(This article is the exclusive contract draft of Bisheng.com. It is strictly prohibited to reprint. Relying on the blanket to transfer the ink of the printing plate to the surface of the substrate is an important feature that distinguishes offset printing from other printing methods. Offset printing is also named after that. Types and choices of blankets The commonly used offset blankets are divided into air cushion blankets and ordinary blankets. The ordinary rubber cloth is composed of a base cloth, an elastic rubber adhesive, and a 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. Because 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 rubber cylinder imprinting area. The convex hull makes the maximum radius (convex hull) on the arc of the front and back arcs of the contact area The minimum radius (the midpoint of the arc) is greater. As a result, the amount of slippage during imprinting between the rubber cylinder and the plate cylinder and between the rubber cylinder and the impression cylinder increases, resulting in the expansion of the dots and the accuracy of the image and text transfer. . In addition to the base fabric, elastic rubber adhesive, and surface adhesive layer, the air cushion blanket also has a micro-foam sponge layer 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. The properties of blankets in offset printing and the corresponding operations. Let's analyze some of the properties that offset blankets must have (change the direction to consider, only when the characteristics of the blanket are not destroyed, the operation may be reasonable, in addition, in operation and maintenance As long as the blanket is used for these properties, it can effectively extend the life and quality of the blanket.) 1. The blanket used in elastic printing must have good elasticity (the property that the deformation can be restored in time after the pressure is removed), so that it can make good contact with the substrate whose surface flatness is not very good during the imprinting process, thus It can transfer the ink of the printing plate to the surface of the substrate relatively completely. Second, lipophilicity Because the blanket is mainly used as an intermediate carrier of ink in offset printing, the blanket must always maintain a certain wettability to the ink (ie, lipophilicity) during the printing process, and an excellent blanket should also have Certain water repellency. In order to better absorb ink, the blanket must also have a certain capillary structure, which can ensure good reproduction of graphic details in printing. 3. Surface smoothness and flatness In order to ensure good contact between the blanket and the substrate or the surface of the printing plate, no matter in the process of manufacturing or use, the thickness of the blanket must be uniform and the surface flatness must be good. Fourth, oil resistance, acid and alkali resistance and solvent resistance During printing, the blanket must be continuously exposed to some organic solvents (gasoline, ink, kerosene, alcohol, etc.). In order to ensure the stable use of the blanket, the blanket must have good oil resistance, Acid and alkali resistance and solvent resistance. Fifth, impact resistance The blanket must withstand the mechanical impact and rolling between the rollers in use. In order to prevent the serious plastic deformation of the blanket, the blanket must have a certain impact resistance. Sixth, light resistance The rubber cloth needs to have strong light resistance, which requires slow aging under the action of light. This is because the main component of the rubber, the rubber itself, is not very good in light resistance, and it is easy to age the conjunctiva under the action of light. This conjunctiva is generally dense and smooth, and the rubber on the surface of the blanket will cause the capillary pores to disappear and the surface adsorption to decrease. . It is for the above considerations that we require blankets to have good light resistance. But even so, blankets that have not been used for a long time should be kept away from light to avoid aging the conjunctiva under long-term light. 7. Heat aging The blanket itself is sensitive to heat. After the blanket is heated, it causes severe aging, which makes the surface adhesive layer hard, tacky, and even cracked, and cannot be used. Some precautions that should be paid attention to when using the blanket: 1. The padding size of the blanket The padding of the blanket (the soft pad has woolen cloth and pad paper; the pad paper in the hard pad) must be slightly narrower than the size of the blanket. The purpose of this is to avoid Oils, impurities, powders, cleaning agents, water, etc. infiltrate inside and cause corrosion of the rollers, delamination of the blanket and bubbling, and the liner becomes hard and loses its elasticity. 2. Cleaning the rubber cloth Try to use cotton fabric as the cleaning cloth. There should be no hard objects on the cleaning cloth (especially pay attention to metal objects) to avoid unnecessary damage to the surface of the rubber cloth. 3. Repairing the blanket When the blanket is accidentally crushed during the printing process, it can be treated with a rubber reducing agent. When the pressure injury is serious, it can be repaired with rubber repair agent. In addition, when the area is large and the effect of reducing agent is not obvious, it can be remedyed by local cushioning. The specific method is as follows: 4. Accurate measurement of blanket thickness In offset printing production, the size of the printing pressure is usually indirectly expressed by the maximum compression of the liner at the closing pressure of the cylinder. To accurately measure the thickness of the liner is to calculate the printing pressure The key to size. 5. The tension of the rubber cloth The rubber cloth fastened on the rubber roller will generate internal stress and its tensile force will be counterbalanced, but the internal stress will gradually attenuate with the extension of time, so that the degree of tension will be reduced. Blanket stress relaxation. 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. Use air-cushion blanket to make it: after the new blanket is tightened, it must be pressed and idle for about 10 minutes and then tightened again, so that the blanket is in the required degree of tension. Some other solutions to extend the service life of the blanket In the original and auxiliary materials, the consumption of the blanket is large and the replacement cycle is short, so increasing the service life of the blanket means a reduction in production costs.
