Influencing factors and improvement measures of printing plate printing durability
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In offset printing, there has been a problem of insufficient printing durability of the printing plate. Increasing the print durability of the printing plate plays an important role in reducing production costs, ensuring print quality, and shortening the printing cycle. It is more complicated to improve the printing durability of offset printing plates. It involves the processes of plate making and printing, so it is worthy of the attention of our printing workers. The reason for the analysis can be as follows:
First, the characteristics of the plate itself
Different plates have different printing durability, which is determined by the surface structure of the plate, the graphic portion and the material properties of the blank portion. The printing plate's resistance to printing is inseparable from the inherent mechanical strength and chemical stability of the printing plates. They are the decisive internal factors.
1. PS plate: Its plate is mainly aluminum. The mechanical strength of aluminum is relatively low. In order to increase its mechanical strength, it is necessary to add a small amount of metal such as iron, magnesium or copper to make it an alloy. The thickness of the oxide layer of the PS plate is generally controlled within 3~4μm. The process of electrolytic oxidation and other processes can form a complex sand surface on the surface of the aluminum plate, which can enlarge the surface area by 2~6 times and the depth of the sand is about 3~6μm. within. The diazo photosensitive layer of the PS version and the blank layer of the aluminum oxide layer have high mechanical wear resistance and good corrosion resistance and affinity adsorption properties, so its printing durability is significantly higher than other versions. Materials, such as PVA flat intaglio. Under normal circumstances, the PS version has a printing durability of about 50,000 to 100,000 sheets. After the high-temperature barbecue PS plate, the hardness and wear resistance of the diazo photosensitive layer in the graphic image are improved, and the printing durability can reach about 200,000 sheets.
2. Multi-layer metal plate: the graphic part and the blank part are respectively composed of copper and chrome with excellent wear resistance, corrosion resistance and affinity adsorption performance, so their printing durability is higher, up to 500,000 sheets.
3. CTP version: At present, the common CTP plate materials include thermal plate, silver salt diffusion transfer plate and other major categories. The plate material is the same as the PS plate material. It still uses aluminum as the plate base. After roughening, it can be coated with different types. Photosensitive adhesive. Due to the different imaging methods in the later stage, the printing durability is generally higher than the printing durability of ordinary PS plates, up to 100,000 to 200,000 impressions, and up to 1 million impressions after baking.
Second, the plate making process is not standardized, causing the printing durability to drop
For the traditional offset printing plate, there are processes such as imposition and printing in the plate making stage, and the specification of these operation operations will have no small influence on the size of the printing resistance.
1. Imposition process: Due to the limitation of the format of the imagesetter, most of the film should be completed by imposition. If the operation specification is not implemented carefully during the imposition, when the film is dirty, scraps and other film traces, film overlap, etc. When the phenomenon occurs, the exposed version will have a word deficiency, the dot is not real, and the like, resulting in a decrease in the printing durability in printing.
2. The problem of film density: film density has a great influence on the quality of the printing plate. Some film density is too large, and some film density is small. If the density of the film is innumerable during printing, it is easy to appear in the normal exposure time. The local printing dots of the printing plate are lost or the partial text lines are missing and the words are false. This will directly lead to a decrease in the printing durability of the printing plate.
3. Exposure time and vacuuming problem: In the drying of the PS plate, proper control of the exposure time is an important aspect to effectively improve the printing durability. Long exposure time, overexposure, will cause the printing plate to become imaginary, lost or incomplete; exposure time is too short, that is, underexposed, the photosensitive layer on the surface of the printing plate will not be cleaned during development, causing printing failure during printing. . This will directly affect the size of the printing resistance. Another important aspect is that you should pay attention to the vacuuming of the printing machine when exposing. If the vacuum is not pumped, there will be bubbles left between the film and the printing plate or between the film and the glass, so that the dots on the plate are exposed. Loss or false words, affecting the quality of the printing plate, so that its printing durability is reduced.
