Photopolymer: The photopolymer coating consists of an epoxy resin which is sensitized with an organic compound. An organic solvent is used to process the plates after exposure to ultra-violet light. The coating on the unexposed areas of the plate are dissolved with the solvent, but the exposed areas become insoluble and are receptive to ink. The plates are capable of long press runs of up to 250,000. The plates can also be put through a baking process that allow the plates to be used for press runs of up to a million or more. Temperature and humidity swings do not affect the plates which can be stored for extended periods before they are actually used. There are also dye sensitized photopolymers that are used for digital plates. The plates are exposed by lasers for use with computer to plate systems.
Presensitized Diazo: A presensitized diazo plate is contacted with film negatives that have been stripped into flats and are exposed to ultra-violet light. After exposure, the plate is treated with a lacquer and a gum-etch solution. The lacquer is deposited on the exposed areas of the plate which makes these areas ink receptive and the gum is deposited on the unexposed areas which makes them water receptive. The presensitized diazo plate is known as an additive plate because lacquer and gum are added to make the plate ink and water receptive. They are capable of handling press runs of up to 150,000 impressions.
There are also prelacquered diazo plates which require a solvent wash after the exposure. The washing removes lacquer from the unexposed areas which become water receptive. The areas of lacquer that are exposed by the ultra-violet light are insoluble in the solvent and become ink receptive. The prelacquered diazo are subtractive plates because some of the prelacquered coating is removed from the plate. The prelacquered plates are able to produce runs of up to 250,000.
Bimetal: A bimetal plate consists of a metal base with a different metal adhered to it. Usually copper is plated onto a base of stainless steel, aluminum, or chromium. The copper has the property of being very ink receptive and the metals used for the base are receptive to water. Bimetal plates can be exposed using negatives or positives, but both types require an electro-chemical treatment to complete the processing. Bimetal plates are the most expensive type of plate, but they can produce press runs in the millions. The plates have optimum performance features because of the difference in the ink and water receptive properties of the metals used. The ink receptive metal is also know as image metal and the water receptive metal is known as non-image metal.
Photo Direct: A photo direct plate does not require the use of film in order to produce an image on the plate. A special type of platemaking apparatus is used which is like a large camera and plate processor all in one. Copy or artwork is place on a copyboard and an overhead camera records an image which is transferred to a plate contained within the machine. The machine automatically processes the plate so that it is ready for use on the press. A photo direct camera platemaker is convenient when the plates will be used on duplicator presses where many short runs are produced.
Electrostatic: A process that is also referred to as Xerography with plates produced with a process much like a photocopy. The plate is coated with a positively charged, light sensitive photoconductor. Like photo direct plates, artwork is photographed by a camera which transfers the image to a plate. The photoconductor on the plate is eliminated in the areas that have been struck by the light and the unexposed areas of the plate (areas blocked by the image on the artwork) retain their positive charge. A powdered toner with a negative charge is applied to the plate and is attracted to the remaining positively charged areas of the plate, resulting in an image. The powdered image is then fused and made permanent by radiated heat.
The quality of the dots on electrostatic plates may not be as good as with other plate processes, so they are not used for color work or any other type of high quality printing. They are also not intended for use with coated papers because the inks used for coated stocks dry primarily by oxidation and the press chemistry required for electrostatic plates inhibits the oxidation drying of the ink. The press chemistry for electrostatic plates is more suited to uncoated papers because the inks for uncoated stocks dry primarily by absorption.
Deep-Etch: The deep-etch offset plate is different than other offset plates in that the image area is slightly below the surface of the plate. The etching allows the plate to carry more ink than surface coated plates. The plate is made from an aluminum, zinc, or stainless steel base. Film positives are used to produce the image on the plate. After the plates are exposed and processed, the unexposed or image areas are etched and chemically treated to form a copper finish. The etching and chemical treatment makes the image areas more receptive to ink. The exposed areas (non-image areas) remain receptive to water (fountain solution). Deep-etch plates have a much longer press life than surface plates and so they are used for press runs of 500,000 and over. They are not used for short runs because there are many other types of plates that are more economical to produce for short runs.
Waterless: As the name implies, a waterless plate does not require water (fountain solution) like the other types of offset plates. Eliminating the need for water is accomplished with the use of silicone rubber for the non-image areas. Silicone rubber repels ink so fountain solution is not necessary to keep the non-image areas of the plate clean. Waterless printing requires the use of special inks, which are thicker than conventional ink used for offset printing. The inks must also be kept at a consistently cool temperature for proper results. The cool temperature is required because the normal heat build up from offset printing can gradually cause conventional ink to adhere to the non-printing areas of the plate. It is a good idea to use better quality paper grades to help reduce the amount of paper dust and debris that may accumulate on the plate, printing blanket, and inking system.
Aqueous: An aqueous plate is different than other offset plates in that after the exposure is made, the plate is developed in a water base solution rather than with a solvent base. An aqueous plate has come about due to environmental concerns. The solvent based developers contain many toxic substances and for several years it has been required to handle these substances as hazardous waste instead of simply dumping the solution into the sewer system. It is usually very costly to have an EPA approved waste hauler pick up and dispose of hazardous waste. Many water based solutions are safe to dispose of directly into the sewer system, but some solutions still contain traces of organic solvents. It is required that local waste treatment facilities give their approval before any substance is put into the sewage system.
Even though the chemistry has changed, aqueous plates still work the same as plates developed with solvent solutions. Film is put into contact with the plate using a vacuum frame and then it is exposed to ultra-violet light. Aqueous plates have improved so much that most of them produce better results than the old solvent based plates.
Processed on Press: Plates processed on the press are one of the newest types of surface plates. Like many other photomechanical processes, the plates are exposed in a vacuum frame with ultra-violet light. The plates are not processed after the exposure, but instead are processed after they are mounted on the press. The processing occurs when the plate comes in contact with the dampening solution on the press. After a few revolutions, the coating on the non-image areas of the plate is washed away and is dispersed into the fountain solution.