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FLEXO Magazine : October 2013
“plate”—a sheet that had a thin coat of acid-resistant mate- rial. When light was exposed to the plate, the acid-resistant layer hardened where the non-negative areas of the film existed. The material that was not exposed remained soft and washed away after the exposure process. The plates were subsequently etched to remove the remainder of the unprotected areas and a mold was made of this plate. When a warm sheet of rubber was fit into the mold and cooled, it became a flexo plate. This process was extremely involved, labor-intensive and full of variables. Of course, the process also made it impos- sible to create the type of work we produce with flexo today. Imagine if there was an error on the plate? The entire process would have to be reproduced again. PHotoPoLYMer revoLution Photopolymer platemaking technology changed the world of flexo—and packaging—when it was introduced in the early 1980s. No longer did we have to rely on rubber flexo plates and the idea of actually printing reasonable 4-color jobs on a flexo press was quite exciting. This process also made it easier for new companies to open their doors as suppliers to produce plates that were made within reasonable standards of tolerance. Expectations of consistent performance could be achieved on a regular basis. Beside the noticeable quality advantages, it is much easier to produce photopolymer plates than rubber plates. The first step is still the same: The creation of a negative. By the time the 1990s arrived, this was typically done in a film imagesetter. On a light frame, a plate is exposed to harden the back of the plate. Then, after it is flipped over, so the unexposed side of the plate is facing up, a negative is placed over the plate and light is again exposed to the plate. The areas of the plate that are exposed then harden. From there, the plate is placed in a processor, removing the unexposed mate- rial, which can be safely discarded. After going through the processor, the plate is dried and given a post-exposure of light to cure the plate. Most people who produce flexo plates without an imager still create plates using this process. In fact, the computer-to- plate (CTP) processes that use nitrogen to purge oxygen from plates still utilize pretty much this same process to create their plates. Some also use a clear lamination layer or high bursts of ultraviolet (UV) light. Early processing was beset with chemical issues and recycling was not yet a concern. iMAging on PLAte The introduction of ablation in 1995—the process of burn- ing off the top layer of a flexo plate to, in essence, create the negative image directly on the plate—has significantly changed the flexo industry by offering direct imaging and much higher—at times, 4000 dpi—resolution. Direct imaging from the laser to the top plate layer has extended the imaging of dots from zero to 100 percent at high linescreens and has made flexo competitive with other forms of printing, such as offset and gravure. The process is similar to photopolymer plate processing, except that the top plate layer that is imaged on a CTP device is, indeed, the negative. Portions that are “imaged” are ablated, or removed from the plate with laser power. From there, the exposure processes are similar. One thing has changed in this process: There are now plates available that eliminate solvents in plate processing. For example, the thermal process melts away the unexposed material. Water-washable plates (as opposed to solvents) are also available. consistent eXPosure There is, however, a challenge that persists: Getting consistent light exposure throughout the plate from the frame is still difficult. Bank lights are not consistent over their lives, degrading from 250 hours to their expected 5,000-hour life- times. Fluorescent tubes were also designed to give very wide light dispersion for office space, rather than to deliver light to specific, focused areas. While compensation is typically done a couple times each year, the degradation is different for each individual lamp and is related to the number of hours the frame is used. This lack Inline UV exposure also provides a consistent light source. 78 FLeXo OCTOBER 2013 www.flexography.org FLEXO WITHOUT COMPROMISES Natural image reproduction Strong colors in contrast with light tones Vibrant colors Smooth transitions to zero Solid colors, strong homogenity The unique Full HD Flexo solution allows you to achieve fine highlight printing, smooth transitions to zero and an outstanding gravure-like solid ink lay down. Learn more about Esko’s innovative flexo solutions on esko.com/flexo. firstname.lastname@example.org