Computer to plate (CTP) – The method 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 subjected to UV light by way of a a film (positive or negative) and in the case of’ positive’ litho plates the uncovered area is washed away but in true of photopolymer the unexposed material is washed away while the totally exposed portion is hardened, therefore film negatives are used.
Photopolymer can be purchased in a wide variety of forms and offset ctp platesetter with different characteristics, the idea feature for letterpress is the’ shore hardness’ that may vary from low 20 ‘s to around eighty five for many steel backed plates, the harder plates (sixty upwards) being perfect for deeper impression work. There are particular problems to hold in your head – every single element of the processing cycle is crucial and some varying is essential. Each plate style according to it’s own specification will require different exposure times, washout times and temperatures, oven temperatures for drying out as well as post exposure and drying times. It may sound difficult but it’s surprisingly straight forward.
A film negative is made of the desired design or image being printed or’ letterpressed’. A percentage of photopolymer plate is lower corresponding to the photograph size and then placed in the exposure tray. The film negative is overlayed making sure the film (emulsion side down) is in contact that is good devoid of air bubbles or pockets between the film and plate that’ll cause UV leakage and a blurred image. The vacuum blanket is rolled over the movie and plate, drawer closed and the time of exposure begins beginning the vacuum and UV lighting.
After exposure the plate is put into the washout unit for many minutes (depending on plate type) in water around 20c. Soft brushes rotate to clean away waste material and the plate is right away dehydrated to remove excess water and placed in the drying device for the proper time at a temperature between 60c and 80c. After initial drying is complete plates are post exposed to UV light without the vacuum (as no movie is needed only at that point) and also placed once more in on the dryer, the second drying time is crucial to ensure the plates are properly’ detacked’.
he plate is now completed and can be mounted on double sided adhesive able to place holding a precision ground metal base on the press, the entire operation taking around thirty – forty minutes. For letterpress the preferred plates are’ foil’ (meaning plastic) backed rather than steel backed that are difficult to cut and work with, especially for multi colour work. Of the foil backed plates available the KF range by Toyobo is one of the most well known and widely used and particularly the KF95 (0.95mm plate) and the KF152 (1.52mm plate). It has to be recalled that the greater plates which include the KF152 need extra length of exposure so the UV is able to penetrate to the floor belonging to the plate and correctly cure or perhaps harden the polymer.
Failing to accomplish this can result in weak plates that don’t survive the print run with wonderful details gradually disappearing from the inked impression. The plate should then be loaded behind to compensate but this’s tricky and not appealing. Even during well made plates you will discover limits to the amount of good detail achievable in ctp machine laser diodes, lines below 0.3 pt may well not keep through the production system.
Important developments in technology have made the polymer plate system even more doable in the recent past at both entry level and for large lithographic businesses both experiencing advancements towards a more’ computer to plate’ (CTP) process. In lithography this’s a slightly different process using a variation on the photopolymer plate system called Flexography which focuses more on accurate halftones required by modern presses. For equally Flexography and Photopolymer for Letterpress, CTP is actually forwarded by the development of new polyester based films.
Developments in laser films do not seem to be successful because of this type of high end work but inkjet films achieve consistent industry standard results with DMAX > four though it’s important to employ a program RIP to achieve this. The success of the polyester films is based on the greater precision of modern inkjet printers (the minimum requirement would be an anhubg such as Epson 4900 which is still a relatively modest investment) and also within the science around the film product.
We’ve tested a variety but endorse the Folex product Reprojet P Hd situated on thirty meter rolls or slice sheets. The film runs not by holding enough ink to be a dense black and thus get to the DMAX objective but rather by the filament inside the structure of the movie dealing with the ink to deflect light and cut it away from the polymer. We have found in tests that exposure times in excess of required could cause UV leakage (particularly if the ink is just too light) but then plate makers must be working on the guidebook times specified by plate makers so this is not a concern.
The movie will hold an amazing amount of ink which together with the film ‘s properties provide exceptional results. Attempting to print film with not a RIP like Waasatch, Efi or Filmgate simply using the cron ctp will result in floating (ink literally drifting on the surface) and wastage. These RIP’s are and also additional expense to small print outlets but there’s a less costly option in Accurip which we have analyzed running at droplet size 13 out of 15 and the results are excellent. We have also used EFI and are about to test Waasatch. Any of these RIP’s perform the main job of taking control of the way ink is laid down and also the amount whereas onboard printer drivers will set up the ink down, in terms that are simple, an excessive amount 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 which allow a deeper perception in to heavy paper of the luxury stationery market. Although polymer plates have been out there for a while the KF152 for profound impression work hasn’t been distributed in the Uk of late. There is now a distributor and Lyme Bay Press are providing KF152 plates as the single distributor and a plate making program along with tech support team for all those with printing problems, encouraging brand new expansion in the letterpress community.