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Blanket
First, the development of blankets
The first generation of blankets was made from natural rubber, the second generation was synthetic rubber, and the third generation was air-cushioned blankets that are commonly used nowadays. This eraser has been used today and has become the standard material for most of today.
Second, the type and application of blankets
Printing blankets are a kind of polymer rubber chemicals, and they are one of the indispensable materials for printing. They play a role in transferring printing plates onto substrates. The accuracy of the printing network and graphic reproduction can reflect the quality of the blanket to a certain extent.
With the advent of high-speed, high-efficiency and high-quality printing, ordinary blankets have basically been eliminated. In recent years, air-cushion blankets have been widely used.
Compared with conventional blankets, air-cushion blankets have less sliding and convex bulges, better linear velocities for printing plates and blanket cylinders, good compensation for rough substrates, high dot reduction, and reduced distortion of substrates. , Reduce the friction on the layout (off version) and other advantages.
Third, the structure of offset air blanket
Air-cushion blankets are generally divided into four layers, surface layers, fiber layers, air cushions, and cotton fabric layers. At present, about 80% of the rubber is 4-layer structure. The main difference between the air-cushion blanket and the first and second-generation blankets is that the micro-bubble sponge microporous layer is added under the surface adhesive layer, and there is also sandwiched between them. A gas-filled layer forms an elastic layer.
Air cushion rubber surface layer
This layer is the use of a good synthetic rubber and rubber polymer polymer as a surface layer of rubber blanket. The surface layer is mostly embossed, ground or highly ground. Most of the printing quality depends on the surface layer of the blanket. In order to ensure good ink transfer, the surface layer must have the characteristics of good ink transfer performance, non-stacking, stacking, and easy cleaning.
The surface roughness of different air-cushion blankets is different, and also has a great influence on the quality of the printed products. The surface roughness is different, and the dot reproduction accuracy, ink transfer performance, and paper peeling performance are also different. The difference.
1 When printing dot products, use a blanket with low surface roughness to ensure accurate dot reduction.
2 When printing products in the field, blankets with a moderate surface roughness should be used to ensure that they are on the ground.
3 When the surface of the paper is rough, you should use a blanket with a high surface roughness to reduce the stripping rate to avoid some printing failures.
2. Air-cushion blanket fiber layer
This layer is an anti-compression blended cloth base with high stretch resistance, puncture resistance and better stability.
Air cushion of air-cushion blanket
The air cushion layer is in the state of closed particles. It consists of synthetic elastomers and particles. The particles contain air. The compressibility between the layers is determined by the air cushion bubbles and rubber compounds. As shown on the left, the types of cushions can be divided into three categories.
1 Blow-type processing sealed air cushion
The cushion layer of the air-cushion blanket is basically an air-blowing type. Through the vulcanization of the rubber, sponge-like holes appear in the rubber layer, and these holes continuously exhaust and inhale during the process of pressure and pressure separation. However, like the sponge, in the high-speed state, the vent cannot perform the complete exhaust and inhalation cycle, and it encounters the case where the exhaust has been compressed before the end of the intake. The operating speed of the printing press at 6000 sheets/hour or more may cause dot distortion caused by incomplete contacting of the blanket surface on the surface of the blanket, poor ink transfer, and unstable printing pressure due to the above factors. Some imported equipment can already achieve a production speed of 18,000 sheets per hour. If air-cushioned rubber is used for blow-type production, it is not suitable for high-speed production.
2 Salt filter type open air cushion
This rubber blanket is filled with high-resilience solid rubber particles in the rubber. In the rubber vulcanization process, closed-type rubber particles with an internal pressure higher than the external pressure of the printing are uniformly added. In this way, under the pressure of the printing, the inside of the closed rubber is pressed. Just changing the shape and volume, without the process of inhaling and exhausting, because the elasticity of the rubber ball is good, the state of the ball can be restored quickly after being pressurized, and its structure is basically the same as the microsphere structure, and the difference is that The ball type is more capable of providing continuous elastic force, and can keep the pressure of the blanket uniform and the reduction of the dot when the print size changes. From the cost point of view, though the cost of the blanket has been increased, it can be very large as a whole. To save on the degree of production interruption of production of blankets and increase production capacity. The disadvantage is that it is less environmentally friendly in manufacturing.
3 MSI air cushion rubber
Today's printing presses generally use such blankets because a certain amount of glass spheres and plastic spheres are added to the rubber layer so that the air cushion layer can provide sufficient support for the rubber under pressure and pressure during high speed operation. The force to restore the dots and ink transfer, air cushion layer has sufficient resilience during the operation cycle. MSI rubber can maintain its stability in printing pressure, but its use of glass balls and plastic balls will produce insufficient elasticity after long-term use. When the size of the printed product changes, the elastic force of the originally unpressed area on the blanket is larger than the area where the paper has been contacted, which will cause the unevenness of the local dot enlargement and affect the fine reduction of the entire printing web.
4. Cushion rubber cotton fabric layer (cloth layer)
The air-cushioned blanket cloth is composed of multiple layers of fabrics, which are generally divided into three layers and four layers. The fabric layer is composed of synthetic and natural fibers, and its strength and durability are different on each floor. At the time of manufacture, fabrics and rubber are combined with heat and chemical energy in precisely controlled processes. The layers of the fabric are not the same in every layer.
