Correct use and maintenance of the blanket
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The use of a blanket to transfer the ink from the graphic portion 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 it.
A blanket is a medium that transfers graphics on a plate to a substrate. Due to the use of the blanket in offset printing, the pressure required for the imprinting is reduced, and also due to the elastic deformation of the blanket, the surface of the substrate and the ink of the image portion to be transferred can be sufficiently contacted to make up for the surface of the substrate. The unevenness also increases the transfer rate of the ink.
Because the blanket has such an important role in offset printing, the correct use of the blanket becomes very important in printing, which is of great significance for improving the quality of the printed product and reducing the production cost.
Types and selection of blankets
Currently commonly used offset blankets are divided into air cushion blankets and ordinary blankets. The ordinary blanket is composed of a base fabric, an elastic rubber adhesive, and a surface rubber 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 in the axial direction, the convex hull is formed on the front and rear sides of the embossing area of the blanket cylinder, and the convex hull makes the maximum radius (convex bulge) of the embossing contact area on the front and back arcs and The minimum radius (the midpoint of the arc) is larger, which results 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 and a decrease in the correctness of the image transfer. . In addition to the base fabric, the elastic rubber adhesive, and the surface adhesive layer, the air cushion blanket also adds a microbubble sponge 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 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 ordinary blanket. For high speed printing presses, the frequency of alternating response In high cases, the use of air-cushion blankets can not make the internal consumption of the blanket too large, while maintaining the stability of printing pressure, it can also reduce the speed of aging and extend the service life of the blanket. However, the cost 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 they are printed with high-grade products with rigid or neutral hard-lining. 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.
The nature of the blanket in offset printing and the corresponding operation
Let's analyze some of the necessary properties of offset blankets. (Considering the direction, it is only reasonable to operate without destroying the characteristics of the blanket. In addition, in the operation and maintenance, as long as the blanket is applied to these properties. Use in order to effectively extend the life of the blanket and the quality of use).
First, flexibility
The blanket used in printing must have good elasticity (the property that the deformation can be recovered in time after the pressure is removed), so that it can be in good contact with the substrate whose surface flatness is not good during the imprinting process, so that it can be printed. The ink in the layout portion is relatively completely transferred to the surface of the substrate.
In addition to ensuring that the blanket has good elasticity in 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), wherein the elastic deformation includes sensitive elastic deformation (when the external force is removed, the deformation caused by the external force can be recovered immediately) and the elastic deformation ( When the external force is removed, the deformation caused by the external force can be recovered within a certain period of time). Among these kinds of deformations, the deformation which is most favorable for printing is sensitive to elastic deformation, and the abnormal plastic deformation of the blanket is avoided as much as possible, thereby maintaining the good elasticity of the blanket, which is a special concern in the use of the blanket.
Plastic deformation is related 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, the more likely this uneven plastic deformation occurs, so when printing, in the case of good ink transfer, printing pressure Should not be too large, should use the best pressure printing.
2. The tightness of the blanket
The tighter the stretch, the thinner the rubber layer of the blanket, the worse the sensitivity and elasticity, the more prone to uneven plastic deformation, and the lower the elasticity. Therefore, the tightness of the blanket should be moderate in use (hitting the blanket at the bite or dragging a little near the blanket, giving a crisp knocking sound, not too tight).
(In addition, if the blanket is too loose, when the blanket is imprinted, it will be displaced under the action of the pressing force and cannot be reset in time, which will lead to the occurrence of ghosting failure. If the blanket is stretched too tight, it will bring many Disadvantages: thinning of the rubber layer; resulting in uneven printing pressure; affecting the synchronous rolling state; accelerating the creep and aging of the blanket).
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. Air cushion blankets should be considered for high-end prints.
4. Number of prints
Long-term large-volume printing of a certain sheet of paper will result in a large local plastic deformation.
5. Stacking of paper and ink
After printing for a period of time, the surface of the blanket adheres to a lot of paper wool and paper powder due to physical adsorption. If it is not cleaned in time, thick paper wool 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 near the edge of the cardboard tip under the action of oil and water. Because the cardboard is not cut, the printing is slightly longer. When a piece of cardboard is used, the accumulated paper scraps cause partial depression at that place. Therefore, when printing paper or cardboard that is easy to lose hair, powder, or scrap, 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.
Second, lipophilic
Since the blanket is mainly used as an intermediate carrier for the ink in the offset printing, the blanket must always maintain a certain wettability to the ink during the printing process (ie, lipophilicity), and the excellent blanket should also have a certain water repellency. . In order to better absorb the ink, the blanket must also have a certain capillary structure, which can ensure the good reproduction of the details in the printing.
