What changes will occur to the printing press when the paper thickness changes
When the paper thickness of the printing machine changes, there are two main changes:
1. The pressure between the rubber drum and the impression drum has changed, and the original pressure state cannot be maintained. When the paper becomes thinner, the pressure between the rubber roller and the embossing roller increases; when the paper becomes thinner, the pressure between the rubber roller and the embossing roller decreases.
2. The size (imprint length) of the image and text printed on the substrate has changed. When the paper changes from thin to thick, the length of the image and text obtained by the imprinting roller is relatively extended; when the paper changes from thick to thin, the length of the image and text obtained by the imprinting roller is relatively reduced.
In order to adjust the printing press accurately when the paper thickness changes, let's first understand the adjustment of the printing pressure and the relationship between the radius of the two rollers and the size of the image and text.
1. Drum pressure
The process of offset printing is to transfer the graphic ink on the plate to the blanket first, and then to the substrate from the rubber. In this transfer process, the surface of the plate cylinder and the rubber cylinder, and the rubber cylinder and the imprinting cylinder (substrate) must be in full contact, that is, there must be a certain contact pressure between the contact surfaces, so that the ink can be fully transferred between them, the contact pressure is the printing pressure. Cylinder pressure: the mutual force between the stampings in the printing process.
Printing pressure is the condition to finish offset printing, and the key to ensure printing quality is whether the pressure between rollers is appropriate. Appropriate printing pressure means that the printing pressure between rollers can be reduced as much as possible on the basis of ensuring a good transfer of ink (that is, to ensure that the printed matter is firmly imprinted, the dot is full but not spread out, and the gradation is clear and rich). Whether the pressure can be low (i.e. using appropriate pressure) is one of the important indicators to measure the level of technology, and also an important condition to ensure good and high yield.
Too much or too little printing pressure has a great influence on product quality. Too much pressure is easy to cause dot deformation, paste, thick product ink color, even image distortion and other colors; too much pressure will accelerate plate wear and machine wear. Too small pressure will cause ink transfer volume is insufficient or unstable, resulting in impression emptiness, light ink, unclear image outline, inconsistent ink depth, etc.
How to adjust the printing pressure:
① Adjust the printing pressure by adjusting the center distance of the drum;
② The printing pressure can be adjusted by adjusting the thickness of the lining (plate cylinder, rubber cylinder lining).
Note: the center distance is related to the meshing condition of the drum gear. It should be adjusted according to the actual needs of the printing process. It is not suitable to change the consumables frequently. Moreover, it must be adjusted within the range indicated by the machine operation and use, and the pressure on both sides must be consistent.
2. The relationship between the radius of three rollers and the size of pictures and texts
General offset press three drum drive: by three teeth, module, spiral angle of the same gear mesh drive. Therefore, whether the radius of the two rollers is equal or not, the angular velocity of the three rollers is completely equal.
In general, the drum radius after lining shall meet the requirement that the linear speed of the drum surface is equal in the printing process, that there is no sliding rolling between the rollers, and that each particle between the printing surfaces is accurately overlapped after each operation; the dot is not deformed or duplicated. If the three rollers are rigid bodies, as long as the radius of the three rollers is equal, the surface linear velocity of the three rollers can be ensured to be equal.
However, the materials covered by the three rollers are not necessarily the same, and the radius of the three rollers after lining is also different. In order to keep pure rolling and no sliding state, the radius of rubber drum after lining should be less than the radius of plate drum and imprint drum, 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 image and text, we regard the three rollers as rigid Manroland, assuming that the image and text area accounts for 180o after the installation of the version, now we will study the following situations:
① If the running diameter of the plate cylinder and the embossing cylinder is the same, and the running diameter of the rubber cylinder is relatively small (or large), because the angular speed is equal to the layout design, when the plate is rotated 180o, the other two rollers also rotate 180o, that is, the image and text area occupies the same position of 180o on the embossing cylinder. Because the running diameter of the rubber drum and the imprinting drum is equal, the imprint length on the surface of the plate drum and the imprinting drum is equal (there is no change in the text).
Conclusion: the diameter of the rubber drum does not affect the length of the image imprint, but it will affect the printing quality.
② If the running diameter of the imprinting cylinder is smaller than that of the plate cylinder, when the plate cylinder turns 180o, the other two rollers also turn 180o, that is, the image and text area occupies the same position of 180o on the imprinting cylinder. Because the running diameter of the imprinting cylinder is smaller than that of the plate cylinder, the actual image and text length of the imprinting cylinder is relatively reduced (the image and text are shortened).
Conclusion: when the diameter of the printing cylinder is smaller than that of the plate cylinder, the image and text are shortened.