Exploring the Reasons for the Decrease in Printability of Offset Printing Plates

In offset printing, there has always been a problem that the printing plate is insufficient in print impact resistance. The reason is mainly the following aspects.

First, the imprinting process is not standardized and the imprint resistance of the printing plate is insufficient. Such as imposition did not seriously do "three no" Yin D (that is, no traces, no treatment, no pollution); in the printing plate did not carefully check the density of the film hook uniform reality; exposure in the printing time The control is not accurate enough, the stencil machine vacuum is not dense enough; the developing time during development is not accurate enough, and the proportion of the developer is not reasonable; the baking time and the temperature of the baking plate are not properly controlled during the baking.

1. Problems with the layout process. Some film editions are squeezed by the imposition workers themselves. If the imposition workers are not responsible in the layout, they cannot seriously implement the operating procedures. When the film is present, there are dirty activities, debris, glue marks, and film overlaps (especially When Gao Wenwen is particularly close to the time, etc., phenomena such as false appearance of printed plates and inaccurate outlets will be caused, which will make printing plates less resistant to printing.

2. The problem of film density. The density of the film plays a major role in the quality of the printing plate. Some film density is small, and some film density is high. If the printing personnel counts the density of the film when printing, countless dots in the normal exposure time will easily appear in the printing plate, or the local text lines will be missing. The appearance of these phenomena, such as the delineation and misrepresentation of words, will also affect the printing plate's resistance to printing forces, and will reduce the printing plate's resistance to printing forces. This requires the printer to carefully check the density of the film before printing, place the film on the layout table, and use a 10-20x magnifying glass to analyze the density, and then use different exposure methods according to the film density. Exposures) and exposure time. 3. Exposure time and pumping vacuum problems. Correctly controlling the exposure time of the printing plate is an important aspect of improving the printing plate's imprinting resistance. If the exposure time is too long, that is, overexposure, it will cause the printing plate network to be imaginary, lost or incomplete; if the exposure time is too short, ie, underexposure, it will make the surface photosensitive layer of the printing plate not clean during development, causing printing Dirty. Overexposure or underexposure can cause the printing plate to resist imprinting force. In the exposure, attention should be paid to the situation of the stencil vacuum. If the vacuum is not true, there will be air bubbles between the film and the printing plate or between the film and the glass. When the printing plate is exposed, the internet point is lost or the word is false. Affect the quality of the printing plate, so that the printing plate's resistance to printing force is reduced. Therefore, in order to improve the imprint resistance of the printing plate. It is required that the plate-making workers should correctly control the exposure time during the printing and pay close attention to the vacuum condition of the plate-making machine. When vacuuming, neither the rush nor the slow-moving person should be drawn.

4. Developer solution ratio, development time issues. At present, most of the printing solutions used in the printing industry are made of salt (such as sodium silicate, sodium metasilicate), alkali (such as sodium hydroxide) and water in proportion to the flowers. If the ratio is not good at the time of matching the flowers, it will affect the reproduction quality of the printing plate graphic or network. Similarly, the developing time should be strictly controlled under the condition that the developer has a reasonable ratio and the temperature is appropriate. If the development time is too long, it will cause the printing plate picture and network part to be imaginary or missing, so that the adhesion of the surface of the picture and text will be reduced; if the development time is too short, the surface of the printing plate will show incomplete images and dots, resulting in offset printing and paste. Versions and other phenomena affect the quality of printed products. In order to improve the imprint resistance of the printing plate, the printing plate maker must strictly control the development time and pay attention to the ratio and temperature of the developer. In addition, it should also be noted that the developed plate should be “thin and scratched” when it is applied with a protective rubber (such as rubbed gum arabic). The rag must be “clean and clean”, otherwise it is protected. After the rubber is blown dry, it will appear as a “Louisher” stripping plate, which also makes the imprinting force of the printing plate lower.

