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 (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.
A film 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 60Âºc and 80Âºc. 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.
Important developments in technology have made the polymer plate system more feasible in recent years at both entry level and for large lithographic businesses both enjoying advancements towards a more ‘computer to plate’ (CTP) process. In lithography this is a slightly different process using a variation of the photopolymer plate system known as Flexography which focuses more on accurate halftones required by modern presses. For both Flexography and Photopolymer for Letterpress, CTP has been forwarded by the development of new polyester based films. Developments in laser films do not seem to be successful for this kind of high end work but inkjet films achieve consistent industry standard results with DMAX >4 though it is necessary to use a software RIP to achieve this. The success of the polyester films lies in the greater accuracy of modern inkjet printers (the minimum requirement would be an appliance like the Epson 4900 which is still a relatively modest investment) and in the science of the film product. We’ve tested a variety but endorse the Folex product Reprojet P HD available on 30 meter rolls or cut sheets. The film works not by holding sufficient ink to be a dense black and thus reach the DMAX target but rather by the filament within the structure of the film working with the ink to deflect light and cut it out from the polymer. We have found in testing that exposure times greater than required can result in UV leakage (particularly if the ink is too light) but then plate makers should be working to the guide times specified by plate manufacturers so this is not an issue. The film will hold a remarkable amount of ink which combined with the film’s properties give excellent results. Trying to print film without a RIP like Waasatch, Filmgate or EFI just using the Epson onboard drivers will result in floating (ink literally floating on the surface) and wastage. These RIP’s are and added expense to small print shops but there is a cheaper option in Accurip which we’ve tested running at droplet size 13 out of 15 and the results are excellent. We’ve also used EFI and are about to test Waasatch. Any of these RIP’s perform the important task of taking control of the way ink is laid down as well as the amount whereas onboard printer drivers will put the ink down, in simple terms, too much too quickly.
With the resurging interest in letterpress and particularly the artform side of this printing process, photpolymer plates have been in increasing demand in the UK and in particular plates that allow a deeper impression in to thick paper for the luxury stationery market. Although polymer plates have been available for a while the KF152 for deep impression work has not been distributed in the UK in recent times. There is now a distributor and Lyme Bay Press are providing KF152 plates as the sole distributor and a plate making service as well as technical support for those with printing problems, encouraging new growth in the letterpress community.
It may seem a little odd that Europe seems to be at the front of this technology, one normally thinks of Japan or the USA for innovation but for once legislation may have been a help rather than a hindrance. Europe has increased it’s expectations of industry meeting environmental requirements in a way which is forcing improvements in the workplace. Traditional film making uses chemicals which are hazardous in use and difficult to dispose of, perhaps this has increased the demand for earlier film products to improve. Many litho printers (us included) will have used laser plates at one time or another for simple short run one colour jobs, it’s quick and easy, printed straight from the desktop. The problems are damping issues which make these plates difficult to run but now it’s possible to develop positive metal plates using inkjet film. Each year the report from DRUPA is the progress the printing industry is making in CTP. There are now printers which inkjet metal plates directly, though the investment is significant the time saving element is very desirable. There is much talk about the ‘digital presses’ which can inkjet 11,000 sheets per hour with ultra sharp dots. The inevitable question is the cost per impression which still means the digital route has not eclipsed the traditional press but it will come. To get back on target, our conclusion has been after vigorous research and testing, recent developments have made it possible to produce Litho standard Flexographic or Letterpress Photopolymer plates of a very high standard through CTP processes at a very affordable rate both in terms of initial investment and production, sufficient to make this a viable commercial interest.
Source by Bruce Maurward