Influencing Factors of Printing Force of Printing Plate and Its Improvement Measures

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 “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, 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 uniform”, the baking time is controlled between 8 and 10 minutes, and the baking temperature is controlled between 200-230°C.

Third, the printing process is not standardized caused by insufficient printing plate resistance

In the printing process, the printing plate can be affected by many factors, causing the printing plate to be damaged and reducing the printing plate's resistance to printing force:

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 inevitably brings about compression deformation, speed difference and friction, and destroys the original structure of the plate. And adsorption performance and damage the plate. The first is the drum pressure, that is, the pressure between the plate cylinder and the blanket cylinder. If the pressure is improperly adjusted, the phenomenon of excessive pressure will exacerbate the friction between the printing plate and the eraser, which will gradually wear the surface of the plate. Causes damage to the dots of the printing plate graphic part, causing the printing plate's resistance to printing force to drop. Again, the pressure between the ink rollers, that is, the pressure between the ink roller and the water roller, especially the pressure between the platen roller and the plate roller and the plate, if the pressure between them and the plate is too large, Will increase the friction, make the printing plate's network point be damaged, appear "falling edition", make the printing plate's resistance to printing force drop.

2. 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 between the blank part and the graphic part, and to protect the blank part of the printing plate from the ink. Encroachment does not allow it to expand into graphic parts. In printing, first try to balance the water and ink as much as possible. Generally, when the ink film is 3μm, the water film of 1μm can completely prevent the ink from expanding on the blank area. If the amount of fountain solution is too large, the phenomenon of ink emulsification will occur, the density of blotting ink will not meet the requirements, and the corrosion degree of the printing plate will be deepened; if the water is small, the printing plate will be “burned” or “stacked”, affecting the printing product. quality. At present, the main ingredient of fountain solution is "Lyde powder" and water in accordance with a certain percentage ratio. Whether the fountain solution is reasonable or not, the pH of the fountain solution is too high or too low, which will directly affect the printing plate's resistance to printing. The pH of the fountain solution is too high, and the blank part of the plate cannot form a dense and firm hydrophilic inorganic salt layer and the hydrophilic colloid layer adsorbed thereon in time, and will cause the unexposed diazonium compound in the graphic part of the PS plate to be dissolved. The incompleteness of the picture and text has weakened its strength and lipophilicity, which has led to a reduction in its useful life and even to its inability to print. If the pH of the fountain solution is too low, the aluminum of the main component of the PS plate base is unstable in a strong acid or strong alkali solution, the plate base will be corroded, and the blank part cannot form a stable hydrophilic inorganic salt layer. Therefore, the graphic part The oleophilic base is eroded and "flowered".

3. Substrate surface appears rough, off powder, pull hair, sand and other paper contained in the printing process, these substances will increase the pressure, accelerate the printing plate graphic wear and reduce the printing plate resistance to printing force.

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 a carrier for pigments, the coefficient of friction between the printing plate and the blanket is increased during ink transfer. , Accelerate the wear and tear of the graphic part, producing a "flower pattern."

5. The effect of the remaining ink layer in the printing plate graphics section. The printing plate graphic portion maintains enough residual ink layer to prevent the graphic text from directly contacting the fountain solution, thereby protecting the graphic portion of the printing plate. If the remaining part of the graphic and ink layer is insufficient for a long period of time, the photosensitive layer is directly subjected to abrasion or acid-alkali erosion, which can easily cause a “flower pattern” phenomenon and significantly reduce the printing plate's resistance to printing force.

6. The effect of failure caused by printing machinery and equipment on the resistance to printing force: Part of the reason is closely related to the improper operation of the equipment. For example, due to improper installation of the machine, excessive printing speed, excessive printing pressure, vibration of the machine, and accelerated printing plate wear. The sleeve sleeves at both ends of the blanket cylinder are worn badly, causing the rubber cylinder to jump when printing, which will inevitably cause the printing plate to wear and reduce its service life.

7. Longer aging of blankets, rubber hardening, loss of elasticity, poor sensory performance, poor ink transfer performance, light prints, and many printers believe that the pressure is low and increase the pressure of the blanket and plate cylinders. , causing a vicious circle and accelerating plate wear.

Fourth, improve the printing plate resistance to printing process measures

1. Standardization of the plate making stage: carefully check the density of the film before printing, and use different exposure modes and exposure times flexibly according to the density of the film. During the printing stage, the amount of exposure is properly controlled, especially before each new batch of PS is used, it is best to do exposure experiments first to determine the appropriate exposure time for the batch of PS plates. In addition, we must pay close attention to the stencil vacuum conditions, do not pump too quickly, do not be too slow, to ensure the quality of the squeegee vacuum. In the development stage of the printing plate, strictly control the length of the development time, pay attention to the ratio of the developer and the temperature. Otherwise, after the protective tape is blow dried, “striped” phenomenon will occur, which will affect the resistance to printing force. When baking, pay attention to the correct matching baking solution and control the baking time.

2. Adjustment of pressure: Carefully adjust the center distance of the roller, accurately measure the lining of the roller, carefully correct the pressure of the water roller and ink roller, and make the pressure of the roller, ink roller and water roller long-term under ideal pressure. Reduce friction on the plate surface.

3. Ensure that the ink has good flow properties, so that the plate graphics part maintains enough residual ink layer. During the printing process, the driers are properly used to prevent early drying during ink transfer.

4. Correct use of fountain solution: first accurately formula fountain solution, PH value should be mastered according to the characteristics of different plates. In addition, in the printing process, as far as possible to control the ink balance, you can make the fountain solution on the plate as small as possible, to the principle of the plate is not dirty, do not make the fountain solution encroach on the graphic parts. After shutting down, the printing plate should not rely on the watering roller containing fountain solution for a long time so as not to be damaged by the fountain solution and damage the printing plate.

5. Save the PS version of the management work. General PS version should be stored in the warehouse for more than one week before the library is out of storage, and should be stored in a place protected from light and moisture. If the storage environment temperature is too high or too wet, the chemical molecules of the photosensitive layer are very active, easily absorbing the moisture in the air, causing the photosensitive layer to fall off, causing premature aging and deterioration. In the plate making stage, PS plate management mainly focused on preventing the slow oxidation of the plate due to light leakage and adjusting the temperature and humidity of the workshop environment. Because the photosensitive layer of the PS plate under normal light irradiation, slow oxidation occurs, affecting the final imaging effect of the photosensitive layer. At the same time, the storage period should be shortened as much as possible. The printing plate storage period should not exceed half a year.

6. Wash paper powder, debris, sand, etc. accumulated on the blanket cylinder to reduce the friction coefficient between the printing plate and the blanket and reduce the occurrence of accelerated wear. This can effectively extend the life of the plate.

7. When the plate is coated with a hydrophilic colloid, pay attention to whether the colloid concentration meets the requirements, the coating colloid should be uniform, the thickness is appropriate, and the high-energy surface of the low-energy surface and the blank part of the picture and text should be covered, and the air should not be covered.