Field printing pressure control principle

As we all know, printing can not be separated from the pressure, one of the most important factors in the offset printing process is the offset pressure problem, the mastery of the offset pressure has become the key to the technical level of the offset printing process. This is true for the cable version, as well as for field-less printing without cable. However, the cable version can be observed and mastered through proofs, while the instrument is not used to monitor the pressure of the proofs. The degree of adjustment is often based on their own experience. At the same time, there is not enough emphasis on web-free prints, as long as the field is printed. It does not matter if the virtual color is saturated or not, so the adjustment of the pressure is also used as a standard. Sometimes the effect of the printed product is not good (possibly due to ink or paper). The method of increasing the printing pressure is used to make it The effect of solid and saturated colors on the ground does not know that offset pressure is too great and there is no harm in it, especially the quality of printed products.

Not long ago, we encountered a problem with the longitudinal ghosting of printed products caused by improper adjustment of pressure when printing a non-website version (pre-regulatory overprinting is not allowed). Printing a batch of web-free packaging products with 250g whiteboard paper requires colorful saturation and high overlay precision. This is not difficult for offset printing, and the main problem of ink and overprinting can meet the product requirements. The image content printed first is mainly some fonts and corporate barcodes, and the black version of light and dark tone. Therefore, after the pressure is adjusted according to the conventional standard, the first black is printed, and the front gauge and side gauge lines of the midway sampling are accurate and meet the requirements. , and then change the ink to print the second color ink. The graphic content is the real plot pattern. Several pieces of test printing found that the effect of the field is poor, reflecting the fact that the actual pattern on the ground is not true and the density of the ink is not enough. Therefore, the operator adopts the method of increasing the lining, and then tries and prints several pieces, and the product is ink-colored. It is also basically in line with the proofing effect, so the official opening of the printing, the sample is also taken out on the way, the rules are accurate. When the printed material was printed on more than 300 sheets, the sample was taken out to inspect the rules and the front gauge was slightly misaligned. Immediately after taking out the sample observation, it was found that the symmetry between the two lines at the two ends of the sample was half-line symmetric, and when the line of sight before the sample was viewed, the difference was even greater, and the machine was shut down for inspection.

From this phenomenon, the two ends of the former gauge are not symmetrically positioned. On the other hand, mechanically, on the one hand, it is the cause of paper feeding, and on the other hand, it is the cause of paper jams. However, it was normal to check both of these aspects. This phenomenon also occurred when the 300-plus-page print was resumed. After discovering this kind of phenomenon, it immediately stopped printing, left the pressure, and re-fed the paper and printed it together. The first three hundred overprints were also accurate. When the pre-registration line appeared, the pressure could be immediately released, and then printing and printing could be done for hundreds of copies. This is also a phenomenon we rarely see in our work. Where exactly is the problem, we still return to the initial phenomenon to analyze.

Printing more than 300 pieces of paper will appear in front of the printed product at both ends of the line symmetry is not in place phenomenon. After printing, it can print another 300 prints with standard rules. This shows that the first ink is accurate, which is based on the first ink. If the problem lies in the mechanical equipment, then it is not possible to produce this phenomenon. Because the mechanical fit is too tight, the fatigue recovery rate of the parts is not so fast. Therefore, we moved the line of sight back to the printing process, separating the pressure from the pressure and transferring the printed graphics through the blanket. When printing, the blanket is in a compressed state and naturally away from pressure. In this state, the blanket is only related to the change of the blanket. Therefore, we re-calculated the lining and found that the lining used was too large and was about 0.3mm larger than the normal printing pressure. Therefore, the lining was reduced to the original value (about 0.1mm more than the screen printing pressure). By adjusting the viscosity and fluidity of the ink, when the ink is reprinted, the color, saturation, and density of the ink on the solid pattern meet the requirements, and no inaccurate phenomenon occurs.

