Flexo Plate Sleeves

The world of flexo plating moves from flat to round.

THOUGH IT MIGHT be tough to remember, it was only a few years ago that computer-to-plate (CTP) for flexo was considered a speculative technology. Today, with many hundreds of digital flexo imaging machines installed around the world, it's fair to say that flexo CTP technology is here to stay. And, as is typical of our ever-changing industry, when one technology matures, another is bound to emerge.

The new kid on the flexo platemaking block is in-the-round, or ITR imaging. ITR differs from standard digital flexo imaging in that the photopolymer plate material is actually imaged and processed directly on a sleeve that will subsequently be loaded onto the press versus mounting a flat plate to the printing cylinder. The advantages of ITR are numerous but the logistical and cost factors can be considerable.

One thing moving adoption along is the prevalence of the gearless CI and inline presses that require sleeves or are sleeve compatible. Sleeve systems reduce press downtime, and at today's hourly rates, that's significant. While one job is on press, the next set of sleeves can be ready and waiting. Sleeves may reduce inventory and tooling costs as well. Using sleeves with varying thickness, a printer might only have to buy nine cylinders to outfit an 8-color press instead of 16—eight to fill the press, and one to use as a base for mounting plates on a second set of sleeves.

Sleeves have other inherent pressroom advantages, too, such as savings on sticky back. Plus, since the plates are stored on the sleeve, printers don't run the risk of stretching or ripping them when they're removed from print cylinders.

From a quality standpoint, ITR imaging offers an exponential improvement over flat plate imaging. The distortion factors and skew errors associated with imaging a flat surface and then mounting that flat plate around a cylinder are completely eliminated. Color-to-color registration is achieved far more quickly because all sleeves are imaged at the same origination point and orientation.

One of the quality challenges with ITR imaging, however, has been the problem of edge cure. It is extremely difficult by conventional methods to mask the outermost edges of the sleeve polymer to prevent it from being exposed by the light source. This can cause the edges of the plate sleeve to retain an undesirable relief that will transfer ink to the substrate. Currently, platemakers use a variety of workarounds to mask the plate edges, such as painting, airbrushing, and taping on a UV blocking agent. These methods work to varying degrees of success, but hand-masking is often time-consuming and is by no means an exact science.

According to Tom Cassano, technical services manager for MacDermid, the problem of edge cure "might be the single most important technical issue for ITR flexo platemaking." Cassano says MacDermid, a manufacturer of photopolymer plates and ITR processors, is currently working on a new mechanical solution for uniformly applying a UV blocking agent to the edges and sides of the plate sleeve to prevent edge cure. While Cassano wouldn't give a release date for the new technology, he made it clear that MacDermid considers solving the problem a "top priority for enabling ITR flexo platemaking."

Next generation ITR technology

Another enabling technology for the use of sleeves in flexo is direct ablation photopolymer imaging. Solvent washable, water washable, and even the newer dry thermal plating systems all share a similar preparation cycle requiring multiple exposures, washout, drying, and post-processing. In the direct ablation process, lasers etch away the non-image areas of the photopolymer and the residue is simultaneously collected by vacuum and disposed of as solid waste. This imaging process—consider it like a reverse gravure engraving process—doesn't require the additional preparation steps of washable or dry thermal platemaking.

In 2000, Zed Instruments launched one of the first direct ablation systems for flexo and has since reported significant success with the equipment in Europe and Japan. Holland-based Stork has also entered the flexo direct ablation market with two new machines, the single-beam Morpheus (which can also image rotary screen cylinders) and its big brother, the three-beam Agrios.

Stork is no novice in the flexo ITR arena having pioneered the endless, seamless plate almost 20 years ago under the trade name Seamex, which is now owned and successfully sold by OEC Graphics. Nor is the company unfamiliar with laser engraving. Says Harry McKay, head of sales and marketing for the graphics business unit of Stork Print Group, with U.S. headquarters in Charlotte, N.C., "The largest part of our company has traditionally serviced the textile industry [where engraved ceramic and rubber cylinders are routinely used]. It makes perfect sense to take all of that technology and translate it to the flexo industry."

Direct ablation holds the promise of a truly "clean" plating operation. In addition, the process allows for imaging at variable relief depths across the plate—making it possible to "shorten" highlight dots for example so as to reduce dot gain on press. The greatest challenge of the process has been to develop plate material that can be imaged at the high screening resolutions (133, 150, and 175 lpi) required for top-quality flexo process printing.

And, of course, as a new process requiring capital equipment, cost remains a significant barrier. McKay sees large printers with multiple plants and in-house plating as the most likely early adopters.