Flexographic plates have the printing image in relief, which means the image area is raised relative to the non-image area. As the name implies, flexographic plates are very flexible and are typically composed of rubber type compounds. Technological evolution has brought changes in flexographic plates. Plates made from light sensitive photopolymers are now commonly being used throughout the industry. Photopolymer plates are similar to traditional rubber plates in that they are flexible and resilient, but their use poses different environmental concerns.
Using a photographic film process, a film negative is produced. The film is then used to expose the plate material, which is coated with light-sensitive chemicals, to UV light. The plate is then chemically developed using water or solvent to remove the unexposed image areas of the plate ultimately producing a plate with raised image that is identical to the artwork. One plate is made for each ink color used on the job.
Types of Plates:
- Rubber Plates
The original flexographic printing plate was composed of rubber. There are several steps to make molded rubber flexographic printing plates. A master pattern (or engraving) is made by exposure through a photographic negative. A photographic negative is imaged and developed by using traditional black and white silver halide film and chemistry (i.e., developer, fixer, and wash water).
Photopolymers are ultraviolet (UV) light sensitive materials. Photopolymer plates used for flexographic printing plates are similar to rubber plates in that they are flexible and resilient. Photopolymer plates are either viscous liquids or solid sheets of appropriate thickness.
Dry Thermal Photopolymer Plates
Dry thermal plate processing eliminates the use of solvents, reduces plate making time and improves plate quality. This system eliminates the need for conventional chemical solvent or aqueous washout.
Digital Photopolymer Plates
Digital photopolymer flexographic plate imaging uses no film, and is not actually part of the traditional prepress process. Conventional plate making generates chemical waste from film processing and solid waste from the films. Multiple generations of films are usually produced during job approval and for conventional analog proofing. Digital plate making only generates a thin black integral mask as waste. It promotes the transition to an all-digital workflow that employs digital proofing for job approval and final proofs.
Health & Safety
Perchloroethylene (or perc, for short) is a known carcinogen and should be replaced with a perc alternative solvent (PAS). Water washout or dry thermal systems are other alternatives.
PASes may be flammable and must be handled with care to avoid ignition and fire. Fires can be caused by heat from friction, static electricity and sparks. Good work practices involving the use of PASes in plate processing include the following:
- Do not eat, drink or smoke in the work area
- Use of chemical splash goggles when handling drums and processed plates
- Wear nitrile gloves and protective apron, when necessary, to protect against chemical contact
- Use well designed and maintained local ventilation and source enclosure to control airborne chemicals at the source.
Manufacturers recommend careful handling of the waste resin from liquid photopolymer plates as it can act as a skin irritant.
Other safe work practices for the platemaking area:
- Safety glasses and protective gloves should be used when cutting and trimming plate material
- Purpose-made ultraviolet-blocking glasses designed to block specific wavelengths and diminish intensity, to be used when working around plate exposure and finishing equipment.