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The blanket is the medium that transfers the pictures on the printing plate to the substrate. Due to the use of blankets in offset printing, the pressure required for imprinting is reduced. At the same time, due to the elastic deformation of the blankets, the ink on the surface of the substrate and the graphics and text parts that need to be transferred can fully contact, making up the surface of the substrate The unevenness also improves the ink transfer rate.
Because the blanket has such an important role in offset printing, the correct use of the blanket has become very important in printing, which is of great significance for improving the quality of printing and reducing production costs.
For printing high-screen color products, 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 and color reproduction. In addition, due to the existence of the micro-foam sponge layer, the air cushion blanket has better sensitivity, resilience and impact resistance than ordinary blankets. For high-speed printing machines, the frequency of alternating response Under very high conditions, the use of air-cushion blankets keeps the internal friction of the blankets not too great. While 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 the production cost. Air cushion blankets are only used on high-speed machines and when printing high-end products with hard or neutral hard liners. 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.
In addition to ensuring that the blanket has good elasticity during production, care should be taken to protect the elasticity of the blanket during use. Here we briefly introduce the deformation of the blanket in printing to discuss the protection of the elasticity of the blanket.
There are two main types of deformation of the blanket, namely elastic deformation and plastic deformation (which cannot be recovered after deformation). Among them, elastic deformation includes sensitive elastic deformation (when the external force is removed, the deformation caused by the external force can be restored immediately) and hysteresis deformation ( When the external force is withdrawn, the deformation caused by the external force can be restored within a period of time). Among these types of deformation, the most favorable deformation for printing is sensitive elastic deformation, and try to avoid the abnormal plastic deformation of the blanket, and then maintain the good sensitive elasticity of the blanket, which needs special attention in the use of the blanket.
Plastic deformation is related to the following four factors:
1. The greater the printing pressure, the greater the pressure on the blanket at the paper, and the greater the plastic deformation, the more likely this uneven plastic deformation will occur. Therefore, when printing, the ink can be transferred well. The printing pressure should not be too high, and the best pressure should be used for printing.
2. The tightness of the rubber band The tighter the band, the thinner the rubber layer and the worse the sensitive elasticity, the more likely to be uneven plastic deformation and the elasticity will decrease. Therefore, the tightness of the blanket in use should be moderate (knock the blanket near the blanket at the bite or a little bit, and make a crisp knocking sound, it should not be too tight).
(In addition, if the blanket is stretched too loose, it will be displaced by the squeezing force and cannot be reset in time when the blanket is imprinted, which will cause ghosting failure. If the blanket is stretched too tight, it will bring many Disadvantages: thinner adhesive layer; uneven printing pressure; affect the synchronous rolling state; accelerate the creep and aging of the blanket).
3. Performance of the rubber blanket Because of the good sensitivity and recovery ability of the micro-foam sponge layer, the air-cushion blanket is not prone to uneven plastic deformation. For high-end printed products should consider using air cushion blanket.
