The success of most security printing features lies in their difficulty to be reproduced. This can be accomplished by using novel and difficult-to-source materials like color-shifting inks, unique watermarks or holograms. Another approach is by employing designs and patterns that are difficult to reproduce, like guilloché patterns and microprint.
In this, the final report on my research funded by the 2017 FFTA Rossini North America Flexographic Research Scholarship, I will discuss a technique employing print cylinders with varying repeat lengths to create designs and patterns that have exceptionally long print lengths. In this research, a job was printed with 12-in., 18-in. and 24-in. cylinders that created 72-in. of unique print before the pattern repeats. While this technique was investigated for the purpose of security printing, it also has direct application to products like wallpaper and giftwraps.
Materials & Methods
Standard flexographic printing practices use the same plate repeat size across print stations. The only time a print station is likely to have a different-sized cylinder, or plate repeat, is for a white or varnish station in which registration is less likely to matter because the cylinder would be seamless. A practice few printers in the flexographic industry have put to use is printing with different-sized plate repeats on the same pressrun. In the past, printer and FTA member International Paper has used this technique to print variably designed cups, with different design repeats having varying colors and elements. This is a fairly unique practice, but one that also enables printers to get multiple, variable design repeats from a single pressrun, instead of many.
In order for the design repeats to register to one another, variable repeat printing requires the use of greatest common divisors when choosing plate sizes. For my first design (see Figure 1), the three cylinder sizes I utilized were 12-in., 18-in. and 24-in., which have a common factor of 6-in. Six inches was the size I used for the repeat of both my designs. The least common multiple of 12, 18 and 24 is 72, my larger variable repeat size over which the design repeats completely. I could fit two 6-in. cars on the 12-in. cylinder, three on the 18-in. cylinder, and four on the 24-in. cylinder.
When printed together, these cylinders produced 72-in. of unique print for a total of 12 differently colored cars. This is because the first combination of cyan, magenta, yellow and black (the first car on each plate), did not line up or register again until after 72-in. of print had been completed.
The second design concept I tested was a bit more abstract. My original inquiry was as to whether or not variable repeat printing could be a valuable asset to the security printing industry. I decided to formulate an idea to make text change over a much larger design repeat. The idea I came up with was to break characters into different elements, specifically breaking the letters for the word “SECURE” into vertical, horizontal and diagonal elements. For example, the letter “S” would have two horizontal elements and one diagonal element. The letter “E” would have three horizontal elements and one vertical element. Vertical elements were placed on a 12-in. repeat plate, again with 6-in. smaller design repeats. Diagonal elements were placed on an 18-in. repeat plate with 9-in. smaller design repeats, and horizontal elements were placed on the 24-in. repeat plate with 6-in. smaller design repeats. Each plate was again output through a separate Adobe Illustrator file. By separating elements of each character onto different plate repeats, the word “SECURE” was only formed after every 72-in. Figure 2 illustrates this second design.
Variable repeat printing requires meticulous prepress work. Whether the printer is using variable repeat printing to print different shapes and images in a single pressrun or different colors or patterns, the design must be very well thought out. My designs included both variable patterns and colors. Each color separation had to be ripped as a separate LEN file because of the distortion each plate repeat required. I actually had a different Illustrator file for each separation. Each of those files had to line up and match exactly to the others in order to ensure the artwork would register on press.
I was gifted print cylinders by RotoMetrics and utilized CS6 paperboard and 3M 1020 stickyback. The print cylinders had a 0.164-in. undercut and the stickyback was 0.015-in. thick. Before I completed my pressrun, I ran a test using my cylinders to ensure the Nilpeter FB3 press would handle different-sized cylinders. I also completed inkjet proofs on an Epson SureColor proofer to make a series of clear overlay proofs to ensure my color separations would print the way I intended.
In order to print using varied repeats, the press must be able to handle different-sized cylinders being run in tandem. I found that so long as the cylinders I used had the same tooth size, the same gear ratio and the same undercut, the various cylinder sizes worked in tandem. My print trial took place on a servo-driven Nilpeter FB3 press, in which I was able to change the plate repeat size setting station by station.
My pressrun was limited by the fact that the largest cylinder size I could fit on press and still register to other cylinders was 24-in. One way to achieve a much, much greater overall variable design repeat would be to add a prime number or even non-integer (decimal) sized cylinder into the mix. For example, if I had switched one of my 12-in. cylinders to a 13-in. cylinder, instead of having a 72-in. overall design repeat, that number would change to 936-in.—13 times larger.
The problem with throwing prime numbers or non-integers into the mix is that they do not share common factors, making registration difficult. Artwork in general, in order to change color or elements, needs to share a greatest common divisor. For example, two 6-in. design repeats would not fit evenly across a 13-in. cylinder like they would across a 12-in. cylinder, or three would across a 19-in. cylinder
For a pattern, or another design where registration is not a concern, adding a prime numbered or non-integer plate repeat length would be an excellent way to achieve greater print variation. Using in-the-round (ITR) plates, or plates with no seam, would be a better solution for patterns printed using variable plate repeats. This would minimize any possible breaks in the pattern and in print. In my cars design, plate breaks were not crucial because I had dead space, or non-imaged space in between each car. For a pattern where lines and elements are continuous, ITR plates would help prevent breaks in print.
The main applications I theorized variable repeats could be utilized for include wallpapers, gift wraps, tapes and other markets that would benefit from extended, less-repetitive patterns. As an application to the security printing industry, I found variable repeats could be an asset, as the prepress and pressruns would be harder to replicate. Cutoffs of a variable patterned product would be nearly impossible to duplicate.
Another benefit of variable repeats is a cost savings on plate materials. The example I will use is a narrow web, 12-in. wide design, utilizing my first car design scenario. To achieve the same product that I did, a printer would have to use four 72-in. repeat plates. Each plate would have an area of 864 sq. in., a total of 3,456 sq. in. between all four plates. By splitting the design among one 12-in. plate, two 18-in. plates and one 24-in. plate, the printer would only consume 864 sq. in. of material. This is a material savings of 2,592 sq. in.—75 percent less material! If printers are able to print more product with less plate material, they should save money.
This research was funded and made possible by the FFTA Rossini North America Flexographic Research Scholarship and through donations by RotoMetrics. I would like to give a special thank you to FTA, my advisor Dr. Liam O’Hara, Bobby Congdon of the Sonoco Institute for Packaging Design and Graphics at Clemson University, professor Kern Cox, Paul Teachout of Nilpeter, Christine Ott of RotoMetrics, and most of all the Rossini family for enabling me to pursue this research.
About the Author: Thomas Koester is a fourth-year student at Clemson University majoring in graphic communications with a packaging science minor. He has worked in the Sonoco Institute for Packaging Design and Graphics at Clemson University for four semesters as a press assistant, interning with Printpack and DuPont Cyrel in the summer of 2016 and fall of 2017, respectively. Throughout his education at Clemson, Thomas has become interested in the more technical aspects of printing and packaging, especially with print and packaging security. Thomas is the recipient of the 2017 FFTA Rossini North America Flexographic Research Scholarship and presented the results of his research at Forum 2018 in the session titled “Flexo Quality Consortium: Cultivating Our Future Through Research.”