Discussion on the application of roller lining
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The drum liner refers to various coverings wrapped around the surface of the drum outside the drum casing. Offset printing plate cylinders and blanket cylinder surfaces are lined. In addition to printing plates and blankets, common lining materials are insulating paper, kraft paper, rubber mats, felts and other papers.
The roller lining generally refers to the lining of the blanket cylinder. The size of the lining determines whether the three rollers of the offset printing machine can be properly rolled, and the transfer of the graphic and the realistic reproduction restores the hue of the printing plate and the image is accurately performed. Multi-color overprinting, to improve the quality of printed matter, is of paramount importance.
The purpose of the pad using the pad: Firstly, the elastic deformation of the pad material is used to produce the required printing pressure; the second is to compensate for the precision error of the machine manufacturing and the defects between the printing surfaces; the third is to reduce the impact and load. , to avoid the "hard hit" contact of each drum shell, to protect the mechanical equipment.
First, the classification of roller lining
According to the size of the various types of offset press blanket cylinders (also called the amount of cutting), the selected cushioning material, the quality requirements of the product, the performance difference of the offset press, etc., the drum liner can be divided into hard packs. Lining, neutral lining, soft lining three categories.
l. Hard lining
The rigid lining consists of a liner paper and a blanket, that is, a few sheets of paper under the blanket form a rigid lining. The roller pressure line width is 4-6 mm, and the diameter reduction amount λ is 0.004 to 0.08 mm. This kind of lining is relatively hard, the elastic modulus of the lining is large, and a small amount of deformation can produce a large printing pressure, and most offset printing machines use contact rollers (ie, shoulder iron), so that the drum runs smoothly.
The rigid roller lining has a small amount of shrinkage, that is, the deformation of the lining is small, and the dot enlargement and deformation are small. Under the condition of high precision of the printing press, the use of rigid lining, the transmission of the dots is better, the fine dots can be clearly reproduced, and the reproduction of color and level is facilitated, and the smooth and realistic high-quality printed matter can be obtained, which is suitable for the textured printing. . When using hard lining, the friction of the surface of the printing plate is large, the printing plate wear is obvious, the printing plate printing rate is low, and the manufacturing precision and the wear resistance of the material are very high. Otherwise, it may be due to printing pressure. The unevenness of the ink sticks. Domestic JJl02A, JJ204, JJ201 web offset presses, imported Miller, Heideberg, and Komori machines in Japan are mostly hard-lined.
2. Neutral lining
The neutral lining consists of thick paper, edgy and blanket. The roller pressure line has a width of 8 - 10 mm and a reduced diameter λ = 0.1 to 0.2 mm.
The printing performance of the neutral lining is between the hard and soft lining, the elastic modulus is centered, and the dot reproducibility is better. The wear on the printing plate is not very large, the printing suitability of the paper and the manufacturing precision of the offset printing machine The requirements are not very demanding and the printing adaptability is strong. Most domestic sheetfed offset presses and some imported mid-range offset presses mostly use neutral lining.
1 neutral hard lining
A domestic blanket with a thickness of 1.85 to 1.90 mm, plus a rubber mat or cork mat, can be found with a certain thickness of paper.
2 neutral soft lining
A domestic blanket with a thickness of 1.85 to 1.90 mm, plus a felt, and finally a piece of paper with a certain thickness can be found.
3. Soft lining
The soft lining is usually a rag on the underside of the blanket plus a few sheets of tissue. The roller pressure line has a width of 10 - 12 mm and a reduced diameter λ = 0.2 to 0.4 mm. The soft lining is opposite to the rigid lining. The elastic modulus is small, and the amount of shrinkage required for the same printing pressure is relatively large, so the soft lining has poor transferability to the dots. When the soft liner is embossed, the contact area has a large width, and the contact is relatively good. The ink color of the printed product is relatively thick and full, especially when the product is printed, which is more suitable for printing characters, color blocks and solid printed materials. .
The soft lining buffering capacity is strong, 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, and the printing plate has a long service life and a high printing durability. However, due to the large deformation and displacement of the blanket, it is easy to be deformed, and the printed product dots are not smooth enough to be printed, and it is not suitable for printing fine products. The requirements for mechanical manufacturing precision and printing pressure adjustment accuracy are relatively low.
The domestic J1202, J2101, and German PZO-5 models are lined with soft lining.
Second, the printability of the roller liner
l. Hard and neutral hard lining
The outlets are clear, smooth and complete, and have good reproduction reproducibility, suitable for printing of fine products and high-grade packaging products;
The geometry of the graphic changes little, especially the width increment of the graphic and the printing plate is relatively small; the amount of ink received on the paper surface is small, and the paste phenomenon is not easy to occur;
The printing plate is easy to wear and has low printing durability;
The amount of pad change is small, and when the blanket is applied, it is easy to play a "hard mouth";
When the paper is folded and skewed, it is easy to break the blanket when there are multiple sheets;
The machine manufacturing precision is high. Otherwise, it is prone to "ink bar" or dirty plate, paste and other defects.
2. Soft and neutral soft lining
The surface friction is less, the printing plate has a long service life; the ink layer is thicker, suitable for medium-volume printing of large-volume lines and solid products; the displacement and deformation of the blanket are large, the dots are easily deformed and not smooth; the width of the graphic width is increased. Larger; it is not easy to "hard" when padding the blanket, but the amount of pad change is large; the blanket is not easily crushed.
