Correct use of offset printing blanket

The printing pressure process technology is one of the three major processes of lithography, and the more complicated process technology in offset printing. The pressure of offset printing comes from the elastic force generated by the compression deformation of the blanket (including blanket and pad) on the blanket cylinder. The correct use of blanket is an important point of the printing pressure process, which is important for ensuring the printing quality of the product influences. Because the blanket is also an important consumable material in the production of offset printing, the correct use of the blanket can extend its service life and reduce production costs. This article discusses several issues that should be noted when using blankets correctly.

1. Choose the right blanket

The commonly used offset printing blankets are divided into air cushion blankets and ordinary blankets. Zengtong blanket is composed of base fabric, elastic rubber adhesive and surface adhesive layer. Because the rubber itself has the characteristics of incompressible volume, when the surface adhesive layer is compressed, it will form a convex hull around the compressed area. Since the rubber cylinder is pressed at the same time in the axial direction, convex hulls are formed on the front and back sides of the imprinting area of ​​the rubber cylinder, and the convex hull makes the circumference of the imprinting contact area the maximum radius on the arc line (convex hull) ) And the minimum radius (the midpoint of the arc) is greater, resulting in an increase in the amount of slippage between the rubber cylinder and the plate cylinder and between the rubber cylinder and the impression cylinder during imprinting, resulting in the expansion of the dots and the transfer of graphics and text The accuracy is reduced. In addition to the base fabric, elastic rubber adhesive, and surface adhesive layer, the air-cushion blanket also adds each layer of microbubbles under the surface adhesive layer. The micro-foam 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 compressed, the air sphere reduces its volume under the pressure, so the blanket produces vertical compression, and the deformation no longer develops to the front and back sides of the imprinting area, and will not be imprinted during the imprinting process. Convex hulls are formed on the front and back sides of the area, reducing the slippage of the imprinting gap, and the accuracy of image and text transfer is improved. For the printing of color products with high screen counts, the use of air cushion blankets reduces the expansion rate of dots, reduces the loss of layers, and improves the clarity of the product, and the accuracy of tone reproduction and color reproduction. In addition, due to the presence of the micro-foam sponge layer, the air cushion blanket has better sensitivity, resilience and impact resistance than the Yintong blanket. For high-speed printing machines, the frequency of alternating response In very high conditions, the use of air cushion blankets can keep the internal friction of the blankets not too large. In addition to maintaining the stability of the printing pressure, it can also reduce the speed of aging and extend the service life of the blankets. However, the price of air-cushion blankets is much higher than that of ordinary blankets, which increases production costs. Air-cushion blankets are only used on high-speed machines and when printing high-end products with rigid linings or neutral hard claddings. The advantages of high precision of image and text transfer and good sensitivity are brought into full play. If the soft lining is used to print ordinary products, because of the large compression of the soft lining, the wide embossed surface, and the easy expansion of the dots, which offsets the advantage of the high accuracy of the graphic transfer of the air cushion blanket, it is not worth using the air cushion rubber Clothed.

2. Prevent twisting and deformation during use of the blanket

During the imprinting process of the blanket, if the surface axial position is subjected to different directions or different magnitudes of force, the shape of the blanket will be twisted and deformed, resulting in graphic shapes, ink transfer, accurate overprinting, and blankets. The adverse effects of the service life, etc. There are two main reasons for the torsional deformation of the blanket:

1. The roller axis is not parallel. Because of carelessness when adjusting the center distance between the cylinders or because of the long-term use of the printing press, the regulator appears to be self-propelled, making the center distance between the two ends of the cylinder unequal, it is possible to make the rubber cylinder relative to the impression cylinder or the plate cylinder The axis is not parallel, and the squeezing force on one end of the blanket is large, pushing forward, the squeezing force on the other end is small, and the speed of pushing forward is slower, causing the blanket to twist.

2. The operation method is not standardized when the blanket is cut or mounted on the blanket holder. When the blanket is cut, if the two sides to be loaded into the plate holder are not parallel, then the tension in different axial positions during tension will be uneven, which will inevitably make the blanket tight on one side and loose on the other. The upper tensile stress is not uniform and causes torsional deformation. When the blanket is attached to the plate clamp, if the blanket is inserted into the plate clamp at a different depth or when the clamp is clamped, the individual screws are not tightened, so that when the blanket is tightened, a small amount of the blanket that the screw is not tightened will come off, which will cause the rubber The tension of the cloth at different positions along the axial direction is different, causing local torsional deformation.

