Damage Control

The key to a high-performance, flawless flexo printing plate is time—imaging time, processing time, and drying time.

by Kate Sharon, Associate Editor

EMERIL LAGASSE IS a thorough chef who doesn't take shortcuts when preparing his mouth-watering cuisine. He never skips over the most tedious of cooking chores, and he never forgets to "take it up a notch" or add a "bam!".

As flexo platemakers take it up a notch to meet today's demands for faster turnarounds, shortcuts can quickly spoil the key ingredients consumer product companies need most to build their brands: accurate and consistent color.

The platemaker's role is more important than ever in today's fast-paced production environment and good platemakers have learned that taking shortcuts in the platemaking process often produces plates that fail, said Charlotte Cushing, senior technical service analyst for DuPont Imaging Technologies.

"It comes down to process control," Cushing said. "You have to follow all the steps for quality control—for consistency, to construct a quality plate every time."

Step by step

Making flexo plates is a fairly simple process. According to the Cyrel Process-of-Use Manual, there are six basic steps to platemaking: back exposure, main exposure, processing, drying, finishing, and post exposure.

On the imager, a plate undergoes both back and main exposure. During back exposure, "a photopolymer (UV light-sensitive, extruded plastic) plate is exposed to UV light through the dimensionally stable polyester backing to build/polymerize a predetermined floor on which the image will secure to," explained Jim Vertullo, president of JV Imaging Systems, Inc.

Main, or front, exposure is a bit more complicated. In this step, "a film negative with a matte emulsion containing the desired image is placed in contact with the surface of the plate after the protective sheet is removed," Vertullo said. "… An exposure cover sheet is then placed over the negative and plate, and the air is removed by introducing vacuum to the underside of the sheet, thus removing any air trapped between the plate, negative, and exposure cover sheet. … At this point, UV light is introduced to the plate and negative 'sandwich' and the polymerization/curing process begins." When front exposure is complete, the image will have transferred from the negative to the plate.

Then the plate is put through the processor where wash out takes place. With analog and digital plates, the unexposed photopolymer is washed out by a solvent or water, and aided by a brushing action. Thermal plates and digital plates can be processed with DuPont's Cyrel®FAST system, which uses no solvent to wash out the plate, but instead uses a nonwoven web material and controlled heat sources to "wick out" the remaining unexposed portions of the relief layer.

Next is drying, the longest of the steps for analog and digital plates processed using solvents. "Plates swell during the washout process and must be dried back to their original thickness," explains Ronnie Scogin, technical service representative for MacDermid Printing Solutions. Drying can take anywhere from one to three hours.

Once drying is complete, the plate needs to be finished and post exposed, which can be done at the same time, Scogin said. Light finishing removes any tackiness from the plate surface, which is done by exposing the plate to a UV Germicidal light. Post exposing completes the curing process, Vertullo said. "The entire face of the plate is exposed to light to maximize its longevity on press and its resistance to solvents that are used in the inks and coatings the plate will be used to print," he added.

However, before any plate undergoes this platemaking process, it's necessary to test the plate material for optimum exposure, processing, and drying times. "It is immensely important that exposure testing is performed on each new batch of material, both back and face exposure tests," said Rob Smithson, CEO of Trinity Graphics USA.

Scogin stressed the importance of determining an exact washout time. He explained: "To make sure the entire unexposed polymer is washed away from the plate, it's necessary to establish the wash rate. This is done by putting a piece of unexposed polymer through washout for 'x' amount of times, then measuring it with a micrometer to see exactly how much polymer was washed away. It can then be calculated how long the washout [period] will need [to be]."

Drying tests are also important. Under-drying the plate will leave it swollen and out of compliance with the design requisites. On the other hand, over-drying the plate will leave it brittle.

Plate problems

Four of the most frequently occurring plate problems are plate cracking, uneven plate thickness, pinholing, and marks from a plate imaged "out of contact." All of these predicaments can result from improper exposing, processing, or drying.

Plate cracking: A probable cause of this prevalent problem is ozone, Scogin said. Cushing further explained that ozone is mostly a seasonal problem, occurring during the summer months with extreme heat. However, she said, while ozone is most often generated outside, an extruder located in the plant could have the same effect.

Exposure to light is another factor that could result in plate cracking. In the same vein, said Vertullo, plates that are over-finished are subject to cracking as well. "As with other steps in the process, more is not better," he said. "Light finishing is different for each plate material and should be done within the guidelines set forth by the manufacturer."

Storage is the No. 1 factor in protecting a plate from ozone, Cushing said. She suggests keeping the plate out of the light and using a UV filter to protect plates from overhead shop lights, hot sun coming in windows, and the environment.

Uneven plate thickness: There are several reasons why plates end up with uneven thickness. Cushing lists insufficient exposure, excessive washout, too low of a drying temperature, and incompatibility of the ink and/or solvent on press. Careless handling of the plate is another possible cause, Scogin said.

Foremost, it's important to gauge the thickness of the starting material using a micrometer, Scogin said. That way, when establishing the optimal process times, platemakers have a criterion on which to compare an imaged and/or processed plate.

If the plate has been produced properly, then handling procedures can be addressed if need be, Scogin said.

Pinholing: Pinholing is created by what is sometimes called "white static." If dust and dirt are not removed from the negative and plate before the exposure cover sheet, or the kreen, is pulled across and vacuum is applied, the contaminants can appear as marks on the plate. These marks will also show up on the printed substrate, Cushing said.

One possible solution to pinholing is to "change or clean the kreen or opaque the film negative on the non-emulsion side," Scogin said. In addition, Cushing suggests keeping the exposure bed clean, and eliminating any dust under the negative sheet and on the exposure unit. Implementing static control in the platemaking room is a possible remedy, she said.

Out of contact: This problem refers to improper drawdown of the plate and negative during expsoure, which results in print flaws like fat type, shiny spots, and filled-in reverses, Cushing said. It is important to use the correct film specifications and equipment vacuum.

Removing any air between the negative and plate is essential, Vertullo said. "This is an important step because pockets of air will disperse light as it passes through the emulsion of the negative, thus affecting the clarity of the exposure," he said. "Also, entrapped oxygen will inhibit the formation of small dots and characters."

Training is critical in the successful operation of the platemaking room. Operators have to be skilled in running the imager and processor, as well as the dryer. They also have to be knowledgeable about the photopolymer plates they use. But most importantly, operators have to understand the correlation between following the steps of platemaking from "A" to "Z" and high-quality, well-performing plates. Once this happens, platemakers will have earned the right to throw in a "bam!" for thoroughness that would make Lagasse proud.

Resources

Anderson & Vreeland www.andersonvreeland.com

BASF Printing Systems www.basf.com/printingsystems

Container Graphics www.containergraphics.com

DuPont Cyrel® Packaging www.dupont.com/cyrel

JV Imaging www.jvimaging.com

Kodak Polychrome Graphics www.kpgraphics.com

K. Walter-Hell Gravure Sys. www.hell-gravure-systems.com

MacDermid www.macdermid.com

RBCor www.rbcor.com

Trinity Graphic www.trinitygraphic.com