The so-called drum lining refers to various coverings wrapped on the surface of the drum outside the drum shell. Both the printing plate cylinder and blanket cylinder of the offset printing machine are covered with a liner. In addition to the printing plate and blanket, commonly used liner materials include insulating paper, kraft paper, rubber pads, felt, and other papers.
The roller lining generally refers to the lining of the rubber cylinder. The size of its lining determines whether the three rollers of the offset printing machine can be rolled correctly. The transfer of the graphics and the realistic reproduction of the color tone of the printing plate and the text are carried out accurately. Multi-color overprinting is extremely important to improve the quality of printed matter.
The purpose of using liners for offset presses: first, the elastic deformation of the liner material is used to generate the required printing pressure; second, it is to compensate for the accuracy errors of the machine manufacturing and the defects between the printing surfaces; third, to reduce impact and load , Avoid the "hard-to-hard" contact of each drum shell, and protect the mechanical equipment.
1. Classification of roller lining
According to the size of the diameter reduction (also called cutting amount) of the blanket cylinder of various types of offset presses, the choice of liner materials, the quality requirements of the products, the performance differences of the offset press, etc., the roll liner can be divided into rigid packages There are three types of lining, neutral lining, and soft lining.
l. Hard lining
The rigid bag liner is composed of a liner paper and a blanket, that is, a few sheets of paper under the blanket form a rigid bag liner. The width of the roller pressure line is 4-6mm, and the diameter reduction λ = 0.04 ～ 0.08mm. This type of liner is relatively hard, the elastic modulus of the liner is large, and a small amount of deformation can produce a large printing pressure, and most offset presses use a contact roller (ie, shoulder iron) to make the cylinder run smoothly.
The amount of shrinkage of the lining of the hard roller lining is small, that is, the deformation of the lining is small, and the expansion and deformation of the dots are small. Under the condition of high precision of the printing machine, the use of hard lining, good transmission of dots, can make small dots reproduce clearly, is conducive to the reproduction of colors and gradations, and can obtain smooth, realistic high-quality prints, suitable for textured prints . When using a hard liner, the friction of the printing plate surface is large, the printing plate wear is more obvious, the printing plate printing rate is lower, the manufacturing accuracy of the machine and the wear resistance of the material are very high, otherwise, it may be due to printing pressure The unevenness appears with ink bars. The domestic JJl02A, JJ204 and JJ201 web offset presses, the imported Miller, Heideberg, and Komori machines in Japan mostly use rigid wrapping.
2. Neutral lining
The neutral pack is made of thick paper, serge and blanket. The width of the roller pressure line is 8-10 mm, and the diameter reduction λ = 0.1 ～ 0.2mm.
The printing performance of the neutral liner is between the rigid and the soft liner, the elastic modulus is in the middle, and the dot reproducibility is good. The abrasion of the printing plate is not very large, the printability of the paper and the manufacturing accuracy of the offset press The requirements are not very strict, and the printing adaptability is strong. Most domestically produced sheetfed offset presses and some imported mid-range offset presses mostly use neutral wrapping.
â‘ Neutral and hard lining
A piece of domestically produced rubber cloth with a thickness of 1.85 to 1.90mm, plus a rubber pad or cork pad, and finally use a certain thickness of paper to align.
â‘¡Neutral softer lining
A piece of domestically produced blanket with a thickness of 1.85 ~ 1.90mm, plus a felt, and finally find it with a certain thickness of paper.
3. Soft lining
The soft lining is usually a woolen cloth under a blanket and a few sheets of tissue paper. The width of the roller pressure line is 10-12mm, and the diameter reduction λ = 0.2 ～ 0.4mm. The soft lining is just the opposite of the hard lining, its elastic modulus is small, and the shrinkage of the lining required to produce the same printing pressure is relatively large, so the transmission of the soft lining to the dot is poor. The width of the contact area is large when the soft liner is embossed, and the contact is relatively good. The ink color of the printed product is relatively thick and full, especially when printing on solid products, it can reflect its advantages, so it is suitable for printing text, color blocks, and solid printed products. .
The soft lining has strong cushioning capacity, the blanket is not easy to be crushed, and the friction effect on the printing plate is small. Most of the sliding amount in the roller rolling can be eliminated by the displacement of the blanket, the printing plate has a long service life and a high printing resistance. However, due to the large deformation and displacement of the blanket, it is easy to deform, and the printed product dots are not smooth enough to print fine products. The requirements for machine manufacturing accuracy and printing pressure adjustment accuracy are relatively low.
Domestic J1202, J2101, PZO-5 and other offset printing machines in Germany adopt soft wrapping.
Second, the printing suitability of the roller lining
l. Hard and neutral hard lining
The dots are clear, clean and complete, with good reproducibility and reproduction, suitable for the printing of fine products and high-end packaging products;
The geometric shape of the graphics and text changes little, especially the width increment of the graphics and the printing plate is relatively small; the amount of ink on the paper is small, and it is not easy to cause the phenomenon of pasting;
The printing plate is easy to wear, and the printing resistance is low;
The amount of change in the cushion is small, and it is easy to play a "hard mouth" when cushioning the blanket;
When the paper is cornered or skewed, it is easy to damage the blanket when there are multiple sheets;
High requirements for machine manufacturing accuracy, otherwise, it is easy to appear "ink stick" or dirty version, paste and other ills.
2. Soft and neutral soft lining
The plate has less friction and the printing plate has a long service life; the ink layer is thicker, suitable for the printing of large-scale lines and solid products with moderate requirements; the displacement and deformation of the blanket are large, and the dots are easily deformed and unclean; the graphic width and size increase Larger; it is not easy to make a "hard mouth" when cushioning the blanket, but the amount of pad change is large; the blanket is not easy to be damaged.
