Offset Printing Pressure Effect and Adjustment (3)

From the above formula, it can be seen that there is a linear relationship between Lpb or Lbi and their influencing factors, that is, how much the influencing factor increases or decreases, and how much Lpb or Lbi also changes. From the analysis in 1, it can be seen that the paper thickness (Tz), the center distance between the blanket cylinder and the impression cylinder (Abi) only affect the pressure between the blanket cylinder and the impression cylinder; the thickness of the plate (Tp), The thickness of the plate liner (Tpc) and the center-to-center distance (Abp) between the plate cylinder and the blanket cylinder only affect the pressure between the blanket cylinder and the impression cylinder; the thickness of the blanket (Tb) and its liner Thickness (Tbc) affects both pressures. This guides the adjustment of printing pressure. When it is only necessary to change the pressure between the impression cylinder and the blanket cylinder, the center distance (Abi) can be adjusted or the paper can be changed. It is practically impossible to replace the paper, so the most common is to change the center distance of the cylinder, but the center distance The change should consider the effect of paper thickness. When only the pressure between the blanket cylinder and the plate cylinder needs to be changed, the center pitch (Apb) is adjusted, or the printing plate is changed, or the printing plate is changed, or the thickness of the printing plate liner is changed, and the actual process of replacing the printing plate is encountered. Very few, adjust center distance to use not too much, the main reason is that there is only ink between rubber and printing plate, and the ink is basically all the same. So it is relatively common to change the thickness of the liner below the plate. When both pressures need to be adjusted, the thickness of the blanket or the thickness of the liner below it can be changed. When the pressure at both places needs to be adjusted, the thickness of the blanket or the thickness of the liner below it can be changed, and the thickness of the blanket is rarely encountered in the actual process.

It is generally used to change the thickness of the liner below it. To summarize the above analysis:

When adjusting the pressure between the blanket cylinder and the impression cylinder, the most common is to adjust the center-to-center distance between the two cylinders. When adjusting the pressure between the plate cylinder and the blanket cylinder, the most common is to change the thickness of the liner under the plate. When both pressures need to be adjusted, the most common is to change the thickness of the liner under the blanket cylinder.

2.7.4 The best printing pressure

In the previous section, the general method for the formation and adjustment of printing pressure was analyzed in detail, but in the actual process, the printing pressure must be able to ensure the good transfer of ink, so there is a problem of the selection of the optimal printing pressure. The best printing pressure should be: full imprinting, smooth, bright, clear, colorful, minimal deformation. To obtain the best printing pressure, it is necessary to make the paper, ink, fountain solution, printing plate, printing equipment, printing process, printing environment, and lining the eight factors out of the best adaptability.

From a paper point of view, the value of Lbi is different for different papers. Lbi for coated paper is small, and Lbi for offset paper is large. Because the surface roughness of offset paper is large, Lbi does not increase, and some areas may not be printed on. However, the surface roughness of coated paper is small, Lbi is large, and the deformation of the imprint is increased rapidly, and it is easy to paste.

From the point of view of the ink-and-wash balance, the ink-and-wash balance is the guarantee of the quality of printed products. The greater the printing pressure, the more likely it is that the print will spread, requiring more water to stop the ink from spreading. Therefore, the pressure of printing changes, and the ink-and-wash balance also changes accordingly. Considering the ink alone, it can be seen from the pressure curve that the pressure must be within the BC segment. The transfer of ink is the most ideal and is also the most sensitive to control.

From the point of view of the printing apparatus, the smaller the printing pressure, the greater the periodic impact and vibration. Considering only printing equipment, the printing pressure should be zero, but in reality this is impossible. It is best to leave it in the state described in section 2.7.3.

From the perspective of the printing process, the first color ink has the best transfer effect, and the subsequent color transfer effect becomes more and more difficult. According to the separation rule of the ink, the value of Lbi should be larger the later, and from the ink point of view, the more viscous the ink, the greater the pressure; the thinner the ink, the lower the pressure.

From the point of view of the printing plate, the surface should have good hydrophilicity, where the graphic and text should have good ink and water repellency, and the other is to obstruct the deformation of the print. From this point of view, the intaglio plate is better than the plano plate. From the point of view of the printing environment, when the temperature and humidity in the environment changes, paper, ink, and fountain solution will be affected, resulting in pressure changes.

In summary, the optimal printing pressure is adjusted by making the gears in a standard mesh state and by varying the thickness of the liner to achieve the best printability.

When printing thick paper, the best printing pressure adjustment method is:

1 reduce the thickness of the liner under the blanket cylinder;
2 increase the thickness of the liner under the plate cylinder;
3 Minor difference can be compensated by changes in center distance.