Correct use of offset blankets
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Printing pressure process technology is one of the three major processes of lithography, which is a relatively complicated process technology in offset printing. The pressure of the offset printing comes from the elastic deformation caused by the compression deformation of the lining (including the blanket and the liner) on the blanket cylinder. The correct use of the blanket is an important part of the printing pressure process and is important for ensuring the printing quality of the product. influences. Since the blanket is also an important consumable material in offset printing, proper use of the blanket can extend its service life and reduce production costs. This article discusses several issues that should be noted about the proper use of blankets.
First, choose the right blanket
Currently commonly used offset blankets are divided into air cushion blankets and ordinary blankets. The Zengtong blanket consists of a base fabric, an elastic rubber adhesive, and a surface adhesive layer. Since the rubber itself has the characteristics of being incompressible in volume, when the surface rubber layer is pressed, a convex hull is formed around the pressure receiving area. Since the rubber cylinder is pressed at the same time, the axial direction is simultaneously pressed, so that the convex and convex portions are formed on the front and rear sides of the embossed area of the blanket cylinder, and the convex hull makes the maximum radius of the circumference of the embossed contact area on the front and back arcs (convex hull The difference between the minimum radius and 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 enlargement of the dot, the transfer of the image The correctness is declining. In addition to the base fabric, the elastic rubber adhesive, and the surface adhesive layer, the air cushion blanket also adds microbubbles per layer under the surface adhesive layer. The microbubble sponge layer is formed by a myriad of tiny, closed air spheres dispersed in a subbing layer. After the rubber layer on the surface of the blanket is pressed, the air sphere acts under pressure to reduce its volume, so the blanket is vertically compressed, and the deformation no longer develops to the front and rear sides of the embossed area, and is not embossed during the embossing process. The front and rear sides of the area are convexly convex, which reduces the slippage of the embossing gap, and the correctness of the graphic transfer is improved. For color products with high number of screen lines, the use of air-cushion blankets reduces the enlargement rate of the dots, reduces the loss of layers, and improves the clarity of the products, and improves the correctness of the tone and color reproduction. In addition, due to the existence of the microbubble sponge layer, the air cushion blanket has better sensitivity, recovery and impact resistance than the sound-passing blanket. For high-speed printing presses, the alternating response frequency In high cases, the use of air-cushion blankets can not make the internal consumption of the blanket 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 cost of the air-cushion blanket is much higher than that of the ordinary blanket, which increases the production cost. The air-cushion blanket can only be used on high-speed machines and high-grade products with hard lining or neutral hard-packing villages. The advantages of high accuracy and flexibility of graphic and text transfer are well developed. If the soft lining is used to print common products, the soft lining has a large compression capacity, the embossing surface is wide, and the dot spreads easily, which offsets the advantage of the high precision of the airbag blanket image transfer, and it is not worth using the air cushion rubber. Cloth.
Second, prevent the use of the blanket during torsional deformation
During the embossing process, if the surface axially different positions are subjected to the force of different directions or different sizes, the shape of the blanket will be twisted and deformed, resulting in the shape of the graphic, the transfer of ink, the overprinting and the blanket. The adverse effects of the service life, etc. There are two main reasons for the torsion deformation of the blanket:
1. The drum axes are not parallel. Because the centering of the rollers is careless or because the presser is self-propelled during the long-term use of the press, so that the center distance between the two ends of the drum is not equal, it is possible to make the blanket cylinder relative to the impression cylinder or the plate cylinder. The axis is not parallel, and one end of the blanket is subjected to a large pressing force, and the other end is subjected to a small pressing force, and the forward speed is slower, so that the blanket is twisted.
2. The method of operation is not standardized when the blanket is cut or mounted on the blanket holder. When the blanket is cut, if the two sides of the clip are not parallel, the tension in the axial position will be uneven when tightening, and the blanket will be tight on one side and loose on one side. The upper tensile stress is unevenly reported to cause torsional deformation. When the blanket is loaded onto the clip, if the depth of the blanket is inserted into the clip or the individual screws are not tightened when the clip is clamped, when the blanket is tightened, a small amount of the blanket is not pulled out, which will cause the eraser. 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 latitude of the blanket is parallel to the binder. The woven fabric of the blanket is interwoven by the warp and the latitudinal lines. When subjected to the same tensile force, the warp elongation is much smaller than the latitudinal elongation. When the blanket is stretched, the more the elongation, the more serious the thinning of the rubber layer, resulting in a decrease in the sensitivity of the blanket and a more serious pressure unevenness. Generally, the base fabric of the blanket has a mark indicating the direction of the tightening, and it can also be tried by hand. The one that is not easy to stretch is the warp direction, and should be used as the tightening direction, especially in the case of scrapping due to hard damage. Or when the full sheet of blanket is cut for use as a 4 open blanket, attention should be paid to the directionality of the warp and weft.