4. The ratio of developing solution and development time: At present, in many printing companies, the developing solution used for plate making is mostly made by mixing salt, alkali and water in a certain ratio. If the proportion is not well mastered during the redemption, it will affect the reproduction quality of the printed 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, the printing plate and the dot part will be imaginary or lost, so that the adhesion of the surface of the graphic surface will decrease, and the development time will be too short, which will make the surface of the printing plate and the dot appear incomplete, resulting in dirty printing during printing. Paste and other phenomena affect the quality of printed products. In addition, coating the protective adhesive in time after development can effectively protect the PS layout surface and prevent further oxidation. When applying the protective adhesive, try to be “thin and even”, and “clean and clean” when raging.
5. Baking stage: In order to make the graphic part resistant to chemicals and effectively improve the printing durability, the baking plate is indispensable. Pay attention to the size setting of the baking temperature when baking. There is also the use of baking solutions. If the 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, which will cause "soft" or "peeling", resulting in The oxide film layer of the printing plate is brittle; the mechanical strength and chemical stability of the blank portion are deteriorated, which affects the printing durability of the printing plate. Therefore, the correct blending solution is used to control the baking time. Under normal circumstances, the baking solution should be coated with “thin and uniform”. The baking time is controlled between 8 and 10 minutes, and the baking temperature is controlled between 200 and 230 °C.
Third, the printing process is not standardized, resulting in insufficient printing durability
In the printing process, the printing plate will be affected by many factors, 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: it is well known that the printing pressure refers to the mutual force between the embossing bodies during the printing process, which inevitably brings about compression deformation, speed difference and friction, and destroys the original structure and adsorption of the layout. Performance damages the plate. The first is the pressure of the drum, that is, the pressure between the plate cylinder and the blanket cylinder. If the pressure is improperly adjusted, the phenomenon of excessive pressure will increase the friction between the printing plate and the rubber, and the surface of the printing plate will gradually wear out. The dot of the printing plate is damaged, resulting in a decrease in the printing durability of the printing plate. Again, the pressure between the ink rollers, that is, the pressure between the ink roller and the water roller, especially the pressure between the ink roller and the water roller and the printing plate, if the pressure between them and the printing plate is too large, The friction will be increased, and the dot of the printing plate will be damaged, and the "disappearance" will appear, which will reduce the printing durability of the printing plate.
2. The effect of the use of dampening solution on the printing plate: We know that the use of dampening solution is unique to offset printing, the purpose of which is to maintain the balance between the blank portion and the graphic portion, and to protect the blank portion of the printing plate from ink encroachment. Do not let it expand into the graphic part. In printing, first of all, we must try to achieve the balance of ink and water. When the ink film of the printing plate is 3μm, the water film is 1μm, which can completely prevent the expansion of the ink to the blank part. If the amount of printing plate dampening solution is too large, the ink emulsification phenomenon is likely to occur, the density of the imprinted ink is not up to the requirement, and the degree of corrosion of the printing plate base is deepened; if the water is small, the printing plate is “paste plate” and “stacked ink”, which affects the printed matter. quality. At present, the main component of the commonly used dampening solution is "Lide powder" and water is blended according to a certain ratio. Whether the ratio of dampening solution is reasonable, the pH value of dampening solution is too high or too low, it will directly affect the printing durability of the printing plate. The pH of the dampening solution is too high, and the blank portion of the plate cannot form a fine and firm hydrophilic inorganic salt layer and the hydrophilic colloid layer adsorbed thereon, and the unexposed diazonium compound in the PS plate image portion is dissolved. The resulting image is incomplete, which weakens its strength and lipophilicity, resulting in reduced service life or even inability to print. If the pH of the dampening solution is too low, the aluminum of the main component of the PS plate is unstable in a strong acid or strong alkali solution, the plate base is corroded, and the blank portion cannot form a stable hydrophilic inorganic salt layer. Therefore, the graphic part The oleophilic base is corroded and "flowered".
3. The surface of the substrate is rough, powdered, and pulled, and sand is trapped in the paper. During the printing process, these substances will increase the pressure, accelerate the wear of the graphic part of the printing plate, and reduce the printing durability of the printing plate.
4. Some ink pigment particles are coarser, such as gold and silver powder inks, coloring media and white inks. Because they use coarse media as the carrier of the pigment, the coefficient of friction between the printing plate and the blanket is increased during the transfer of the ink, and the acceleration is accelerated. The wear and tear of the graphic part produces a "flower plate".
