The correct use and maintenance of the blanket

Relying on a blanket to transfer the ink on the graphic part of the plate to the surface of the substrate is an important feature of offset printing that distinguishes it from other printing methods. Offset printing is therefore also named.

Blanket is the medium that transfers the graphic on the plate to the substrate. Due to the use of rubber blanket in offset printing, the pressure needed for imprinting is reduced, and due to the elastic deformation of the blanket, the surface of the substrate and the ink in the graphic part that needs to be transferred can be fully contacted, thus making up for the surface of the substrate. The unevenness also increases the ink transfer rate.

Because the blanket has such an important role in offset printing, the correct use of blankets becomes very important in printing, which is of great significance for improving the quality of printed products and reducing production costs.

Blanket type and selection
The commonly used offset blankets are classified into air-cushion blankets and ordinary blankets. Ordinary blanket consists of a backing, an elastic rubber adhesive, and a surface adhesive layer. Because the rubber itself has a volume incompressible characteristic, when the surface adhesive layer is pressed, a convex bulge will be formed around the pressure area. Since the blanket cylinder is pressed axially at the same time, the convex and convex bulges appear on both the front and rear sides of the embossing area of ​​the blanket cylinder. The convex hull makes the maximum radius of the embossed contact area on the front and rear arcs (bump) and The difference between the minimum radius (the midpoint of the arc) is greater, resulting in an increase in the amount of slippage between the blanket cylinder and the plate cylinder and between the blanket cylinder and the impression cylinder, resulting in an enlarged dot and a decrease in the correctness of graphic and text transfer. . In addition to the backing cloth, elastic rubber adhesive, and surface adhesive layer, the air-cushioned blanket also adds a micro-foam sponge layer under the surface adhesive layer. The microbubble sponge layer is formed by numerous tiny, closed air spheres dispersed in the adhesive layer. After the rubber layer on the surface of the blanket is pressed, the air sphere reduces its volume under pressure, so the blanket will be compressed vertically. The deformation will no longer develop on the front and rear sides of the embossing area, and will not be imprinted during the imprinting process. The front and rear sides of the zone are raised with convex bulges, which reduces the slippage of the embossing gap and improves the correctness of graphic and text transfer.

For color products with a high number of screen printing lines, the use of air-cushioned blankets reduces the expansion rate of the dots, reduces the loss of gradation, improves the definition of the product, and improves the accuracy of tone adjustment and color reproduction. In addition, due to the presence of the microbubble sponge layer, the air-cushion blanket has better sensitivity, elasticity, recovery performance, and impact resistance than the conventional blanket. For the high-speed printing press, the alternating frequency is suitable for the frequency. Under high circumstances, the use of air-cushion blankets can make the internal friction of the blanket not too large, while maintaining the stability of the printing pressure, but also reduce the speed of aging and extend the life of the blanket. However, air-cushioned blankets are much higher in price than ordinary blankets, which increases the production cost. Air-cushion blankets are used only on high-speed machines and are used when rigid liners or neutral parasol liners are used to print high-end products. The advantages of high image transfer accuracy and sensitivity are well developed. If soft liners are used to print common products, because of the large amount of compression of the soft liners, the wide embossing area, and the easy spread of the outlets, which offsets the advantage of high precision of the graphic transfer of the air-cushion blanket, it is not worth using the air-cushioned eraser. Cloth up.

The nature of rubber blankets in offset printing and the corresponding operations
Let's analyze some of the necessary properties of offset blankets (considering the other direction, it may be reasonable to operate only without damaging the properties of the blanket. In addition, in operation and maintenance, only the blankets for these properties should be used. Use it to effectively extend the life and quality of the blanket.)

First, elasticity
The blanket used in printing must have good elasticity (because the deformation can be promptly restored after the pressure is withdrawn) so that it can be in good contact with the substrate with poor surface flatness during the imprinting process. The ink in the plate section is relatively intact and transferred to the surface of the substrate.

In addition to ensuring good elasticity of the blanket in production, care should be taken to protect the elasticity of the blanket during use. Here we briefly describe the deformation of the blanket in the printing to discuss the protection of the elasticity of the blanket.

There are two main types of deformation of rubber blankets, namely elastic deformation and plastic deformation (which cannot be recovered after deformation). Among them, elastic deformation includes sensitive elastic deformation (when external force is removed, deformation caused by external force can be immediately recovered) and elastic deformation ( When the external force is removed, the deformation caused by the external force can be restored within a certain period of time). Among these kinds of deformations, we are most interested in the deformation of printing is sensitive to elastic deformation, and try to avoid abnormal plastic deformation of the blanket, and then maintain the blanket good sensitivity and elasticity, the use of blankets need special attention.

Plastic deformation is related to the following four factors:

1. Printing pressure
The greater the pressure, the greater the pressure on the blanket at the paper, and the greater the plastic deformation that occurs, the more likely this non-uniform plastic deformation will occur. Therefore, when the ink is printed, the printing pressure can be well transferred. Should not be too large, should use the best pressure printing.

2. The tightness of the blanket
The more tightly stretched, the thinner the rubber layer of the blanket and the worse the sensitivity and elasticity, the more easily the non-uniform plastic deformation occurs and the elasticity decreases. Therefore, the tightness of the blanket during use should be moderate (knocking the blanket near the bite or near the blanket and issuing a crisp knocking sound should not be too tight). (In addition, if the blanket is too loose, when the blanket is imprinted, it will be displaced under the effect of the pressing force and cannot be reset in time, which will cause ghosting failure. If the blanket is too tight, it will bring many Disadvantages: thinning of the adhesive layer; uneven printing pressure; affecting the state of simultaneous rolling; accelerating the creep and aging of the blanket).

3 blanket performance
Due to the good sensitivity, elasticity, and recovery ability of the microbubble sponge layer, the air-cushion blanket is less prone to non-uniform plastic deformation. For high-grade prints, air-cushion blankets should be considered.
4. Printing quantity Long-term large-volume printing of a certain size of paper will cause local plastic deformation.

5. Piles of paper and ink
After printing for a period of time, the surface of the blanket adheres to many paper and paper powders due to physical adsorption. If it is not cleaned in time, the thick paper and paper powder increase the local printing pressure, causing more plastic deformation locally. . When paperboard is printed, the paper scraps produced by the trimming of the trailing edge are likely to accumulate on the blanket relative to the edges of the cardboard trailing edge under the action of oil and water. Since the cardboard is not cut regularly, the printing is slightly longer. In the case of a little cardboard, the accumulated scraps of paper cause partial depressions in the area. Therefore, when printing papers or cardboard that are easy to lose hair, powder, or chip, special attention should be paid to timely cleaning of the blanket. The dry ink is adhered to the impression cylinder, and the partial pressure is uneven during printing, resulting in the partial depression of the blanket. Therefore, after each type of product is printed and after printing is completed, the blanket should be cleaned in time. And impression cylinders.
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