Blanket and pad application problems in offset printing (20-29)

20. What is the rolling contact arc?

When two adjacent rollers are pressed together, there is no contact if there is no squeeze deformation. Only the squeezing deformation, there is contact, the greater the squeezing force, the tighter the contact. Therefore, there is the existence of squeezing deformation values ​​to generate contact arcs. The blanket is squeezed by the rigid plate cylinder and the impression cylinder, and the blanket is compressed and deformed. At the contact between two adjacent rollers, there appears a compressed compression band with a certain width, which is referred to as a contact arc in printing: a printing plate, a pressure-compressing belt of a simple rubber roller, a rubber roller, and a pressure-compressing belt of an impression roller. .

When the extrusion deformation value is constant, the larger the roller radius, the wider the contact embossing arc; the smaller the roller radius, the smaller the contact embossing arc width.

twenty one. How to distribute the squeezing deformation value when the buckling pressure is?

The pressure of two adjacent rollers is squeezed under the effect of additional extrusion to form a pressure. However, when balancing the drum diameter, there is a distribution problem in the extrusion deformation value. The diameter of the three rollers must be equal and the surface line speed can be consistent. However, when the three cylinders are pressed together, the diameters of the plate and the impression cylinder are changed due to the compression deformation, and the diameter of the blanket cylinder becomes smaller. In order not to make the rubber reduction diameter too small, or to say that there is too much difference between the diameter of the other two cylinders after the blanket cylinder is pressed, there is a problem of distribution of the extrusion deformation value. The extrusion deformation is previously assigned to the other two rollers, that is, the diameters of the three rollers before the closing are not equal. After the pressing, they are still not equal. How is not equal is good? To verify the rationality of this distribution, two points must be met. in principle:

First, we must obtain the same speed between the roller surface radius of the roller, the dot area is not deformed, to achieve the best pressure;

Second, we must obtain the balance of the drum radius, no change in arc length, so that overprint graphics accurate.

From the lithographic offset process, depending on the structure, type, and conditions of the lithographic machine, there are generally two distribution methods: equal-diameter distribution and inequality distribution.

twenty two. What is the equal diameter distribution method?

The squeezing deformation value is distributed proportionately, the purpose is to ensure that the three drum radius is equal, resulting in the same linear velocity of the roller surface particle, which is called equal-diameter distribution method. Equal diameter distribution method for...

Take the deformation value 1/3 of the extrusion and distribute it to the plate cylinder;

With a squeeze deformation value of 2/3, it is allocated to the squeegee.

This assigns a specific value of the pressing deformation variable of the blanket to one third of the printing plate roll, so that the radius of the three rollers is more nearly equal, which is favorable for the line speed, and the effect on the domestic machine is better. The main advantages are:

(1) Various failures due to excessive friction due to poor roll speed are avoided;

(2) It is helpful to improve the printing plate's resistance to printing force, which has universal significance for improving the printing plate's resistance to printing force;

(3) It will help improve the quality of printed materials, maintain the integrity of outlets, and have a clear hierarchy, especially for the correct reproduction of higher nets, so as not to deform the outlets.

(4) The dimensional change of the image arc length does not occur, and overprinting is guaranteed to be accurate, and often the graphic size of the finished paper is slightly larger than that of the plate.

twenty three. What is the method of distribution of inequality?

The method of distribution of inequality is also called the theory of distribution of large, middle and small schools. The arguments for this doctrine are: the radius of the printing plate is the largest, the radius of the embossing roller is the second, and the radius of the blanket roller is the smallest. The specific allocation method is:

First, the entire compression value of the extrusion deformation value is distributed to the plate cylinder.

This method of distribution has not been widely adopted in China. Its advantages are:

(1) Adjust the thickness of the printing plate to adapt to the change in the size of the printing paper caused by the change in thickness of the printing paper, and to achieve accurate overprinting;

(2) It can maintain the original size of printed graphic (the same size as the original graphic). At the same time, it can make up for the adjustment effect of the paper's dimensional change caused by the absorption of moisture or the elongation after embossing.

(3) Maintaining a solid and clear dot network and a small deformation value of the dot expansion;

(4) It is helpful to improve the printing plate's resistance to printing force.

twenty four. Why is there a speed difference and friction between rollers?

The pressure distribution in the contact arc of the roller is uneven (because it is a circle with a round pressure, and the pressure of the straight lines of the two centers is maximum). From this microscopic point of view, the line speed of the roller is not absolutely equal, but is a relative balance, to a certain extent. The slight difference in speed is always present. In addition, there are several types of distribution of the three-roller pressure deformation value (two types have already been introduced in this lecture). However, no matter which type, the final conclusion is that the three-roller radius will not be equal ( Although very small, as the printing time grows and accumulates, it can reflect the cumulative amount of speed difference.

25. What is the concept of friction between rollers?

There is a speed difference and friction in the roller contact arc. The physical meaning of friction is the force that hinders motion when a moving object comes into contact with another surface.

The friction between the rollers only occurs on pressure belts of a few millimeters or even more than ten millimeters. Each dot of the embossing tape follows the principle of deformation of the blanket, under pressure, the force in the middle of the pressure bandwidth. Pointing to the center of the blanket cylinder becomes a positive pressure on the blanket cylinder and plays the role of the transfer of the ink in the image and drawing; the two forces on the edge of the pressure belt decompose two forces and then follow the direction of the tangent of the printing plate to make the eraser. The cloth is subject to tangential friction and acts as a rolling friction between the blanket and the plate and the impression cylinder. The conclusion is that the greater the roller pressure, the greater the frictional force. Therefore, due to the presence of pressure, speed difference, and radius lining during the pressing of the roller, the tangential friction force cannot be equal.

