What happens to the printing press when the paper thickness changes
When printing presses change paper thickness, two main changes occur:
1. The pressure between the rubber cylinder and the impression cylinder has changed, and the original pressure state cannot be maintained. When the paper becomes thinner and thicker, the pressure between the rubber cylinder and the impression cylinder increases; when the paper becomes thinner and thicker, the pressure between the rubber cylinder and the impression cylinder decreases.
2. The size (print length) of the graphic printed on the substrate has changed. When the paper becomes thinner and thicker, the graphic length obtained by the impression roller is relatively elongated; when the paper becomes thinner and thicker, the graphic length obtained by the impression roller is relatively reduced.
In order to make a more accurate adjustment of the printing machine when the paper thickness changes, let's first understand the adjustment of the printing pressure and the relationship between the radius of the two cylinders and the size of the text.
The process of lithographic offset printing is to transfer the graphic ink on the printing plate to the blanket first, and then transfer the rubber ink to the substrate. In this transfer process, the surface of the plate cylinder must be in full contact with the rubber cylinder, the rubber cylinder and the impression cylinder (substrate), that is, there must be a certain contact pressure between the contact surfaces in order to make the ink fully in between. Transfer, this contact pressure is the printing pressure. Drum pressure: the interaction force between the imprint bodies during the printing process.
Printing pressure is the condition for completing offset printing. Whether the pressure between the cylinders is appropriate is the key to ensuring printing quality. Appropriate printing pressure refers to the pressure to reduce the printing pressure between the rollers as much as possible on the basis of ensuring that the ink can be transferred well (that is, the print is strong on the print, the dots are full but not spread, and the tone level is clear and rich). Whether or not the pressure can be reduced (that is, using appropriate pressure) is one of the important indicators for measuring the level of technology, and it is also an important condition for ensuring excellent and high yield.
Excessive or small printing pressure has a great impact on product quality. Excessive pressure can easily cause dot distortion, paste, heavy product ink, and even image distortion. Excessive pressure can also accelerate plate wear and machine wear. Too little pressure can cause insufficient or unstable ink transfer, which can cause imprints, pale ink, unclear image outlines, and inconsistent ink shades.
Method of adjusting printing pressure:
① Adjust the printing pressure by adjusting the center distance of the cylinder;
② Adjust the printing pressure by adjusting the thickness of the lining (the lining of the plate cylinder and the rubber cylinder).
Note: The size of the center distance is related to the meshing status of the drum gear. It should be adjusted according to the actual needs of the printing process. It is not appropriate to change consumables frequently. It must be adjusted within the range indicated by the machine operation and the pressure on both sides must be consistent.
2.The relationship between the radius of the three rollers and the size of the graphic
Transmission of the three cylinders of general offset printing machines: it is driven by three gears with the same number of teeth, modulus, and helix angle. Therefore, regardless of whether the radii of the two rollers are equal, the angular speeds of the three rollers are completely equal.
Generally, the radius of the cylinder after lining should meet the linear velocity of the cylinder surface during printing, keep the rollers rolling without slippage, and ensure that the dots between the printing surfaces overlap accurately after each operation. No distortion or ghosting. If the three rollers are all rigid bodies, as long as the three rollers have the same radius, the surface linear speed of the three rollers can be guaranteed to be equal.
However, the materials covered by the three rollers are not necessarily the same, and the radius of the three rollers after the lining is different. In order to maintain pure rolling and no slippage, the radius of the rubber cylinder lining should be smaller than the radius of the plate cylinder and the impression cylinder, and in principle should be equal to the pitch circle radius.
In order to facilitate the study of the relationship between the radius of the three rollers and the size of the graphics, we consider the three rollers as rigid manroland. It is assumed that the graphic area occupies the 180o position after the version is installed. Now we will study the following:
① If the running diameter of the plate cylinder and the impression cylinder are equal, and the running diameter of the rubber cylinder is relatively small (or larger), because the angular velocity is equal to the layout design, when the printing plate is rotated 180o, the other two cylinders are also rotated. 180o, that is, the graphic area occupies 180o on the impression cylinder. Because the running diameter of the rubber cylinder and the impression cylinder are equal, the length of the imprint obtained on the surface of the plate cylinder and the surface of the impression cylinder is the same (the same image does not change).
Conclusion: When ignoring the deformation of the graphic caused by the elastic deformation of the rubber roller, the size of the rubber roller's running diameter does not affect the length of the graphic print, but it will affect the print quality.
② If the running diameter of the impression cylinder is smaller than the diameter of the plate cylinder, when the plate cylinder is rotated 180o, the other two cylinders are also rotated 180o, that is, the graphic area occupies the position of 180o on the impression cylinder. The running diameter of the printing cylinder is smaller than the diameter of the plate cylinder, and the actual length of the graphic obtained by the impression cylinder is relatively reduced (the graphic is shortened).
Conclusion: When the running diameter of the impression cylinder is smaller than the diameter of the plate cylinder, the picture and text are shortened.