With the advancement of science and technology, in the field of sheet-fed offset printing, CTP plate making has almost replaced traditional CTF plate making. In the production process, we will encounter long orders with a production task of more than 100,000 prints, and the delivery cycle is very tight. However, it often occurs that the product quality cannot meet customer requirements due to low printing resistance of the CTP plate during the printing process, and It is necessary to shut down the machine to change the plate and restart the machine for printing. Therefore, how to improve the printing strength of CTP plates to ensure product quality and delivery cycle, and at the same time reduce the waste of plate materials, paper, and ink caused by restarting printing after plate change has become a must for printing companies with many long-run jobs. subject.
There are many factors affecting the printing resistance of CTP plates, such as inappropriate imaging time, developer concentration, developing temperature, and developing time in the process of CTP plate making; inappropriate ink volume control and printing pressure adjustment in the printing process; paper is easy to fall Powder shedding, poor water, and ink absorption performance; too high ink viscosity, poor emulsification resistance, too large particle size; poor surface tension of fountain solution, too small pH value of fountain solution; uneven printing plate base, electrolytic sand Inappropriate thickness, etc. Here, the author only uses a limited space to discuss the problem of the decline in printing plate durability caused by the CTP plate itself in the printing process, hoping to be helpful to readers.
Effect of plate base on printing plate durability
Simply put, all CTP plates are composed of base and imaging coating. At present, most of the CTP plate bases on the market are aluminum plate bases, whose aluminum content is above 99%, and also contain alloy components such as silicon, manganese, copper, and magnesium. Because the aluminum plate base contains alloy components, which improves its metal crystal structure, it is better than a pure aluminum plate base in terms of hardness, tensile strength, and corrosion resistance. This is also a prerequisite for improving the printing plate durability in sheetfed offset printing. However, the content of other metals in the aluminum base should not be too high, otherwise, the aluminum base will become brittle, and the tensile strength and corrosion resistance will decrease.
Another important factor in the plate base that affects the printing force of the printing plate is the flatness of the plate base. In the initial stage of CTP's development in my country, the CTP plates produced by domestic CTP plate manufacturers are 70% of the imported CTP plates in terms of printing resistance, and sometimes even less than 50%. In addition to the difference, the main reason is that the content of magnesium, silicon, manganese, copper, and other metals in the plate base exceeds the standard and the flatness of the plate base is too large. The author did a test in 2007 and measured the base thickness of imported CTP plates and 8 different positions on the four sides of domestic CTP plates. The base plate thickness deviation of the plates exceeds 0.005mm.
The basic requirements of CTP plate production for the aluminum plate base are clean, flat, with no cracks, corrosion pits (points), vent holes, scratches, breaks, no marks, peeling, pine-like patterns and oil marks, and other disadvantages; There should be no bulges, ruffles, etc.; the surface of the plate base is not allowed to have defects such as non-metallic pressing, sticking, horizontal skin, and horizontal lines. In addition, the production of CTP plates has some specific requirements for the flatness of the plate base: the longitudinal and transverse thickness tolerances of the aluminum plate base should be ≤±0.005mm; The gap between the planes is ≤1mm and the number of waves per meter of the unfolded surface is not more than 3; the aluminum coil should be tight, and the end faces should be neat, and no cracks or burrs are allowed. The thickness of the plate is set to 0.270mm or 0.275mm in most plate-making machines of various models.
It can be seen that the CTP plate should be made of the high-quality aluminum plate so that the plate base has a uniform and fine grain structure and a dense oxide film, which can not only ensure the bonding fastness of the imaging coating and the plate base but also make the printing plate have high durability. SCPG and accurate dot reproducibility guarantee high-quality printing.
Effect of the plate production process on printing plate durability
Regardless of whether it is a traditional PS plate or a CTP plate, the production process can be divided into the following steps: ① Supply the aluminum coil to the CTP plate production equipment; ② Pre-purification treatment to remove the naturally formed oxide layer on the aluminum surface; ③ Electrolysis Roughening, so that the surface of the plate base forms grains; ④Remove the film layer produced by post-etching; ⑤Anodizing treatment, harden the roughened aluminum oxide layer with a thickness of 0.5-3.0μm by mechanical and chemical methods; ⑥For The plate base is sealed so that the plate base can better coat the photosensitive layer and retain moisture; ⑦coat the imaging material and dry; ⑧coat the matte layer and dry, and thus form a small air channel to Improve the vacuum degree during printing; ⑨ use laser beam to detect the surface of the plate, record the plate with surface defects, mark it, and remove it during the cutting process; ⑩ cut the coated plate on the cutting machine The required size, and insert the isolation protective paper, so that the package can be stacked. It should be noted that the production date and shelf life date must be printed on each box.
It can be seen that plate base treatment generally includes processes such as purification, electrolytic roughening, oxidation, sealing, and drying. Among them, electrolytic roughening has a significant impact on the performance of the CTP plate, such as printing force and dot reduction. Electrolysis is the process of forming the grains of CTP plates. Different electrolysis processes will result in different grain shapes. The quality of the grains not only directly affects the sensitivity of CTP plates in pre-press plate making, but also directly affects printing. Process water-ink balance and plate durability. The ideal surface roughness of the CTP plate, that is, the grain value Ra of the plate base is 0.45-0.55 μm. The surface roughness Ra of CTP plates currently on the market is generally 0.45-0.60 μm, while the surface roughness Ra of traditional PS plates is generally 0.60-0.80 μm. If you want to get finer grains, the aluminum base must have good electrolytic performance. In addition, the thickness of the sand mesh is also related to the electrolysis process conditions of plate production. The smaller the surface roughness, the fewer deep pits on the surface of the plate, the more uniform the grain size, and the richer the reproduced image layer, but too small surface roughness will cause poor adsorption of the imaging coating coated on the surface of the CTP plate. Firm, thus reducing the printing plate's durability.
Conversely, the greater the surface roughness, the more deep pits on the surface of the plate, which will cause difficulty in developing the blank part of the CTP plate, or even loss of dots. Higher laser energy is required for plate making, and bottom dust is easy to occur. Therefore, in addition to requiring good electrolytic performance of the aluminum plate base, it is also necessary to strictly control the selection of the electrolyte in the production process of the CTP plate, and control the current, voltage, and power during the electrolysis process.
After electrolytic roughening, the plate needs to be oxidized and sealed. Oxidation treatment can increase the hardness of the plate base, form a firmly adhered oxide film layer on the surface of the grain, improve the corrosion resistance of the printing plate, and further improve the printing durability of the printing plate. Sealing is actually to close extremely small holes, reduce their adsorption capacity, and thicken the walls of larger holes, thereby improving the wear resistance and corrosion resistance of the oxide film, and at the same time improving the ink affinity of the oxide film on the surface of the printing plate. Prolong the storage time of the printing plate and enhance the printing durability of the printing plate. But if the sealing operation is not done properly, it may be counterproductive.
To sum up, if you want to improve the printing durability of the printing plate, for the CTP plate itself, you must not only strictly select the aluminum plate base, but also strictly control the electrolytic roughening, oxidation, and sealing during the production process of the CTP plate. and other process links
Huida Print-All Technology company is an advanced and professional manufacturer of producing printing plates, mainly selling 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/.