When cutting the blanket, it is also worth noting that the weft of the blanket should be parallel to the plate holder. The woven fabric of the rubber cloth is formed by interweaving the warp and weft threads. When the same tensile force is applied, the warp elongation is much smaller than the weft elongation. When the blanket is stretched, the more it stretches, the more serious the thinning of the adhesive layer, resulting in a reduction in the sensitive elasticity of the blanket and uneven pressure. Generally, the bottom of the blanket has a mark indicating the direction of tension. You can also try it by hand. Those who are not easy to stretch are in the warp direction, which should be used as the direction of tension. Especially in the case of scrapping due to hard damage. Or when cutting a full-length blanket for 4-fold blankets, pay attention to the direction of warp and weft.

3. Correct measurement of blanket thickness

In the production of offset printing, the size of the printing pressure is usually indirectly expressed by the maximum compression of the liner when the cylinder is pressed. Correctly measuring the thickness of the liner is the key to calculating the size of the printing pressure.

During offset printing, the blanket is stretched tightly on the blanket cylinder. The blanket is stretched after being pulled in a warp direction. The amount of extension is proportional to the amount of pull. However, in the circumferential direction of the blanket, due to friction, the pulling force at different points is different. The pulling force at both ends near the plate holder is the largest, and the pulling force at the middle position is the smallest, from the middle to the plate. The pulling force at both ends of the clamp increases sequentially. As the blanket is stretched and stretched, the adhesive layer will be thinned and the thickness will be reduced. Therefore, the thickness of the blanket measured in the free state between the rubber blanket and the rubber roller is not equal to the thickness after tension. To correctly measure the thickness of the liner, a barrel caliper should be used to measure the thickness of the liner after tightening at different positions in the circumferential direction of the rubber cylinder, and use this thickness to calculate the printing pressure. However, at present, many small printing plants are not equipped with a tube diameter gauge, and they usually use a micrometer to measure the thickness of the blanket and liner. Since the micrometer can only be measured in the free state before the liner is not installed, the operator should deal with the blanket tight. The difference between the thickness after the thickness and the thickness in the free state is correctly estimated so as not to cause a large deviation in calculation. For soft linings, this difference may have little effect, while for hard linings, due to the large elastic modulus of the hard linings, a small amount of error in the calculation of compression will cause a large brush pressure Changes, improper printing pressure will seriously affect the correct transfer of ink.

4. Tighten the blanket correctly

The blanket must be tightened on the rubber roller under the proper tension. If the blanket is stretched too loosely, when the blanket is imprinted, it will be displaced under the action of squeezing force and cannot be reset in time, which will cause ghosting failure. If the blanket is stretched too tight, it will bring many disadvantages:

1. The adhesive layer is thinned and the elasticity is reduced. If the printing pressure is calculated by the thickness measured by a micrometer in the blanket's self-field state, the printing pressure may be insufficient due to the excessive thinning of the adhesive layer. The thinning of the ply layer reduces the elasticity of the blanket. Even if the center distance is reduced and the amount of compression is increased, the phenomenon of poor ink transfer may occur. If it is an air-cushion blanket, stretching it too tightly may also invalidate the micro-foam sponge layer and lose the good performance that the air-cushion blanket should have.

2. Cause uneven printing pressure. Due to the tension, the tensile force received by the blanket at different positions in the circumferential direction is uneven, resulting in the thickness reduction is uneven in the circumferential direction, resulting in uneven printing pressure. The tighter the tension, the more severe this unevenness. The unevenness of the pressure causes the unevenness of the ink transfer, which makes the solid or flat screen background color that should be uniform becomes uneven, and the color image has a hue shift.

3. Affect the synchronous rolling state. Fine fruit under the normal blanket tension and blanket thickness of the rubber cylinder, the three rollers have a good synchronous roll zone state, then under excessive tension, the rubber cylinder is synchronized with the plate cylinder and the impression cylinder The rolling ratio will rise, destroying the synchronous rolling state, slipping between the three rollers, affecting the correct transfer of the dots, and destroying the restoration of gradation and color.

4. Accelerated the creep and aging of the blanket. The tighter the stretch, the easier it is to cause conformational changes in the rubber molecules, stiffen the molecular chains, and reduce the elasticity of the rubber, affecting the printing quality of the product and shortening the life of the blanket.

Five, pay attention to the deformation of the use force of the blanket

The rubber cloth fastened to the rubber roller will generate internal stress and the tension to tighten it, but the internal stress will gradually attenuate with the extension of time, and the tension will decrease, which is called the stress relaxation of the rubber cloth. The phenomenon of stress relaxation is most likely to occur on the newly changed blanket, so on the one hand, the newly changed blanket cannot be stretched too tightly at one time, and on the other hand, it must be stretched again after a period of co-press printing. After that, the stress relaxation phenomenon can be weakened, and the tightness of the blanket remains stable for a long time.

