To bring gravure technology into

Transcription

1 Gravure Research Agenda: The Journey of a Test Form from Engraving to Proofing Bob Chung, Rochester Institute of Technology Abstract To bring gravure technology into the classroom and laboratory, the author visited a gravure engraving plant. With photographic illustrations, he describes an automated engraving operation and a manual-oriented cylinder proofing operation. Introduction Gravure imaging by means of cylinder engraving and proofing requires special-purpose equipment, skilled operators, and capital investment in process automation and environmental compliance. This makes an engraving company, such as Keating, a valuable supplier to many packaging printers. The same reason makes teaching gravure technology with hands-on components almost impossible unless there is a business model to help sustain the financial viability of these resources within a university system. I have the privilege and the challenge to offer a gravure process course to print media students at Rochester Institute of Technology. The course is designed to analyze the infrastructure as well as the print production workflows in the gravure printing industry. Other than lecturing on gravure materials, components, and technology, students perform gravure-related laboratory assignments for process comprehension. One of the lab assignments focuses on the use of a monochrome digital test form to study the effect of cylinder engraving and proofing on tone reproduction. After I designed the test form (Appendix A), I needed an engraver to do the rest of it. With the help of Walter Siegenthaler of Max Daetwyler Corp. (MDC), I visited Keating in Charlotte, North Carolina, during my school break. The purpose of the visit was to follow the monochrome test form from engraving to proofing using the Ballard shell method. In doing so, I would record the steps involved in cylinder engraving using an Ohio electromechanical engraver [Ohio is a part of MDC since 2000] and proofing with a JM Heaford proofing machine. By providing the digital file, the engraved surface, and the proof to students in a laboratory session, they will be able to relate the digital data of the file to cell volume in the stripped Ballard shell and density of the ink-on-paper proof. Figure 1. A computer-integrated engraving environment

2 the test form from an InDesign CS file to a PostScript Level 3 file using an appropriate PPD (PostScript Printer Description) setting was executed beforehand. An electromechanical engraver works like a lathe that cuts copper with a diamond stylus at very high frequency, e.g., 8,100 cycles per second. The indented cut forms a cell on the cylinder. The cell volume, varying in cell width and depth, determines the amount of ink it will hold and transfer to the substrate upon printing. Figure 2. A crane with robotic arms transports cylinders Keating at a Quick Glance Keating ( is the largest cylinder engraver in the packaging printing industry in England. Keating (www. keatinglobal.com), a Sonoco-owned company, established its North American operation in Based in Charlotte, North Carolina, Keating is strategically located near MDC ( and manufactures cylinder preparation, engraving, and automation equipment. With multiple engraving stations and full automation from copper plating, chrome plating, and cylinder finishing, Keating is capable of processing 250 cylinders per week using electromechanical and laser engraving systems. Phil Pimlott, President of Keating, was my host during my visit. After I put on the toe-protecting gear and the eye goggles, I was introduced to Chuck in production control, Colin in engraving, and Tim in the proofing department. The plan was to observe the engraving and proofing of the test form in one day. With good planning, skilled operation, and computer-integrated manufacturing, the cycle time for engraving and proofing a job can be as short as five hours, and not five days. Below describes the journey of a test form from engraving to proofing. arms transport cylinders. Figure 1 shows a computer screen in the foreground that monitors in real-time the location and treatment of all cylinders that are work-inprogress. Figure 2 shows the use of a crane to move a cylinder from Point A to Point B. A motion sensor (upper left) monitors any object at its height, e.g., an operator s head, and will stop the crane while sending out an alarm. Cylinder Engraving All high-end raster output devices, e.g., platesetter, inkjet printer, electrophotographic printer, are driven by PostScript code. This is also the case for an electromechanical engraver. The conversion of There are a number of cylinder preparatory steps prior to electromechanical engraving. If the cylinder is to be repurposed, it is washed, dry ink removed, dechromed, and surface copper removed. The cylinder is then copper plated, polished to the correct circumference, and mounted on the engraver. We want to preserve the engraved surface so that we can examine engraved cells up close in this project. The Ballard shell with surface copper of 80 to 100 μm thick, divided from its base copper by a nonadhesive electroplating method, was used. It allows the separation of the thin copper layer after proofing is done. Figure 3 shows the operator putting a thin layer of lubricating oil on the surface of the cylinder to allow the shoe of the engraving head to ride on the surface freely. Cylinder Transporting A cylinder weighs anywhere from 200 to 600 lbs. In a computer-integrated engraving environment, cranes with robotic Figure 3. Cylinder surface lubricating prior to engraving GRAVURE/February

