Abstract

To bring gravure technology into the classroom and laboratory,

the author visited a gravure engrav-ing plant. With photographic illus-trations, he describes an automated engraving operation and a manual-ori-ented cylinder proofing operation.

IntroductionGravure imaging by means of cylin-der engraving and proofing requires special-purpose equipment, skilled operators, and capital investment in process automation and environmen-tal compliance. This makes an engrav-ing company, such as Keating, a valu-able supplier to many packaging print-ers. The same reason makes teaching gravure technology with hands-on components almost impossible unless there is a business model to help sus-tain 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, stu-dents perform gravure-related labora-tory assignments for process compre-hension. 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 engrav-ing to proofing using the Ballard shell method. In doing so, I would record the steps involved in cylinder engrav-ing using an Ohio electromechani-cal engraver [Ohio is a part of MDC since 2000] and proofing with a JM Heaford proofing machine. By pro-viding 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.

Gravure Research Agenda:The Journey of a Test Form from Engraving to Proofing

Bob Chung, Rochester Institute of Technology

Figure 1. A computer-integrated engraving environment

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the test form from an InDesign CS file to a PostScript Level 3 file using an appropri-ate 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, deter-mines the amount of ink it will hold and transfer to the substrate upon printing.

There are a number of cylinder preparatory steps prior to electromechanical engrav-ing. 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 cor-rect 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 nonad-hesive 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 engrav-ing head to ride on the surface freely.

Keating at a Quick GlanceKeating (www.keatings.co.uk) is the largest cylinder engraver in the packaging print-ing industry in England. Keating (www.keatinglobal.com), a Sonoco-owned company, established its North American operation in 1997. Based in Charlotte, North Carolina, Keating is strategically located near MDC (www.daetwyler.com) 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 engrav-ing, 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.

Cylinder TransportingA cylinder weighs anywhere from 200 to 600 lbs. In a computer-integrated engrav-ing environment, cranes with robotic

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-in-progress.

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 send-ing out an alarm.

Cylinder EngravingAll high-end raster output devices, e.g., platesetter, inkjet printer, electrophoto-graphic printer, are driven by PostScript code. This is also the case for an electro-mechanical engraver. The conversion of

Figure 2. A crane with robotic arms transports cylinders

Figure 3. Cylinder surface lubricating prior to engraving

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inder, and takes it to the next station for degreasing and cleaning (Figure 7). The automated process takes about ten min-utes, 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 depos-its 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 finish-ing station (Figure 8). The buffing action produces smooth and fine crisscross pat-terns 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 ProofingTo verify and to document that correct engraving procedures have been carried out, cylinder proofing is required at Keat-ing before a cylinder is shipped to a con-verter (printers are known as converters in the packaging industry).

There are two levels of interest when it comes to proofing—i.e., proofs for inter-nal use and proofs for external use. An

Figure 6. Engraver shield is closed once the engraving starts

When image carriers for offset lithog-raphy 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 engrav-ing process hands-off. Figure 6 shows the engraver being shielded to reduce engrav-ing 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 TreatmentAs soon as the engraving is completed, the crane comes over, picks up the cyl-

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 cali-brated 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 pro-duce just printable inked dots. The width of the shadow cell is set at 160 μm. The

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 calibra-tion using the edge of the cylinder as the test area. The engraving head is equipped with machine vision at high magnifica-tion. 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 specifica-tions.

Figure 4. A schematic of gravure cells (courtesy of GAA)

Figure 5. Engraving head is equipped with machine vision

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internal proof helps verify that no engrav-ing-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 inter-ested in the mechanism of gravure print-ing. A cylinder proofer has the same com-ponents as a gravure press. Thus, a color-managed digital proof is outside the scope of this paper.

As shown in Figure 9, the RIT mono-chrome test cylinder has been manually placed in the JM Heaford proof press (http://www.jmheaford.co.uk/). 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 craftsman-ship. It is fascinating to watch the opera-

tor going from step to step to transform a number of engraved surfaces into an ink-on-paper proof.

Electrostatic assist (ESA) facilitates the production of a paper-based cylinder proof. To do so, (1) the impression cylin-der 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 cyl-inder is 3 meters long while the circum-ference of the engraved cylinder is only 1 meter long. We can produce three repeats of the engraved images per proofing oper-ation. 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).

Multicolor ProofingA good packaging design builds brand identity, adds value to the package, and promotes customer loyalty. A good pack-aging 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-to-image registration, inking, printing, cyl-inder 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 8. Cylinder cleaning and buffing

Figure 9. A cylinder proofer

Figure 7. Cylinder degreasing, buffing, and cleaning

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inders (foreground) to be proofed on a proofer next to the proofer

running the RIT monochrome job.

Engraving as a Business Solution in a Specialty MarketMy observation of engraving and proof-ing 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-the-art engraving equipment and automa-tion serves as a barrier for others to enter into this specialty market. Finally, it is the people, management, and produc-tion together who face the challenge of continuously improving the processes and procedures of engraving and proofing so that first-pass yield increases while manu-facturing bottlenecks reduce.

About the Monochrome Test FormA 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 12640. It provides known digital values to an imaging system. The output can be assessed visually and benchmarked regarding print quality with other imag-ing 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

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 char-acterize the tonal relationship between widths of the gravure cells and density of the image. The tone reproduction study is key to defining, verifying, and document-ing standard operating procedures (SOP) from engraving to printing.

The vignette or continuous-varying gray-scale, 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.

The 100-step grayscale, element (4), is a refined representation of both the 15-step grayscale and the vignette. Finally, the L-

Figure 10. Pouring ink on the cylinder in front of the doctor blade

Figure 11. Proofs from a step-and-repeat cylinder

Figure 12. Proofing a multicolor job

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shape test target, element (5), is a repeat of full-tone solid and 50 percent dot. By situ-ating 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.

AcknowledgmentsThe 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 fol-lowing people at Keating for their generos-ity and courtesy in cylinder engraving and proofing: Phil Pimlott, Charles Forshey, Colin Pratt, and Tim Bateman. He fur-ther 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 AuthorBob Chung is a professor in the School of Print Media, Rochester Institute of Tech-nology. Bob was named the RIT Gravure Research Professor in 2004 with the charge to explore research, develop curriculum, and promote career opportunities in gra-vure 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 www.rit.edu/~gravure or e-mail him at rycppr@rit.edu.

Appendix A. RIT Monochrome Test Form