In the printing process of sheet-fed offset printing presses, especially for fine prints that use coated paper and print large-volume screens, in order to prevent the ink from becoming dirty, powder spraying is usually used when printing the last color. At present, the new type of offset printing machine does not use or use less powder spraying, and the light irradiation drying device is gradually becoming popular. However, most of the paper drying system is high-level paper collection on the offset press, and its price is very expensive. Few people in domestic printing companies will purchase this system when ordering printing machines. Therefore, most printing companies have to use powder spraying to prevent the paper and ink from sticking, and powder spraying is still widely used. The following is a brief summary of the precautions for controlling the amount of powder sprayed. 1 Powder used for dusting The printing powders sold on the market are mainly based on pure plant materials. Commonly used materials are flour, corn flour (millet flour), vegetable starch, cassava flour, etc. Powder spraying powder should be correctly selected according to the quantity of paper and other factors. The particle diameter should be at least 15nm. When printing rough paper, the powder diameter can be larger. The fine dust content in high-quality powder spraying is very small . If the process requires secondary printing, starch-based powder spraying should be used to avoid abrasion of the printing plate. Never use powders such as talc. 2 Control of dusting operation The amount of powder spraying should be determined according to the specific printing conditions and requirements, and it is compatible with the specific printing speed, paper characteristics, paper format, and printed graphics. Generally, the appropriate amount of powder should be selected according to the size of the product ink and the paper quality of the paper used, and the number of nozzles used and the size of powder spray should be adjusted according to the size of the paper. [next] 3 The dangers of dusting 1) Impact on human health Although most of the powders currently sold do not have chemical compositions that are extremely harmful to humans, operators who stay in fine dusty workshops for a long time may also cause some lung tracheal diseases. Be careful to avoid inhaling large amounts of powder. 2) Impact on equipment After powder spraying, there are dust particles sticking to the paper, which not only affects the quality of printing during the second printing, but also particles from the paper to the ink roller, which not only affects the transfer of ink but also greatly reduces the service life of the ink roller At the same time, it also seriously reduces the service life of the blanket, and a considerable part of the powder is flying to the machine's paper collection chain, gripper and roller, which has a serious impact on the machine's life. The resulting maintenance work has virtually increased production costs. 3) Impact on the product In front and back printing, if the amount of powder on the front side is too large, it will directly affect the ink transfer on the back side. The powder spray deposits on the printing blanket, causing severe discoloration. In some cases, dusting will be transferred to the printing plate, causing dampening and paste / dirty problems. In severe cases, the blanket needs to be cleaned every few hundred sheets, and the ink color of the product will also be affected. The ink color is inconsistent before and after, which restricts the quality and efficiency of production. It can be solved by three measures: removing the powder on the paper; printing with pressed powder without dampening; printing with ink and oil if necessary. When printing some high-end products, the operator always wants the ink color saturation to be higher. In this way, the printed product is both bright and bright, and at the same time, the amount of powder is increased to prevent smudging. The powder spray is absorbed by the ink on the ink layer that has not been dried, and will form a layer of mist on the color, which reduces the gloss of the product. In packaging and printing, the post-processing status is becoming more and more important, but because of the powder spraying in the printing process, the powder particles stick to the paper, which greatly increases the rate of waste products such as glue and laminating. At the same time, the paper in the packaging industry is heavier and the amount of ink is larger. In this environment, the equipment often has mechanical failures in the printing and delivery. [next] 4 How to reduce the amount of powder 1) Strictly control the density value. Obtain the best density value through proofing, and master the best range of paper to bear the ink amount. Controlling the amount of ink in this range can reduce the sticky dirt on the back side, which can reduce the amount of powder sprayed. 2) When printing some products with large dark areas and thick ink layers, it can be alleviated by means of plate making. Thin the dark tone ink layer appropriately within the allowable range. 3) The density of proofing proofs should be consistent with the state that can be successfully achieved by printing. Do not blindly increase the amount of ink in order to sign the sample, which will cause serious trouble to the printing. After increasing the amount of ink, the dot will be seriously expanded, and the powder must be increased to avoid sticking. 4) Control the amount of fountain solution, maintain a good ink emulsification state, and ensure the reasonable drying of the ink itself, which can reduce the amount of powder spraying. 5) Set the correct printing pressure. If the pressure is too light, the ink cannot be transferred normally, and it cannot be fully combined with the paper. If the transfer is not firm, the back will be dirty. Increase the printing pressure within the allowable range of the dot expansion to avoid unnecessary sticking and reduce the amount of powder sprayed. 6) In front and back printing, you can first analyze the size relationship between the front and back ink volume, print the ink with a small ink volume and not easy to be dirty, and print the side with a large ink volume that may be sticky. The amount of powder sprayed can be increased appropriately. If a large amount of powder is sprayed on the side with the largest amount of ink printed first, it will inevitably be gradually deposited, which may result in discoloration, dirty version, and paste version. 7) Add a paper rack. For products with a large amount of powder spraying, adding a drying rack can reduce the amount of powder spraying. Products with a small amount of powder spraying may no longer need to be sprayed. 8) Use new ink additives. The additives are mixed with the ink in proportion to form a polymer film on the ink surface of the paper after printing, which prevents direct contact between the wet ink and the paper, thereby preventing sticking.
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