COMPUTER TO PLATES (CTP)

COMPUTER TO PLATES (CTP)

Computer-to-plate (CTP) is an advanced technology used in the printing industry to streamline the process of transferring digital designs directly to printing plates, eliminating the need for traditional film-based intermediate steps. It has revolutionized the prepress phase of printing by improving accuracy, efficiency, and overall print quality.

In the traditional plate-making process, designs were first created on a computer and then transferred to photographic film through a series of steps involving exposure and chemical processing. The film was then used to create printing plates, which were ultimately mounted onto the printing press for producing the final prints.

 

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.

Top of Form

 

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.