During the printing process of a sheet-fed offset printing press, especially for fine prints that use coated paper and for printing a large amount of ink, in order to prevent the ink from becoming dirty, dusting is usually used when printing the last color.
At present, new type offset printing presses do not use or use less powder, and light irradiation drying devices gradually become popular. However, most of the paper drying systems on the offset printing press are high-stage delivery, which is expensive. Domestic printing companies rarely order this system when they order printing presses. Therefore, most printing companies have to use dusting to prevent the problem of sticky paper and ink, dusting is still widely used. The following is a brief summary of the precautions needed to control the amount of dusting.
1 powder used for dusting
Printed dusting powders sold on the market are mainly based on pure vegetable substances. Commonly used materials include flour, corn flour (potato powder), plant starch, and tapioca flour. The dusting powder should be properly selected according to the paper's quantification and other factors. The particle diameter must be at least 15nm. When printing coarser paper, the diameter of the powder particles can be selected more. The fine dust content in the high-quality dusting powder is very small. . If the process requires secondary printing, starch-based dusting should be used to avoid wear on the printing plate, and talcum powder must not be used.
2 Control of dusting operations
The amount of dusting should be determined according to the specific printing conditions and requirements, and with the specific printing speed, paper characteristics, paper format, printed products and graphics. Generally, the appropriate amount of powder to be sprayed should be selected according to the size of the ink of the product and the paper quality of the paper used, and the number of nozzles used and the size of the sprayed powder should be adjusted according to the size of the paper web.
3 Dust Hazard
1) Impact on human health
Although most of the dusting powders currently sold do not have a chemical composition that is extremely harmful to the human body, the operator's long stay in dusty workshops may also cause some lung tracheal diseases. Care should be taken to avoid inhaling large amounts of powder.
2) Impact on the equipment
After dusting, dust particles adhere to the paper, which not only affects the quality of the printing during the second printing, but also causes the powder to pick up from the paper onto the ink roller. This not only affects the ink transfer but also greatly reduces the service life of the ink roller. At the same time, the service life of the blanket is severely reduced, and a considerable part of the powder is flying to the machine's delivery chain, grippers and rollers, which have a serious impact on the life of the machine. The resulting maintenance and repair work virtually increases the cost of production.
3) Impact on the product
In positive and negative printing, if the amount of powder on the front face is too large, it will directly affect the reverse ink transfer. Dust deposits on the printing blanket, causing serious discoloration. In some cases, dusting will be transferred to the plate, resulting in dampening and scumming/dirty plate problems. In severe cases, every hundred sheets need to be cleaned once, and the ink color of the product will also be affected. The ink color will be inconsistent before and after, restricting the production quality and efficiency. It can be solved by three kinds of measures: the process of removing dust from the paper; printing with powder under the condition of no dampening; overprinting with ink oil if necessary.
When printing some high-end products, operators always hope that the ink color saturation is higher. The products printed in this way are both bright and shiny, and at the same time they increase the amount of powder in order to prevent dirt. The dusting is absorbed by the ink on the ink layer that has not been dried, and a layer of fog is formed on the color, and the gloss of the product is reduced accordingly.
In the packaging and printing, the position of post-processing is more and more important, but because the printing links are all sprayed with powder, the particles adhere to the paper, and the rate of rejects such as glue and film is greatly increased. At the same time, the paper in the packaging industry is heavy and the amount of ink is large. Equipment in this environment often suffers from mechanical failures in the delivery of printed paper and transmission parts.
4 How to reduce dusting
1) Strictly control the density value. The best density value is obtained by proofing, and the optimal range of the paper to withstand the ink volume is grasped. By controlling the amount of ink in this range, the phenomenon of sticking on the back surface can be reduced, and the amount of dusting can be reduced.
2) When printing some products with a dark tone and a thick layer of ink, they can be eased by means of plate making. Appropriately darken the ink layer in the allowable range.
3) The density of the proofs should match the state that the printing can successfully reach. We must not blindly increase the amount of ink for the sake of signing. This will cause serious trouble for printing. After increasing the amount of ink, it will cause a serious expansion of outlets, and we must increase the amount of dust to avoid sticking.
4) Control the amount of fountain solution, maintain a good state of ink emulsification, and ensure that the ink itself is reasonably dry, which can reduce the amount of spray powder.
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, it can also cause back-scratching. Increase the printing pressure within the allowable range of outlets to avoid unnecessary stickiness and reduce the amount of dusting.
6) In the case of positive and negative printing, the relationship between the positive and negative ink volumes can be analyzed first, and the ink can be printed in small quantities and can not be easily stained. The ink can be printed on the side with a large amount of ink and may be dirty, in a larger amount of ink. Can increase the amount of dusting. If a large amount of ink is sprayed first, a large amount of powder will inevitably be gradually deposited, which may cause discoloration, dirty prints, and stencils.
7) Add and put paper holders. Adding a paper holder to a product with a large amount of dusting can reduce the amount of powder to be sprayed, and a product with a small amount of dust may no longer need to be powdered.
8) Use a new ink additive. The additive is mixed with the ink according to the ratio, and a layer of polymer film is formed on the ink surface of the printed paper, which prevents the direct contact between the dry ink and the paper, thereby preventing stickiness.