Offset printing cylinder impression

2.2 The impression cylinder

The impression cylinder is the most important roller in the printing unit. Its role is:

1 support paper, forming pressure to complete the printing.
2 Pass the paper.
3 is the benchmark for installing and debugging the printing unit.

Through the role of the impression cylinder, we can see its important position. The level of precision of the surface of the impression cylinder directly determines the height of the stencil. If the accuracy of its surface is low, the dots will become imaginary and deformed. The accuracy of the transfer of the impression cylinder directly determines the overlay accuracy of the printed product. Therefore, it must always be stable during the printing process. The time and displacement of the impression cylinder directly affect the time and displacement of the other two cylinders. In a nutshell, there are two requirements for the impression cylinder:

(1) The surface has high processing accuracy, and it can be consistent at work. When the impression cylinder is shipped from the factory, its surface needs to be ground. Under conditions, the surface of the drum needs to be treated to prevent it from being corroded by the acid and alkali to destroy its accuracy.

(2) The operation is accurate and stable. All drums are statically balanced before they leave the factory, and they can also be balanced by conditions. The goal is to first eliminate the imbalance. Choose high-quality materials, make the drum with high rigidity and strength, resist impact and vibration, and optimize the design to minimize the stress. The accurate and smooth operation of the impression cylinder can create good conditions for the balance operation of the other two rollers.

2.2.1 Analysis of the movement of paper on the impression cylinder

What does the movement of paper in this part rely on? Is it relying on the pressure of the dental chip? This concept is incorrect. Paper from the registration roller to the impression cylinder, in order to ensure the accuracy of overprinting, it must maintain its original positioning accuracy on the roller. On the roller, as long as the position of the front mouth of the paper is always the same, its positioning accuracy will not be destroyed. The tooth chip and the tooth pad clamp the paper so that the front of the paper does not appear to have a good shape, even if it is in a horizontal plane. Therefore, the pressure of the dental sheet is such that the positional relationship of the paper in the radial direction of the drum does not change. So how can we guarantee the position of the paper before and after it? If you want to change the relationship between the front and the back, you must have the force that hinders the relative movement of the paper and the drum. No obvious pulling force or thrust can be seen from the roller. The only force that can impede the relative movement of the paper can only be friction. There are two conditions for generating friction: pressure and friction coefficient. The pressure (n) of the pad is only one of the conditions. The friction coefficient (u) on the surface of the pad has an important influence on the formation of the friction force, ie f=n·u is the maximum static friction that hinders the relative movement of the paper on the surface of the roller. force. Above this value, the paper will slide relative to the surface of the roller; below this value, the paper will remain relatively stationary with the surface of the roller. In normal work, the frictional force that prevents the relative motion of the paper must be less than its critical value. The pad wear in actual work is one of the most powerful arguments of friction drive.

There are two ways to make the paper and roller do not move relative to each other. One is to increase the pressure on the die. Increasing the pressure of the tooth chip can increase its critical value f, but the greater the pressure of the tooth chip, the greater the impact of the open and close teeth, so the f is limited by the pressure adjustment of the tooth chip; the second is to increase the friction coefficient. Increasing the friction coefficient to increase the critical value f is a very effective measure. At present, many manufacturers have made a fuss about increasing the friction coefficient.

2.2.2 Analysis of the role and requirements of impression cylinder in transferring paper

The accuracy of the transfer of the impression cylinder must be absolutely guaranteed, which requires that its critical value f can always maintain the relative stationary state of the paper. Since the impression cylinder needs to “peel” the paper from the blanket cylinder, the magnitude of this force is related to the graphic distribution of the surface of the printing plate, but in the design, it can only be calculated on the full version. This requires that there must be sufficient friction at any point along the axis of the drum. According to the above analysis, from the perspective of transferring paper, there are the following requirements for the impression cylinder:

1. All tooth pads have the same level. This is a prerequisite for the positioning of the paper on the roller. Therefore, the pad of the impression cylinder cannot be adjusted or replaced at will. In general, the impression pad of the impression cylinder is made not adjustable. At the same time, after the pad is installed, the horizontal direction needs to be leveled with a grinder, and the horizontality error is usually required to be within 0.3 mm.

2. All tooth pads have the same pressure. This is one of the conditions for ensuring that the paper does not slide relative to any place on the roller. At the same time, the pressure is consistent and stress concentration can also be prevented. Otherwise, when the pressure is inconsistent, the force of the individual spring is too large, and it may be ejected during the process of opening and closing the teeth, resulting in an accident of the machine.

3. All tooth pads have the same coefficient of friction. This is another prerequisite for ensuring that the paper does not slide relative to the drum. To ensure this prerequisite, it is required that the surface of the tooth pad is not damaged, but often the grinding amount of all the tooth pads is not the same when grinding on a grinding machine. Some are much worn and some are worn less because there is inconsistency in the level of the tooth pads and there is an error, which may make the friction coefficient of the surface of the tooth pad inconsistent. This is a prerequisite for ensuring that the printed product does not exhibit partial ghosting, slippage, or pleating.

4. The opening and closing teeth of all tooth pads are consistent. If the opening and closing teeth of the dental chip do not move in a consistent manner, there may be some friction between the teeth and the tooth pad, while others may not. This may cause uneven force on the front mouth of the paper and cause printing failure. In summary, to ensure the accurate transfer of paper on the surface of the impression cylinder, these requirements should be basically met, so this part must be checked regularly. The way to check is:

1 Use a dial gauge to check the level of the pad and see if it is the same level.
2 Visually inspect the indentation of the pad on the surface of the pad with an offset paper and see if the indentation is even.
3 The time to open and close the teeth is measured visually to see if the movements are consistent.