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).

Chemical transfer

In this process, also known as transfer diffusion, a silver-halide negative and original are exposed together. Light passes through the negative from the back and reflects from the original onto the negative coating to obtain a latent image. Exposure can be obtained through transmitted light for translucent or clear originals. The original is placed over a light box, the side to be copied facing upwards and the negative with emulsion side up.

The resulting negative is then fed into a processor with a specially coated plate where it is developed and then rolled into contact with the plate. The unexposed emulsion areas on the negative are therefore transferred onto the plate where they diffuse into the surface coating. The plate is then developed and fixed to produce an image of firmly adhered silver.

The plate processes used incorporate both exposing and processing units, thereby eliminating the need for dark-rooms and special plumbing (print run: approx 10,000-100,000).