Discussion on the application of roller lining

Discussion on the application of drum lining

the so-called drum lining refers to all kinds of wrappings wrapped on the surface of the drum outside the drum shell. The surface of the plate cylinder and blanket cylinder of offset press are lined. In addition to the plate and blanket, the commonly used lining materials include insulating paper, kraft paper, rubber pad, felt and other papers. Drum lining generally refers to the lining of the rubber drum. The size of its lining determines whether the three rollers of the offset press can be rolled correctly, which is of great significance for the transfer of pictures and texts, the realistic restoration and reproduction of the hue of the pictures and texts of the printing plate, the accurate multi-color overprint and the improvement of the quality of the printed matter

the purpose of using pads in offset press: firstly, the elastic deformation of pad material is used to generate the required printing pressure; The second is to make up the precision error of machine manufacturing and the defects between printing surfaces after the friction is clean; The third is to reduce the impact and load, avoid the "hard contact" of the drum shell, and protect the mechanical equipment

I. classification of drum lining

according to the size of the diameter reduction (also known as the cutting amount) of the blanket drum of various types of offset printing machines, the selected liner materials, the quality requirements of products, the performance differences of offset printing machines, etc., the drum lining can be divided into three categories: hard lining, neutral lining and soft lining

l. hard lining

hard lining is composed of backing paper and blanket, that is, a few pieces of paper under the blanket form a hard lining. The width of the roller pressure line is mm, and the diameter reduction is λ= 0.04～0.08mm。 This kind of liner is relatively hard, and the elastic modulus of the liner is large. A small amount of deformation can produce a lot of printing pressure, and most offset presses use contact roller pillows (i.e. shoulder Irons) to make the drum run smoothly

the shrink diameter of the hard drum lining is small, that is, the deformation of the lining is small, and the point expansion and deformation are also small. Under the condition of high precision of the printing machine, the use of hard lining can deliver better point-to-point transmission, which can make the fine dots reproduce clearly, which is conducive to the reproduction of color and hierarchy, and can obtain smooth and lifelike high-quality prints, which are suitable for textured prints. When hard lining is used, the friction on the surface of the printing plate is large, the wear of the printing plate is obvious, the printing plate resistance is low, and the requirements for the manufacturing accuracy of the machine and the wear resistance of the material are very high. Otherwise, ink bars may appear due to uneven printing pressure. Domestic jjl02a, jj204 and jj201 web offset presses, imported Miller, heideberg and Komori machines in Japan mostly use hard liners

2. Neutral lining

neutral lining is composed of thick paper, Serge and blanket. The width of the roller pressure line is mm, and the diameter reduction is λ= 0.1～0.2mm。

the printing performance of neutral liner is between hard and soft liner, the elastic modulus is in the middle, and the point reproducibility is good. The wear of the printing plate is not great, and the requirements for the printability of the paper and the manufacturing accuracy of the offset press are not very strict, so the printing adaptability is strong. Most domestic sheet fed offset printing machines and some imported mid-range offset printing machines use neutral liner

① neutral and hard lining

a piece of domestic blanket with a thickness of 1.85 ~ 1.90mm, plus a rubber pad or cork pad, and finally align it with a certain thickness of paper

② neutral soft lining

a piece of domestic blanket with a thickness of 1.85 ~ 1.90mm, plus a piece of felt, and finally align it with a certain thickness of paper

3. Soft lining

soft lining is usually padded with woolen cloth and several sheets of tissue paper under the rubber digital display, which is controlled by a single chip microcomputer, that is, a microcomputer. The width of the roller pressure line is mm, and the diameter reduction is λ= 0.2～0.4mm。 The soft lining is opposite to the hard lining. Its elastic modulus is small, and the amount of lining shrinkage required to produce the same printing pressure is relatively large, so the soft lining has poor point-to-point transmission. The width of the contact area is large and the contact property is relatively good during the soft lining embossing. The ink color of the printed matter is relatively thick and full, which can better reflect its advantages, especially when printing field products. Therefore, it is suitable for printing words, color blocks and field printed matter

the soft lining has strong cushioning capacity, the rubber 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 rubber blanket, and the printing plate has long service life and high printing resistance. However, due to the large deformation and displacement of the blanket, it is easy to deform, and the printed product points are not bright and clean enough, which is not suitable for printing fine products. The requirements for mechanical manufacturing accuracy and printing pressure adjustment accuracy are relatively low

domestic j1202, J2101, and German pzo-5 offset presses use soft liners

II. Printability of roller lining

l. hard and neutral hard lining

points are clear, bright and clean, complete, and have good reproducibility. It is suitable for the printing of fine products and high-end packaging products

the geometric shape of graphics and text changes little, especially the width increment of graphics and text and printing plate is relatively small

the ink on the paper surface is small, which is not easy to produce paste phenomenon

the printing plate is easy to wear and the printing resistance is low

the amount of pad change is small, and it is easy to have a "hard mouth" when padding rubber blanket

it is very easy to damage the blanket when encountering paper corners and skews

high requirements for machine manufacturing accuracy, otherwise, it is prone to "ink bar" or dirty version, paste and other ills

