1. Classification of the lining
Both the printing plate cylinder and blanket cylinder (rubber cylinder) of the offset printing machine are covered with a liner. The blanket cylinder liner can be generally divided into the following three categories.
1. Rigid lining
Generally, a few sheets of paper under the blanket form a rigid wrap. This type of liner is relatively hard, and the elastic modulus of the liner is large, and a small amount of deformation can produce a large printing pressure. Therefore, when using this type of liner, the compression of the liner is small, that is, the deformation of the liner is small, and the expansion and deformation of the dot are also small. Therefore, the rigid liner has better transmission of dots, which is conducive to the reproduction of colors and gradations, and is suitable for printing textured prints. However, it is especially important to note that when using this type of liner, the compression of the liner should be controlled within 0.2 mm, preferably about 0.1 mm. If the compression amount of the lining is too large, it will not only be detrimental to the machine, but also cause other failures, and it will not be good for the transmission of the dots, and the advantage of using a rigid lining will be lost. Therefore, the use of rigid lining requires high machine accuracy and precise adjustment of printing pressure, otherwise, not only can not reflect the advantages of rigid lining, but will bring new shortcomings. In other words, the rigid lining has its unique use conditions and application occasions, and cannot be used everywhere. At present, it is unscientific for some printing houses to blindly use rigid wrapping.
2. Soft lining
The soft lining is usually a woolen cloth under a blanket and a few sheets of tissue paper. The software lining is just the opposite of the hardware lining, and its elastic modulus is relatively small. The compression amount of the lining required to produce the same printing pressure is relatively large. Generally, the compression amount is controlled between 0.2mm and 0.3mm, so The soft liner has poor transmission to the dots, but the contact area of ​​the soft liner is wider during imprinting, and the contact is relatively good. The ink color of the printed product is relatively full and full, especially when printing solid products, it can reflect its advantages. Therefore, it is suitable for printing text, color blocks, and field prints. In addition, the soft lining has strong buffering ability, the blanket is not easy to be crushed, and the requirements on the accuracy of the machine and the adjustment of the printing pressure are relatively low, so the soft lining is suitable for use on old machines or new machines with low accuracy .
3. Neutral lining
The neutral lining is generally composed of rubber under the blanket, and its elastic modulus is in the middle, so its performance is also in the middle, and its use occasion is on the machine between the new and old machines, and can also be used on new and old machines. This kind of lining is more versatile, and the performance is ok.
Second, the determination of drum lining
The determination of the type of lining can be specifically determined according to the above, and the following principles should be considered in determining the total thickness of the lining:
1. Determination principle
â‘ The principle of relative sliding does not occur on the surface of the drum (synchronous rolling conditions)
There are three cylinders in each color group of the offset printing machine. When printing, the three cylinders are in contact with each other. To ensure the quality of the product, it is required that the cylinder contact surface is a pure cylinder, and there can be no relative sliding friction. However, because the three cylinders pass through the gear For the transmission, the angular velocity is equal. To ensure that the surface does not slide, the rolling radius (R roll) of each roller is required to be equal. When the cylinder is a rigid body, the rolling radius of the cylinder is equal to the free radius of the cylinder (R self), and the blanket cylinder is equipped with an elastic lining and cannot be treated as a rigid body. The impression cylinder and the plate cylinder can be regarded as rigid bodies, so the blanket The rolling radius of the drum is not equal to the free radius of the drum. Research shows that the rolling radius of the elastic roller is related to the compression of the lining on the elastic roller, and the general relationship is as follows:
R roll = R self + λ, λ is the compression amount of the lining of the elastic roller.
â‘¡ Principle of tangent of pitch circle of roller gear (condition of center distance)
The transmission of power between the cylinders on the offset press is through gears. To ensure the stability of the transmission and reduce the wear of the gears, this requires the gears to be tangentially meshed in the pitch circle, that is, the center distance of the cylinder is the dividing circle of the two cylinder gears with. However, in practical applications, some people often adjust the center distance of the drum too much, resulting in a large gear tooth backlash, the transmission stability is greatly reduced, and the gear wear is also increased at the same time, which will not only reduce product quality, but also Reducing the service life of the machine is a problem that should be avoided.