Through the above examples, it is proved that no network printing pressure is neglected. Generally, the printing pressure should be adjusted according to the printed paper and the content of the printed product before printing. The printing without network printing is slightly more stressful than the printing pressure of the network cable, because the pattern is real. Plots make up. Due to the effect of the smoothness and smoothness of the printed paper surface, the surface of the paper is uneven, and if we use a high power microscope, we can see the unevenness. It is because the surface of the paper has a certain degree of roughness, some parts of the ink are printed when the ink is printed, and some parts have less ink. It is obvious that the degree of ink is expressed through the surface of the paper, and the effect is completely different, and the overall effect is. The impression is that the ink is not uniform, the graphics are not evenly imprinted, and in the field it is necessary for the entire pattern to achieve even ink density, bright colors, if the pressure is not enough due to the impact of paper surface smoothness is difficult to achieve This is achieved only by increasing the printing pressure and adjusting the two major characteristics of the ink (fluidity and viscosity).

The average operator judges the pressure increase and decrease based on experience and printed samples. This is not scientific enough and not standardized, but there is no better solution at present. As in the previous example, only the ink adhesion density of the printed sample was taken into consideration, blind linings were added blindly, and the danger of overstress was ignored. Blanket is a kind of elastic object with certain elasticity. It will produce deformation under the action of external force (pressure and friction in printing), such as compression deformation and tangential deformation. When the external force is removed, the deformation immediately disappears and returns to its original state. In the printing, it receives the pressure from the plate cylinder and the impression cylinder, and is affected by the factors of pressure and friction at the same time in the printing zone. When a rigid roller and a rubber roller with a certain elasticity are in direct contact and rotating under pressure, it is impossible to achieve a pure rolling state. The presence of friction causes bumps of different sizes to appear at both ends of the imprinting section. The greater the pressure, the greater the compressive deformation of the blanket and the greater the friction in the imprinting zone.

Therefore, the difference in rotation speed during the printing of the roller is also greater. From the knowledge of physics, it can be known that the velocity of each point in an object rotating on a fixed axis and within an object is equal to the product of the angular velocity and the rotation radius, and the azimuth is perpendicular to the rotation radius. The direction of rotation, ie V=ω·R. According to this formula, we can analyze the tangential velocity of the plate cylinder and the blanket cylinder in the imprinting interval. Since the surface of the plate cylinder is a metal plate, each point R does not change during the imprinting interval, so the tangential velocity at each point is equal. The blanket cylinder is compressed and deformed by the pressure from the plate cylinder within the embossing zone. The amount of compression at each point in the embossing area is not the same, that is, the R of each point is not the same, so according to the formula V:O. R, the tangential velocity at each point is different, which creates a speed difference. This speed difference changes with the change in the amount of compression of the blanket, and the generation of a speed difference also causes the generation of relative slip, which is the surface of the plate cylinder and the blanket cylinder in the contact area, resulting in a frictional sliding movement. This results in varying degrees of tangential deformation of the blanket surface. This tangential deformation causes the surface of the blanket to have a peristaltic shape. Eventually, convex and convex bulges of different sizes appear at both ends of the imprinting section. As the imprinting time prolongs, the peristaltic amount of the rubber increases, resulting in printing. There is a phenomenon that the symmetry of the circumference of the front gauge is not in place on the product.

The pressure blanket was restored to its original shape, and the combined pressure printing was manifested after a period of time. At the same time, the pressure is too great to increase the wear and tear of parts and components of machinery and equipment, and it also reduces India's resistance to printing plates. It is useless for the equipment and printing quality. Therefore, we should also pay attention to the adjustment of printing pressure in the printing screenless version. The pressure is generally adjusted according to the flatness and smoothness of the printed paper surface. In the case of printing without a net version, the printing pressure should be slightly larger than the flat screen printing pressure by about 0.5-0.15mm. According to the nature of the paper, the nature of the blanket can be slightly adjusted, and special attention should be paid to not achieving the actual printing effect. Unrestricted increase in liners and increased pressure should consider other factors. (author/Wei Xin)

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