3. The theory of roller lining
l. Isometric lining theory
The equal-diameter lining theory is based on the linear velocity uniformity theory, that is! = Rω. The theory believes that when the roller is rolled, the radius of the three rollers must be kept equal to ensure that the linear speed of the surface of the three rollers is consistent, and there will be no slip during rolling, so as to obtain the ideal rolling and ink on the printing plate After two transfers, it will be faithfully reproduced on the printed matter.
Because the blanket cylinder is not a rigid body, but an elastic body, during the printing process, the imprinting point must be depressed to reduce the radius, that is to say, the radius of other points in the imprinting area is not equal, so the equal diameter theory The radius of the three rollers is not exactly equal, but the radius of the blanket roller is slightly larger.
2. Reducing lining theory
â‘ Geometric speed difference theory
When a rigid body contacts an elastic body, they have only one or two points with the same radius, and the other points cannot have the same radius. Therefore, the theory holds that when the two cylinders are in contact, only one or two points have the same linear velocity, and the other points are inconsistent, so the surface slip of the plate cylinder and blanket cylinder is generated.
â‘¡Sites (Sits) Reducing Theory
When the equal-diameter rigid body and the elastic body roll against each other, the elastic body undergoes a slightly longer stroke than the rigid body due to the compression deformation of the elastic body. It is precisely because of the fact that the radius of the elastic cylinder will increase after being compressed. It is proposed that the radius of the blanket cylinder must be smaller than the radius of the impression cylinder and the plate cylinder. That is, the plate cylinder is the largest, the impression cylinder is centered, the blanket cylinder is the smallest, or the blanket cylinder is the smallest, and the other two rolls are equal.
Fourth, the determination of the drum lining
The determination of the so-called cylinder lining is to cover the shell of the plate cylinder and blanket cylinder with a certain thickness of cushion material as required to adjust the gap between the cylinders and the printing pressure, so that the gears are tangent to the pitch circle The position is engaged to meet the needs of the offset printing process.
l. Determination principles
â‘ The principle of relative sliding does not occur on the surface of the drum
The principle of synchronous rolling. Each color group of the offset printing machine has three cylinders. When printing, the three cylinders are in contact with each other. To ensure the quality of the product, there is no relative sliding friction between the contact surfaces of the cylinders. However, since the three rollers are driven by gears, their angular velocities (ω) are equal. To ensure that their surfaces do not slide, the rolling radius (R roll) of each roller is required to be equal. When the roller is a rigid body, the roller's rolling radius and free radius (R self) are equal. Because the blanket cylinder is equipped with an elastic lining, it is an elastic body, and its rolling radius is not equal to its free radius, and the plate cylinder and the impression cylinder can be regarded as rigid bodies.
The rolling radius of the elastic roller is related to the shrinkage (λ) of the lining on the elastic roller,
That is: R roll = R from + λ
R Roll: Rolling radius R From: Free radius λ: Shrinkage of lining
â‘¡The principle of the tangent of the pitch circle of the drum gear
That is the principle of the center distance of the drum. Gears are used to transmit power between the cylinders on the offset press. In order to ensure the stability of the transmission and reduce gear wear, etc., this requires the gears to be tangentially meshed in the pitch circle, that is, the cylinder center distance is equal to the two cylinder gear indexing The sum of the radius of the circle. However, in practical applications, this is not the case. If the rolling center distance is adjusted too large, resulting in a large gear mesh clearance, the transmission stability is greatly reduced, the gear wear is also increased, and the product quality and machine life will be reduced to varying degrees. .
â‘¢ The principle of proper pressure
The contact pressure between the plate cylinder (P) and the blanket cylinder (B) is called plate pressure (λP-B), and the contact pressure between the blanket cylinder and the impression cylinder (I) is called platen pressure (λB- I), the printing pressure. Generally, the printing pressure should be greater than 0.05-0.1mm, and the appropriate pressure is the first principle to determine the roller lining. Care should be taken to prevent excessive pressure, especially the plate pressure is the most sensitive to the expansion of the dot. The plate pressure will cause the dot to expand seriously, which will affect the color and gradation of the printed matter.
2. Determine the method
In practical applications, the above principles cannot be satisfied at the same time, and only some of them can be satisfied. Therefore, we should take some technological measures to meet the above principles as much as possible, so as not to make too much difference.
â‘ Emphasizing the rolling ratio of synchronous rolling
According to the synchronous rolling conditions, keep the radius of the three rollers equal during rolling. However, when the thickness of the printed material changes, the drum lining must also be properly adjusted. However, in actual production, it seems inconvenient, so the liner data determined by this method is suitable for printing products with a fixed paper thickness or a small change in paper thickness for a long time or in most cases.
â‘¡Prioritize printing plate and blanket cylinder
This method can meet the conditions of synchronous rolling and center distance between the printing plate and the blanket cylinder. It does not change with the change of the thickness of the printed matter, but does not meet any conditions between the impression cylinder and the blanket cylinder. The advantage is that when the thickness of the printed product changes, the lining of the printing plate and the blanket cylinder does not need to be changed, and the center distance between the plate cylinder and the blanket cylinder does not need to be changed, only the center distance between the impression cylinder and the blanket cylinder can be changed. , This operation is more convenient and less troublesome.
5. Inspection of the thickness of the lining
Both the rubber cloth and the cushion material are elastomers, and the compression and tensile deformations are relatively large. When measuring with a micro-card, a few more points should be measured and the average value should be measured. It is best to use a tube caliper to check. Once the lining thickness and center distance of the cylinder are determined, it is best not to change it frequently in order to realize the dataization and standardization of printing process parameters, so as to ensure the stable quality of the printed matter, save production time, and create conditions for the gradual realization of production automation.
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