â‘¢ The principle of proper pressure
The contact pressure between the plate cylinder and the blanket cylinder is called the plate pressure, which is denoted by λp, and the contact pressure between the blanket cylinder and the impression cylinder is called the platen pressure, that is, the printing pressure, which is expressed by λi. It is larger than the platen pressure by 0.05mm ～ 0.1mm. Appropriate pressure is the first principle to determine the roller lining. With regard to pressure, special attention should be paid to preventing excessive pressure. Some people blindly increase the pressure when they encounter problems, so that the pressure is severely excessive. The platen pressure is the most sensitive to the expansion of the dots. If the plate pressure is too large, the dots will expand seriously, which will affect the color and gradation of the printed products.
2. Determination method
The above principles cannot be satisfied at the same time. In practical applications, only some of the conditions can be met, or the above conditions can be met as much as possible, so as not to be too different. There are two specific determination methods:
â‘ The method of determining the lining that emphasizes the synchronous rolling conditions
This method can meet the synchronous rolling conditions under a fixed paper thickness, but does not meet the center distance conditions, such as paper thickness changes, does not meet any conditions, so this method is only applicable to the long-term printing of fixed paper thick printed matter. Set the thickness of the printed paper as h, the plate pressure as λp, the printing pressure as λi, λp, λi set specific data in advance according to the type of liner, rigid liner: λp = 0.1mm, λi = 0.15mm, package liner: λp = 0.2mm, λi = 0.25mm, neutral lining: λp = 0.15mm, λi = 0.2mm, according to the synchronous rolling conditions: RP roll = RB roll = Ri roll, the following formula can be derived:
RP since = Ri + h
The plate cylinder lining is δp = RP self-Rp
RB self = RP self-λ λ can be replaced by λp calculation ie λ = λp
The rubber drum liner is δB = RB self-RB
Plate-rubber cylinder center distance A (PB) = 2R B from + λ-λp = 2R B rubber-imprint cylinder center distance A (IB) = 2R B self + λ-λi≈2R B from plate-rubber Roller pillow clearance H (PB) pillow = A (PB)? RP pillow-RB pillow rubber-embossed roller pillow clearance H (IB) pillow = A (IB)? RI pillow-RB pillow
It can be known from the above computer formulas that δP and δB are functions of the paper thickness h. When h changes, the thickness of the liner must be changed. In actual production, it is not convenient to change the liner when the paper thickness is changed. Therefore, the bag determined by this method Lining data is suitable for printing long-term or most cases of fixed paper thickness or products with little change in paper thickness. In addition, it can also be seen from the formula that the center distance A (PB) and A (IB) of the roller are approximately 2R B, which is very close to the sum of the graduation circles of the two rollers, that is, the center distance condition is approximately satisfied.
â‘¡Prioritize the determination method of printing plate and rubber roller
This method can meet the conditions of synchronous rolling and center distance between the printing plate and rubber cylinder, and can not change with the paper thickness, but does not meet any conditions between the printing and rubber cylinder. The advantage of this method is that when the thickness of the printed paper changes, the lining of the printing plate and the rubber roller does not need to be changed, and the center distance between the plate and rubber roller does not need to be changed, only the center distance between the stamping and rubber roller can be changed. This method is more convenient and labor-saving, so this method is more commonly used, but it also has disadvantages: that is, when the paper thickness changes greatly, the degree of synchronous rolling between the imprint and the rubber roller and the center distance condition are also not satisfied. The friction on the surface of the two rollers is intensified, and the backlash of the gear teeth is increased. It is not suitable for long-term use in this state. According to the printing plate-rubber roller synchronous rolling condition RP roll = RB roll and center distance condition A (PB) = RP self + RB self-λp = C, the following formula can be obtained:
RB self = (1/2) A (PB)
δB = RB to -RB
RP self = RB self + λ λ = λp
δP = RP from -RP
A (IB) = 2R B since + Ri + h-λi
A (PB) = 2R B = CC is a constant
H (PB) pillow = A (PB)? RP pillow-RB pillow
H (IB) pillow = A (IB)? RI pillow-RB pillow
From the above formula, δP and δB are independent of h, A (PB) is a known constant, and A (IB) is a function of h, so H (IB) should be adjusted with h.