3 A diamond stylus with a cutting angle of 120 degrees was selected to produce a screen angle at 45 degrees with a 70 lines/cm (180 lines/in) screen frequency. Figure 4 shows a schematic of gravure cell at highlight (right), midtone, and shadow (left) with a screen angle at 45 degrees. Specifically, the cell widths have to be calibrated to predetermined specifications at highlight and shadow. In this project, the width of the highlight cell is set at 45 μm. The highlight cell at this width should produce just printable inked dots. The width of the shadow cell is set at 160 μm. The Figure 4. A schematic of gravure cells (courtesy of GAA) shadow cell at this width should produce maximum inked area while still providing a 5 μm wall to support the doctor blade to remove ink from the nonimage area of the cylinder during printing. In addition, a gradation (gamma) curve is selected to produce a desired midtone. Figure 5 illustrates the engraving calibration using the edge of the cylinder as the test area. The engraving head is equipped with machine vision at high magnification. It displays the cell width on the display and allows the operator to make fine adjustments in microns until the cell widths conform to engraving specifications. Figure 5. Engraving head is equipped with machine vision When image carriers for offset lithography and flexography were prepared with films in the 1980s, gravure cylinder engraving had already gone digital. As a matter of fact, the computerized cylinder transport system made the entire engraving process hands-off. Figure 6 shows the engraver being shielded to reduce engraving noise. To maximize the number of test forms per proofing cycle in this project, a large cylinder is used. The engraving time is two hours. The operator can either start another job or monitor the magnified cell shapes on the display while the cylinder is being engraved. Post Engraving Treatment As soon as the engraving is completed, the crane comes over, picks up the cyl- inder, and takes it to the next station for degreasing and cleaning (Figure 7). The automated process takes about ten minutes, and there is no human intervention other than watching. Under computer control, the crane then takes the cylinder for chrome plating. Twenty minutes later, the tank deposits a 6 to 7 μm thickness of chrome to increase the hardness of the image carrier and prolong the life of the cylinder. The color of the cylinder changes from copper to silver-like chrome. The crane takes the cylinder to the cleaning and finishing station (Figure 8). The buffing action produces smooth and fine crisscross patterns on the chromed surface to facilitate ink flow during printing. The crane then places the cylinder on a trolley ready for proofing. The engraving operation takes three hours to complete. Cylinder Proofing To verify and to document that correct engraving procedures have been carried out, cylinder proofing is required at Keating before a cylinder is shipped to a converter (printers are known as converters in the packaging industry). There are two levels of interest when it comes to proofing i.e., proofs for internal use and proofs for external use. An Figure 6. Engraver shield is closed once the engraving starts 44 GRAVURE/February 2006

4 Figure 7. Cylinder degreasing, buffing, and cleaning internal proof helps verify that no engraving-related artifact e.g., broken diamond or wrong screening or missing printer s marks exists in finished cylinders. An external proof helps communicate the color appearance of a gravure-printed product prior to printing. A cylinder proof fulfills the need as an internal proof. A color-managed inkjet proof is a popular medium for an external color proof. In this project, we are interested in the mechanism of gravure printing. A cylinder proofer has the same components as a gravure press. Thus, a colormanaged digital proof is outside the scope of this paper. As shown in Figure 9, the RIT monochrome test cylinder has been manually placed in the JM Heaford proof press ( A large drum 1.75 meters wide and 3 meters long and covered with a (blue) blanket is the impression cylinder. Substrate, either paper or plastic, is to be wrapped around the impression cylinder. The doctor blade is situated between the engraved cylinder and the impression cylinder. Compared to cylinder engraving, cylinder proofing is a manually intensive task that requires physical strengths and craftsmanship. It is fascinating to watch the operator going from step to step to transform a number of engraved surfaces into an inkon-paper proof. Electrostatic assist (ESA) facilitates the production of a paper-based cylinder proof. To do so, (1) the impression cylinder is first covered with a paper-backed foil and followed by the coated paper; (2) the doctor blade and the cylinder are cleaned with solvent; (3) the cylinder is brought in contact with the doctor blade; (4) the ink is poured on the cylinder in front of the doctor blade (Figure 10); and (5) as the cylinder rotates, ink in the nonimage area of the cylinder is doctored (or scraped) off its surface and ink in the recessed (image) area is transferred onto the coated paper under ESA and impression. The circumference of the impression cylinder is 3 meters long while the circumference of the engraved cylinder is only 1 meter long. We can produce three repeats of the engraved images per proofing operation. The first repeat usually is discarded as the cylinder is initially inked from a dry state. From the second repeat and on, the proofs usually are printed without any proofing/printing-related flaws. In our project, we produced more than 50 proofs from the step-and-repeat cylinder for use in the lab assignment (Figure 11). Figure 8. Cylinder cleaning and buffing Multicolor Proofing A good packaging design builds brand identity, adds value to the package, and promotes customer loyalty. A good packaging design often means multicolor printing. Producing multicolor proofs are routine challenges faced by personnel in the proofing department. When proofing a multicolor job, the process of cylinder mounting, image-toimage registration, inking, printing, cylinder cleaning, and cylinder removal has to be repeated for each and every cylinder according to job specifications. This is essentially how colors are separated into different cylinders and reconstructed by means of multiple printing impressions in registration. Figure 12 shows a row of cyl- Figure 9. A cylinder proofer GRAVURE/February

