Flexographic Plates
Rubber plates are solid rubber pieces made from a mold. The mold can be used over and over to make new plates. Photopolymer plates have taken an increasing share of the flexo plate market. As the name suggests, photopolymers are light sensitive polymers that crosslink when exposed to UV light. Photopolymer flexo plates are flexible and resilient like rubber plates.
There are many candidate materials for photopolymer plates. They may be viscous liquids or solid sheets. The steps for preparing photopolymer plates are
1. Back exposure of base to UV light to harden (cure) the floor and establish relief depth.
2. Face exposure of surface to UV light through a negative to cure the image area.
3. Washout of unexposed polymer with appropriate solvent.
4. Drying to remove solvent and restore gauge thickness.
5. Post expose to final cure of floor and character shoulders.
6. Finish plates with chemicals or UV light to remove residual tackiness.
Negatives for flexo platemaking may be produced either digitally or photomechanically, although most are made digitally today. Photopolymer plates have made it possible to print process color jobs with halftone screens up to 150 lpi. They produce accurate multicolor registration and hold fine halftone dots.
Solid photopolymer plates are generally produced by crosslinking a rubber, or elastomer, with a photoreactive material Because of their low activation energy, acrylates are commonly used. Even these require the use of a photoinitiator to achieve curing in a reasonable time. The photopolymer layer in an unexposed flexo plate contains a thermoplastic elastomer, a polyfunctional acrylate monomer, a photoinitiator and additives. UV light activates the photoinitiator, which starts the crosslinking.
Many different elastomers may be used: even natural rubber can be crosslinked with acrylates. A popular photopolymer plate is produced by crosslinking poly(2-chloro-1,3-butadiene, commonly known as chloroprene) with trimethylolpropane triacrylate. Upon exposure the trimethylolpropane triacrylate crosslinks the polymer rendering it insoluble. Other flexo plates employ styrene-isoprene rubber with a polyacrylate. The photoinitiator accelerates the reaction by UV light.
When the photoinitiator absorbs UV light, it splits apart into free radicals. The unpaired electrons on the free radicals attack the carbon-carbon double bonds of the elastomer or the acrylate, which open up and and react with other double bonds. This process crosslinks the elastomer. Typical photoinitiators are benzophenone, benzoin and benzil. The photoinitiators generally become part of the polymer chains and can therefore initiate very long polymer chains.
The washout solvents are frequently chlorinated solvents, such as perchloroethylene. Because of environmental hazards of chlorinated solvents, substitutes are being sought. Introducing carboxyl groups into the elastomer improves the solubility of the non-cured elastomers in washout solvents. Copolymerizing nitrile rubbers with acrylic acid allows them to be washed out with glycol ethers. Liquid photopolymer plates are made from partially polymerized acrylates or other unsaturated products.
Most photopolymer plates work well with alcohol and water based inks. Oil based inks tend to swell natural rubber. Sheet photopolymer for oil based inks generally consist of butadiene-acrylonitrile elastomers. Overall plate thickness and relief height vary with the substrate. Plates for corrugated printing tend to thicker with greater relief than for printing film or paper. For corrugated a typical thickness is .28" with a plate thickness of .25" and a carrier of .3".