According to the lining data determined by the above method, the roller pressure, the roller pillow gap, and then the full version of the trial printing, test the pressure condition, generally speaking, there will be no problem, if the pressure is not enough, you can add a piece of paper in the rubber lining That's it. After the rubber lining is used for a long time, the lining will change, and the pressure may be insufficient. At this time, add a piece of paper to the lining, but it should be noted that when the new lining is replaced, the original data should still be given.
3. The relationship between wrapping and overprinting
Roller lining has an important impact on product overprinting, mainly in the following aspects
1. The relationship between the drum liner and the length of the printed image
The three cylinders of each color group of the offset printing machine rotate at an equal angular speed. The arc length traversed by the surface of each cylinder at the same rotation angle is proportional to its free radius. The length of the graphic on the printed sheet has the following relationship with the length of the graphic on the printing plate:
Lprint = (Rprint / Rp)? Lp
Rprint is the free radius of the impression cylinder (Rprint = Ri + h), and Rp is the free radius of the plate cylinder.
It can be seen from the formula that the free radius of the blanket cylinder has nothing to do with the length of the printed image. That is, changing the blanket cylinder liner has no effect on the length of the printed image. The length of the printed image is only related to the free radius of the plate cylinder and the impression cylinder.
The increase or decrease of the change of the plate cylinder lining or the impression cylinder radius on the circumferential length of the printed image can be directly derived from the above formula:
ΔL / ΔR = Lp / Rp
ΔL The amount of change in the circumferential length of the printed image, ΔR is the amount of change in the radius of the cylinder, that is, ΔR = R before-R after. When ΔL is a negative value, the graphic length decreases, and when ΔL is a positive value, the graphic length increases. That is to say, when the plate cylinder liner is increased or the impression cylinder radius is reduced, the printed image length will be reduced, otherwise the opposite.
2. The relationship between multicolor overprint and pressure
According to the formula Rroll = Rfrom + λ, it can be seen that if the pressure changes, that is, λ changes, the rubber roller rolling radius changes. When the plate pressure and printing pressure are increased or decreased at the same time, it is equivalent to increasing or decreasing the rubber liner, and will not change the printed product map. The length of the document, if only the plate pressure is increased without changing the printing pressure, this is equivalent to increasing the printing plate liner, and the printed image will be shortened. If only the printing is increased without changing the printing pressure, this is equivalent to increasing the radius of the printing cylinder , The printed graphics will increase, otherwise the opposite. The amount of change can also be obtained from the previous formula, where ΔR is replaced by Δλ.
3. The relationship between wrapping and overprinting when printing with monochrome machine
It is assumed in advance that there is no change in the size of the paper, that the offset press passes the paper accurately and that the paper is positioned accurately. According to the above discussion, as long as the monochrome machine does not change the cylinder parameters (mainly refers to the printing plate liner), the length of the picture and text will be the same after each printing, so the overprint will completely overlap. From this perspective, the monochrome machine is more than the multicolor machine. Accuracy is better.
4. The relationship between lining and overprinting in multi-color machine printing
For the same reason, it is assumed in advance that the paper has no size deformation, and the offset press passes the paper accurately. According to the analysis of the liner and the length of the picture and text, the latter color must completely register with the previous color. It is a copy of the previous color group, but in fact, it is impossible to make the same color group in the machine manufacturing, there will always be a little error, so the registration of the multi-color machine is not as good as the monochromatic machine, but as long as the manufacturing accuracy meets the given requirements , Registration is not a problem. If the roller is corroded, worn, crushed or the processing accuracy of the roller itself is low, the difference between the rollers is large. It is no longer possible to register each color. Therefore, the plate cylinders of the multi-color machine, the impression cylinder and the liner are consistent. It is particularly important, and special attention should be paid to the protection and maintenance of the impression cylinder.
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