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Description | Offset Plate Composition | Offset Plate Types | Equipment

 

Description

Analog plates for offset lithography are manufactured with a surface coating of photosensitive material. The plates are contacted with an image on film and exposed to ultra-violet light in order to activate the photosensitive coating.

The film is stripped into sheets called flats and the flats are held into contact with the plate with the use of a vacuum frame. The film, which can be either a negative or a positive of the image depending on the type of plate or process used, is then exposed to the ultra-violet light. The image areas of the plate become ink receptive and the non-image areas become water receptive.

The plates work on the principle that oil and water do not mix. An offset press utilizes a fountain solution to wet the plate during the printing process. The non-image areas of the plate attract the fountain solution and repel oil base ink and the image areas attract the ink and repel the fountain solution.

 

Offset analog plates can be either negative working or positive working. The coating on a negative working plate becomes the image area, or ink receptive area, after the plate has been in contact with a film negative of the image and exposed to a measured amount of ultra-violet light. With a positive working plate, the areas not exposed by the ultra-violet light are the areas that become the image, or ink receptive area. Exposing a positive working plate requires the use of a film positive in contact with the plate which is then exposed to ultra-violet light.

Offset Plate Composition

Substrates: Plates for offset lithography are manufactured from a number of different substrates to accommodate the requirements of different presses and different types of offset printing. Some of the materials that are used include:

  • Cellulose based paper.
  • Paper that has been coated with plastic.
  • Polyester.
  • Acetate.
  • Aluminum coating adhered to a paper base.
  • Aluminum sheets.
  • A steel base with a surface of plastic.
  • A base of chromium, aluminum, or stainless steel with a surface of copper.

Plate End Types: Offset plates are manufactured with several different types of plate ends to accommodate the different types of presses on which they will be mounted. The most common type of plate end is a straight cut end which is used on most types of offset web presses. The straight cut end is also used on some sheet fed and smaller duplicator types of presses. More commonly used on the sheet fed and small duplicator presses are slotted, pinbar, and serrated edges.

Graining: Many of the metal plate types are manufactured with a textured surface called graining. The grain is not visible but is sufficient to create a surface that more easily attracts water than grainless plates. The grain allows a consistent ink and water balance to be maintained at the press. Grainless plates are treated with chemicals during the manufacturing process in order to increase their water receptivity. There are some metal plates that are grainless, but all paper and plastic plates are manufactured as grainless.

Grain is applied to plates during their manufacture using one of the following methods:

  • Ball Graining: A method of graining accomplished by covering the plate with an abrasive powder. The plate is inserted into a graining machine where marbles made of steel or glass are vibrated and rolled over the powdered surface at high speed to produce the grain.
  • Brush Graining: A graining technique that is better than ball graining in that the process provides the plate with increased water receptivity. Soft brushes are used to roughen the plate surface to make sharper and more numerous peaks and valleys, creating greater surface area for moisture to adhere.
  • Sandblast Graining: The surface of the plate has grain similar to ball graining, but it is created using fine particles of sand sprayed on the surface of the plate under high pressure.
  • Chemical Graining: The chemical is applied during the manufacture of the plate and usually consists of an etching acid or anodizing solution.

Offset Plate Types

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.

Equipment

Various types of equipment are necessary in order to prepare analog offset plates for press work:

Vacuum Frame: The vacuum frame is used to provide contact between film and plate and to produce the ultra-violet light exposure required for offset plates. A popular type of vacuum frame is the flip top model. It consists of a vacuum frame which pivots within a large box shaped 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 entire frame is then flipped upside down on it's pivot point which brings it in line with the ultra-violet light source. The equipment is totally sealed so that none of the concentrated ultra-violet light can cause eye damage. 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. Many flip top models have vacuum frames on both sides of the frame so that while one side is being exposed, the other side can be prepared with another set of plates and film.

Pin Register System: There are times when a plate may have the same image repeated on it to accommodate the requirements of the press. For example, a press with a 17" printing cylinder can print one 17" sheet or two 8 1/2" sheets with one revolution of the cylinder. If a business form with a total width of 8 1/2" is required, then the 17" press can print two forms from one revolution of the cylinder. The plate mounted on the cylinder would then require two identical images which means that two negatives must be be stripped into a flat and exposed onto the plate.

An alternative method is to prepare one negative and use a pin register system to repeat the image 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 into 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 image (the first image is covered so that it does not receive any further ultra-violet exposure). The result is that any point on the first image is exactly 8 1/2" from the same point on the second image.

Many other configurations are possible with the pin register system to match the requirements of the job and the press. If a print job with a finished size of 4 1/4" were being run on a 17" press, then the plate would require four identical images (4 x 4 1/4 = 17). The process of repeating images on a plate is known as step and repeat and the pin register board is also referred to as a step and repeat board. The number of images on a plate is referred to as the number up, such as 2 up or 4 up.

Step and Repeat Platemaker: The manual step and repeat process described above, can also 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 required positions or steps on the plate. When one image 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 accurate than the manual pin register method because the steps are controlled by computerized equipment. It is very helpful when many 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 expensive 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 manually. Although the manual method was used for many years, it was difficult to maintain consistency especially 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 many models, the exposed plate is simply inserted into the processor and the correct amount of developer and gum is applied to the plate. The plate is clean and dry and ready to be mounted on the press. The processors require regular cleaning and fresh chemicals periodically to operate correctly. Because of health and environmental requirements, many automated processors use water based chemicals designed for aqueous plates.

 


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