Fourth, other offset printing blanket
In addition to air cushion blankets can be divided into different types of air cushion, salt filter and micro-star type, there are some different types of air-cushion blankets are often used in printing.
1, low viscosity air cushion blanket
The peeling force of the substrate on the surface of the low-viscosity air-cushioned blanket is lower than that of the ordinary air-cushioned blanket. The surface releasability of the ordinary air-cushioned blanket is about 50 mm, and a good low-viscosity air-cushioned blanket can reach 15 mm. The advantage of the low-viscosity blanket is that the paper is not easy to fall when the surface of the substrate is relatively loose, and the paper and the blanket can be rapidly peeled off during printing, and the paper surface is not easily pulled down, and is more suitable for the texture of the paper. The printing of loose and color-colored groups or thin papers solves the problem of dragging and ghosting of printed products, improves the stability of paper and paper stretching deformation, and improves the quality of full-page printing.
2, thick air cushion blanket
The average air-cushioned blanket has an air-cushion thickness of about 0.5mm, but there are blankets with air-cushion thickness of 0.9mm or even more than 1mm in the market. The main reason for the air-cushion thickness is that the equipment is relatively old and is prone to mechanical vibration. When the bar is suitable for air cushion blanket thicker, because the general air cushion blanket compressibility at 1350Kpa deformation in the 0.16mm ± 0.02mm range, and thick air cushion blanket under the same conditions deformation can reach 0.20 The range of mm±0.025mm ensures that the dot change is normal. The thick blanket has a large amount of compression and can absorb a certain amount of machine vibration.
3, UV air cushion blanket
With the constant change and improvement of printing requirements, many new techniques and new technologies have emerged. For example, UV printing is used continuously today and has achieved good results. UV printing must be equipped with a special UV blanket, because non-UV blankets are made of synthetic cyanide rubber. Synthetic rubber particles are fine, and dot-reduction properties are good, but they are not resistant to corrosion. UV-specific blankets not only contain EPDM rubber, but also contain natural rubber and are resistant to corrosion. When UV blankets are used, they should be matched with UV inks and UV car wash water. Otherwise, the printing effect, finished product rate, and production cost will cause problems.
E.g:
1 The portion of the blanket surface in contact with the plate pattern ink is chemically deformed by the ink.
2 New blankets are needed to print UV products for other prints.
3 Because the ink particles are coarse, the gloss is poor, and the printing dot enlargement value is large.
4 The effect is not ideal when printing in full version.
V. Reasonable use and storage of air-cushion blankets in printing production
1, choose the principle of blanket
Choose blankets from the models, product requirements, the speed of the printing press, the size of the printed graphics area, the field, flat net, stickiness and other aspects of comprehensive consideration, but also from the economic factors, such as quality requirements are not very strict or Substrates are rough and expensive rubber can be considered.
2, the new blanket inspection and measurement
1 check the validity period
The blanket is valid for one year to one and a half years and is calculated starting from the finished product date. If the blanket is stored for a long time, the surface rubber naturally ages, and its mechanical properties, chemical properties, and printing performance will decrease. Therefore, the new replacement blanket should pay attention to view the production date, use during its effective period.
2 check the appearance of the blanket
Check the appearance quality of the new blanket before using it on the machine. The surface of the blanket should be flat and free from damage, scratches, bubbles, indentations, and hard debris. The surface of the cotton fabric on the back of the blanket should not have any defects such as wrinkles, tears, hard debris, threads and seams.
Both sides of the rubber are coated with a sealer to prevent swelling on both sides after entering the water.
The surface of the new blanket has a smooth anti-oxidation film. This film must be wiped with clean water before use to ensure that the surface of the blanket has good ink absorption properties.
3 Check blanket pattern
The blanket is textured. Basically, printers generally ignore the texture of the blanket because most blankets have been cut. However, if the large size is cut to a small size, it is necessary to pay extra attention: the line on the back of the blanket is usually used as the printing direction, and the back specification is used as the mouthpiece.
4 Measure the thickness of the blanket correctly.
The actual height of the lining must be carefully measured. In printing, the size of the printing pressure is usually expressed as the amount of compression when the blanket covering the drum is pressed. Therefore, the correct measurement of the thickness of the lining is the key to calculating the printing pressure. As the blanket is pulled on the roller, after stretching, it will cause the adhesive layer to thin and the thickness to decrease. Therefore, when the blanket is not mounted on the blanket cylinder and is mounted on the blanket cylinder, the thickness values ​​measured in these two states are not the same. An appropriate measurement tool should be used to accurately measure the blanket thickness.
5 Correctly select, calculate and measure the thickness and size of blanket lining
The correct calculation of the required blanket lining thickness can improve the dot reducibility, improve the resistance to printing and many other advantages, but also can avoid many defects in the printing process.
According to the blank size of the product cut rubber blanket lining, lining size should be smaller than the white paper size, so as not to affect product quality, printed product specification line is appropriate. Of course, this also requires accurate positioning and placement.
