Printing pressure process technology is one of the three major processes of lithographic printing. Outcome is a relatively complex process technology in offset printing. The pressure of offset printing comes from the elastic force generated by the compression deformation of the blanket (including the blanket and liner) on the blanket cylinder. The proper use of the blanket is a key point in the printing pressure process, and it is important to ensure the printing quality of the product. influences. Since the blanket is also an important consumable material in offset printing production, the correct use of the blanket can extend its useful life and reduce production costs. This article discusses several issues that should be noted regarding the correct use of the blanket.
First, choose the right blanket The current commonly used offset blanket is divided into air blanket and ordinary blanket. Zengtong rubber cloth consists of a base fabric, 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. As the blanket cylinder is imprinted, the axial pressure is applied at the same time, so that the front and back sides of the embossing area of ​​the blanket cylinder are convex. The convex hull makes the maximum radius of the circumference of the embossing contact area on the front and rear arcs (projection bulge). There is a greater difference between the minimum radius (the midpoint of the arc) and results 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, which results in an increase in the dots and the transfer of images. The correctness dropped. In addition to the backing cloth, elastic rubber adhesive, and surface adhesive layer, the air-cushioned blanket also adds micro-bubbles per cotton 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 that print high-add-on-line numbers, the use of air-cushioned blankets reduces the expansion rate of the outlets, reduces the loss of gradation, improves the definition of the product, and improves the correctness of the reproduction of tones and colors. In addition, due to the presence of the microbubble sponge layer, the air-cushion blanket also has better sensitivity, elasticity, recovery performance, and impact resistance than the sound-pass blanket. For high-speed printing presses, the frequency of alternating response is Under high circumstances, the use of an air-cushioned blanket can make the internal friction of the blanket not too large. In addition to maintaining the stability of the printing pressure, it can also reduce the speed of aging and extend the life of the blanket. However, the price of air-cushion blankets is much higher than that of ordinary blankets, which increases the production cost. Air-cushion blankets are used only when they are used on high-speed machines and when high-grade products are printed on hard-lining or neutral-neutral hard-packed villages. 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.
Second, to prevent the blanket in the use of torsional deformation blanket in the imprinting process, if the axial surface at different positions by the direction of different or different size of the role of the force, it will make the shape of the blanket torsion deformation, resulting in the graphic shape The adverse effects of ink transfer, overprinting, and the life of the blanket. There are two main reasons for the torsional deformation of the blanket:
1. The axis of the drum is not parallel. Because when the center distance between the rollers is adjusted carelessly or because of the long-term use of the printing machine, the pressure regulator appears to be self-propelled, so that the center distance between the two ends of the roller is unequal, and it is possible to make the blanket cylinder relative to the impression cylinder or the plate cylinder. The axis is not parallel, the end of the blanket is subjected to a large squeezing force, and it is pushed forward. The other end is subjected to a small squeezing force, and the forward speed is slower, causing the blanket to twist.
2. The method of operation when the blanket is cut or mounted on the blanket plate is not standardized. When the blanket is cut, if the two sides of the clamping plate are not parallel, the tightness at different axial positions will be uneven when tightening, and the blanket will be tight on one side and the other side will be loose. The upper tensile stress is published unevenly and causes torsional deformation. When the blanket is mounted on the platen, if the depth of the blanket is different from the platen or the individual screws are not tightened when clamping the platen, when the blanket is tightened, a small amount of the rubber that has not been screwed out will cause the rubber to escape. The tension of the cloth at different positions in the axial direction is different, resulting in local torsional deformation.
When the blanket is cut, it is worth noting that the blanket should be parallel to the platen. The woven fabric of the blanket is formed by interweaving warp and weft threads. When subjected to the same tensile force, the elongation in the warp direction is far smaller than the elongation in the weft direction. When the blanket is stretched, the more it stretches, the more severe the thinning of the adhesive layer will result in the lower elasticity of the blanket and the more uneven the pressure. Generally, the base cloth of the blanket has a mark indicating the direction of tightening, and it can also be tried by hand. The person who does not easily stretch is the longitudinal direction and should be used as the direction of tightening, especially in the pair that will be scrapped due to hard damage. Or when full-length blanket is cut and used as 4 open blankets, attention should be paid to the directionality of warp and weft.
Third, the correct measurement of the blanket thickness In the offset printing production, the size of the Indian pressure is usually expressed indirectly by the maximum amount of compression when the liner lining the drum. Accurately measure the thickness of the liner is to calculate the printing pressure key.
In offset printing, the blanket is stretched on the blanket cylinder. The blanket is stretched after being pulled by the warp. The amount of stretching is proportional to the amount of tension. However, in the circumferential direction of the roller, due to the friction force, different points are subjected to different pulling forces. The two ends near the plate are subjected to the greatest pulling force, and the middle position is subjected to the smallest pulling force. From the middle to the plate The pulling force in the direction of the clamp increases in sequence. Since the blanket is stretched for a long time, the thickness of the adhesive layer is reduced and the thickness is reduced. Therefore, the thickness measured by the blanket in a free state not attached to the blanket cylinder is not equal to the thickness after the blanket is stretched. The correct measurement of the thickness of the lining should be done using a caliper gauge. The thickness of the lining after the tensing is measured at different positions in the circumferential direction of the squeegee and the printing pressure is calculated from the thickness. However, many small-scale printing plants do not have a cylinder meter. Usually, the micrometer is used to measure the thickness of the blanket and liner. Since the micrometer can only be measured in a free state before the liner is installed, the operator should tighten the blanket. There is a correct estimate of the thickness difference between the post-thickness and the free state, so that there will be no large computational deviation. For soft-filled linings, this differential effect may not be significant, and for hard-lining, due to the large elastic modulus of the hard-lining, a small amount of error in the calculation of compression will result in greater brush pressure. Change, improper printing pressure will seriously affect the correct transfer of ink.