4. Printing quantity Long-term large-volume printing of a certain size of paper will cause local plastic deformation.
5. The accumulation of paper wool and ink skin After a period of printing, the surface of the blanket is physically adsorbed and adheres to many paper wool and paper powder. If it is not cleaned in time, the thick paper wool and paper powder increase the local printing pressure. , So that more plastic deformation occurs locally. When printing cardboard, the paper scraps generated by the cutting edge of the trailing edge are easily accumulated on the blanket relatively close to the edge of the cardboard trailing edge under the action of oil and water. Due to the uneven cutting of the cardboard, the printing is slightly longer. When the cardboard is a little bit, the accumulated paper scraps cause the local depression, so when printing paper and cardboard that are prone to lint, powder, and scraps, special attention should be paid to cleaning the rubber cloth 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.
During the printing process, due to the continuous contact of the blanket with the printing material (mainly paper), ink, and the dust carried on the surface of the printing material (including dust, anti-rubbing powder, etc.), the pressure and the viscosity of the ink are common Under the action, fine fibers, fillers, rubber compounds, pigments and dust on the surface of the paper will penetrate into the capillary holes on the surface of the blanket. Because these substances contain a large number of polar molecules and are very hydrophilic, they will absorb moisture when they come into contact with water (foaming solution) and begin to repel the ink, so the blanket must be thoroughly cleaned during the printing process. The cleaning will remove the polar substances adsorbed on the surface of the blanket, otherwise the dots transferred from the blanket to the substrate will have quality defects such as emptiness and false edges.
The blanket participates in printing under the pressure of the cylinder, and is constantly affected by some mechanical forces such as the rolling pressure (positive pressure applied to the blanket between the cylinders), the tension applied when installing the blanket, and so on to maintain the pressure during the printing process In addition to choosing an offset blanket with a certain degree of form stability, the following points should also be noted in the printing operation:
1. Under the effect of tension, the blanket is not uniformly pulled at different positions in the circumferential direction, resulting in uneven thickness in the circumferential direction (near the mouth and the position of dragging) The thickness of the blanket is severely reduced, and the thickness in the middle of the blanket is relatively large), 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 background color of the solid or flat screen that should be uniform becomes uneven, and the color image has a hue shift.
2. When installing and replacing the blanket, first pay attention to the size of the blanket and cut it while ensuring a reasonable size of the blanket (although the current high-end blanket is already a cut size, and even has its own card Clips, but when using domestically produced blankets on many domestic printing presses, you still need to cut them by the printing house). Pay attention to the following two points:
(1) Ensure that the blanket is parallel to both sides, and that the two adjacent sides of the blanket are perpendicular, otherwise it will cause uneven tension on both ends of the blanket, which will result in uneven stretch of the blanket and uneven thickness. .
(2) When clamping the clips at both ends of the rubber cloth, make sure that the amount of rubber cloth extending at both ends of the clip is the same. In the case of inconsistency, the situation mentioned above also occurs.
3. To prevent foreign objects from entering the embossing area, rolling injuries or rolling concave blankets. And it is absolutely forbidden to press, smash, or scratch the blanket with hard objects.
When cleaning the blanket, the organic solvent with faster volatility should be used as the detergent, and the volatile kerosene or other solvents should be used to wash the blanket, which is easy to make the blanket swell. When washing, the blanket should be washed and dried to leave no traces. On the one hand, the traces are easily oxidized and dried, which causes the blanket to age in advance. On the other hand, when printing other products at the traces, unevenness in ink color is likely to occur at the beginning. From the cost point of view, it is generally possible to use highly volatile gasoline on a daily basis, and then regularly use blanket cleaning fluid (currently domestically produced can fully meet the requirements) to thoroughly clean the blanket.
Although the blanket itself has a certain impact resistance, it is only within a certain range. In the adjustment of printing pressure, the ideal pressure between the blanket and the adjacent cylinder should be ensured as much as possible. From the perspective of using the blanket itself, the impact and rolling strength of the blanket can be reduced to a minimum.
