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 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.
Computer-to-plate plates
(1) Bank of China's transfer of silver plate transfer type
This plate imaging feature is the formation of positive silver images in the rinse core layer by reducing the amount of silver. There are two types of internal transfer and external transfer.
a. Internal transfer type plate
A typical plate among such plates is the Silvalith plate supplied by Agfa. The plate consists of aluminum plate base, rinse core layer and silver emulsion emulsion layer. In the imaging process, a silver metal superficial coating is formed on the surface of the unexposed aluminum substrate by the diffusion of the silver ink after the laser exposure, which is the image part. Since the unexposed part is the final printed part, it is also a positive sun-type plate and should be exposed under red safe light. The photosensitive wavelength range is from 630nm to 680nm (the wavelength range of argon ion lasers and laser diodes), and the printing plate can withstand 10,000 impressions.
b. External transfer type plate
A typical example of this type of media is the Silver Digi-Plate provided by Mitsubishi, which consists of a substrate (paper or PET), a silver emulsion layer, and a rinse core layer. During the exposure imaging process, the non-sunlight portion of the silver supervised device is diffused, and the metallic silver is deposited on the topmost developing and rinsing core layer to form a graphic printing portion. The non-graphic area is the exposed area (using a hydrophilic, stable gelatin emulsion). This plate is a positive sun-type plate, its resistance to India can reach 10,000 to 20,000 India.
(2) Photopolymer plate
Typical examples of such plates include the Brilia LPA plate supplied by Fujifilm Co., Ltd. and the Diamond LA-2 plate supplied by Mitsubishi. This plate is composed of aluminum plate, radical polymerization layer and outer layer.
The imaging process of the photopolymer type plate material is such that the exposed portion of the photopolymer layer undergoes a cross-linking reaction under the influence of the stimulated polymerization, is insoluble in the developer, and forms a graphic printed portion. Unexposed areas are washed away by the developer to form a non-image area.
Due to the sensitivity enhancement effect, the sensitivity of this plate is equivalent to 1 thousand times the sensitivity of ordinary PS plates. In order to prevent the generation of radicals in the photopolymer layer caused by contact with oxygen in the air, an oxygen-barrier outer protective layer is provided. Plates should be washed with alkaline aqueous developer under a red safety laser. This kind of plate can withstand up to 100,000 impressions.
(3) Thermal reactive media
a. Thermal reaction plate
Typical of this type of plate is Kodak's 830 thermal plate, consisting of an aluminum plate substrate and a heat-sensitive cross-linking reaction layer. The imaging process is that the dye component capable of absorbing infrared light in the heat-sensitive cross-linking reaction layer generates heat after absorbing infrared laser light, and generates an acid by co-existing an acid generator through thermal decomposition. When the plate is heated at a temperature of 130-140 degrees Celsius for about 30 seconds, a cross-linking reaction takes place at the exposed portion of the laser, and the soluble phenolic resin acts together with the generated acid to expose the solution to the developer, thereby accelerating the graphic printing. The formation of parts. The plate is a negative sun-type plate. The unexposed areas are washed away by the developer to expose the aluminum substrate, forming a non-graphical portion.
The heating process before exposure is not easy to control. The plate was processed with an aqueous developer under bright room conditions. This plate has a reproducible resolution of 4.6 microns, which is equivalent to a 600 line 1% range. When printing is not baked, the printing force is 200,000 impressions. After the baking version, the printing resistance can reach more than 1 million.
When the heat-reactive plate was first introduced, it was a negative-image type material for negative images. Later when the second-generation version appeared, it was replaced with a negative-heated negative-type pattern.
b. Mortar type plate
A representative type of plate is the Pearl Plate plate.