In the offset printing, there has been a problem that the printing plate has insufficient printing resistance. Improving the printing plate's resistance to printing forces plays an important role in reducing production costs, ensuring print quality, and shortening the printing cycle. The factors that make offset printing plates more resistant to printing forces are more complicated. It involves plate making, printing and other processes. Therefore, it is worthy of our printers' attention and serious treatment. The reasons for this can be analyzed from the following aspects:
First, the characteristics of the plate itself
Different media have different print resistance, which is due to the surface structure of the printing plate, the graphic part and the material properties of the blank part. The level of resistance of printing plates is inseparable from the inherent mechanical strength and chemical stability of plates. They are the internal factors that determine the role.
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, and copper to make it an alloy. The thickness of the oxide layer of the PS plate is generally controlled within 3~4μm. Through electrolytic oxidation and other processes, the surface of the aluminum plate base is formed with a complex structure of sand, which can increase the surface area by 2 to 6 times, and the depth of the sand hole is about 3~6μm. within. The diazo photosensitive layer and the blank aluminum oxide layer of the PS plate and text part have high mechanical abrasion resistance and good corrosion resistance and affinity adsorption performance, so its printing capability is significantly higher than other versions. Material, such as PVA gravure. Under normal circumstances, the PS version is about 5-10 million prints. The PS plate after high-temperature grilling increases the hardness and abrasion resistance of the diazo photosensitive layer in the graphic and textural parts, and the resistance to imprinting force can reach about 200,000 sheets.
2. Multi-layered metal plate: The graphic part and the blank part are respectively composed of copper and chromium, which have a wide range of wear resistance, corrosion resistance and affinity adsorption performance. Therefore, their resistance to printing is higher and can reach more than 500,000 sheets.
3.CTP version: At present, common CTP plates include thermal plates, silver salt diffusion transfer plates, and other major categories. Plates are the same as PS plate plates, and aluminum is still used as the plate base. After coating, different types are applied. Photosensitive adhesive. Due to the different imaging methods in the later period, its resistance to printing force is generally higher than that of ordinary PS plates. It can reach up to 10 to 200,000 impressions, and can reach over 1 million impressions after baking.
Second, the plate making process is not standardized and causes a decrease in the printing force resistance
For traditional offset printing plates, there are processes such as imposition and printing in the platemaking stage. The specification of these operations will have a significant impact on the size of the resistance force.
1. Imposition process: Due to the limitation of the format machine of the imagesetter, most films are to be completed by imposition. If the film is not carefully implemented during the imposition, the film is dirty, scraps, glue marks, and film overlaps. When the phenomenon occurs, it will cause the printed version to appear imaginary, the network point is not true and so on, resulting in the decline of the printing resistance.
2. The problem of film density: The film density has a great influence on the print quality. Some films have a high density and some films have a low density. If the density of the film is countless in the printing, it will easily appear in the normal exposure. During the period of time, the dried printing plate has lost some of its local printing sites, or the local text lines are missing strokes, and the words are not true. This will directly lead to a decrease in the printing plate's resistance to printing forces.
3. Exposure time and vacuum problem: In the printing of the PS plate, correct control of the exposure time is an important aspect of effectively improving the resistance to printing. Excessive exposure time, overexposure, will cause the printing plate network to send virtual, lost or incomplete; exposure time is too short, that is, underexposure, will make the photosensitive layer of the surface of the printing plate when developing is not clean, causing printing problems such as dirty . This will directly affect the size of the resistance force. Another important aspect is that during exposure, attention should be paid to the situation of the stencil vacuuming. If vacuum evacuation is not true, bubbles will appear between the film and the printing plate or between the film and the glass, and the dots on the printing plate will be exposed when the printing plate is exposed. Loss or misrepresentation of the word affects the quality of the printing plate, making it less resistant to printing forces.
4. The proportion of developer and development time: At present, in many printing companies, the developer used in plate making is mostly made up of salt, alkali and water with a certain proportion. If the ratio is not good at the time of matching, it will affect the reproduction quality of the printing plate graphic or network. Similarly, under the condition that the ratio of developer is reasonable and the temperature is appropriate, the development time must be strictly controlled. If the development time is too long, it will cause the printing plate graphics and dots to be missing or missing, so that the adhesion of the surface dots on the graphic surface will be reduced, and the development time will be too short, which will cause the surface and the dots of the printing plate to appear incomplete, resulting in dirty printing. Paste and other phenomena have affected the quality of printed products. In addition, the timely application of protective glue after development can effectively protect the PS plate surface and prevent further oxidation. In the case of rubbing and protecting the glue, try to achieve “thin and uniform†and clean and clean the rag.
5. Baking stage: In order to make the graphic part produce chemical resistance and effectively improve the printing resistance, baking version is indispensable. When baking, you need to pay attention to the baking temperature setting. There is also the use of baking solutions. If the proportioning of the baking solution is not reasonable, the coating is not uniform, the baking temperature is too high or too low, and the baking time is too long or too short, which can cause “soft†or “peelingâ€, resulting in The oxide film layer of the printing plate is brittle; the mechanical strength and chemical stability of the blank portion are deteriorated, and the printing plate is resistant to the printing force. So correctly match the baking solution and control the baking time. Under normal circumstances, it is required that the baking liquid should be coated with “thin and uniformâ€, the baking time is controlled between 8 and 10 minutes, and the baking temperature is controlled between 200-230°C. (to be continued)