During the printing process, the blanket is in constant contact with the substrate (mainly paper), the ink, and the dust carried on the surface of the substrate (including dust, anti-smudge dusting, etc.), and the pressure and the viscosity of the ink are common. Under the action, fine fibers, fillers, rubber compounds, pigments and dust in the ink surface will penetrate into the capillary pores on the surface of the blanket. Since these substances contain a large amount of polar molecules, they are highly hydrophilic. When they come into contact with water (dampening solution), they absorb moisture and start to repel the ink. Therefore, the blanket should be thoroughly used during the printing process. The cleaning removes the polar substances adsorbed on the surface of the blanket, otherwise the dots transferred from the blanket to the substrate may have quality defects such as emptiness and edge failure.
Third, the surface is smooth and flat
In order to ensure good contact between the blanket and the substrate or the surface of the printing plate, it is necessary to ensure that the thickness of the blanket is uniform and the surface is flat during the manufacturing or use process.
The blanket participates in the printing under the pressure of the drum, and is constantly subjected to the rolling pressure (positive pressure applied to the blanket between the rollers), the tension applied by the rubber when the blanket is applied, and the like, in order to maintain the pressure during the printing process. Stabilization, in addition to the choice of offset printing blankets with a certain degree of stability, the following points should be noted in the printing operation:
1. Under the action of the tension, the tension of the blanket in the circumferential direction and the different positions is uneven, resulting in the thickness reduction being uneven in the circumferential direction (close to the mouth and the position of the drag) The blanket is severely thinned, and the thickness of the intermediate position of the blanket is relatively large), 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 solid or flat background color which should be uniform is made uneven, and the hue of the color image is shifted.
2. When installing and replacing the blanket, first pay attention to the size of the blanket and cut it while ensuring the reasonable size of the blanket (although the current high-grade blanket is already cut size, even has its own card) Clip, but when using a domestic blanket on many domestic printing presses, you still need to pay attention to the following two points when you need to cut it yourself:
(1) Ensure that the blanket is parallel to the two sides, and ensure that the adjacent sides of the blanket are vertical. Otherwise, the tension at both ends of the blanket will be uneven, which will result in uneven stretch of the blanket and uneven thickness. .
(2) When clamping the clips at both ends of the blanket, make sure that the amount of blanket at both ends of the card is consistent. In the case of inconsistency, the above mentioned situation will also occur.
3. Prevent foreign matter from entering the embossed area, crushing or rolling the blanket. And it is absolutely forbidden to use a hard object to press, smash, or scratch the blanket.
Fourth, oil resistance, acid and alkali resistance and solubility
In the printing, the blanket should be in constant contact with 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 solubility.
When cleaning the blanket, the organic solvent with a relatively high volatility should be used as the detergent, while the kerosene or other solvent with a slow volatility should wash the blanket and easily swell the blanket. Wash and dry the blanket when washing, leaving no residue, the residue is easy to oxidize and dry on the one hand, so that the blanket is aging ahead of time. On the other hand, when the residue is printed on other products, the unevenness of the ink is likely to occur at the beginning. From the cost point of view, it is generally possible to use gasoline with strong volatility on a daily basis, and then use the blanket cleaning liquid regularly (the domestic production can meet the requirements at present) to thoroughly clean the blanket.
Fifth, impact resistance
The blanket should constantly withstand the mechanical impact and rolling between the rollers during use. In order to prevent the plastic deformation of the blanket, the blanket must have a certain impact resistance.
Although the blanket itself has a certain impact resistance, it is only within a certain range. In the adjustment of the printing pressure, the ideal pressure between the blanket and the adjacent roller should be ensured as much as possible. From the point of view of the use of the blanket itself, the impact and rolling strength of the blanket can be minimized.
Sixth, light resistance
The blanket needs to have a strong light resistance, requiring slow aging under the action of light. This is because the rubber of the main component of the blanket 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 property to decrease. . It is because of the above consideration that we require the blanket to have good light resistance. Even so, blankets that are not used for a long time should be stored away from light to avoid aging the conjunctiva under long-term illumination.
For the blanket that has been aged under the light, it can be properly compensated by using a mixture of high-boiling kerosene and pumice powder on a cotton cloth repeatedly scrubbing 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 then wipe it with a dry cloth. Although the blanket thus treated can be used continuously, it can only be used for printing of prints of low quality, so it is still very important to protect the blanket from light during storage.
Seven, heat aging
The blanket itself is sensitive to heat. After the blanket is heated, it causes severe aging, which makes the surface rubber layer hard, sticky, and even cracked and cannot be used.
At work we try to make the blanket work at a relatively low temperature. 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 conditions permit.
In addition, since the blanket is subjected to the periodic stress of the plate cylinder and the impression cylinder during the printing process, the frequency of the alternating stress is high, and the internal friction of the rubber is also large, and the heat released also makes the blanket. The temperature of the glue layer rises, which in turn exacerbates heat aging. From this perspective, we should use an ideal printing pressure during the printing process.
