Description

The film is stripped in to sheets called flats and the flats are held in to contact with the plate with the use of a vacuum frame. The film, which can be either a negative or a

positive of the picture depending on the type of plate or method used, is then exposed to the ultra-violet light. The picture areas of the plate become ink receptive and the

non-image areas become water receptive.
Analog plates for offset lithography are manufactured with a surface coating of photosensitive material. The plates are contacted with an picture on film and exposed to ultra-

violet light in order to activate the photosensitive coating.
The plates work on the principle that oil and water do not mix. An offset press utilizes a fountain solution to wet the plate in the work of the printing method. The non-image

areas of the plate attract the fountain solution and repel oil base ink and the picture 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 picture area, or ink receptive area, after the plate

has been in contact with a film negative of the picture & 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 picture, 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 plenty of different substrates to accommodate the requirements of different presses & different types of offset

printing. A number 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 Finish Types: Offset plates are manufactured with several different types of plate ends to accommodate the different types of presses on which they are going to be

mounted. The most common type of plate finish is a straight cut finish which is used on most types of offset web presses. The straight cut finish is also used on some sheet fed

& smaller duplicator types of presses. More often used on the sheet fed & small duplicator presses are slotted, pinbar, & serrated edges.
Graining: Lots of of the metal plate types are manufactured with a textured surface called graining. The grain is not visible but is to generate a surface that more basically

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 in the

coursework of the manufacturing method in order to increase their water receptivity. There's some metal plates that are grainless, but all paper and plastic plates are

manufactured as grainless.

Grain is applied to plates in the coursework of their manufacture using of the following methods:
Ball Graining: A process of graining accomplished by covering the plate with an abrasive powder. The plate is inserted in to 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 method that is better than ball graining in that the method 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, generating 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 in the coursework of the manufacture of the plate and usually consists of an etching acid or anodizing solution.