The Effect of Offset Printing Pressure on the Quality of Printed Products

In the offset printing process, each adjacent two of the three cylinders are in contact with each other to perform the transfer function. In the process of printing plate, eraser and embossing three rollers, in order to get the normal amount of ink transferred on the printing paper, the continuously adjusted images, characters, and lines of the graphic part are faithfully reproduced; halftone dots are reproduced. In addition to the contact between the two rollers and the surface of the rollers, the corresponding pressure must be additionally applied. The pressure of the printing is light and heavy, and the degree of adjustment is accurate. It will directly determine the graphic transfer and printing dots. The degree of reproduction of the pros and cons.

Offset printing is characterized by the unique printing method of indirect elastomers. Toppan printing is a printing method in which the printing plate is directly in contact with the paper. The graphic conversion of offset printing is the sulfuric acid paper output from the negative electricity of the negative image and the laser printer. , After the imposition, the printing machine sunning PS version presents a positive image, the positive image on the printing plate is being transferred to the indirect elastic body blanket showing the reverse image, the image layer of the ink layer of the rubber roller v, in the pressure The role of the printing cylinder is the positive image transferred to the paper. Therefore, the pressure between the three rollers must be in the clearance of the roller, so that they increase the thickness of the elastic body by adding a thickness greater than this gap. This is the source of the offset pressure, that is, the compression value of the blanket lining. Required force.

What kind of printing pressure is the ideal pressure? The pressure should be based on the premise that printed materials have strong dots, clear graphics, bright colors, and adequate darkness, and the smaller the application, the better.

If the printing pressure is too large or too small, the quality of the printed product will be reduced. And the pressure of printing is too great, and the more serious problems it brings.

When printing pressure is too high, it will cause:

1. Graphic lines are distorted and the prints are enlarged.
2. The ink is “overfull”, resulting in inconsistent hue in the prints.
3. The paper is pulled.
4. The main pressure parts and transmission parts of the printing press are greatly deformed, affecting the working life of the parts.
5. Accelerates plate wear compared to normal pressure and friction conditions.
6. The print dot is easy to expand; the print level is unclear, and the durability of the blanket is affected.
7. Printing pressure will reduce the printing plate's resistance to printing.

8. As the machine load increases, the power consumption will also be more.

When the printing pressure is too small, it will appear:

1. The transfer of printing graphics is not complete.

2. The dot is not true, and the color is grayish.

3. Make the original defects and defects of the printing plate, lining, paper and printing machine more exposed.

The size of the printing pressure can be adjusted, and the adjustment method is

1. Roller center distance adjustment method: By adjusting the center distance between two rollers to increase or decrease the printing pressure, this is the most common and the most important adjustment method.

2. Lining thickness adjustment method: To increase or decrease the printing pressure by increasing or decreasing the thickness of the drum liner, this is a method that can only be used if it is a last resort.
It should be noted that easily increasing or decreasing the lining thickness of the blanket can often cause the diameter of the cylinder to change, resulting in an unequal peripheral speed. The result will inevitably lead to inaccurate dot deformation and overprinting. "The phenomenon of"

As mentioned earlier, there are two main ways to adjust the pressure between the rollers: Adjust the center distance between the rollers, thicken and thin the backing paper below the plate and the blanket. Judging from the current situation in China, most printers and operators are willing to use the latter to meet the requirements of imprinting force.

Printing pressure assessment

1 is expressed in kg/cm2. (often called surface pressure)

Its advantage is that it can reflect the distribution of contact pressure, but it is more difficult to master and measure in actual production.

2 With line pressure, kg/cm.

The force per unit length is represented by the linear pressure, which is the total pressure of the two rollers in contact with the pressure, that is, the pressure acting on the bearing divided by the effective length of the roller, that is, the average pressure per unit length. This method has limitations and does not accurately represent the pressure distribution over the width of the contact zone, and it is difficult to measure and actually use it.

3 The pressure of printing is often expressed as "compressed amount".

The amount of compression is the sum of the radii of the two-phase rollers, plus the difference between the thickness of the pad and the like and the actual center distance of the two rollers.

The way to increase or decrease the printing pressure by changing the center distance between the two rollers is called pressure regulation. In fact, the ideal basis to truly be able to fully express the printing pressure is the size of the compression deformation of the blanket cylinder liner. That is, the compression deformation of the blanket cylinder liner is proportional to the pressure to which it is exposed.

The ideal printing pressure, there is no one quantitative. It is not a fixed concept, but it should be adjusted according to various objective printing conditions on the premise of favorable product quality, and the pressure between the impression cylinder and the blanket cylinder is greater than that of the plate cylinder and the blanket cylinder. The pressure between the two is about 0.5-0.10mm. In production practice, the ideal pressure is generally controlled at about 0.15-0.25mm. The ideal printing pressure must be emphasized. That is, under the condition that the printed surface is compressed and deformed, the imprint of the graphic part of the printing plate is sufficiently strong on the paper, and the minimum pressure is evenly applied on the basis of the clear and complete dot.

(Wang Xincheng)

Reprinted from: ("China Packaging News·Printing Weekly")