5. The effect of insufficient ink layer remaining in the graphic part of the printing plate. The remaining part of the printing plate maintains enough residual ink layer to prevent the graphic text from directly contacting the dampening solution, thereby protecting the graphic portion of the printing plate. If the remaining ink layer in the graphic part is insufficient for a long time, the photosensitive layer is directly subjected to abrasion or acid-base erosion, which may cause a "flower plate" phenomenon and greatly reduce the printing durability of the printing plate.
6. The impact of printing machinery equipment on the printing resistance: Part of the reason is closely related to the improper operation of the equipment. For example, due to improper machine installation, the printing speed is too fast, the printing pressure is too large, causing the machine to vibrate, and the printing plate will wear faster when printing. The bushings at both ends of the blanket cylinder are seriously worn, causing the blanket cylinder to jump during printing, which inevitably causes the printing plate to wear and deteriorate its service life.
7. The blanket is used for a long time aging, the rubber becomes hard and loses elasticity, the fat-sensing ability is degraded, the ink transfer performance is poor, and the printed ink is light, many printers think that the pressure is small and the pressure of the blanket cylinder and the plate cylinder is increased, resulting in Vicious cycle to accelerate plate wear.
Fourth, improve the printing process of printing plate resistance
1. Standardization of the plate making stage: carefully check the density of the film before printing, and use different exposure methods and exposure time according to the density of the film. In the printing stage, the exposure is controlled correctly. Especially before each batch of PS is used, it is best to do an exposure experiment to determine the appropriate exposure time of the PS version of the batch. In addition, we must pay close attention to the vacuuming of the printing machine. Do not take too much or too slow to ensure the quality of the vacuuming machine. In the printing and developing stage, the length of development time is strictly controlled, and the ratio and temperature of the developing solution are noted. Otherwise, the phenomenon of “peeling off” occurs after the glue to be protected is dried, which affects the printing durability. Pay attention to the correct blending solution when baking, and control the baking time.
2. Pressure adjustment: carefully adjust the center distance of the roller, accurately measure the lining of the roller, 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 an ideal pressure for a long time, and the printing is minimized. The friction of the surface of the plate.
3. Ensure that the ink has good flow properties so that the remaining portion of the ink remains in the graphic portion of the plate. During the printing process, the drier is used reasonably to prevent early drying during the ink transfer process.
4. Proper use of dampening solution: First, accurately prepare dampening solution, PH value should be mastered according to the characteristics of different plates. In addition, in the printing process, try to control the balance of ink and water, so that the dampening solution on the printing plate can be made as small as possible, so that the printing plate is not dirty, and the dampening solution does not encroach on the graphic parts. After the machine is stopped, it is not advisable to rely on the watering roller containing the dampening solution for a long time to avoid the erosion of the dampening solution and damage the printing plate.
5. Do a good job of saving and managing the PS version. Generally, the PS version should be stored in the warehouse for more than one week before leaving the warehouse. It 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, and it is easy to absorb the moisture in the air, thereby causing the photosensitive layer to fall off and prematurely deteriorate. In the plate making stage, the PS version management mainly focuses on preventing the slow oxidation of the plate material due to light leakage, and adjusting the temperature and humidity of the workshop environment. Because the photosensitive layer of the PS plate is slowly oxidized under ordinary light, it affects the final imaging effect of the photosensitive layer. At the same time, the storage period should be shortened as much as possible, and the plate preservation period is generally not more than half a year.
6. To wash the paper powder, debris, sand, etc. accumulated on the blanket cylinder, 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.
7. When applying the hydrophilic colloid on the printing plate, pay attention to whether the concentration of the colloid meets the requirements. The coating colloid should be uniform and the thickness should be appropriate to ensure that the low-energy surface and the blank part of the high-energy surface are covered. Oxidation damages the plate.
We know that there are many other external and internal factors in printing that have an impact on the printing durability of the plates. For the printing staff, to maintain a high sense of responsibility, improve and improve the various printing specifications and technical level, we will be able to fundamentally improve the printing durability of the printing plate, and can print high quality, high quality printing .