Text 5 Film assembly, imposition and platemaking

VI. Write a short description of electronic page planning process.

V. Discuss the advantages and disadvantages of CAD systems.

Film assembly This is the assembly of all the type and pictorial elements within a page in the correct position. Prior to the development of electronic page planning and desk-top publishing, this was done by combining film of the type with film of the individual pictures. All the pieces in turn were fixed to a sheet of plastic foil in position with one foil for each colour. These foils could either be used directly for platemaking or converted to one piece of film per colour, by making a page negative and contacting back to positive prior to platemaking.

This technique was slow, expensive and accuracy depended on the skill of the individual planner. Although this method is still used, particularly where existing film is being re-used for a job, repro houses firstly started doing page assembly using electronic page planning and then desk-top publishing, where the document files contain all the page assembly instructions and all film produced is in page form.

Imposition is the positioning of the pages in the right places on the plate, so that they come out in the right order and in the correct positions, with the intended margins, where printed.

Most printing machines print 8, 16 or 32 pages (or multiples of these) at a time, and the printer will work out an imposition scheme that will print the required number of pages in the most economical way on the particular machine being used.

The imposition itself is nowadays done by electronic imposition devices such as the Opticopy, which consists of a camera which exposes onto a large piece of film, the size of the printing plate. The pages are photographed one at a time and the machine moves the film, so that each page is exposed in the correct position. Other systems such as Rachwal photograph pages on to microfilm and project each page on to the plate in the correct position. Now, desk-top publishing files can use imposition software to produce imposed film to the full plate size or expose direct to plate.

Once the imposition has been done, the printer makes ozalids (called 'blueprints' in America) from the imposed foils, Ozalid proofs are produced by a process similar to that used for dye-line prints, with the imposed foil placed over the ozalid paper, which is photosensitized. This assemblage is then exposed to light and the ozalid developed to give a white image on black or blue. Ozalids can usually be produced two-sided and are ruled up and folded so that the client can see exactly how the job will be printed.

Platemaking Once the ozalid has been approved by the client, the printer makes any necessary corrections and then produces plates from the imposed foils. The metal plate, which is usually presensitized (that is, the manufacturer has coated it with a chemical which reacts to light), is put in a vacuum frame and the imposed foil placed over it. Light from an artificial source is shone through the clear parts of the film, and this has the effect of hardening the image area. Exposure is critical and can be checked with an exposure control step wedge. When the plate is developed and washed (normally on automatic machines), the image area will attract ink and repel water while the non-image (background) area will repel ink and attract water. Plates can be baked to give longer life.

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