Discussion on the most popular offset printing pre

As we all know, printing is inseparable from pressure. One of the most important factors in the printing process is printing pressure, which is the key to smoothly transfer the ink on the printing plate to the substrate. Offset pressure is measured by elastic deformation (compression) of the blanket. In offset printing production, the deformation of elastomer is formed by adjusting the gap between two adjacent rollers, which is the source of offset printing pressure

1 Relationship between printing pressure and ink transfer

during the printing process, keep the printing pressure stable. To adjust the roller pressure regulating mechanism, we should avoid the change of printing pressure caused by the change of the clearance of the pressure regulating mechanism, which will affect the ink transfer rate and make the ink color of the printed matter unbalanced

the relationship between pressure and ink supply can be divided into many segments. The AB segment indicated by the dotted line can be called "insufficient ink" segment. This is because within the range of p0-p1, the printing pressure can not make the distance between the printing surfaces small enough for the ink and solid to attract each other. The imprint cannot be completely copied, and there is a "void" phenomenon

in BC section, starting from point B, the ink transfer rate increases significantly with the increase of pressure, which can replicate the imprint better. That is, under P1-P2 pressure, the imprint with unsmoothed contour can be printed. However, the roughness of the paper often changes, so it is difficult to ensure the same thickness of the ink layer, and some impressions will have the phenomenon of "emptiness". Generally, on the BC section, the ink supply varies with the pressure (the thickness of the ink layer that often changes cannot exceed the normal thickness of printing), and the ink supply of the printing plate is in direct proportion to the pressure. Therefore, the BC section is called the ink proportional transfer section, and P1-P2 is called the proportional transfer pressure

in the CD section, the pressure changes within P2-P3, but the ink supply is approximately the same, which can print unsmoothed impressions under certain objective conditions and maintain the same ink volume between printing sheets. At this time, we can get the careful promise of Jinan Shijin with relatively stable printing quality: printed matter

in the de section, due to excessive pressure, the imprint spreads to the non graphic part, and the ink transfer tends to decrease. Because the ink spreads under pressure, the point is deformed, the ink layer in the middle of the imprint is thin, and the ink layer on the edge is thick. At this time, the imprint is out of shape, and the replication effect becomes worse

when the pressure exceeds e, the point deforms sharply and the image and text are distorted. At this time, the ink transfer amount increases slightly with the increase of pressure. Under the action of excessive pressure, the ink spread more seriously, increasing the final area of ink; The point spreading and merging are serious, which greatly reduces the replication effect. Therefore, we call P2 the process necessary pressure, P3 the appropriate pressure, and P4 the critical pressure

2 the influence of inappropriate pressure on printing quality

in actual production, if the printing pressure is inappropriate, the quality of the printed product is bound to be low. 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 printing sheet is light and unclear, and the field is uneven; If the printing pressure is too high, the printing plate will wear out quickly, and the printing resistance of the printing plate will drop sharply. Various new materials continue to appear. Moreover, the ink spreading is severe, and the image and text points are deformed, which makes the levels of the middle tone and the dark tone part level, the tone reproduction and color reproduction of the printing crystal are not ideal, and the image is distorted. The main faults caused by improper printing pressure are as follows

1) paper stripping

when the printing pressure is too high, the paper surface friction increases, the separation power of paper stripping from the blanket is large, and the paper wool is seriously stripped

2) ink bar

the pressure between the printing plate cylinder and the blanket cylinder is too large, and the blanket cylinder will produce large friction and sliding after being extruded, resulting in point deformation and the formation of ink bar. At this time, the pressure between the printing plate and the blanket cylinder can be adjusted so that sepiolite as a needle reinforced filler has both incremental and modified effects, which is within the range of 0.1 ~ 0.15mm. In addition, the inking roller has too much pressure on the printing plate, and the pressure between the inking roller and the inking roller is too large, which will also produce ink bars

3) white bar

the ink cannot be adsorbed in this area because the landing roller has too much pressure on the layout

4) ghosting

when the pressure between the rollers is too large, especially when the pressure between the blanket roller and the embossing roller is too large, the blanket displacement is too large. After each transfer, it cannot be completely reset, resulting in a large area of longitudinal ghosting

5) imprint extrusion

when the printing pressure is too large or the roller radius is too different, the points will be stretched into ellipses or lines, and the imprint will expand in a direction, resulting in imprint extrusion

6) paste plate

the pressure of 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 pressure is increased, and the sand mesh of the plate is gradually ground down, reducing its water storage capacity, resulting in paste plate

7) the pressure between the plate cylinder and the blanket cylinder is too high, which increases the friction. At first, it is a point to expand and produce a paste version. After a long time, it is a point to polish the foundation, and then it begins to spend the version

8) greasy

the pressure between the printing plate cylinder and the blanket cylinder is too large, which makes the printing plate sand mesh polished and the layout shiny, seriously destroying the hydrophilic layer on the printing plate surface and making it lipophilic, resulting in greasy

9) down fluffing phenomenon

when printing field, line or text, burrs will appear before and after the edge of the text, which is down fluffing phenomenon. In the production process, we often judge whether the liner of the drum is correct or not according to the friction and bright position of the connection between the printing plate text and the blank part and the accumulation of residual ink, paper powder and paper wool on the surface of the blanket. If paper wool and paper powder accumulate on the side of graphics and text or point bite, this is the phenomenon of fuzzing. At this time, it shows that R printing> R rubber, which can reduce the pad of the printing plate cylinder or increase the pad of the blanket cylinder. If the paper wool and powder accumulate on the side of the point trailing tip during printing, it is a fluffy phenomenon. At this time, it shows that printing> R rubber can be solved by increasing the pad of the printing plate cylinder or reducing the pad in the blanket cylinder

10) escape paper escape

when the pressure is too high, there is a large friction between the rollers, which is far more than the friction on the back of the blanket. The lining and lining paper will move downward (in the direction of trailing the tip) or upward (in the direction of biting)

11) one side point is not full

due to long-term wear of the gears on both sides of the drum, the center distance on both sides of the drum will move on one side, resulting in light pressure on one side, resulting in one side point is not full

12) through printing

the pressure is too high, causing the ink to penetrate into the back of the printed matter

in addition to the quality of printed matter, improper printing pressure will also have a negative impact on the printing plate durability, printing machine and blanket. When the printing pressure is not adjusted properly, the printing plate cylinder and the blanket cylinder are rolling each other, the ink layer on the printing plate cannot be broken in the middle, and there is no residual ink layer on the graphic part of the printing plate surface or the residual ink layer is very thin, the graphic Foundation of the printing plate is exposed, which is easy to be worn and eroded by the fountain solution, resulting in "plate loss". In addition, the pressure of water roller and ink roller is too large, the printing plate is not resistant to printing, and the imprint is spread; The pressure between the water roller and the printing plate is too large, the lining loses elasticity, and increases the surface friction, which will cause the ink roller to jump, making it frequently hit the plate, affecting the printing plate durability. Excessive printing pressure will also cause excessive friction, increase the wear of roller bearings, and the rubber blanket will "creep" early, lose elasticity, and reduce the service life of the printing machine