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 to the printing plate has also emerged and quickly entered the stage of practical production. CTP plates must not only be able to image on aluminum substrates, but also have many physical and chemical properties related to the printing structure, ink, fountain solution and paper.
This article first explains the various physical and chemical properties that CTP plates should have, and then introduces the structure, image formation principles and characteristics of CTP plates.
Physical and chemical properties of CTP plates
(1) Ink affinity
Ink affinity refers to the characteristic that the graphic and text parts of the printing plate are selectively inked through the ink transfer roller. When a certain thickness of ink film is attached to the surface of the ink transfer roller, the amount of ink transferred to the printing plate depends on the ink affinity of the printing plate. If the ink affinity of the graphics and text part of the printing plate is low, the remaining ink will accumulate on the ink transfer roller, thickening the ink film of the ink transfer roller, and excessive ink will accumulate at both ends, resulting in ink splashing and reducing the ink of covering power.
In order to improve the ink affinity of the plate, the photosensitive material in the graphic part of the printing plate should have chemical properties that are close to the pigments, resins and solvents in the ink.
(2)Hydrophilicity
The hydrophilicity of the plate refers to the characteristic of selectively retaining water on non-image and text parts of the printing plate. At present, aluminum is the main base material of CTP plates. It is hydrophilic, and the surface has been grinded to make it rougher. It is then anodized and coated to form a porous surface, increasing the total surface area. Finally, Improve hydrophilic properties. However, the amount of fountain solution that comes into contact with non-graphic parts should be as small as possible, as long as the non-graphic parts do not stick to ink. This can prevent non-graphic parts from being stained due to excessive emulsification and improve the ink-receptive properties of the graphic parts of the printing plate. After the surface is ground and anodized, it helps to enhance the adhesion between the photosensitive coating and the substrate, thereby improving the printing durability of the printing plate.
(3)Print durability
Print durability refers to the number of prints that can be printed on a printing plate. The rotation speed of the water fountain roller of the printing machine is 5-10% slower than the rotation speed of the printing plate, in order to remove fine dirt on the printing plate at any time and prevent the printing plate from decreasing in printing durability.
Another factor that affects print durability is that the printing plate and blanket must be separated and pressed each time they rotate.
Some substances from the paper (such as additives and debris) are sometimes mixed into the ink and fountain solution, and are then brought to the ink transfer roller and blanket, causing friction and reducing the printing durability of the printing plate.
To increase the printing durability of the printing plate. 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 corrosion-resistant film used for LSI as an example, it can form images with a resolution of half a micron. However, for most PS plates, the resolution depends on the positive image and its contact with the plate. The photosensitive coating Thickness and flushing process, etc. In the Negative/Positive type platemaking method, the area around the graphic part of the printing plate is often semi-solidified into a slope due to light reflection on the surface of the aluminum substrate and light scattering in the photosensitive coating, resulting in poor resolution. Decline. In the positive/positive type plate making method, the reflected light is irradiated on the graphic parts of the printing plate, and the non-graphic parts are washed away, so that higher resolution can be obtained. Most PS plates use the positive printing method, which can produce 3% dots and a screen line count of 175. However, since the CTP plate uses direct exposure method, it can copy 1% of the dots and the number of screen lines is also 175. The resolution of thermal CTP plates is considered to be the highest resolution among all CTP plates. This is because the laser has high heat transfer performance, and the contrast in the lateral position is very large during heating, making it easy for differences in thermal curing between graphic and non-graphic parts.
(5) Sensitivity and color sensitivity
Sensitivity refers to the intensity of response to a certain energy, while Color Sensitivity refers to the energy range that causes a reaction.
Compared with ordinary PS plates (10-100 mJ/cm2), the CTP plate with the highest sensitivity is the CBRC diffusion transfer CTP plate (1-100 mJ/cm2), followed by photopolymer plates (100-200 mJ/cm2), and finally heat-sensitive plates (150-200 mJ/cm2).
In terms of CTP laser light source, argon ion laser source (488nm) and FD-YAG laser light source (532nm) are used for photopolymer plates; FD-YAG laser source (532nm) and Red LD laser source (630nm) are used for silver salt Diffusion transfer plate; IR-LD laser source (830nm) and YAG laser source (1,046nm) are used for thermal plates. The color sensitivity of each of the above plates is strongest at the main wavelength of the laser used, but from a practical point of view, the sensitivity should appear in the ultraviolet or infrared light region outside the visible light range (400-700nm).
(6)Photosensitive properties
Photosensitive properties refer to the characteristics of Negative/Posititive plates that are cured by a certain amount, and also refer to the characteristics of positive-type plates that are decomposed by a certain amount of energy.
The CTP output recorder can perform graphic exposure and non-graphic exposure. However, when using negative and sunny graphic plates to expose graphic parts, because the laser intensity of the exposure head changes, it is easy to cause damage to the solid or colored parts. Uneven coating or banding. When using a sun exposure pattern plate to expose non-image and text parts that need to be rinsed out, the image and text parts will remain uniform and no adhesion will occur.
(7) Friction resistance
When bumps occur during transportation, or when a plate is removed from a pile of plates, scratches may be left on the plate. In order to enhance the friction resistance of the plate, materials with better friction resistance should be used as the light layer of the plate, and efforts should be made to reduce friction on the surface of the plate. If friction is unavoidable, backing paper should be inserted between each plate so that the metal and photosensitive layer are not in direct contact. However, in the automated plate making process, removing each plate and backing paper may become a more time-consuming problem. The top silicone layer of the watermark-free version is easily scratched. If a scratch is left, the ink will get into the center of the scratch and cause staining. Therefore, a protective film needs to be applied to the plate, and the protective film must be removed after exposure and rinse. Due to the above reasons, high-gloss dots are often easily lost when printing with positive images.
(8)Output imaging
Output imaging refers to the change or discoloration of the exposed part of the printing plate to facilitate proofing or prevent secondary exposure. To do this, special pigments are added to the photosensitive material to show the exposed areas by changing or fading.
(9) Rinse
Processing is the process of separating the exposed areas from the non-exposed areas on the printing plate. To reduce environmental impact, flushing agent waste should be minimized. Where possible, the flushing process should be dispensed with so that no flushing agent waste is generated.
(10)Other aspects
The monomers contained in UV ink can easily cause the graphics and text parts to swell and reduce the printing durability of the printing plate. Therefore, efforts should be made to prevent the occurrence of such phenomena caused by monomers. When using a positive image to expose on an ordinary PS plate, the contact between the film and the plate will affect the resolution of the image. In order to increase the degree of contact, the vacuum treatment time should be extended which can affect the processing efficiency. In order to solve this problem, the surface of the plate must be sanded to improve the air flow under the film. However, the above characteristics are not required in computer-to-plate making, because the graphics and text are directly transferred to the plate.
Huida Print-All Technology company is an advanced and professional manufacturer of producing printing plates, mainly sales CTP, CTCP, and PS plates as well as offering ODM/OEMs, especially for the offset. For more information, please click the official website link here: https://www.huidaoffsetplate.com/.