26. What is the frictional force when the plate cylinder radius is smaller than the radius of the blanket cylinder?

When the plate squeezing radius is smaller than the radius of the blanket cylinder, the blanket is subjected to the pressure of the slewing middle wave cylinder, and the resulting elastic deformation of the extrusion elongation is called extrusion deformation. The rubber is deformed under pressure to extend the deformation of the rubber blanket. Due to the action of the grinding control force, the edge of the contact arc pushing against the reversal direction of the roller is piled up into a bulge shape. This deformation is commonly referred to as a bulge in printing.

The analysis of serious bulge and bulge bags is very valuable for dealing with certain accidents in printing.

The reason for the bulge pack is that when the radius of the plate cylinder is smaller than the radius of the blanket cylinder, the linear velocity of the surface of the printing plate cylinder is less than the linear velocity of the surface of the blanket cylinder. Then, the blanket cylinder must be pushed against the surface of the plate cylinder that runs more aggressively. The rolling friction force (because of the same angular speed of the two rollers, the rubber roller does not allow the plate cylinder to slow down). Faster rubber cylinders - rolling on the sides and the surface of the plate cylinder, while also sliding forward, removing the fast line speed by continuously sliding forward and keeping each revolution - week, plate cylinder , rubber roller surface quality of the coincidence. Since the plate cylinder has a small radius, it only fluctuates when it comes into contact with the blanket cylinder. While the rubber roller has a large radius and a large linear velocity, it needs to eliminate the higher linear velocity. In addition to the simple surface rolling of the printing plate, The front slides, so that the deformation of the blanket toward the front area reduces the force and shifts backward, creating a bulge.

27. What is the friction force when the plate cylinder radius is larger than the radius of the blanket cylinder?

When the plate cylinder radius is larger than the radius of the blanket cylinder, the blanket is subjected to the extrusion roller elongation due to the action of the roller pressure during operation. Due to the reverse friction force, the extension blanket is twisted toward the roller in the direction of rotation and deformation, which is called submarine deformation. .

Because the linear speed of the surface of the plate cylinder is greater than the linear speed of the surface of the blanket cylinder, the plate cylinder will produce a pushing rolling friction force on the surface of the rubber cylinder. This pushing force causes the blanket to be piled on the front line of the contact arc. , called bulging package.

28. What is the direction of friction between rollers?

Controlling the direction of the frictional force on the surface, we can infer from various phenomena the speed between the scrolls, or the size of the radius from the drum lining, predict the various phenomena that occur from the speed difference, and can observe a roller's Friction direction, inferred the direction of friction of the other roller.

The direction of sliding friction is always the opposite of sliding.

(1) When the linear speed of the blanket cylinder is greater than the linear speed of the surface of the printing cylinder, the surface slides in a clockwise direction (the person stands on the operation surface of the machine and observes the running direction of the blanket cylinder). The direction of friction.

(2) When the radius of the plate cylinder is larger than the simple radius of the rubber roller, the surface sliding direction is counterclockwise, and the friction force on the surface of the plate cylinder will be the reverse direction. The general conclusion:

(3) When the plate roll radius is greater than the rubber roll radius

"Indian" direction is reversed,

The "rubber" friction direction follows the direction of rotation.

Blanket deformation, front bulge;

When the plate cylinder radius is less than the radius of the blanket cylinder

"Print" friction direction along the direction of rotation,

"rubber" friction direction reversed direction

Deformation of the blanket, bulge;

The friction between the two rollers' surfaces is the same, the direction is opposite, and they are generated at the same time and disappear.


29. What is the rubbing direction of the imprinting roller and rubber roller?

This group of rolling under the pressure, there are also friction and its direction, but with the plate - rubber roller there is a certain difference.

(1) The speed difference and friction force when the radius of the impression cylinder is larger than the radius of the blanket cylinder: the linear speed of the impression cylinder is greater than the linear speed of the blanket cylinder, and the direction of the friction force of the impression cylinder is the direction of rotation of the counter cylinder, resulting in speed Poor, part of the paper may be eliminated in the radial compression, friction force is buffered. The paper in the embossing is controlled by the roller bite and the bite force. When the frictional force acts, the paper will be stretched. If the sensitive elastically deformed paper, the extrusion will disappear as the force disappears; the plastic deformation becomes larger. After the paper is squeezed, it cannot be widened again.

(2) When imprinting the speed difference and friction force when the radius of the simple radius is smaller than the radius of the blanket cylinder: the linear speed of the surface of the impression cylinder is less than the linear speed of the surface of the blanket cylinder, and the direction of the friction force of the impression cylinder is the direction of rotation of the rotary cylinder. The coefficient of friction of the blanket is much larger than that of steel, and the slipper can be eliminated by the friction of the back surface of the paper and the surface of the impression cylinder. At this time, there are no significant influences on the shape and dot shape of the printed sheet. Therefore, if the thickness of the pad thickness of the plate cylinder is moved into the blanket cylinder during the process, the width of the graphic text will vary. However, adding the same thickness to the blanket cylinder alone does not have as much variation in the size of the graphic shape.

Source: Guanhua Screen Printing