In offset printing, there is always the problem of insufficient durability of the printing plate. Improving the durability of the printing plate plays an important role in reducing production costs, ensuring printing quality, and shortening the printing cycle. Factors to improve the durability of offset printing plates are more complicated. It involves plate-making, printing and other processes, so it is a problem that our printing workers should pay attention to and take seriously. The analysis of the reasons can be from the following aspects:
First, the characteristics of the plate itself
Different plate materials have different printing resistance, which is determined by the surface structure of the printing plate, the material properties of the graphic part and the blank part. The strength of the printing plate is inseparable from the inherent mechanical strength and chemical stability of the printing plate. They are the internal factors that play a decisive role.
1.PS plate material: its plate material is mainly aluminum. The mechanical strength of aluminum is relatively low. In order to improve its mechanical strength, a small amount of iron, magnesium, copper and other metals need to be added to make it an alloy. The thickness of the oxide layer of the PS version is generally controlled within 3 ~ 4μm. Through electrolytic oxidation and other processes, the surface of the aluminum plate is formed with a complex structure of sand, which can expand the surface area by 2-6 times, and the depth of the sand is about 3 ~ 6μm within. The diazo photosensitive film layer in the graphic part of the PS version and the aluminum oxide layer in the blank part have high mechanical wear resistance and good corrosion resistance and affinity adsorption performance, so its print resistance is significantly higher than other versions Materials, such as PVA gravure. Under normal circumstances, the PS version is about 50,000 to 100,000 sheets. The PS version after high-temperature grilling improves the hardness and wear resistance of the diazo photosensitive layer in the graphics and text, and the printing resistance can reach about 200,000 sheets.
2. Multi-layer metal version: the graphic part and the blank part are respectively composed of copper and chromium with a wide range of wear resistance, corrosion resistance, and affinity adsorption performance, so their durability is higher, up to 500,000 sheets.
3.CTP version: At present, common CTP plates include thermal plates, silver salt diffusion transfer plates, etc. The plates are the same as PS plates, and aluminum is still used as the base. After roughening, different types of coating Photoresist. Due to the different imaging methods in the later period, its printing resistance is generally higher than that of ordinary PS plates, which can reach 100,000 to 200,000 prints, and can reach more than 1 million prints after baking.
Second, the plate making process is not standardized, resulting in a drop in print resistance
For traditional offset printing plates, there are processes such as imposition and printing in the plate-making stage. Whether the specifications of these processes will affect the size of the printing resistance is not small.
1. Imposition process: Due to the limitation of the size of the imagesetter, most of the film should be completed by imposition. If the operation specifications are not carefully implemented during imposition, there will be dirt, waste and other glue marks on the negative film and film overlap When the phenomenon occurs, the printed version will have false characters and false dots, etc., resulting in a decrease in printing durability.
2. The problem of film density: film density has a great influence on the quality of printing, some film density is too large, some film density is too small, if the density of the film is countless during printing, it is easy to appear in normal exposure The partial dots of the printing plate lost within a period of time or the partial text lines are missing pen and broken strokes, and the characters are false. This will directly lead to a reduction in the durability of the printing plate.
3. The problem of exposure time and vacuum: In the PS version, it is an important aspect to effectively control the exposure time to effectively improve the durability. Long exposure time and overexposure will cause the printing plate dots to become dull, lost or incomplete; too short exposure time, that is, underexposure, will make the photosensitive layer on the surface of the printing plate not cleaned during development, causing printing problems such as dirty printing . This will directly affect the size of the durability. Another important aspect is that you should pay attention to the vacuum of the printing machine during exposure. If the vacuum is not true, there will be bubbles left between the film and the printing plate or between the film and the glass, which will expose the dots on the printing plate. Missing or false characters affect the quality of the printing plate and reduce its durability.
4. Mixing ratio of developer and developing time: At present, in many printing companies, the developer used in plate-making is mostly made from salt, alkali and water in a certain proportion. If the ratio is poorly mastered during blending, it will affect the reproduction quality of the printing plate graphics or dots. Similarly, under the conditions of reasonable developer ratio and suitable temperature, the development time needs to be strictly controlled. If the development time is too long, it will cause parts of the printing plate and text dots to become false or lost, which will reduce the adhesion of the dots on the surface of the graphics and text. Phenomenon and other phenomena have affected the quality of printed products. In addition, coating protective glue in time after development can effectively protect the surface of the PS layout and prevent further oxidation. When applying the protective glue, try to be "thin and even" and "clean and clean" when wiping.
