Correct use of offset blankets (below)

Fourth, tighten the blanket properly

The blanket must be tight on the blanket cylinder with proper tension. 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 to the blanket cylinder will inherently contend with the internal stress and the tension that tightens it, but the internal stress will gradually decay with time, causing the tightness to decrease, which is called the stress relaxation of the 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. Paper and ink piles. After printing for a period of time, the surface of the blanket adheres to many papers and papers 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 printing the cardboard, the paper scraps produced by the trimming of the trailing edge are likely to accumulate on the blanket over the edge of the cardboard with respect to the edge near the tip edge of the cardboard. Because of the uneven cutting of the cardboard, some of the printing is slight. When the cardboard is longer, piled paper pieces cause local depressions in the area. Therefore, it is necessary to pay special attention to cleaning the blanket in a timely manner when printing paper or cardboard that is susceptible to lint-free, powder-off, or chip-off. 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.

Six, blanket maintenance

Due to the hot spots of photoaging and heat aging, the rubber blankets to be used after purchase are wrapped in black paper and stored in a cool place.

When cleaning the blanket, the organic solvent with faster volatiles should be used as the detergent, and the slow evaporation of kerosene or its solvent can make the blanket swell, and it should be washed to dry the blanket without leaving any residue. On the one hand, residual traces are easily oxidized and dried, causing the blanket to age prematurely. On the other hand, when the residual traces are printed on other products, the unevenness of the ink is likely to occur at the beginning.

After a certain product is printed, if the down time is long, the tensioning device of the blanket can be loosened, the blanket can be relaxed to obtain the opportunity of internal stress recovery, and the effect of actively preventing stress relaxation can be achieved.

Source: Chinese Printing Community