Third, the theory of roller lining
l. Equal diameter lining theory
The equal-diameter lining theory is based on the theory of linear velocity uniformity, namely !=Rω. According to the theory, when the roller is rolled, the radius of the three rollers must be kept equal to ensure that the surface speed of the three rollers is uniform, and no slip is generated during rolling, thereby obtaining ideal rolling, ink on the printing plate. After two transfers, it will be faithfully reproduced on the printed matter.
Since the blanket cylinder is not a rigid body, but an elastic body, during the printing process, the embossing point is inevitably depressed to reduce the radius, that is, the radii of other points in the embossed area are not equal, so the theory of equal diameter The radius of the three rollers of the liner is not exactly equal, but the radius of the blanket cylinder is slightly larger.
2. Different diameter lining theory
1 geometric speed difference theory
When a rigid body is in contact with an elastomer, they have only one or two points with equal radii, and the remaining points cannot have equal radii. Therefore, the theory is that when the two rollers are in contact, only one or two points have the same linear velocity, and the other points are inconsistent, so that the surface slip of the plate cylinder and the blanket cylinder is generated.
2Sites (Sizes) theory of different diameters
When the equal-diameter rigid body and the elastic body are rolled together, the stroke of the one-length movement of the elastic body is slightly longer than that of 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 is increased after being pressed, 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 liner
The so-called roller lining is determined by coating the casing of the plate cylinder and the blanket cylinder with a certain thickness of cushion material as required to adjust the gap between the rollers and the printing pressure, so that the gears are tangent to the pitch circle. Engage in position to meet the needs of the offset process.
l. Determine the principle
1The surface of the drum does not produce a relative sliding principle
That is, the principle of synchronous rolling. There are three rollers in each color group of the offset printing machine. When the three rollers are in contact with each other during printing, in order to ensure the quality of the product, it is required that there is no relative sliding friction between the contact surfaces of the rollers. However, since the three rollers are driven by gears and their angular velocities (ω) are equal, to ensure that the surface does not slip, the rolling radii (R rolls) of the rollers are required to be equal. When the drum is a rigid body, the rolling radius and free radius (R from) of the drum are equal. Since the blanket cylinder is provided with an elastic lining, it is an elastic body whose rolling radius is not equal to its free radius, and the plate cylinder and the impression cylinder can be similar to a rigid body.
The rolling radius of the elastic roller is related to the reduced diameter (λ) of the lining on the elastic roller.
Namely: R roll = R from + λ
R Roll: Rolling radius
R from: Free radius
λ: the amount of shrinkage of the liner
2 drum gear section circle tangency principle
That is, the drum center distance principle. The gears on the offset printing machine are transmitted by gears. In order to ensure the smoothness of the transmission and reduce the wear of the gears, the gears are required to be tangentially meshed and driven, that is, the center distance of the drum is equal to the index of the two drum gears. 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 meshing gap, the transmission stability is greatly reduced, the gear wear is also increased, and the product quality and machine life are reduced to varying degrees. .
3 pressure suitable principle
The contact pressure between the plate cylinder (P) and the blanket cylinder (B) is called the plate pressure (λP-B), and the contact pressure between the blanket cylinder and the impression cylinder (I) is called the stamping pressure (λB- I), that is, printing pressure. Generally, the printing pressure is greater than the plate pressure of 0.05-0.1mm. The proper pressure is the first principle to determine the drum lining. Care should be taken to prevent excessive pressure, especially the plate pressure is most sensitive to the expansion of the dot. The overpressure of the plate causes the dot to expand greatly, thus affecting the reproduction of the color and level of the printed product.
2. Determine the method
In practical applications, the above principles cannot be met at the same time, and only some of them can be met. Therefore, we should adopt some technological measures to try to satisfy the above principles, and the difference will not be too great.
1 emphasizes the rolling ratio of synchronous rolling
According to the synchronous rolling condition, the radius of the three rollers is kept equal when rolling. However, when the thickness of the printed material changes, the lining of the drum must also be properly adjusted. However, in actual production, it is not very convenient, so the lining data determined by this method is suitable for products with long-term or most cases of printing fixed paper thickness or paper thickness.
2 Priority to plate, blanket cylinder
In this way, the condition of the synchronous rolling and the center distance between the printing plate and the blanket cylinder can be satisfied, and it does not change with the change of the thickness of the printed matter, but the condition between the impression cylinder and the blanket cylinder is not satisfied. The advantage is that when the thickness of the printed matter 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, and only the center distance between the impression cylinder and the blanket cylinder can be changed. This makes it easier to operate and saves trouble.
V. Inspection of the thickness of the lining
Both the blanket and the cushioning material are elastomers. The compression deformation and the tensile deformation are relatively large. When measuring with a thousand-cent card, several points should be measured and the average value should be obtained. It is best to use a cylinder diameter gauge for inspection. Once the thickness and center distance of the lining of the drum are determined, it is best not to change it frequently to achieve the dataization and standardization of the printing process parameters, so as to ensure the quality of the printed matter is stable, save production time, and create conditions for the gradual realization of production automation.