Fourth, correctly taut the blanket The blanket must be tight under the appropriate tensioning force on the blanket cylinder. If the blanket is too loose, when the blanket is imprinted, displacement occurs under the action of the pressing force and cannot be reset in time. This can cause ghosting failure. If the blanket is stretched too tightly, there will be many drawbacks:
1. The adhesive layer is thinned and its elasticity is reduced. If the printing pressure is calculated by measuring the thickness obtained by the micrometer in the field state of the blanket, then the printing pressure may be insufficient due to the excessive thinning of the adhesive layer. The thinning of the strand reduces the elasticity of the blanket, and even if the center-to-center distance is decreased to increase the amount of compression, poor ink transfer may occur. If it is an air-cushioned blanket, too tight a stretch may also cause the microbubble sponge layer to fail and lose the good performance that the air-cushion blanket should have.
2. Causes uneven printing pressure. Due to the tightening force, the pulling force of the blanket at different positions in the circumferential direction is not uniform, resulting in a non-uniform thinning of the thickness in the circumferential direction, resulting in uneven printing pressure. The tighter the tension, the more serious this unevenness is. The nonuniformity of the pressure causes inhomogeneity of the ink transfer, so that the background color of the solid or flat net, which should otherwise be uniform, becomes non-uniform and the hue of the color image shifts.
3. Affects the synchronous rolling state. Fine fruit under normal blanket tightness and rubber blanket lining thickness, the three rollers have a good state of synchronization of the rolling area, then in the case of excessive tension, the rubber roller relative to the plate cylinder and the impression roller synchronization The rolling ratio will rise, destroying the state of simultaneous rolling, sliding between the three rollers, affecting the correct transfer of dots, destroying the reduction of levels and colors.
4. Accelerated the creep and aging of the blanket. The tighter the tension, the more likely it is to cause a conformational change in the rubber molecules, which will stiffen the molecular chain and deteriorate the elasticity of the rubber, affecting the printing quality of the product and shortening the service life of the blanket.
Fifth, pay attention to the deformation of the use of the blanket The rubber blanket fastened on the blanket cylinder, will produce its own internal stress and tension to pull it against, but the internal stress will gradually decay with the extension of time, so that the degree of tension is reduced, Stress relief called a blanket. The phenomenon of stress relaxation is most likely to occur on newly replaced rubber blankets. Therefore, on the one hand, the newly replaced blanket cannot be stretched too tightly. On the other hand, after printing for a certain period of time, it must be tightened again. This will be repeated three or four times. After that, the stress relaxation phenomenon can be weakened and the tightness of the blanket can be kept stable for a long time.
Under the effect of pressure during the combined printing, the blanket undergoes plastic deformation in the thickness direction, resulting in a decrease in the thickness of the blanket. The law of plastic deformation is that the newly-replaced rubber blanket has a large amount of plastic deformation. In each subsequent printing cycle, the amount of plastic deformation gradually decreases until it is negligible, making the thickness of the blanket stable for a certain printing time. . Due to the plastic deformation of the newly replaced rubber blanket, the thickness is reduced more, and the thickness of the blanket is successively reduced when the blanket is tightened in order to correct the reduction of the tightness caused by the stress relaxation phenomenon, which may lead to a decrease in the thickness of the blanket. Insufficient pressure. For software linings, the pressure reduction can reach 20%~30%, and the pressure of the hard lining can be reduced by 10%~15%. Therefore, it is necessary to increase the thickness of the liner or reduce the center appropriately after a period of printing. To make up for it. The large deformation of the new blanket at the beginning of the thickness is adjusted, the thickness of the blanket will remain stable during the longer normal use, but this stability is relative, after a very large number of printing Afterwards, this tiny but permanent micro-deformation, together, caused the conformation of the blanket to change, resulting in the loss of elasticity and eventually the blanket was scrapped.
The pressure applied to the blanket during its use will cause different plastic deformations in the thickness of the blanket, which will lead to uneven thickness of the blanket. For example, after printing a product on the boot for a long time and then open the product, the product will then be reprinted. When printing 4 open products, the pressure between blanket cylinder and plate cylinder is evenly distributed over the entire blanket. However, the pressure between the blanket cylinder and the impression cylinder is not uniform due to the presence of the printed paper, the corresponding position of the four sheets of paper, the pressure on the blanket is large, and the pressure on the position other than the four sheets is small or If there is no pressure, the plastic deformation of the blanket will occur at the position corresponding to the 4th paper, and the thickness will decrease. When the folio is reprinted, this position is prone to underpressure. Since this lack of pressure is caused by the partial depression of the blanket, even if the overall pressure between the rollers is increased, there can be no significant improvement, and the rubber can only be remedied by the method of repairing the rubber. This type of blanket cannot be used. High quality products are required. This non-uniform plastic deformation occurs with 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 printing, the ink can transfer well. Brush pressure 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 likely it is that the plastic deformation is uneven.
3. The performance of the blanket. 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.
4. Printing quantity. Long-term printing of a certain number of pages with a large number of prints will lead to greater local plastic deformation.
5.