be accomplished by calculating relative density as shown in Equation 1 (Williams, 2007).
Drel = Di+p (abs) – Dp (abs)
where Drel =Relative Density
Di+p (abs) = Absolute density of ink and paper
Dp (abs) = Absolute density of paper
Dot surface area is calculated in square microns (μm²). Average dot area (in pixels) and pixels per micron square are the metrics used in calculating the dot surface area. The formula for calculating the dot surface area is given by Equation 2.
Hybrid Chrome Replacement (HCR) technology was developed by RotoHybrid to replace chrome plating. The Hybrid Chrome Replacement (HCR) coating process has its foundation in Diamond-Like Carbon (DLC) formed from a material containing carbon atoms as a major constituent and a trace amount of hydrogen atoms (RotoHybrid Coatings, n.d; HybridCylinders, 2017, para. 5; U.S. Patent No. 8,691,386, 2014). DLC combines the properties of graphite and diamond. DLC films made with a high concentration of sp2 bonded carbon atoms behave more like graphite during tribological (friction) tests. DLC films made with high concentrations of sp3 bonded carbon atoms behave are more like diamond, are super hard, and perform impressively in tribological environments (Erdemir & Donnet, 2006). Prior research indicates that Plasma Enhanced Chemical Vapor Deposition (PECVD) is commonly used (Vetter, 2014; Singh & Jatti, 2015) for coating DLC on the metal surfaces.
The methodology consists of the steps required to generate samples for measurement followed by the steps required to analyze these samples and draw conclusions. The methodology is divided in to nine steps which are outlined in the flowchart shown below (see Figure 1).
Process color test layout designed for the print test include a Tone Step Wedge, Control Strip, ECI2002 Random i1iO Single and Two page chart, Continuous Gradients and ISO Images. This layout was identically engraved on Chrome and HCR Type 1 cylinders. These cylinders are used on the Mondi Production Press to print on a plastic substrate at a standard press conditions using solvent based ink. The print samples collected were measured for density, lightness and chroma data using a Spectro Dens device, dot surface area using BetaFlex3Pro device, gamut volume using ColorThinkPro and Esko Color Engine software.
This section discusses the analysis of Density data, Dot surface area data, Lightness and Chroma data, Gamut Volume data, Image Quality of gradients and ISO images. It first discusses the quantitative density data generated by measurement of the print samples. The second, third and fourth section presents and discusses quantitative Dot area data, Lightness and Chroma data, and Gamut volume data respectively. The fifth section presents and discusses qualitative data generated from a visual assessment of Continuous Gradients and ISO images.
Quantitative Density Data. Figures 2 compares Chrome and HCR Type 1 print density data measured on production press prints.
For each print, the data source used is a Tone Wedge. Examination of Figure 2 shows that the HCR Type 1 print densities using black, yellow, magenta, and cyan data ink are higher than the corresponding Chrome print densities. Consistently higher HCR Type 1 print densities observed (compared to Chrome) warrant statistical analysis to investigate the significance of this observation. Statistical analysis of black, yellow, magenta and cyan density data from press prints of Chrome and HCR Type 1 Cylinders are shown in Tables A1 to A4 of Appendix A. The results reveal that the difference is significant at p < 0.001 for all tone percentages.
Quantitative Dot area (μm2) data. Dot Area (μm2) data was generated by measuring the tone step wedges using BetaFlex3Pro device and software. Bar charts for measured Dot Area were prepared to summarize Chrome and HCR Type1 results.
Examination of Figure 3 leads to observation that HCR Type 1 print Dot Area values are higher than Chrome print Dot Area values. Chrome and HCR Type 1 values for Black 2% dots are missing because neither cylinder could reproduce these dots. Whereas both HCR Type 1 and Chrome cylinders could reproduce Yellow 2% dots. In the 2% Magenta and Cyan tone areas, HCR Type 1 cylinders were able to reproduce dots on the production press whereas Chrome cylinders could not reproduce the same dots. See Figure A1 and A2 in Appendix A.
Quantitative Lightness L* and Chroma C* Data of KYMC. L*C*h data is generated by measuring the control strip patches in a gravure production press printed samples of conventional and HCR Type1 cylinders. The resultant average data of the L*C* are presented in the line charts with Chrome and HCR in side by side format in Figure A5 and A6 in Appendix A. Figure A5 and A6 show higher Chroma values and low Lightness values for black, yellow, magenta and cyan tones % in gravure production print sheets printed using HCR Type 1 cylinders (compared to Chrome). This is a result of increased density values of HCR Type1 and it warrant gamut analysis to investigate the significance of this observation on the gamut volume. HCR black Chroma value at 100% tone is less than that of Chrome. The reason could be higher ink film thickness transferred by HCR causing lower Chroma value especially in 100% tones.
Gamut Volume Data. Density and Chroma analysis shows that small increase in ink density results in the increase in Chroma values. The sensitivity of chroma values in relation to ink density values warrant gamut volume analysis. Gamut Volume data is generated by measuring the ECI2002 Random i1iO two page chart sample sheets printed on the gravure production press using conventional gravure and new technology gravure cylinders. The charts were measured using i1Profiler software connected to i1iO device to generate ICC profiles of Chrome and HCR Type1 cylinders. The resultant ICC profile is used in ColorThinkPro application to visually present the gamut boundary and compare both the cylinder gamut. It is observed from Figure 6 that the gamut boundary of gravure production press with HCR Type1 cylinders is noticeably bigger than that of gravure production press with Chrome cylinders.
