Offset lithographic plates have the printing image in reverse. Offset Lithographic plates are typically composed of thin plates of aluminum. Technological evolution has brought changes in offset lithographic plates. Plates are coated from light sensitive materials, which eliminates the steps of making a film positive.
Using a modern film & plate process, printers are able to produce plates that reduce materials consumed & time required to produce the final plate. A photopositive plate is engraved with printing image either using film to transfer the image or directly from computer to plate. When using film, it is 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. One plate is made for each ink color used on the job.
There are eight different types of litho plates common to the commercial printing industry. The predominant surface plate in use today is termed a "presensitized" plate. Most printers will primarily use one or two types of plates. It is highly unlikely that a printer uses a few of each type of plate, nor is it easy to switch to a different type of plate due to equipment, expense and application resasons.
Eight Offset Lithographic Printing Plate Types:
- Silver Halide
- Electrophotographic (Electrostatic)
Diazo plates are coated with organic compounds and are developed with a special solvent. They have a shelf life of about one year. These are used for print runs of about 150,000 impressions.
Photopolymer plates are coated with organic compounds, which are very inert and abrasion resistant. This makes them last much longer than diazo plates. They are used for print runs of up to 250,000 impressions.
Silver halide plates use photosensitive coatings similar to photographic film, except that the silver halide emulsions are slower and for color reproduction are coated on anodized aluminum. The processing solutions contain silver, which must be recovered with the proper equipment before being discharged to the sewer. Film based silver halide plates are used for single color printing and metal based silver halide plates are used in computer-to-plate systems.
Electrostatic plates are based on the principle of the electrostatic copier. There are two types, inorganic photoconductors on a drum, and the second is organic photo conductor on a substrate. These are used mostly in quick printing jobs of 100,000 impressions or less.
Bimetal plates use presensitized polymer coatings. There are two types of bimetal plates; copper plated on stainless steel or aluminum and chromium plated on copper. These are the most expensive, but rugged plates are used for very long print runs. In fact, they are capable of print runs in the millions.
Waterless plates, used on waterless presses only, consist of ink on aluminum for the printing areas and a silicone rubber for the non-image areas. These systems require special inks and high grade paper to avoid debris accumulating on the blanket.
Ablation plates are imaged by digital data and requires no chemical processing. These plates are digitally imaged by selectively burning tiny holes in thin coatings of a polyester or metal base. These types of plates are used on the new computer-to-plate imaging systems and the brand new computer-to-press system. The cost of equipment and materials is high and the technology is relatively new.
Heat-sensitive plates are exposed by infrared diodes in a special imagesetter and processed in water-based chemistry. This is relatively new technology and requires the printer to invest in new equipment that can be quite costly.
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.