Some precautions that the blanket should pay attention to when using it:
1. Blanket size of the blanket
The blanket pad (soft padded and padded paper; the pad in the hard pad) must be slightly narrower than the size of the blanket. The purpose of this is to avoid oil, impurities, powder, The cleaning agent, water, etc. penetrate into the inside to cause corrosion of the drum, and the blanket is delaminated and foamed, and the gasket becomes hard and loses elasticity.
2. Cleaning the blanket
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 blanket.
Choose a solvent with good volatility. It is best to use a special blanket cleaner if possible. Try not to use solvents with poor volatility (such as kerosene). Wash and dry the blanket when washing, leaving no residue. On the one hand, the residue is easy to oxidize and dry, so that the blanket is aging in advance; on the other hand, when the residue is printed on other products, the ink color unevenness is likely to occur at the beginning.
When a hydrophilic substance such as a paper, a fiber, or a rubber on a paper is adhered to the surface, water is appropriately mixed in the cleaning liquid to improve the cleaning efficiency.
When cleaning the blanket, try not to touch the cleaning liquid to the side of the blanket to prevent the cleaning solution from swelling the blanket from the side and lifting the edge of the blanket.
3. Repair blanket
When the blanket is accidentally crushed during the printing process, the rubber reductant can be used to treat the wound. When the crush is more serious, you can use a rubber repair agent for repair. In addition, when the area is large and the effect of the reducing agent is not obvious, the method of partial padding can be used to remedy. The specific method is as follows:
Remove the liner of the blanket from the amount of 0.1mm; make a full version (do not discharge the ink roller to smooth the surface of the printing plate); try to press the pressure (print the surface of the blanket with a uniform full-page ink); Draw a map (accurately draw the area on the back of the blanket that needs to be padded); padding blanket (preferably use a pad of 40g or less, if the pad is only slightly weaker than other places, sometimes only After a small amount of peach gum is applied, it can be lightly coated with some powder.) Re-print the full version of the test pressure (if the pad area and other parts of the ink color are the same, if not, repeat the above process until the ink color is consistent); When the padding is completed, the removed pad will be re-backed and printed.
Although the above method is troublesome, it is worthwhile to change a piece of blanket.
4. Correct measurement of blanket thickness
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 for a long time, the rubber layer is thinned and the thickness is lowered. Therefore, the thickness measured by the blanket in a free state between the blankets and the blanket cylinder is not equal to the thickness after the tension. The thickness of the blanket and pad is usually measured with a micrometer. Since the micrometer can only be measured in a free state before the liner is not installed, the operator should correctly estimate the difference in thickness between the blanket after the tension and the free state, so that there is no large deviation in calculation. For the use of soft lining, the difference may not be significant, and for rigid lining, due to the large elastic modulus of the rigid lining, a small amount of error in the calculation of the compression amount will lead to a large change in brush pressure. Improper printing pressure can seriously affect the correct transfer of ink.
5. The tightness 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. Use the air cushion blanket to make: After the new rubber blanket is tightened, it must be pressed and idling for about 10 minutes and then tightened once to make the blanket in the tightness required for working.
Some other options to extend the life of the blanket
In the raw and auxiliary materials, the consumption of the blanket is large and the replacement period is short, so increasing the service life of the blanket means lowering the production cost.
1. When the blanket is not used for a long time, the original stretched blanket should be relaxed. Even when in use, tighten the blanket should be moderate.
2. Ensure that the plate cylinder, the blanket cylinder and the three cylinders of the impression cylinder are parallel to avoid distortion of the blanket.
3. The use of the wheel-off method is an effective way to extend the life of the blanket. Because, after using the blanket for a period of time, there is always a partial over-pressure condition, in order to restore its elastic fatigue, it is necessary to give a certain recovery time. Practice has proved that for two-shift users, they will be replaced once in ten days. The high-speed printing press is replaced about 1 million prints. The specific method is: use two sets of blankets to serially number according to the unit, and exchange the counterparts. Remove the blanket, use a blanket cleaner to clean the ink, shield it from light, avoid harmful chemicals, and store it in a ventilated place. In this way, in addition to being scratched or crushed by hard objects, the blanket can actually be used for 6 to 9 months, which is about 3 to 4 times longer than when the wheel is not used. Moreover, the pad data does not have to be changed, it only needs to be removed, and when it is replaced, the recording is made, and the tension is in accordance with the original requirements.
If you use a printing press that only prints several types of printed materials all year round (especially book printing houses), it is best to use several special blankets in turn.
The wheel work method is not only applicable to air cushion blankets, but also to solid blankets. After the wheel-off operation method, the elastic recovery of the solid blanket is about 70%, and the elastic recovery of the air-cushion blanket can reach about 90%.