The gamut volume analysis done in Esko Color Engine Pilot application using Gamut Volume tool revealed that the HCR Type1 cylinder gamut volume in Cubic dE is higher than the Chrome counterpart. This shows the gamut sensitivity to small differences in ink density, so higher ink density values of HCR Type 1 cylinders resulted in increased Chroma and reduced Lightness values and hence marginal increase in the output gamut volume by HCR Type1. Further, the gamut volumes of Chrome and HCR are compared with the other industry standard press ICC profile downloaded from Adobe website. The comparisons revealed that the gamut volume of HCR Type 1 is higher than the other standard ICC profiles. See Figure 6.
Reproduction of Pantone spot colors by Chrome and HCR Type1 ICC profiles based on the gamut volume is assessed in Esko Color Engine Pilot application using Gamut Check tool. Assessment revealed that the HCR Type1 cylinder could reproduce 1% to 5 % more Pantone spot colors than conventional Chrome cylinders. See Table A5 and A6 in Appendix A.
Visual Assessment of ISO Images and Continuous Gradients. The ISO 12640-1 “Wine and Tableware” (N4) and ISO 12640-1 “Bicycle” (N5) SCID images (See Figure A3 and A4 in Appendix A) printed using Chrome and HCR Type 1 cylinders on production presses were visually evaluated for legibility, smoothness, sharpness, image detail, and print defects to award ratings. The ratings awarded to Press prints are summarized in Table 1.
In Press prints, HCR Type 1 cylinders continued to demonstrate superior ability to print the ISO Bicycle fine lines. In addition, the number of missing dots in highlight areas was noticeably reduced when using HCR Type 1 cylinders compared to Chrome. Finally, the use of HCR Type 1 (vs Chrome) cylinders enhanced edge sharpness. Images printed with HCR Type 1 cylinders had darker, sharper edges due to the superior ability of these cylinders to print fine black dots. Based on these findings, Chrome Press prints were awarded an overall rating of Good while the HCR Type 1 Press prints were awarded a rating of Excellent.
Gravure production press prints of cyan, magenta, yellow, and black continuous gradients were visually evaluated for gradient smoothness and print defects to award ratings. The ratings awarded to press prints are summarized in Table 2.
Examination of Table 2 leads to two observations. 1) On a production press, HCR Type 1 cylinders produced higher quality gradients than Chrome cylinders. 2) The quality of black gradients printed using HCR Type 1 cylinders is much better than the quality of black gradients printed using Chrome cylinders. HCR’s superior ability to print small dots is a major factor contributing to this improvement.
Summary and Conclusions
In terms of density, HCR solids exhibit superior density compared to Chrome solids. The density values obtained for Chrome samples were typical of conventional gravure printing; the densities achieved in HCR samples were exceptional and impressive. Visual analysis revealed that HCR’s higher black densities resulted in increased sharpness in the highlight and shadow areas. These results strongly support the conclusion that HCR densities outperform Chrome densities for samples printed using the same screens, engraving methods, ink formulations, and press conditions.
Based on the results just discussed, HCR can achieve higher relative density, chroma values compared to Chrome for identically engraved cylinders printed using identical print conditions. This means fully formulated ink could be mixed with extender to match Chrome solid ink densities when printing with HCR cylinders. Extender is significantly less expensive than fully formulated ink, so adding extender simultaneously lowers ink cost. The resulting cost reduction would further improve the overall competitiveness of gravure printing compare to flexographic printing. Higher Chroma (C*) values achieved by HCR cylinders increased the gamut volume on the press resulting marginal Pantone spot color reproduction thus reducing some of the expensive spot color ink cost. Using specially formulated high Chroma process inks the gamut volume could be further increased and thus allowing more number of expensive Pantone spot colors and brand color reproduction at a lower ink cost.
The dot surface area analysis compared identically engraved Chrome and HCR cylinders, the results show that the larger dots were purely a function of the difference in coating thickness. In conventional Chrome cylinders, chrome coating is electroplated using process conditions that create microcracks in the chrome surface. Wet ink in these cracks lubricates the doctor blade/cylinder interface and greatly reduces frictional heating. With HCR cylinders, the coefficient of friction between the doctor blade and cylinder is reduced to 0.1. This means that the doctor blade/cylinder interface no longer requires lubrication to prevent excessive heat buildup and the need for microcracks is eliminated. Since drying conditions are no longer constrained by the need to dry ink in microcracks, the distance between the doctor blade and impression nip can be reduced. This enables ink in small cells to remain wet and printable, thus enabling smaller dots to be printed reliably. Improving the ability of Gravure to print smaller dots would improve its image quality.
Governments, especially in Europe, have been tightening regulatory requirements associated with the use of chrome. This trend is expected to continue and could result in a ban on using chrome to plate gravure cylinders. Because HCR coating is an environmentally friendly process which is not regulated, it provides a viable option for engravers, trade houses, and printers to replace Chrome. The results of this research demonstrate that in terms of image quality, HCR is a better option than Chrome and this could potentially accelerate the adoption of HCR.
Density and dot quality results strongly support the conclusion that HCR Type 1 Cylinder enhanced the quality of image detail and outperform Chrome using the same screens, engraving methods, ink formulations, and press conditions. The adoption of environment-friendly HCR could make in-house cylinder making possible, potentially reduce the cost of cylinder, and accelerate the growth of the gravure industry.
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Density data is statistically analyzed using T-tests for equality of means.
Note. This material is reproduced from ISO 12640-1:1997 with permission of the American National Standards Institute (ANSI) on behalf of the International Organization for Standardization (ISO). No part of this material may be copied or reproduced in any form, electronic retrieval system or otherwise or made available on the Internet, a public network, by satellite or otherwise without the prior written consent of the ANSI. Copies of this standard may be purchased from the ANSI, 25 West 43rd Street, New York, NY 10036, (212) 642-4900, https://webstore.ansi.org