COMPUTER TO PLATES (CTP) |
Computer-to-plate (CTP) is an imaging technology
used in modern printing processes. In this technology, an image created in
a Desktop Publishing (DTP) application is output directly to a printing
plate.
Negative lithographic printing plate Agfa Advantage DL violet laser foto-setter with VPP68 plate processor
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 make a printing plate, in a similar manner to a contact proof in
darkroom photography.
With CTP technology, the intermediate film step is
bypassed entirely. The digital design files are directly processed and
transferred to the printing plates using specialized CTP systems. Here's how
the process typically works:
1.
Design Preparation: The graphic design files are
created or prepared on a computer using graphic design software. These files
can include text, images, colors, and other elements necessary for the print
job.
2.
Prepress Workflow: The design files are then processed
through a prepress workflow that includes tasks such as color management,
imposition (arranging multiple pages on a single plate), and trapping
(adjusting overlapping colors to prevent gaps or overlaps in the final print).
3.
Plate Exposure: The processed design files are sent to
a CTP device, which exposes the printing plates using lasers or LEDs. The CTP
device precisely transfers the digital design onto the plate by selectively
exposing the plate's light-sensitive coating.
4.
Plate Development: After exposure, the plates undergo a
development process to remove the unexposed areas, leaving behind the image
areas that will attract ink during printing.
5.
Plate Mounting: The developed plates are then mounted
onto the printing press, where they are inked and come into contact with the
printing substrate (such as paper or cardboard). The inked image areas on the
plate are transferred to the substrate, producing the final prints.
Overall, computer-to-plate technology has significantly
streamlined the printing process, enabling faster production times, improved
quality, and enhanced efficiency in the prepress phase of printing.
Types of
plates
Photopolymer plates
- Photosensitivity between 380 nm and 550 nm;
- low laser energy of 30 to 100 µJ/cm2 needed for exposure;
- run lengths of up to 300,000 prints;
- negative plate.
Silverhalogen plates
- Silverhalogen aluminum printing plates;
- photosensitivity between 400 nm and 700 nm;
- low laser energy of 1 to 2 µJ/cm2 needed for exposure;
- run lengths of up to 150,000 prints;
- resolution to 450 LPI possible;
- dot reproduction 1–99%;
- positive plate.
Thermal plates
- Photosensitivity between 830 nm and 1.070 nm;
- laser energy between 70 and 200 mJ/cm2 is needed for exposure;
- run lengths from 100,000 up to 500,000 prints;
- resolution to 200 LPI–400 LPI possible;
- dot reproduction 1–99%;
- high process stability;
- many providers;
- processing in the daylight possible.
Comparison
with CTP
Advantages
CTP has several advantages over conventional plate making. In CTP, one generation (transfer of film image to the printing
plate) is removed from the printing process (eliminating the need for film and
related developer chemicals), increasing sharpness and detail. CTP avoids
potential losses in quality that may occur during film processing, including
scratches in the film, and variations in the exposure. An imagesetter usually
has an accuracy rate of ±2%. Plates are produced in less time, are more
consistent, and at a lower cost. CTP can also improve registration and
image-to-edge repeatability over traditional methods. More recent advancements
in CTP plate technology made by companies such as DuPont and Plate Crafters
have further widened the gap with analog plates, allowing for very fine
vignettes and minimal dot gain on press.
In CTP, the media is registered (held in a precise
position) in the plate setter during imaging and does not rely on a
separately-aligned pin grid, as is the case with film. Defects due to dust,
scratches, or other artifacts are minimized.
CTP systems can significantly increase plate production
outputs. Plate setters for newspaper production can output up to 300 12-inch
(300 mm) plates per hour at 1,270 dpi (dots per inch), whilst for
commercial applications a CTP system could output 60 B1 plates at
2,400 dpi for higher screen rulings.
Small portrait presses that typically create one- or
two-color output can use anything from a standard laser printer, for low
quality/low volume, up to a higher-end dedicated plate setter for higher quality
and volume.
The advantages of computer-to-plate technology include:
1. Improved Accuracy: CTP eliminates potential errors and
quality loss associated with multiple film-based steps, resulting in higher
precision and fidelity between the original digital design and the printed
output.
2. Faster Turnaround: CTP reduces the time required for
plate production, as it eliminates the time-consuming film processing steps.
3. Cost Efficiency: By eliminating film and related
processing costs, CTP can be more cost-effective in the long run.
4. Greater Flexibility: CTP allows for easy digital
manipulation and adjustments of the design files, offering more flexibility in
making last-minute changes or corrections.
Disadvantages
CTP is restricted to digital format. CTP productions
require that the basis for the printed matter as well as the imposition be
digital.
In CTP, if for some reason a plate is damaged, if an error occurs when it is ripped, or if something has to be corrected after the plate is exposed, a completely new, imposed plate must be created.
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