For blankets that have been aged under light, the appropriate compensation can be made in the following way: use a cotton cloth dipped in a mixture of high-boiling kerosene and pumice powder to repeatedly scrub the surface of the blanket until the aging film on the surface of the blanket is removed, and then Clean the surface of the blanket with gasoline and water, and finally wipe it again with a dry cloth. Although the blankets treated in this way can continue to be used, they can only be used for printing of prints with low quality requirements, so it is still very important to avoid light during the preservation of the blankets.
In our work, we should try to make the blanket work under a relatively low temperature condition. First of all, we must ensure that the temperature of the environment is stable. It is best to install air conditioners in the workshop if possible.
In addition, because the blanket is subjected to the periodic stress of the plate cylinder and the impression cylinder during the printing process, the frequency of this alternating stress is very high, so the internal friction of the rubber will be very large, and the heat released will also make the blanket The increase in the temperature of the adhesive layer further exacerbates thermal aging. From this perspective, we should use an ideal printing pressure in the printing process.
Choose a solvent with good volatility for cleaning. It is best to use a special blanket cleaning agent if possible. Do not use solvents with low volatility (such as kerosene). Wash and dry the blanket when washing, leaving no trace. On the one hand, the residue is easy to oxidize and dry, which causes 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.
When there are many paper wool, fibers, glue on paper and other hydrophilic substances attached to the surface, appropriate mixing of water in the cleaning solution can improve the cleaning efficiency.
When cleaning the blanket, try not to touch the cleaning liquid to the side of the blanket, so as to avoid the cleaning liquid from swelling the blanket from the side, so that the edge of the blanket is layered.
Remove the pad of the blanket by 0.1mm; hit the full version (do not put the water roller and only the ink roller to smooth the ink on the surface of the plate); try to press (print the blanket surface with even full ink); Draw a map (accurately draw the area to be padded on the back of the blanket); pad pad (preferably use paper under 40g for padding, if the place where you need to pad is only slightly more virtual than other places, sometimes only use After a small amount of peach gum is applied, lightly apply some powder to it); re-apply the full version of the test and press (if the pad area and other parts of the ink color are the same, if it does not work, repeat the above process until the ink color is consistent); When the padding is completed, the removed padding paper can be printed and used again.
Although the above method is cumbersome, it is worthwhile to change a blanket.
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, in the free state between the blanket and the rubber roller, the measured thickness is not equal to the thickness after tension. Usually use a micrometer to measure the thickness of the blanket and liner. Because the micrometer can only be measured in the free state before the liner is not installed, the operator should correctly estimate the difference between the thickness of the blanket after the tension and the thickness in the free state, 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 change in brush pressure , Improper printing pressure will seriously affect the correct transfer of ink.
1. When the blanket is not used for a long period of time, the original tight blanket should be loosened. Even when in use, the taut blanket should be moderate.
2. Ensure that the plate cylinder, blanket cylinder and impression cylinder are parallel to avoid distortion of the blanket.
3. The use of the wheel rest method is an effective method to extend the service life of the blanket. Because, after using the blanket for a period of time, there will always be excessive local compression. In order to recover its elastic fatigue, a certain recovery time must be given. Practice has proved that for users of the two-shift system, it is replaced once every ten days. The high-speed printing machine is replaced about 1 million times. The specific method is: use two sets of blankets according to the sequence number of the unit, and exchange the counterparts. Remove the rubber blanket, wipe off the ink with a blanket cleaning agent, block the light, avoid harmful chemicals, and store it flat in a ventilated place. In this way, in addition to being scratched or scratched by a hard object, the blanket can actually be used for 6 to 9 months, which is about 3 to 4 times longer than when it is not used. Moreover, there is no need to change the pad data. It only needs to be recorded when it is removed and replaced, and the tightness meets the original requirements.
If you are using a printing machine that only prints several types of prints all year round (especially book and printing plants), it is best to use several special blankets in turn.
The shift work method is not only applicable to air-cushion blankets, but also effective for solid blankets. After adopting the wheel rest method, the elastic recovery of the solid blanket is about 70%, while the elastic recovery of the air cushion blanket can reach about 90%.
Correct use and maintenance of blanket