2. Soft and neutral soft lining

less friction on the plate and long service life of the printing plate

the ink layer is thick and suitable for the printing of large quantities of lines, fields and other products with medium requirements

the rubber blanket has large displacement and deformation, and the points are easy to deform and not smooth

the size increment of text width is large

it is not easy to have a "hard mouth" when padding the blanket, but the pad changes a lot

the blanket is not easy to be crushed

III. theory of drum lining

l. equal diameter lining theory

equal diameter lining theory is proposed based on the theory of linear velocity consistency, that is= R ω。 According to this theory, when the roller is rolling, the radius of the three rollers must be kept equal to ensure that the surface linear speed of the three rollers is consistent, and there will be no slip in the rolling, so as to obtain the ideal rolling. After two transfers, the ink on the printing plate will be faithfully reproduced on the print

because the blanket cylinder is not a rigid body, but an elastomer, during the printing process, the imprint point must be concave, which will reduce the radius, that is, the radius of other points in the imprint area is not equal, so the radius of the three cylinders lined by the equal diameter theory is not completely equal, but the radius of the blanket cylinder is slightly larger

2. Different diameter lining theory

① geometric velocity difference theory

when a rigid body contacts with an elastomer, the radii of only one or two points are equal, and the radii of other points cannot be equal. Therefore, the theory believes that when the two rollers contact, only one or two points of linear speed are consistent, and the other points are inconsistent, so there is a surface slip between the plate cylinder and the blanket cylinder

② sites' reducing diameter theory

when a rigid body with equal diameter rolls with an elastomer, its one cycle travel is slightly longer than that of a rigid body due to the compression and deformation of the elastomer. Because of the fact that the radius of the elastic cylinder will increase after being pressed, it is proposed that the radius of the blanket cylinder must be smaller than that of the impression cylinder and the plate cylinder. That is, the plate cylinder is the largest, the impression cylinder is in the middle, the blanket cylinder is the smallest, or the blanket cylinder is the smallest, and the other two rollers are equal

IV. determination of cylinder lining

the so-called determination of cylinder lining is to coat the shell of the printing plate cylinder and the blanket cylinder with a certain thickness of liner material as required to adjust the gap between the cylinders and the printing pressure, so that the gears mesh at the position where the pitch circle is tangent, so as to meet the needs of offset printing process

l. determination principle

① the principle of no relative sliding on the drum surface

that is, the principle of synchronous rolling. Each color group of the offset press has three rollers, which are in contact with each other during printing. In order to ensure product quality, it is required that there should be no relative sliding friction between the contact surfaces of the rollers. However, because all three require saving resources, energy and improving utilization rate, the drum is driven by gear, and its angular velocity（ ω) Equal, to ensure that its surface does not slide, it is required that the rolling radius (r roll) of each drum is equal. When the drum is a rigid body, the rolling radius of the drum is equal to the free radius (r self). Because the blanket cylinder is equipped with an elastic lining, it is an elastomer, and its rolling radius is not equal to its free radius. The plate cylinder and the impression cylinder can appear to be rigid bodies

the rolling radius of the elastic drum and the reduction of the lining on the elastic drum（ λ) About,

that is: R roll =r self+ λ

r roll: rolling radius

there are 10 R from the large plastic Market: free radius

λ： The diameter reduction of the lining

② the principle of tangent of the pitch circle of the drum gear

that is, the principle of the center distance of the drum. In the offset press, the power is transmitted between the rollers through gears. In order to ensure the stability of transmission and reduce the wear of gears, it is required that the gears are tangentially meshed and transmitted in the pitch circle, that is, the center distance of the rollers is equal to the sum of the radius of the graduation circle of the gears of the two rollers. However, in practical application, this is not the case. If the rolling center distance is adjusted too large, resulting in large gear meshing clearance, the transmission stability will be greatly reduced, gear wear will also be intensified, and the product quality and machine service life will be reduced to varying degrees

③ principle of appropriate pressure

the contact pressure between the plate cylinder (P) and the blanket cylinder (b) is called plate pressure（ λ P-B), the contact pressure between the blanket cylinder and the impression cylinder (I) is called impression（ λ B-I), i.e. printing pressure. Generally, the printing pressure should be greater than the plate pressure by 0 1mm, appropriate pressure is the primary principle to determine the drum lining. Attention should be paid to prevent the phenomenon of excessive pressure, especially the plate pressure is the most sensitive to point expansion. The plate pressure will cause serious point expansion, which will affect the restoration of print color and hierarchy

2. Determination method

in practical application, the above principles cannot be met at the same time, but only some of them can be met. Therefore, we should take some process measures to meet the above principles as far as possible without too much difference

① emphasize 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 printing material changes, the lining of the drum must also be adjusted appropriately. However, in actual production, it is not convenient, so the lining data determined by this method is suitable for printing products with fixed paper thickness or little change in paper thickness for a long time or in most cases

② give priority to printing plate and blanket cylinder

this method can meet the conditions of synchronous rolling and center distance between printing plate and blanket cylinder, which does not change with the change of print thickness, but does not meet any conditions between impression cylinder and blanket cylinder. Its advantage is that when the thickness of the printed matter changes, the lining between the printing plate and the blanket cylinder does not need to be changed, and the printing plate