Third, the correct measurement of the thickness of the blanket
In offset printing, the pressure of the printing press is usually indirectly expressed by the maximum amount of compression when the drum is pressed. The correct measurement of the thickness of the liner is the key to calculating the printing pressure.
In offset printing, the blanket is stretched on the blanket cylinder. The blanket is stretched by the warp tension, and the amount of elongation is proportional to the amount of tension. However, in the circumferential direction of the drum, the tensile force of the different points is different due to the frictional force. The tension between the two ends of the clamp is the largest, and the tension in the middle position is the smallest. The pulling force in the direction of the clip ends increases in sequence. Since the blanket is stretched and stretched, the thickness of the rubber layer is reduced and the thickness is lowered. Therefore, the thickness of the blanket measured in the free state between the two without the blanket cylinder is not equal to the thickness after the tension. To correctly measure the thickness of the lining, the thickness of the lining after the tension is measured at different circumferential positions of the blanket cylinder, and the printing pressure is calculated by the thickness. However, many small printing factories do not currently have a diameter gauge. Usually, the thickness of the blanket and the liner is measured with a micrometer. Since the micrometer can only be measured in a free state before the liner is not installed, the operator should tighten the blanket. There is a correct estimate of the thickness difference between the thickness and the free state, so that there is no large deviation in calculation. For the soft lining, the difference may not be significant, and for the hard lining, due to the large elastic modulus of the rigid lining, a small amount of error in the calculation of the compression amount may result in a larger brush pressure. Changes, improper printing pressure can seriously affect the correct transfer of ink.
Fourth, properly tighten the blanket
The blanket must be taut on the blanket cylinder under the appropriate tension. If the blanket is stretched too loose, the blanket is displaced under the action of the squeezing force and cannot be reset in time, which may cause a ghosting failure. If the blanket is stretched too tight, there are many drawbacks:
1. The rubber layer is thinned and the elasticity is lowered. If the printing pressure is calculated by the thickness measured by the micrometer in the field of the blanket, the printing pressure may be insufficient due to excessive thinning of the rubber layer. The thinning of the strand causes the elasticity of the blanket to be lowered, and even if the amount of compression is increased by reducing the center distance, the phenomenon of poor ink transfer may occur. If it is an air cushion blanket, too tightly stretched may also invalidate the microbubble sponge layer and lose the good performance of the air cushion blanket.
2. Lead to uneven printing pressure. Since the tensile force of the blanket at different positions in the circumferential direction is uneven under the action of the tension, the thickness reduction is uneven in the circumferential direction, resulting in uneven printing pressure. The tighter the tension, the more serious this unevenness. The unevenness of the pressure causes the unevenness of the ink transfer, so that the ground or the flat background color which should be uniform is made uneven, and the hue of the color image is shifted.
3. Affects the synchronous rolling state. The fine fruit has a good synchronous rolling state between the three rollers under the normal blanket tension and the thickness of the blanket cylinder. Then, in the case of 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, and sliding between the three rollers affects the correct transfer of the dots, destroying the restoration of the level and color.
4. Accelerated creep and aging of the blanket. The tighter the tension, the more likely the rubber molecules will undergo a conformational change, the molecular chain will be stiff, and the elasticity of the rubber will be deteriorated, which will affect the printing quality of the product and shorten the service life of the blanket.
Fifth, pay attention to the deformation of the use of the blanket
The blanket fastened to the blanket cylinder itself will produce internal stress and the tension of the tension will be counterbalanced, but the internal stress will gradually decay with time, which will reduce the tension, which is called the stress relaxation of the blanket. The stress relaxation phenomenon is most likely to occur on the newly replaced blanket. Therefore, the newly replaced blanket cannot be stretched too tightly on the one hand. On the other hand, after pressing for a period of time, it must be tightened again, so that it is repeated three or four times. After that, the stress relaxation phenomenon is weakened, and the tightness of the blanket is kept stable for a long period of time.
