Impressive Technology

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Photopolymer Plates

The process is a simple theory, photopolymer properties alter under exposure to UV light. A similar technology exists in the coating on aluminium lithographic printing plates, both are exposed to UV light through a film n&k (positive or negative) and in the case of 'positive' litho plates the exposed area is washed away but in the case of photopolymer the unexposed material is washed away while the exposed portion is hardened, therefore film negatives are used. Photopolymer is available in a variety of forms and thicknesses with different features, the principle feature for letterpress is the 'shore hardness' which can range from low 20's to around 85 for certain steel backed plates, the harder plates (60 upwards) being suitable for deeper impression work. There are certain issues to keep in mind - every part of the processing cycle is important and any variable makes a difference. Each plate type according to it's own specification will require different exposure times, washout times and temperatures, oven temperatures for drying and post exposure and drying times. It sounds complicated but it is surprisingly straight forward.

The Process

A film n&k  negative is made of the desired image or design to be printed or 'letterpressed'. A portion of photopolymer plate is cut corresponding to the image size then placed in the exposure tray. The film negative is overlayed ensuring the film (emulsion side down) is in good contact with no air bubbles or pockets between the film and plate which will cause UV leakage and a blurred image. The vacuum blanket is rolled over the film and plate, drawer closed and the exposure time begins starting the vacuum and UV lights. After exposure the plate is placed in the washout unit for several minutes (depending on plate type) in water around 20ºc. Soft brushes rotate to wash away waste material and the plate is immediately dehydrated to remove excess water and placed in the drying unit for the appropriate time at a temperature between 60ºc and 80ºc. After initial drying is complete plates are post exposed to UV light without the vacuum (as no film is used at this point) and placed once more in to the dryer, the second drying time is important to ensure the plates are properly 'detacked'. The plate is now finished and can be mounted on double sided adhesive ready to place on a precision ground metal base on the press, the whole process taking around 30 - 40 minutes. For letterpress the preferred plates are 'foil' (meaning plastic) backed rather than steel backed which are difficult to cut and work with, particularly for multi-colour work. Of the foil backed plates available the KF range by Toyobo is one of the most popular and widely used and particularly the KF95 (0.95mm plate) and the KF152 (1.52mm plate). It has to be remembered that the deeper plates like the KF152 need extra exposure time so the UV can penetrate to the floor of the plate and properly cure or harden the polymer. Failure to do this can result in weak plates that don't last the print run with fine details gradually disappearing from the inked impression. The plate must then be packed behind to compensate but this is problematic and not desirable. Even in well made plates there are limits to the degree of fine detail achievable in a photopolymer plate, lines below 0.3 pt may well not hold through the production process.








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