Blanket liners that are smaller than the white paper size can avoid many printing problems:
When printing a small area of ​​graphics and text, it is not easy to pile up powder and reduce ink.
2 When printing coated paper, the stacking of ink is avoided, and the time for wiping the impression cylinder is saved.
3 improves the cleaning efficiency and saves car wash water.
4 When printing on-site products, reduce the amount of ink accumulated on both sides of the plate to reduce the probability of color aberration.
During 5UV printing, avoid the loss of the blanket due to UV radiation.
6 When the cut paper is small and UV printing needs to be reduced, the occurrence of ink skin is avoided.
7 When the paper has burrs, the generation of paper hair is reduced.
8 It can avoid the edge marks produced on the rubber after the blanket and paper are combined to extend the service life of the blanket.
3, the installation and use of blankets
1 install blankets should pay attention to the correct distinction between the latitude and longitude direction of the blanket
The axis of the blanket must be wrapped around the axis of the roller to achieve the printing effect. This is because the blanket is not easily stretchable. When the blanket is cut, it should be noted that it is a rectangular shape with the same length on both sides. Mount both sides of the splint parallel. Then when the blanket is tight, it will not be tight on one side and loose on one side, which will make the printing outlets change shape and affect the quality of printed products.
2 to choose the right tool to tighten the blanket
A torque wrench must be used to lock the blanket so that the blanket is secured to the blanket cylinder with proper tension. If the blanket is stretched too loosely, displacement under compression will occur at the time of imprinting and cannot be promptly reset, resulting in the occurrence of failure of the dot deformation and ghosting. Of course, blankets can not be stretched too tightly. If blankets are stretched too tightly, the intermediate fabrics will be excessively stretched and damaged, the surface layer will be thinned, the elasticity will be reduced, and the pressure will be insufficient. Moreover, accelerating the aging of the blanket causes poor transfer of the ink, affecting the quality of the product and shortening the service life of the blanket. The new blanket could not be stretched too tightly at the time of replacement. It was necessary to print about 10,000 sheets at the time of pressing and tighten again. The degree of tightening is generally reduced by 5 N/m on the basis of the circumferential length of the paper. For example, if the circumferential width of the paper is 52 cm, the tightening degree is 52-5=45 N/m.
3 "ideal pressure"
The “ideal pressure†advocated by offset printing production not only guarantees the product quality, but also has important significance for extending the service life of the blanket and reducing the creep. If the machine stops for a long time, it is necessary to loosen the tightening device of the blanket to relax the blanket and prevent the stress relaxation effect of the blanket. This is also an effective method of prolonging the high elasticity. In the blanket, the internal stress gradually declines with the continuation of the time, and the blanket will appear elastic. Therefore, it is recommended to use two blanket rotation methods.
4 blanket cleaning, maintenance.
After the blanket has been printed for a period of time, the surface of the blanket has been physically absorbed and a lot of paper, paper powder and ink skin have accumulated. If there is no timely cleaning, local printing pressure will increase. Make plastic deformation of the rubber layout. Over time, the accumulation will be partially depressed. Therefore, special attention must be paid to timely cleaning. Cleaning blankets must be serious, manual cleaning with water and cleaning agents carefully scrub, do not let the ink and paper residue remain in the surface of the blanket pores, otherwise, the oxidized conjunctiva, dry, make its surface smooth, reduce suction The ink capability directly affects the improvement of product quality.
5 durability of the blanket
Durability depends on many aspects such as: dents (paper shreds, sticky papers), suitable cleaners (does not use strong acids, strong base cleaners), work environment (temperature and humidity effects, dust effects) without being pressed by external force, Mechanical abrasion (damage caused by the automatic cleaning device), printing quality requirements (higher quality), and general durability of about 1-3 million sheets.
4) Storage of Blanket
During the storage of the blanket, avoid contact with oil, acid and alkali, salt and other items; do not make it subject to squeezing, so as not to break the blanket and destroy its elasticity. The blanket also has the characteristics of being afraid of light and hot and humid air, these media will make the surface of the blanket sticky, crusted, cracked, hardened and make the fabric layer rotten, so the blanket after purchase should be stored in black paper In a cool place. When the blanket removed from the machine is stored, the ink and oil must be wiped clean with detergent and water and wiped dry. Finally, talc is used to prevent surface aging.
Conclusion
From the point of view of the total production process of printing, the quality of products and the selection of blankets are inextricably linked. The choice of blankets is not the best, only the most suitable. Of course, proper use and proper storage also contribute to the stability of printing operations, and can also reduce production costs.
Use and maintenance of sheet-fed offset air blankets
Blanket - the soul of printing. In recent years, air-cushioned blankets have been widely used, and their characteristics are that good resilience can help reduce the adhesion to paper and good dot reduction. However, due to the large number of brands and their different characteristics, it brings users the right choice. A lot of confusion. Some printing companies only know how to purchase high-quality blankets to help improve the quality of the printing, but do not attach importance to the storage and proper use of the blanket after entering the factory. In fact, the use and maintenance of rubber blankets are quite significant for improving product quality and reducing production costs.