Physico-Chemical Properties of CTP Plates

(1) Ink affinity

The ink-repellent fingerprint image part has a feature of selective inking 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 depends on the ink receptivity of the printing plate. If the ink affinity of the printing plate graphic part is low, the remaining ink will accumulate on the ink transfer roller, making the ink transfer roller film thicker, and the ink depositing at both ends is too much, 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) Hydrophilicity

Hydrophilicity of the 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 prevents the non-image area from sticking due to over-emulsification, and improves 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 force

Resistance to India refers to the number of prints that can be printed on a plate. The speed of the water gate roller of the printing press 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 foreign materials (such as additives and debris) from the paper are sometimes mixed into the ink and fountain solution, and then they are carried to the transfer roller and the blanket, causing friction, and the plate's resistance to printing force is also reduced.

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 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 coating, resulting in resolution. decline. In the Positive/Positive type printing 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 and Sensibility

Sensitivity refers to the intensity of the response to a certain energy, while the color sensitivity (Clolor Sensitivity) refers to the range of energy that causes the reaction.

Compared with the ordinary PS plate (10-100 mJ/cm2), the most sensitive CTP plate material adopts the monitoring diffusion diffusion type CTP plate material (1-100 mJ/cm2), followed by the photopolymer plate. Material (100-200 mJ/cm2), and finally the heat sensitive 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 plate; FD-YAG laser source (532nm) and Red LD laser source (630nm) are used for banking supervision Diffusion transfer type plate; IR-LD laser source (830 nm) and YAG laser source (1.046 nm) are used for the thermal type plate. 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) Photosensitive characteristics

Photosensitivity refers not only to the characteristics of a Negative/Positive type plate that is cured by a certain amount of energy, but also to the characteristics of a positive-type plate that is decomposed by a certain energy.

The CTP output recorder can perform graphic exposure and non-graphic exposure, but when the image part is exposed using the negative sun-type plate, the laser intensity of the exposure head changes, and it is easy to cause the solid or colored part of the exposure. 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 bumps occur 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 photosensitive layer of the plate, and the friction on the surface of the plate should be reduced. If it is difficult to avoid friction, then a kiss should be inserted between the plates so that the metal and photosensitive layer are not in direct contact. However, in the automated platemaking process, removing each piece of media and paper 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 ensure that it is removed 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 Imaging

The output image indicates that the exposed part of the plate has changed or discolored, 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

Flushing is the process of separating the exposed and non-exposed parts 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, which would produce flushing agent waste.

(10) Other aspects

The monomers contained in the UV ink tend to swell the graphic parts and reduce the printing resistance. 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.