5. Baked version of the problem. Roasting plates can increase the imprint resistance of printing plates and make the graphic part chemically resistant. However, if the method used in baking is incorrect, it will also reduce the printing plate's resistance to printing. At present, the baking solutions used by many printing plants are self-matching flowers, such as the use of pentacarboxylic acid, emulsifier OP-10 and water with flowers. If the proportion of the baking solution is not reasonable, 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 “soft plate” or “IJ stripping”, resulting in Plate A ho. The Al2O3 film is brittle. Makes the mechanical strength and chemical stability of the blank part worse. Affect the imprint resistance of the printing plate. and so. We should correctly match the baking solution and control the baking time. Under normal circumstances, the baking solution should be coated with “thin and uniform”, the baking time is controlled between 8-10 minutes, baking temperature is controlled at 200-230. "C between.

Second, the printing process is not standardized caused by the printing plate that is insufficient strength. There are mainly two aspects: improper printing pressure adjustment and improper use of fountain solution.

Questions about printing pressure. As we all know, printing pressure refers to the mutual force between imprinting during the printing process, which inevitably brings about compression deformation, speed difference and friction. The main reasons for the decrease of imprinting impact resistance of printing plates caused by printing pressure are as follows:

1. The problem of the roller pressure is the pressure between the plate cylinder and the blanket cylinder. If the pressure is not adjusted properly, excessive pressure will occur and the printing force between the printing plate and the rubber will decrease. Therefore, you must follow the relevant printing technology requirements before printing. Carefully adjust the pressure to make it ideal.

2. The pressure problem between the rollers, that is, the pressure between the roller and the roller, especially the rollers (usually four rubber rollers with different diameters) and the roller (usually two like rollers) The pressure between the glue or flannel roller and the printing plate is too large, it will increase the friction, make the printing plate's network point suffer loss, appear "off version", make the printing plate's resistance to printing force drop.

Questions about fountain solution. We know that the use of fountain solution is unique to lithography. In the process of lithographic offset printing, the purpose of the use of dampening is to maintain the balance between the blank part and the graphic part, and to protect the blank part of the printing plate from being invaded by ink. It does not let it expand into the graphic part. The following focuses on the use of fountain solution in the printing process.

The main ingredient of the commonly used fountain solution is "Lyde powder" (wetting powder) and water with a certain ratio of cash. If the pH of the fountain solution is too high or too low, it will directly affect the printing plate's resistance to printing. If the pH value of the fountain solution is too high, the unexpanded diazotization hand of the positive part of the picture will be dissolved, resulting in incomplete picture and image, which will weaken the strength and lipophilicity of the printing plate, resulting in a decrease in the life of the printing plate. If the pH of the fountain solution is too low, the plate base will be heavily corroded and the blotting ink will dry slowly, which will affect the quality of the printed product. The main component of the plate base used in the lithographic printing is aluminum, and the metal is very active. Strong acid or alkali solution is extremely unstable, prone to chemical reactions, resulting in the loss of the printing plate base, resulting in decreased printing plate resistance to printing forces.

It is generally believed that the pH of dampening used in offset printing should be controlled between 2.0 and 6.5, which is to make the dampening fluid slightly weaker, because in the weakly acidic dampening fluid, the aluminum substrate is only slightly corroded. A layer of hydrophilic inorganic salt is formed. Therefore, in order to improve the printing plate's resistance to printing force. Must control the fountain solution with the ratio and PH value. Of course, if a hydrophilic colloid is added to the fountain solution to supplement the depleted hydrophilic colloid layer, the printing plate's resistance to printing will be greatly improved.

In the lithographic printing process, the balance between water and ink should also be as far as possible. If the ink balance is not achieved during the printing process, it will also affect the printing plate's resistance to printing forces. When the water is large, the ink is emulsified, and the density of the blotted ink does not meet the requirements. The corrosion degree of the printing plate base is deepened; when the water is small, the printing plates are “glued” and “ink piled”, which affects the quality of printed products.

Of course, there are many other external and intrinsic factors that will also cause the printing plate's resistance to printing forces to decline, such as the quality of the PS plate itself, the access to the printing plate is reasonable and so on. However, no matter what factors, as long as we strengthen our sense of responsibility and improve our printing technology, we will be able to improve the printing plate's resistance to printing and print high quality, high-quality prints.


Qingdao Weinan Printing Factory Li Wei