Litho Platemaking

Most supply companies producing platemaking materials market pre-sensitised plates, and these are normally grouped under two headings - diazo compounds or polymer resin coatings. The basic difference is that diazo images are non-receptive to grease while the polymer group is naturally grease receptive.

Diazo coated plates are supplied with either negative or positive - working coatings. Due to the peculiarities of this coating, it is necessary to use plates that have a fine grain, but this has the advantage of allowing excellent tonal reproduction. When negative - working coatings are exposed they are rendered insoluble to the solutions used on the press; the non-exposed areas are then developed away. The image may subsequently be inked or lacquered and finally treated with a de-sensitising solution. However there is at least one type of plate that needs no lacquering after processing as the image lacquer is applied at the time of manufacture.

Positive - working coatings, unlike other photo-litho coatings which are hardened upon exposure to ultra-violet light, are rendered soluble in the alkaline developer solution, leaving an image of unexposed coating. After development, the image is lacquered or inked and finally de-sensitised. Any unwanted areas, such as film edges, may be removed with a correcting fluid before final de-sensitising.

Polymer-coated plates are negative-working and rely on the action of U-V light to polymerise the coating in the image areas. The unexposed coating is washed away with developer. Because of the grease receptive nature of polymer images, they do not need lacquering or inking if the plate is to go direct to the press.

Apart from diazo compounds and polymer resin coatings used in preparing presensitised plates, there are a number of other surface treatments used in platemaking; they are nearly as varied as the different plate processes used.

Main types of plate materials in current use are divided into five categories; direct image and electrostatic; chemical transfer; surface; deep etch; and bimetal.

Direct image and electrostatic Direct image

This is the simplest form of the plate processes and origination is placed onto a paper plate directly from a typewriter. The plate is then treated with gum and phosphoric acid solution in preparation for printing. Being suitable only for short-run work, this type of plate barely comes within the scope of the commercial printer (print run: approx 1,000).

Electrostatic

There are two types of platemaking process involved here, one is transfer and the other is direct. The transfer system employs a selenium plate; the selenium is photoconductive and can be used more than once. The photoconductive plate is positively charged in special units necessary to the process. It is then exposed in a camera, where reductions and enlargements are possible. The white light reflected from the background areas of the original disperses the positive charge in the corresponding area on the plate, which is then dusted with a negatively charged powder which sticks to the image areas. A suitable printing plate is then placed onto the selenium plate and the image transferred. Finally, the powder is fused onto the printing plate to complete the process.

An alternative method of platemaking using the electrostatic principle is produced directly from copy. The special surface layer is of an organic photo-conductive nature. The plate itself is charged positively and exposed. The latent image is then developed by passing thorough the image-forming pigment. At this stage alterations can be made to copy simply by wiping away the powder. The powder is then heat-fused to the image for 30 sec at 170⁰C. The coating in the non-image areas must be removed with the appropriate de-coating fluid before the process is complete (print run: approx 100,000 on metal plates).