The blanket is plastically deformed in the thickness direction under the pressure during the co-press printing, resulting in a reduction in the thickness of the blanket. The plastic deformation law is: the plastic deformation amount of the newly changed blanket is larger. In each subsequent printing cycle, the plastic deformation amount gradually decreases until it can be ignored, so that the thickness of the blanket remains stable within a certain printing time . The thickness of the newly changed blanket is reduced due to plastic deformation. In addition, when the blanket is tightened in order to correct the decrease in tension caused by stress relaxation, the thickness of the blanket is gradually reduced, which may cause Insufficient pressure. For software package lining, the pressure can be reduced by 20% to 30%, and the pressure of the rigid lining can be reduced by 10% to 15%, so it is necessary to increase the thickness of the liner or reduce the center after a period of printing. To compensate. After the large deformation of the thickness of the new blanket at the initial stage has been adjusted, the thickness of the blanket will remain stable during the longer normal use, but this stability is relative, after a large amount of printing Afterwards, this very small but always small deformation always combined to change the conformation of the blanket, leading to the loss of elasticity, and eventually the blanket was scrapped.

The pressure on the blanket in use is different, so that the thickness of the blanket is plastically deformed by a different amount, which will cause unevenness in the thickness of the blanket. For example, after printing a 4K product for a long time on the boot, then change the 2K product. When printing 4K products, the pressure between the blanket cylinder and the plate cylinder is evenly distributed throughout the 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 4-open paper, the pressure on the blanket is large, and the pressure on the positions other than 4 open is small There is no pressure, so the plastic deformation of the blanket at the position corresponding to the 4 open paper is large, and the thickness is reduced much. When the offset paper is reprinted, this position is prone to insufficient pressure. Because this lack of pressure is caused by the local depression of the blanket, even if the overall pressure between the rollers is increased, there can be no obvious improvement, and only the method of patching rubber can be used to remedy, this kind of blanket can not be used Products with high printing quality requirements. The occurrence of this uneven plastic deformation is related to the following four factors:

1. Printing pressure. The greater the pressure, the greater the pressure on the blanket where there is paper, and the greater the plastic deformation occurs, the more likely this uneven plastic deformation occurs. The brush pressure should not be too large, and the best pressure should be used for printing.

2. The tightness of the blanket. The tighter the stretch, the thinner the rubber layer of the blanket and the worse the sensitive elasticity, and the more likely to be uneven plastic deformation.

3. The performance of the blanket. Because the air cushion blanket has good resilience and recovery ability of the micro-foam sponge layer, uneven plastic deformation is not easy to occur.

4. Number of prints. Long-term large-volume printing of a certain size of paper will cause large local plastic deformation.

5. Accumulation of paper hair and ink skin. After a period of printing, the surface of the rubber blanket adheres to a lot of paper wool and paper due to physical adsorption. If it is not cleaned in time, the thick paper wool and paper powder increase the local printing pressure, causing more plastic deformation in the local . When printing cardboard, the paper scraps generated by cutting at the trailing edge are easy to accumulate on the blanket under the action of oil and water relative to the edge near the edge of the cardboard. Due to the uneven cutting of the cardboard, some printing When the cardboard is longer, the accumulated paper scraps cause local depressions. Therefore, when printing paper and paperboard that are prone to lint, powder and chipping, special care should be taken to clean the blanket in time. Dried ink adheres to the impression cylinder, and local uneven pressure will cause partial depression of the blanket during printing. Therefore, the blanket should be cleaned in time after the printing of each size product and after the daily printing. And impression cylinder.

6. Maintenance of the blanket

Because blankets have hot spots of photoaging and thermal aging, blankets to be used after purchase should be wrapped with black paper and stored in a cool place.

When cleaning the blanket, use organic solvents with faster volatility as the detergent, and wash the deep rubber with slow volatile kerosene or its solvent easily to swell the blanket, wash and dry the blanket when washing, leaving no trace On the one hand, the residue is easy to oxidize and dry, causing the blanket to age in advance. On the other hand, when printing other products at the residue, the ink color unevenness is easy to appear at the beginning.

After a certain product is printed, if the downtime is longer, the tensioning device of the blanket can be loosened to relax the blanket and get the opportunity to recover the internal stress, which has the effect of actively preventing stress relaxation.

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