5. Baking stage: In order to make chemical resistance of the graphics and text, and effectively improve the printing resistance, baking is necessary. It is necessary to pay attention to the setting of the baking temperature when baking the plate. There is also the use of baking solution. If the mixing ratio of the baking solution is unreasonable, the coating is uneven, the baking temperature is too high or too low, and the baking time is too long or too short, it will cause a "soft version" or "stripping", which will The oxide film layer of the printing plate is brittle; the mechanical strength and chemical stability of the blank part are deteriorated, which affects the printing durability of the printing plate. So mix the baking solution correctly to control the baking time. Under normal circumstances, it is required that the baking solution should be coated "thin and uniform", the baking time should be controlled between 8 ~ 10min, and the baking temperature should be controlled between 200 ~ 230 ℃. [page]
Third, the printing process is not standardized, resulting in insufficient durability of the printing plate
The printing plate will be affected by many factors during the printing process, which will damage the printing plate and reduce the printing durability of the printing plate:
1. The printing plate is affected by the printing pressure: As we all know, the printing pressure refers to the mutual force between the stamping bodies during the printing process, which will inevitably bring about compression deformation, speed difference and friction, and destroy the original structure of the plate And adsorption performance and damage the printing plate. The first is the cylinder pressure, that is, the pressure between the plate cylinder and the rubber cylinder. If the pressure is not adjusted properly, excessive pressure will increase the friction between the plate and the rubber, and the sand on the surface of the plate will gradually wear out. The dots of the graphic part of the printing plate are damaged, resulting in a decrease in the printing plate's durability. Once again, the pressure between the ink roller, that is, the pressure between the ink roller and the water roller, especially the pressure between the plate ink roller and the plate water roller and the printing plate, if they are too large It will increase friction, damage the dots of the printing plate, and appear "plate-off", which will reduce the printing plate's durability.
2. The impact of the use of fountain solution on the printing plate: We know that the use of fountain solution is unique to offset printing. Its purpose is to maintain the balance of the blank part and the graphic part, and protect the blank part of the printing plate from ink. Embezzlement, and do not allow it to extend to the graphic part. In printing, we must first try to achieve water-ink balance. Generally, when the printing plate ink film is 3μm, the water film is 1μm, which can completely prevent the ink from expanding the blank part. If the amount of plate fountain solution is too large, ink emulsification is likely to occur, the color density of the blotting ink does not meet the requirements, and the degree of corrosion of the printing plate base is deepened; quality. At present, the main components of the commonly used fountain solution are "Lide powder" and water mixed according to a certain ratio. Whether the ratio of fountain solution is reasonable, and the PH value of fountain solution is too high or too low, will directly affect the printing durability of the printing plate. The pH value of the fountain solution is too high, and the blank part of the layout cannot form a fine and strong hydrophilic inorganic salt layer and the hydrophilic colloid layer adsorbed thereon, and the unexposed diazo compound in the PS graphic part will be dissolved It causes incomplete graphics and text, weakens its strength and lipophilicity, and reduces its service life or even makes it impossible to print. If the pH value of the fountain solution is too low, the aluminum of the main component of the PS plate is unstable in strong acid or alkali solution, the plate will be corroded, and the blank part cannot form a stable hydrophilic inorganic salt layer. Therefore, the graphic part The oleophilic foundation is corroded and "flower version".
3. The surface of the substrate is rough, powdered, brushed, and sand is contained in the paper. During the printing process, these substances will increase the pressure, accelerate the wear of the graphic parts of the printing plate, and reduce the printing endurance of the printing plate.
4. Some ink pigment particles are relatively thick, such as gold and silver powder ink, coloring media and white ink. Because they use the coarse media as the pigment carrier, the friction coefficient between the printing plate and the blanket will increase during the ink transfer process. , Accelerate the abrasion of graphic parts, and produce "patterns".
5. The influence of insufficient ink layer in the graphic part of the printing plate. The graphic part of the printing plate maintains enough residual ink layer to prevent the graphic base from directly contacting with the fountain solution, thereby protecting the graphic part of the printing plate. If the remaining ink layer of the graphic part is insufficient for a long time, the photosensitive layer is directly abraded or eroded by acids and alkalis, which is easy to cause a "pattern" phenomenon and greatly reduce the printing plate's durability.