5 inders (foreground) to be proofed on a proofer next to the proofer running the RIT monochrome job. Figure 10. Pouring ink on the cylinder in front of the doctor blade Engraving as a Business Solution in a Specialty Market My observation of engraving and proofing workflows at Keating and interactions with the people there gave me a sense of what cylinder engraving is all about. On one hand, the quality of the engraving and timeliness of the delivery are viewed as value-added features to packaging customers. On the other hand, the high capital intensiveness of the state-of-theart engraving equipment and automation serves as a barrier for others to enter into this specialty market. Finally, it is the people, management, and production together who face the challenge of continuously improving the processes and procedures of engraving and proofing so that first-pass yield increases while manufacturing bottlenecks reduce. About the Monochrome Test Form A test form is a collection of test targets, either pictorial or synthetic, with known values. The pictorial image, element (1), in Appendix A is a reference image from ISO It provides known digital values to an imaging system. The output can be assessed visually and benchmarked regarding print quality with other imaging systems. There are a number of synthetic images that are also included in the test form shown in the Appendix A. The 15-step grayscale, element (2), situated beneath the pictorial image, consists of digital Figure 12. Proofing a multicolor job values, in terms of percentage dot area. These input values are identified at each step of the gray scale. By means of cell width measurement of the Ballard shell, we can characterize the tonal relationship between percentage dot area and widths of the gravure cells. In addition, if we measure density of the proof, we can characterize the tonal relationship between widths of the gravure cells and density of the image. The tone reproduction study is key to defining, verifying, and documenting standard operating procedures (SOP) from engraving to printing. The vignette or continuous-varying grayscale, element (3), is situated beneath the 15-step grayscale. It shows the degree of smoothness of the tonal transition while identifying the smallest and the largest printable dots. In gravure printing, it also unveils the nature of continuous-tone printing. Specifically, a three-quarter tone (or 75 percent dot) will print not as dot patterns, but as a solid. The first point of a vignette that prints as a solid ink film is also known as the flooding point. The density of the proof continues to increase after the flooding point. This is attributed by the cell depth variation of the gravure cylinder. Figure 11. Proofs from a step-and-repeat cylinder The 100-step grayscale, element (4), is a refined representation of both the 15-step grayscale and the vignette. Finally, the L- 46 GRAVURE/February 2006

6 shape test target, element (5), is a repeat of full-tone solid and 50 percent dot. By situating the test target across the width and along the direction of printing, we can evaluate the spatial uniformity of printing by analyzing density data statistically. Appendix A. RIT Monochrome Test Form Acknowledgments The author wishes to recognize Walter Siegenthaler of MDC for his advice and continuing support of the Ballard shell project. He also wishes to thank the following people at Keating for their generosity and courtesy in cylinder engraving and proofing: Phil Pimlott, Charles Forshey, Colin Pratt, and Tim Bateman. He further wishes to thank his colleagues, Edline Chun and Franz Sigg, for their review and comments. Without the kindness and knowledge sharing of these individuals, the article could not have been written. About the Author Bob Chung is a professor in the School of Print Media, Rochester Institute of Technology. Bob was named the RIT Gravure Research Professor in 2004 with the charge to explore research, develop curriculum, and promote career opportunities in gravure printing and packaging industry. He is interested in your comments regarding this article and any suggestions that you may have to further gravure research and scholarship. Visit his web site at edu/~gravure or him at rycppr@ rit.edu. GRAVURE/February