Computer-to-plate plates and their characteristics

In recent years, with the development of electronic technology (including optoelectronics) and photosensitive material technology, computer-to-plate (CTP) technology that directly outputs digital data directly to printing plates has risen to the forefront and has rapidly entered a practical stage of production. The CTP plate must not only be imaged on aluminum substrates but also have many physical and chemical properties associated with printed structures, inks, fountain solutions, and paper.
This article first describes the various physical and chemical properties that CTP plates should possess, and then introduces the structure, image formation principles and characteristics of CTP plates.

Physico-Chemical Properties of CTP Plates
(1) Ink-printing ink-repellent fingerprints A feature of the ink-printing roller that is selectively inked by the transfer roller. When a certain thickness of ink film has adhered to the surface of the ink transfer roller, the amount of ink transferred to the printing plate is determined by the ink receptivity of the printing plate. If the ink affinity of the printing plate graphics part is low, the remaining ink will accumulate on the ink transfer roller, making the ink transfer roller ink film thicker, and the ink at both ends will be too much ink, resulting in the phenomenon of ink splashing, and also reducing the ink. Covering power.
In order to improve the ink receptivity of the printing plate, the photosensitive material in the printing plate graphic portion should have chemical properties that are close to the pigments, resins, and solvents in the ink.

(2) The hydrophilicity of a hydrophilic plate refers to the property of selectively retaining water on the non-graphic area of ​​the plate. At present, aluminum is the main substrate of CTP plates. It is hydrophilic, and the surface is roughened by roughening. Then it is anodized to form a porous surface and increase the total surface area. Improve hydrophilic properties. However, the amount of fountain solution that is not in contact with the image area should be as small as possible, as long as the non-graphic area can be made non-stick ink. This can prevent the non-image area from being over-emulsified and sticky and dirty to improve the ink-receptive properties of the plate graphic area. After the surface has been grinded and anodized, it helps to increase the degree of adhesion between the photosensitive coating and the substrate, thereby enhancing the printing plate's resistance to printing.

(3) Resistance to India Force India refers to the amount of printed matter that a plate can print. The speed of the printing roller of the printing machine is 5-10% slower than that of the printing plate, so that the fine dirt on the printing plate can be removed at any time to prevent the printing plate from being affected by the printing force.
Another factor affecting the resistance to printing forces is that each time the plate and the blanket are rotated, they must be separated and pressed together.
Some of the material from the paper (such as additives and debris) is sometimes mixed into the ink and fountain solution, and then is brought to the transfer roller and blanket, causing friction, but also make the printing plate resistance to printing force has decreased.
To increase the printing plate's resistance to printing. The plate should be baked at 230-250 degrees Celsius for 1-2 minutes.

(4) Resolution Resolution is the ability to form fine images. Taking the photosensitive resist film for LSI as an example, an image can be formed with a resolution of half a micrometer, but for most PS plates, the resolution depends on the positive picture and its contact with the plate, and the photosensitive coating. Thickness and washing process. In the Negative/Positive type, the plate around the graphic area is semi-cured to a slope due to light reflection on the surface of the aluminum substrate and light scattering in the photosensitive layer, resulting in resolution. Decline. In the Positive/Positive type method, the reflected light is irradiated on the printing plate and the non-graphical part is washed away, and a higher resolution can be obtained. Most of the PS versions use the positive-printing method and can produce 3% dots with 175 screens. However, since the CTP plate adopts a direct exposure method, 1% dots can be copied, and the number of screen lines is also 175. The resolution of the thermal-type CTP plate is considered to be the highest resolution among all CTP plates. This is because the laser has a very high heat transfer performance, and it has a large contrast in the lateral position when heated, making it difficult to create differences in thermal curing between graphic and non-graphic parts. (5) Sensitivity Sensitivity refers to the intensity of a certain energy, and Color Sensitivity refers to the range of energy that causes a reaction.
Compared with the ordinary PS plate (10-100 mJ/cm2), the CTP plate with the highest sensitivity is the silver plate diffusion diffusion type CTP plate material (1-100 mJ/cm2) followed by the photopolymer Plate (100-200 mJ/cm2), and finally the thermal plate (150-200 mJ/cm2).
In terms of CTP laser source, argon ion laser source (488nm) and FD-YAG laser source (532nm) are used for photopolymer plates; FD-YAG laser source (532nm) and Red LD laser source (630nm) are used for silver salts Diffusion transfer type plates; IR-LD laser source (830 nm) and YAG laser source (1,046 nm) were used for the hot plate type. The color sensitivity of each of the above plates is the strongest at the main wavelength of the laser used, but from the practical point of view, the sensitivity should be in the ultraviolet or infrared region outside the visible range (400-700 nm).

(6) Photosensitivity characteristics The photosensitivity characteristics refer to the characteristics of a Negative/Solutitized plate that is cured by some amount, and also refers to the characteristics of a solar chart based on a positive energy decomposition pattern. .
The CTP output recorder can perform both graphic exposure and non-graphic exposure, but when using the negative sun exposure pattern to expose parts of the image, the laser intensity of the exposure head changes, making it easy to create light on the spot or in the colored part. Uneven coating or banding. In the case of using Sun-Yang pattern printing plates for exposure to parts that need to be washed away, the parts of the image are uniform and the adhesion phenomenon does not occur. (7) Friction resistance When a bump occurs during transportation, or when a plate is removed from a stack of plates, it is possible to leave scratches on the plate. In order to enhance the anti-friction property of the plate, a material with better friction resistance should be used as the light for each layer of the plate, and the friction on the surface of the plate should be reduced. If it is difficult to avoid friction, you should insert liners between the plates so that the metal and photosensitive layer are not in direct contact. However, in the automated platemaking process, removing each plate and liner can be a time-consuming issue. The top silicone layer of the unwatermarked version is easily scratched. If you leave a scratch, the ink will enter the center of the scratch and cause it to become dirty. Therefore, it is necessary to cover the plate with a protective film, and then remove the protective film after the exposure and rinsing. Because of the above reasons, when printing with positive pictures, high-light spots are often easy to lose.

(8) Output image output The image of the exposed part of the printing plate changes or discolors, making it easier to proof or prevent double exposure. For this purpose, special pigments are added to the photosensitive material and the exposed areas are revealed by changing or fading.

(9) Flushing is the process of separating the exposed and non-exposed areas of the plate. In order to reduce the impact on the environment, rinse waste should be minimized. Where possible, the flushing process should be dispensed with so that flushing agent waste does not occur.

(10) In other aspects, the monomer contained in the UV ink can easily cause the image part to expand and reduce the printing plate resistance to printing. Therefore, efforts should be made to prevent the occurrence of such phenomena caused by monomers. When using a positive picture to expose on a normal PS plate, the contact of the film with the plate affects the resolution of the image. In order to increase the degree of contact, the vacuum treatment time that can affect the processing efficiency should be extended. To solve this problem, the surface of the plate should be sanded to improve the airflow under the film. However, the above features are not required in computer-to-plate, because the graphic is directly transferred to the plate.