Printing pressure adjustment principle and analysis of common problems (2)

First of all, let's take a look at the factors that affect the adjustment of printing pressure.

(1) The smoothness of the paper

Offset printing has many types of products, including newspapers and periodicals, packaging and decoration products, and the surface conditions of printing papers are also very different. Therefore, the printing pressure required for printing varies. Table 13-1 lists the paper smoothness and printing pressure data on a sheet-fed offset press. The line pressure between the plate cylinder and the blanket cylinder is fixed at 5 kg/cm and printed at 4,000 rpm. Speed, with different smoothness of the paper for printing, when obtaining the best quality prints, measured line pressure between the impression and blanket cylinders.

No. Paper Type Smoothness (sec) Line Pressure (kg/cm)
1 Ultralight coated paper 2472 4.4
2 Coated paper 518 7.8
3 Offset paper 51 11.1
4 Newsprint 17 12.2

As can be seen from the table, the printing pressure required for rough-surfaced newsprint is almost three times that of ultra-smooth coated paper with extremely smooth surfaces. Printing on paper with a low degree of smoothness must appropriately increase the printing pressure, otherwise it will appear. The defects of the printing network were false and the writings were broken.

(2) Printing speed

Many production examples can prove that when the printing speed increases significantly, the printing pressure also increases. Because the printing speed is increased, the contact time between the corresponding dots on the printing surface is also reduced. This requires a greater pressure, so that the degree of incomplete contact between the printing surfaces is reduced, and the ink can be quickly The transfer from the ink supply surface to the ink receiving surface allows the ink to perform the necessary transfer in a short time.

The same is the rotary press. The high-speed machines in modern times must have higher printing pressure than the old ones because the high-speed machines have a larger roller utilization coefficient and a smaller radius than the old ones. The compression width is also relatively reduced, which also affects the ink transfer.

(3) Quality requirements of the product

Different products have different replication requirements. In addition to the use of different raw materials and plates, it is also possible to adapt to the pressure of printing. "Field" graphics can (also allow) a little more pressure to meet the requirements of imprinting. Network line graphics should emphasize "ideal pressure" and avoid network loss and loss. High-line products are more demanding on the quality of copying. In particular, it is necessary to control the printing pressure so as to reach the minimum value. When printing such products, in addition to using coated paper with a smooth surface, it is also necessary to pay attention to the quality of the printing plate. Strict control of ink balance on the printing plate surface.

(4) The elasticity of the blanket and its cushion

The deformation of the blanket and its cushioning material (liner, cushion, and also felt) is closely related to the printing pressure, and blankets and pads that are flexible and that are not easily lost after mass printing should be used. Paper and felt, to meet the basic requirements of the ideal printing pressure. On the contrary, if the domestic poor quality blanket is selected to reduce costs, the absolute value of the minimum printing pressure must be increased. If the use of imported air-cushion blanket printing pressure is generally set to 0.08 ~ 0.10mm, if the domestic blanket with Maoni to use as a gasket, the general printing pressure is set to 0.2 ~ 0.25mm.

After a certain amount of time (inking) is used, the blanket loses or reduces its original elasticity (which is determined by their quality) either quickly or slowly. At this time, the pressure has to be increased to solve the problem of good ink transfer. The problem, which is to say that the new blanket and liner material can achieve a smaller printing pressure.

(5) The number of prints

In the printing process, as the number of printed matters increases, the printing pressure decreases. This is because after the blankets and liners are used for a period of time, a certain amount of printed matter is printed, the original pressure will be lost or reduced. Elasticity therefore reduces printing pressure. According to the experiment: the printing pressure for the soft liner is reduced by 20-30%, and for the rigid liner it is decreased by not more than 10-15%. As the printing pressure drops, the ink transfer rate will decrease. Therefore, in order to obtain stable quality prints, the pressure may be adjusted slightly higher before printing so that the printing pressure after a certain number of printing is still sufficient to ensure quality.

This reduction in printing pressure was most noticeable after the pad was updated. Therefore, after the liner is renewed, it must be run at the working speed for half an hour to an hour so that the liner and the blanket can be deformed and stabilized, and then the printing pressure can be adjusted, so that a good print can be obtained with a substantially constant printing pressure.

Two channels to change the printing method:

Offset printing press mainly adjusts the printing pressure by changing the compression amount of the blanket, and this compression amount is mainly derived from the fact that the center distance of the rollers after printing is smaller than the sum of the roller surface radius. For example, the pressure between the plate and the impression cylinder comes from the fact that the center distance between the two cylinders is smaller than the sum of the free surface radius of the plate cylinder and the free surface radius of the blanket.

There are two main ways to adjust the printing pressure. One is to change the printing pressure by changing the pad under the printing plate or the blanket; the other is to adjust the center distance of the roller. The author believes that the adjustment of the printing pressure should be determined by the circumstances, and adopt different pressure adjustment methods under different circumstances.

When the thickness of the substrate changes, different adjustment methods are used. The method is as follows (increase the thickness of the substrate as an example):

1. Method one: reduce the liner under the blanket to increase the amount of substrate, and at the same time, in order to ensure the pressure between the printing drum and the blanket cylinder, the thickness of the liner in the printing cylinder is reduced by an equal amount.

2. Method 2: Directly increase the center distance between the impression cylinder and the blanket cylinder. The amount of increase should be the amount of substrate thickening.

In the older models such as domestic 01, 03, 08 and other machines, because the machine's pressure regulating device is not reliable, so the general use of a method to adjust the paper thickness changes, unless the thickness of the substrate changes At this time (it must be adjusted by adjusting the center distance of the drum, the specific method behind the specific introduction), try not to change the center distance of the drum has been accurately adjusted, mainly due to labor intensity and production efficiency considerations. At the same time using method one to adjust we also pay attention:

A. Since the thickness of the plate cylinder liner is changed, it is necessary to adjust the plate roller and the plate roller at the same time.

B. Pay attention to the change of the imprint on the substrate after the change of the diameter of the plate cylinder (due to the change of the cylinder diameter, the tangential diameter of the roller does not change. When the plate gasket is thickened, the imprint on the substrate will be Will be shorter; the same reason when the thinning is the opposite), at the same time it is strictly prohibited in a batch of India's printing process in this way to change the printing pressure.


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