Plate advances bring flexo closer to offset

Developments in all aspects of printing plate and platemaking technology continue unabated. This article, because of space constraints, will be limited to flexo and ultraviolet flexo, not because innovations in offset, gravure, or any, of the other more mature technologies are any less important, but because the advances in flexo and UV flexo have, in just a few short years, transformed its capabilities dramatically.

Flexography historically used very thick printing plates. The typical plate five years ago was 0.250 in thick. This extra thickness is not really necessary, because the amount of image depth, or what's often called relief depth, is only 0.025 to 0.030 for narrow web and 0.050 for corrugated. The additional plate thickness was literally used as a shock absorber.

"The problem, if you have a very thick polymer plate, is that it's hard to image and etch that plate properly without losing a lot of the fine detail," says Dan Rosen, product coordinator with Polyfibron Technologies Inc.

Lately, the trend in North American and European markets has been moving toward thinner plates and the use of compressible foam tapes for cushion. On a thinner plate there is physically less image depth that needs to be imaged and etched. A thinner plate requires a considerably shorter processing time, which means less loss of detail. The result is a plate that can hold fine image detail, because the dots are physically smaller.

"When you affix a thinner plate onto a cylinder with a thick piece of foam tape, what you get is the same or even better compression and the ability to use high levels of impression without the loss of fine image detail," says Rosen.

Compressible foam tapes have come a long way as well, he says. "We've learned that not all foam tapes are the same. There are open-cell and closed-cell tapes with different levels of resiliency, and, typically, what printers require to get higher levels of reproduction is a tape featuring a high dynamic resiliency.

"The photopolymers used to make flexographic printing plates today result in harder plates than those used just a few years ago. Softer tapes allow printers to use plates made of a higher-durometer polymer material, and this is another factor that enhances the ability to hold very fine isolated dots and reverse type."

He adds, "Because we are now using thinner, higher durometer plates, it allows us to incorporate the latest developments in printing technology, such as stochastic printing. You couldn't do it with the thick, soft rubber plates. A thinner, harder plate is necessary, because the dots in stochastic printing are very small."

Marcia Cho, senior applications specialist with DuPont Cyrel, explains that it's her company's goal to push the development of flexo plate and platemaking technology to where it will equal offset. "If flexography hopes to compete with offset lithography, the plate material must hold fine images and fine resolutions. In this area, I believe we've made significant improvements."

Dupont recently introduced a plate called EXL, said to feature a very wide exposure latitude, excellent ink transfer, and increased durability.

A problem with flexo plates in the past has been ink compatibility. Solvent-free inks contain monomers and photoinitiators that diffuse into the plate polymer matrix, resulting in swelling and softening. The EXL plate is especially resistant to degradation and is compatible with most of the UV inks available.

"I believe flexo will soon be equal in quality to offset especially thin-plate UV flexo," says Cho.

Rosen agrees, adding. "All aspects of printing are making great strides, such as line screen quality, resolution, and fine detail quality. The harder, thin plate in combination with finer anilox rolls has brought flexography a lot closer to litho than anyone would have imagined just three years ago."

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