Plates / Platesetters

Computer-to-Plate, usually shortened to CTP, is the general term for a range of technologies that use digital data to create a printable image directly onto the surface of a printing plate or cylinder. CTP is replacing earlier platemaking processes where sheets of photographic film were used as an intermediate image carrier.

The machine that carries out CTP imaging is normally called a ‘platesetter,’ in contrast to the earlier ‘filmsetter,’ though ‘imager’ or ‘recorder’ is sometimes used. Platesetter manufacturers include Screen (which also makes imagers sold by Fuji and Heidelberg), plus Agfa, ECRM, Esher-Grad, Highwater, Kodak, Krause, Lüscher and Presstek.

The first platesetters addressed the most common offset press formats of B1 (8-page) and B2 (4-page). As that market started to fill up, B3 (2-page) platesetters were introduced. The latest trend is for giant VLF (very large format) platesetters, generally for the new generation of giant packaging presses, typically able to take 16, 24 or even 32-page formats. For instance Screen’s PlateRite Ultima VLF high speed setters can either take single large plates, or two smaller plates side by side. The top model is the PlateRite Ultima 36000Z, which can output 29 plates per hour sized up to 2,100 mm x 1,600 mm, or up to 58 B1 plates per hour.

‘Conventional’ film is still widely used, but manufacturers are winding down or halting new filmsetting equipment production because almost all new sales are CTP. Film was necessary because plate manufacturers initially struggled to develop a coatings that were both sensitive to the relatively weak laser energy and tough enough to withstand long runs on the press. Suitable plates first appeared in the early 1990s.

Economically, CTP removes the costs and floor-area of film-based processing and printing-down frames (to transfer the image from film to plate) as well as the consumable film and chemistry. Most litho plates still require chemical processing, but ‘processless’ plates are starting to reach the market.

Quality-wise, the digital data is written directly onto the plate surface with no loss of sharpness compared with the inescapable degradation of the previous film copying method, so very sharp dot and line edges are possible. Also there is no problem with dust trapped between film and plate causing unwanted spots.

Today the main choice of CTP metal lithographic plates is between ‘thermal,’ with coatings that respond to the heat energy of powerful but expensive lasers, and ‘violet’, which are sensitive to the weaker blue-violet light of cheap laser diodes. Thermal plates can be handed in daylight, but violet requires a yellow safelight. As a general rule, thermal is favoured for larger formats of B1 and above, while violet is popular with newspapers and smaller printers using B2 presses or smaller.

Alternative, rarer systems start with uncoated but grained aluminium plates and use either thermal transfer of an ink-receptive ribbon material, or a modified inkjet to ‘print’ an ink-receptive fluid onto the plate.

There’s also an intermediate CTP process for flexible plastic plates. These plates can be roll-fed like film and can be imaged by modified filmsetters, although today’s plastic platesetters are purpose-built.

Normally, the CTP and platesetter terms refer to the production of offset litho plates. However, photopolymer flexographic plates or cylinders can also be created by different CTP technologies.

CTP for flexo is often written as CtF, though this may be confused with the use of CtF to mean Computer-to-Film. Here a photopolymer plate is coated with a thin black top layer, which is ablated by a powerful laser in the platesetter, to reveal bare polymer in the areas that will ultimately form the printed image. The plate is then exposed to intense ultra violet light. The bare areas harden and the masked areas remain soft so can be removed by a chemical or mechanical processing stage. Screen has just entered the market with its own design, the PlateRite FX870.