Under the pressure of the pressure-sensitive printing, the blanket is plastically deformed in the thickness direction, resulting in a decrease in the thickness of the blanket. The law of plastic deformation is that the plastic deformation of the newly changed blanket is large. In each subsequent printing cycle, the amount of plastic deformation gradually becomes smaller until it can be neglected, so that the thickness of the blanket remains stable for a certain printing time. . Since the newly-changed blanket is reduced in thickness due to plastic deformation, and the blanket is tightened in order to correct the decrease in the tension due to the stress relaxation phenomenon, the thickness of the blanket is gradually decreased, which may result in Insufficient pressure. For the package lining, the pressure can be reduced by 20%~30%, and the pressure of the rigid lining can be reduced by 10%~15%. Therefore, it is necessary to increase the thickness of the liner or reduce the center after printing for a period of time. To make up for it. The larger 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 After that, the extremely small but always existing minor deformations combined to change the conformation of the blanket, resulting in the disappearance of elasticity, and finally the blanket was scrapped.
The pressure exerted by the blanket in use is different, and the plastic deformation of the thickness of the blanket is different, which may cause the blanket to be uneven. For example, after printing 4 products for a long time on the boot, then change the products. When the product is printed 4, the pressure between the blanket cylinder and the plate cylinder is evenly distributed over the entire blanket. However, the pressure between the blanket cylinder and the impression cylinder is uneven due to the presence of the printed paper. The corresponding position of the 4 sheets of paper, the pressure on the blanket is large, and the pressure at the position other than the 4 sheets is small or There is no pressure, so the plastic deformation of the blanket at the position corresponding to the 4th paper is large, and the thickness is reduced. When the paper is changed, the position is prone to insufficient pressure. Because this lack of pressure is caused by the partial depression of the blanket, even if the overall pressure between the rollers is increased, there is no obvious improvement, and only the method of replenishing the rubber can be used to remedy the rubber. Products with high printing quality requirements. This uneven plastic deformation occurs in relation to the following four factors:
1. Printing pressure. The greater the pressure, the greater the pressure on the paper blanket, the greater the plastic deformation that occurs, and the more likely this uneven plastic deformation occurs, so when printing, in the case of good ink transfer, The brush pressure should not be too large and should be printed at the optimum pressure.
2. The tightness of the blanket. The tighter the stretch, the thinner the rubber layer of the blanket, the worse the sensitivity and the more likely the uneven plastic deformation.
3. The performance of the blanket. The air cushion blanket is less prone to uneven plastic deformation because of the good sensitivity and recovery ability of the microbubble sponge layer.
4. The number of prints. Long-term printing of a certain amount of paper in a large number of prints will result in a large local plastic deformation.
5. The accumulation of paper and ink. After printing for a period of time, the surface of the blanket adheres to a lot of paper and paper 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 paperboard, the paper scraps generated by the cutting edge are easily piled on the blanket with the oil and water at a position other than the edge near the tip end of the cardboard. Since the cardboard is not cut, some printing is slightly When a piece of cardboard is a little longer, the accumulated paper dust causes a partial depression at that place. Therefore, when printing paper or cardboard that is easy to lose hair, powder, or chip, special care should be taken to clean the blanket in time. The ink is adhered to the impression cylinder, and the partial pressure of the blanket is caused by uneven local pressure during printing. Therefore, the blanket should be cleaned in time after the printing of each size of the product and after the printing is finished every day. And impression cylinder.
Six, blanket maintenance
Because the blanket has a hot spot of photoaging and heat aging, the blanket to be used after purchase is wrapped in black paper and stored in a cool place.
When cleaning the blanket, the organic solvent with faster volatility should be used as the detergent, while the slow volcanic kerosene or its solvent should be used to wash the rubber, so that the blanket can be swollen, and the blanket should be washed and dried without leaving any residue. On the one hand, the residue is easy to oxidize and dry, so that the blanket is aging in advance, and on the other hand, when the residue is printed on other products, the unevenness of the ink color is likely to occur at the beginning.
After a certain product is printed, if the downtime is long, the tensioning device of the blanket can be loosened to relax the blanket and obtain the opportunity of internal stress recovery, thereby actively preventing stress relaxation.