6. The impact of printing machinery and equipment on the durability of printing: some reasons are closely related to the improper operation of the equipment. For example, due to improper installation of the machine, the printing speed is too fast, and the printing pressure is too large, causing the machine to vibrate, which will accelerate the wear of the printing plate during printing. The bushings at both ends of the rubber cylinder are seriously worn, causing the rubber cylinder to jump during printing, which will inevitably cause plate wear and reduce its service life.
7. The rubber blanket is aged for a long time, the rubber becomes hard and loses its elasticity, the fat-sensing ability is reduced, the ink transfer performance is poor, and the printed graphics and ink are light. Many printers believe that the pressure is small and increase the pressure of the rubber cylinder and the plate cylinder. , Causing a vicious cycle and accelerating plate wear.
Fourth, the technical measures to improve the printing plate durability
1. Standardization in the plate-making stage: carefully check the density of the film before printing, and flexibly adopt different exposure methods and exposure times according to the density of the film. In the printing stage, the exposure is properly controlled, especially before each batch of PS new version is used, it is best to do an exposure experiment to determine the appropriate exposure time of the batch of PS versions. In addition, we must pay close attention to the vacuuming situation of the plate-making machine, neither too fast nor too slow to ensure the quality of the vacuuming of the plate-making machine. In the printing and developing stage, strictly control the length of development time, pay attention to the proportion and temperature of the developer. Otherwise, after the protective glue is dried, the phenomenon of "stripping" will occur, which will affect the durability of printing. When roasting, pay attention to correct mixing of roasting liquid to control the time of roasting.
2. Pressure adjustment: carefully adjust the center distance of the drum, accurately measure the lining of the drum, carefully correct the pressure of the water roller and the ink roller, so that the pressure of the roller, the ink roller and the water roller are at the ideal pressure for a long time. Reduce the friction of the printing plate surface.
3. Ensure the good flow performance of the ink, so that the graphic part of the plate maintains sufficient residual ink layer. In the printing process, rational use of driers to prevent early drying in the ink transfer process.
4. Correct use of fountain solution: First of all, the fountain solution is accurately prepared, and the PH value should be mastered according to the characteristics of different plates. In addition, in the printing process, try to control the water and ink balance as much as possible to make the fountain solution on the printing plate as small as possible, based on the principle that the printing plate is not dirty, and do not allow the fountain solution to occupy the graphic parts. After the machine is stopped, the printing plate should not lean on the water roller containing fountain solution for a long time, so as not to be damaged by the fountain solution and damage the printing plate.
5. Do a good job of saving and managing the PS version. In general, the PS version should be stored in the warehouse for more than one week before leaving the warehouse, and should be placed in an environment protected from light and moisture during storage. If the storage environment temperature is too high or excessively humid, the chemical molecules of the photosensitive layer are very active, easily absorbing moisture in the air, causing the photosensitive layer to fall off and prematurely aging and deteriorate. In the plate-making stage, PS plate management mainly focuses on preventing the slow oxidation of plate materials due to light leakage, and adjusting the temperature and humidity of the workshop environment. Because the photosensitive layer of the PS version will slowly oxidize under ordinary light, affecting the final imaging effect of the photosensitive layer. At the same time, the storage period should be shortened as much as possible, and the storage period of the printing plate generally does not exceed half a year.
6. Wash the paper powder, debris, sand, etc. accumulated on the rubber roller to reduce the friction coefficient between the printing plate and the blanket, and reduce the probability of accelerated wear, which can effectively extend the life of the printing plate accordingly.
7. When coating the plate with hydrophilic colloid, pay attention to whether the concentration of the colloid meets the requirements. The coating colloid should be uniform and the thickness is appropriate to ensure that the low-energy surface with graphics and text and the high-energy surface of the blank part are covered. Oxidize and damage the printing plate.
We know that there are many other external and internal factors in printing that will affect the durability of the printing plate. For printing workers, maintaining a high sense of responsibility, improving and improving various printing specifications and technical levels, we will be able to fundamentally improve the durability of the printing plate, and be able to print high-quality, high-quality printed products
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