Analysis of Influencing Factors of Offset Printing Pressure

As we all know, printing can not be separated from the pressure, one of the most important factors in the printing process is the printing pressure, it is the key to the smooth transfer of ink on the printing plate to the substrate. The offset pressure is measured by the elastic deformation (compression amount) of the blanket. In the offset printing production, the deformation of the elastic body is constituted by adjusting the gap between two adjacent rollers, which is the source of the offset pressure.

1 The relation between printing pressure and ink transfer

In the printing process, it is necessary to maintain the stability of the printing pressure. Adjusting the drum pressure regulating mechanism should avoid the change of the printing pressure due to the fluctuation of the pressure regulating mechanism, which will affect the ink transfer rate and make the printed ink ink unbalanced.

The relationship between pressure and ink supply can divide the curve into many segments. The AB segment marked with a dashed line can be referred to as an “ink insufficiency” segment. This is because in the range of P0-P1, the printing pressure does not make the distance between the printing surfaces small enough for the ink and the solid to attract each other. It may be completely copied and there is an "emptiness" phenomenon.

In the BC section, starting from point B, the ink transfer rate increases significantly with the increase in pressure, and the print can be better reproduced. That is, under the pressure of P1-P2, it can print the contour without leaving the mark. However, the roughness of the paper often changes, so the ink layer is difficult to guarantee the same thickness, and some of the imprints will have “emptiness”. Under normal circumstances, in the BC section, the ink supply varies with pressure (the thickness of the ink layer often changes cannot exceed the normal thickness of the print), and the ink supply of the printing plate is proportional to the pressure. Therefore, the BC segment is called the ink proportional transfer section, and P1-P2 is called the proportional transfer pressure.

In the CD segment, the pressure changes within P2-P3, but the ink supply is approximately constant and can print non-aliased impressions under certain objective conditions and maintain the same amount of ink between printed sheets. At this time, print products with relatively stable print quality can be obtained.

In the DE section, due to the excessive pressure, the print spreads to the non-graphic part, and the ink transfer amount tends to decrease. Because the ink is stretched by pressure, the dots are distorted, the ink layer in the middle of the print is thin, and the ink layer on the edge is thicker. At this time, the blotting will be lost and the copying effect will be worse.

When the pressure exceeds E, the dots are sharply deformed and the picture and text are distorted. At this time, the ink transfer amount increases slightly with the increase of the pressure. Under the influence of excessive pressure, the spread of ink is more serious and the area of ​​ink receiving is increased; the spread of outlets is severe and the effect of copying is greatly reduced. Therefore, we call P2 the process pressure, P3 the appropriate pressure, and P4 the critical pressure.

2 Effect of inappropriate pressure on print quality

In actual production, the printing pressure is not suitable, and the quality of printed products is not necessarily high. The printing pressure is too small, the amount of ink transferred from the printing plate to the paper is too small, the ink on the printed sheet is pale and unclean on the spot; the printing pressure is too high, the printing plate wears out quickly, and the printing plate printing rate falls sharply. . In addition, the ink spreads badly, and the picture and text dots are distorted, so that the level of the intermediate tone and the dark tone are equal to each other. The tone reproduction and color reproduction of the printing crystal are not ideal, and the image is distorted. There are mainly the following failures caused by improper printing pressure.

1) When the pressure of peeling paper is too high, the amount of friction on the paper surface increases, the separation power when the paper peels from the blanket is large, and the paper hair is severely peeled off.

2) The pressure between the ink plate cylinder and the blanket cylinder is too large. After the blanket cylinder is squeezed, a large frictional sliding occurs, which causes the dot to be deformed to form the ink bar. At this time, the pressure between the printing plate and the blanket cylinder can be adjusted to be in the range of 0.1 to 0.15 mm. In addition, the pressure of the ink roller on the printing plate is too large, the pressure between the ink roller and the ink roller is too large, and ink bars are also generated.

3) The white bar is not able to absorb ink in this area due to the pressure of the water roller on the plate surface.

4) Ghosting When the pressure between the rollers is too large, especially when the pressure between the blanket cylinder and the impression cylinder is too large, the displacement of the blanket is too large. After each transfer, it cannot be completely reset, resulting in a large area. Longitudinal ghosting.

5) When the printing pressure of the imprinting printing and printing is too large or the roller radius is too large, the dots will be elongated into ellipses or lines, and the imprints will expand in directions, resulting in imprinting.

6) The pressure of the stencil with the water roller is too light, or the pressure between the rollers is too large, especially when the pressure between the plate cylinder and the blanket cylinder exceeds 0.15mm, the mutual extrusion force is increased, and the printing plate sand Gradually flattened and reduced its water storage capacity, resulting in paste.

7) Excessive pressure between the plate cylinder and the blanket cylinder increases the friction. At the beginning, it was the expansion of outlets that produced a paste version. After a long time, the bases of the outlets were polished and they started to print.

8) The pressure between the greasy printing plate cylinder and the blanket cylinder is too large, so that the plate grains are polished and the plate surface is shiny, which seriously damages the hydrophilic layer on the plate surface and makes it oleophilic, resulting in greasy.

9) Inverting the hair phenomenon When printing on the ground, lines, or text, burrs occur before and after the edges of the graphic, which is the phenomenon of inverted hairs. In the production process, the padding of the drum is often judged to be correct or not based on the position of the rubbing where the printing plate graphic and the blank portion are connected, and the ink remaining on the surface of the blanket, the chalkiness of the paper, and the accumulation of the paper hair. If the paper hair or paper powder accumulates on the side of the image or mesh, this is the phenomenon of bristling. This indicates that the R>R rubber can reduce the pad of the plate cylinder or increase the pad of the blanket cylinder. If it is printed, paper hair and silt are accumulated on the side of the dot, which is a hair straightening phenomenon. In this case, it can be explained that the printing rubber can be solved by increasing the pad of the plate cylinder or reducing the pad in the blanket cylinder.

10) Escape from paper When the pressure is too high, there is a large friction between the rollers, which is far more than the friction of the back of the blanket. Lining, liner will move down (pull direction) or up (mouth direction).

11) Unilateral outlets Due to the long-term wear of the gears on both sides of the underfill drum, the center distance between the rollers on both sides of the drum will move, resulting in a lower unilateral pressure and a unilateral outlet.

12) The over-printing pressure is so great that the ink penetrates into the back of the print.

In addition to the quality of the printed matter, improper printing pressure can also adversely affect the printing plate's printing rate, printing presses, and blankets. When the printing pressure is improperly adjusted, and the plate cylinder and the blanket cylinder are rolling in each other, the ink layer on the printing plate cannot be broken in the middle, the printing plate surface has no remaining ink layer or the remaining ink layer is thin, the printing plate The graphic base is bare, easily eroded and dampened by fountain solution, and appears "off the plate." In addition, the water roller and ink roller pressure is too large, the printing plate is not tolerant to printing, and the imprint spreads; the pressure of the water roller and plate is too large, the lining loses its elasticity, and the surface friction is increased, which may cause the ink roller to beat, causing frequent impacts. Layout, affect printing plate printing rate. Excessive printing pressure can also cause excessive friction, increase the wear of the roller bearings, and the blanket “creep” prematurely, losing its elasticity and reducing the life of the press.