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 can long press runs of up to 250,000. The plates may even be put through a baking process that permit 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's 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 in to 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 called an additive plate because lacquer and gum are added to make the plate ink and water receptive. They can handling press runs of up to 150,000 impressions.
There are also prelacquered diazo plates which need 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 can 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 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 need an electro-chemical treatment to complete the processing. Bimetal plates are the costliest 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 picture metal and the water receptive metal is called non-image metal.
Picture Direct: A picture direct plate does not need the use of film in order to produce an picture on the plate. A special type of platemaking equipment is used which is like a massive camera and plate processor all in. Copy or artwork is place on a copyboard and an overhead camera records an picture which is transferred to a plate contained within the machine. The machine automatically processes the plate so that it is prepared for use on the press. A picture direct camera platemaker is convenient when the plates will be used on duplicator presses where lots of short runs are produced.
Electrostatic: A method that is also known as Xerography with plates produced with a method much like a photocopy. The plate is coated with a positively charged, light sensitive photoconductor. Like picture direct plates, artwork is photographed by a camera which transfers the picture to a plate. The photoconductor on the plate is eliminated in the areas that have been struck by the light & the unexposed areas of the plate (areas blocked by the picture on the artwork) retain their positive charge. A powdered toner with a negative charge is applied to the plate & is drawn to the remaining positively charged areas of the plate, leading to an picture. The powdered picture is then fused & made permanent by radiated heat.
The quality of the dots on electrostatic plates may not be as nice 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 & the press chemistry necessary 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 picture area is slightly below the surface of the plate. The etching allows the plate to over 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 picture on the plate. After the plates are exposed & processed, the unexposed or picture areas are etched & chemically treated to form a copper finish. The etching & chemical treatment makes the picture areas more receptive to ink. The exposed areas (non-image areas) stay receptive to water (fountain solution). Deep-etch plates have a much longer press life than surface plates & so they are used for press runs of 500,000 & over. They are not used for short runs because there's plenty of other types of plates that are more economical to produce for short runs.
Waterless: As the name implies, a waterless plate does not need water (fountain solution) like the other types of offset plates. Eliminating the necessity 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 tidy. Waterless printing requires the use of special inks, which are thicker than conventional ink used for offset printing. The inks must even be kept at a consistently chilled temperature for proper results. The chilled temperature is necessary 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 nice suggestion to make use of better quality paper grades to help reduce the amount of paper dust & debris that may accumulate on the plate, printing blanket, & inking method.
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 than with a solvent base. An aqueous plate has come about due to environmental concerns. The solvent based developers contain plenty of poisonous substances & for several years it's been necessary to handle these substances as risky waste in lieu of basically dumping the solution in to the sewer method. It is usually pricey to have an EPA approved waste hauler pick up & dispose of risky waste. Plenty of water based solutions are safe to dispose of directly in to the sewer method, but some solutions still contain traces of organic solvents. It is necessary that local waste treatment facilities give their approval before any substance is put in to the sewage method.
Although the chemistry has changed, aqueous plates still work the same as plates developed with solvent solutions. Film is put in to contact with the plate using a vacuum frame & then it is exposed to ultra-violet light. Aqueous plates have improved a lot that most of them produce better results than the elderly solvent based plates.
Processed on Press: Plates processed on the press are of the latest types of surface plates. Like lots of other photomechanical processes, the plates are exposed in a vacuum frame with ultra-violet light. The plates are not processed after the exposure, but in lieu 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 in to the fountain solution.

Equipment
Vacuum Frame: The vacuum frame is used to provide contact between film and plate and to produce the ultra-violet light exposure necessary for offset plates. A popular type of vacuum frame is the flip top model. It consists of a vacuum frame which pivots within a immense box formed piece of equipment containing an ultra-violet light source. The vacuum frame has a rubber surface on which is placed the plate topped by the film flat. A glass door is closed over the rubber surface and sealed. A vacuum pump removes the remaining air between the blanket and glass door which creates a maximum surface contact between the plate and film. The whole frame is then flipped upside down on it is pivot point which brings it in line with the ultra-violet light source. The equipment is sealed so that not of the concentrated ultra-violet light may cause eye destroy. When the exposure is completed, the frame is flipped back right side up, the vacuum is released, and the plate is removed from the equipment to be chemically processed. Lots of flip top models have vacuum frames on both sides of the frame so that while side is being exposed, the other side can be prepared with another set of plates and film.
Various types of equipment are necessary in order to prepare analog offset plates for press work:
Pin Register Process: There's occasions when a plate may have the same picture repeated on it to accommodate the requirements of the press. For example, a press with a 17" printing cylinder can print 17" sheet or 8 1/2" sheets with revolution of the cylinder. If a business form with a total width of 8 1/2" is necessary, then the 17" press can print forms from revolution of the cylinder. The plate mounted on the cylinder would then require identical images which means that negatives must be be stripped in to a flat and exposed onto the plate.
An alternative process is to prepare negative and use a pin register technique to repeat the picture on the plate. A special board is placed on a vacuum frame and the plate and flat are mounted on the board. The board contains holes around the perimeter in to which special pins can be placed. For the 8 1/2" business form, pins would be inserted every 8 1/2" across the top of the board. Special tabs are applied to the film flat that line up exactly with the pins on the board. The plate is placed on the board and fastened so that it cannot move. The flat is place onto the first set of pins on the board and the exposure is made. The flat is then moved to the next set of pins and the exposure is made to produce an identical picture (the first picture is covered so that it does not receive any further ultra-violet exposure). The result is that any point on the first picture is exactly 8 1/2" from the same point on the second picture.
Lots of other configurations are feasible with the pin register technique to match the requirements of the job and the press. If a print job with a done size of four 1/4" were being run on a 17" press, then the plate would need identical images (four x four 1/4 = 17). The process of repeating images on a plate is called step and repeat and the pin register board is also often called a step and repeat board. The number of images on a plate is often called the number up, such as two up or four up.
Step and Repeat Platemaker: The manual step and repeat system described above, may even be performed automatically with the use of a step and repeat platemaker. The step and repeat platemaker can move the flat, which is mounted in a chase, to all of the necessary positions or steps on the plate. When picture is being exposed, all other areas of the plate are automatically covered so that they do not receive any further exposure. The step and repeat platemaker is even more correct than the manual pin register method because the steps are controlled by computerized equipment. It is very helpful when plenty of steps must be made on the same plate and it eliminates much of the labor involved in manual step and repeat applications. The equipment is costly and may not be cost effective for low volume printers.
Automated Plate Processors: Analog offset plates can be developed after exposure by applying the developer and other necessary chemicals by hand. Although the manual method was used for plenty of years, it was difficult to maintain consistency when several people in the same facility where responsible for developing plates. A more consistent and efficient method for processing is with the use of automated equipment. With plenty of models, the exposed plate is basically inserted in to the processor and the correct amount of developer and gum is applied to the plate. The plate is neat and dry and prepared to be mounted on the press. The processors require regular cleaning and fresh chemicals periodically to operate correctly. Because of health and environmental requirements, plenty of automated processors use water based chemicals designed for aqueous plates.