Computer to plate
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Computer to plate (CTP) is an imaging technology used in modern lithographic printing. In this technology, an image created in a Desktop Publishing (DTP) computer file is output directly to a printing plate.
This compares with the older technology, Computer to film (CTF), where the computer file is output onto a photographic film. This film is then used to mask a printing plate, in a similar manner to a contact proof in darkroom photography.
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[edit] Comparison of CTP and CTF
[edit] Advantages of CTP
CTP has several advantages over conventional platemaking. In CTP, one entire generation (transfering the film image to the printing plate) is removed from the printing process. Plates are produced in less time and more consistently. (Film can move on the plate as it is being placed for exposure, or as it is being exposed). CTP ensures better registration, and image-to-edge repeatability.
CTP also has the advantage of being registered (held in precise position) in the CTP machine itself, and does not rely on a separately-aligned pin grid (as is the case with CTF). Defects due to dust and scratches (or any other artifacts) are minimized. CTP reduces press 'make-ready' time, i.e., the process of bringing the press, ink and paper system up to a minimum acceptable quality at the beginning of a press run, which saves a significant amount of time and material, and increases overall productivity.
The chemistry used to develop the plates is generally more environmentally safe compared to older film chemistry. (more environmentally-friendly 'chem-free' plates are just now (Oct 2006) becoming available).
Chemistry-free plates are typically exposed on the CTP unit (thermal wavelength sensitive plates only as of Nov 2006) and then hung on press to develop using the printing press chemistry and ink to wash away the unused emulsion. Some chemistry free plates do require a washout unit with a finishing gum to prepare the plate prior to installing on press.
Chemistry or processing free violet sensitive plates are reported to be in early testing and will be available mid to late 2007.
[edit] Disadvantages of CTP
CTP requires anywhere from $2,000 upward to $200,000 to implement - depending on the needs of the printer. Small portrait presses that typically create 1 or 2 color output can purchase anything from a standard $2,000 HP5000 laser printer, for low quality, up to a $8,000 XANTE Prepress System for higher quality. Either system uses polyester plates which are much less expensive than standard metal plates which require film, chemicals, and a lot of human labor to produce an image. It should be noted that in order to use the HP5000 as a low cost CTP system does require the use and disposal of chemicals and special cleaners to clean excess toning from the plate. Chemicals or cleaning of plates are not typically required of the XANTE Prepress System as it is designed specifically for platemaking and is a chemical free system.
Some printers still use film for long print runs and have adopted a CTF workflow. In the CTF environment, proofing can be done on the computer monitor or by outputting a digital proof on a digital printer or proofing device. The XANTE device noted above incorporates CTP, CTF, and proofing in one device, while the HP5000 cannot produce film.
[edit] See also
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