Analysis: The Influence of Offset Printing Pressure on Print Quality
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In the offset printing process, each adjacent two rollers of the three rollers are in contact with each other for transfer. In the printing process of printing plate, rubber and embossing, in order to obtain the normal amount of ink on the printing paper, the continuously adjusted image, characters and lines of the graphic part are reproduced, and the halftone dots are reproduced. In addition to the contact of the two rollers, these prints and patterns must be additionally subjected to the necessary printing pressure. The printing pressure is light and heavy, and the degree of mastery is accurate. It will directly determine the transfer and printing dots. The degree of pros and cons of the reproduction.
The offset printing feature is a unique printing method using indirect elastomer. The letterpress printing is the printing method in which the printing plate is directly in contact with the paper. The graphic conversion of the offset printing is the electro-chemical film of the reverse image and the sulfuric acid paper output by the laser printer. After imposition, the plate version of the printing machine shows a positive image. The positive image on the printing plate is just like the transfer to the indirect elastic blanket. The ink layer of the blanket cylinder v is pressed. The transfer to the paper under the action of the printing cylinder is positive. Therefore, the pressure between the three rollers is generated in the gap between the rollers, so that they are deformed by the thickness greater than the gap, which is the source of the offset pressure, that is, the compression deformation value of the blanket cylinder liner. A force that is needed.
What kind of printing pressure is the ideal pressure? The pressure should be based on the premise that the printed product has a strong dot, a clear picture, a bright color and a light shade, and the smaller the better, the better.
If the printing pressure is too large or too small, the quality of the printed product will be reduced. And if the printing pressure is too large, the more ills will be caused.
When the printing pressure is too high, it will cause:
1. The graphic lines are distorted and the print is enlarged.
2. The ink is partially "overfilled", resulting in uncoordinated color tones.
3. Paper is pulled.
4. The main pressure-bearing parts and transmission parts of the printing press are greatly deformed, which affects the working life of the parts.
5. Accelerated plate wear compared to normal pressure and friction conditions.
6. The dots of the imprint are easy to expand; the prints are unclear and the durability of the blanket is affected.
7. The printing pressure is high, which will reduce the printing durability of the printing plate.
8. Due to the increase in machine load, the power consumption will be much higher.
When the printing pressure is too small, it will appear:
1. The transfer of printed graphics is not complete.
2. The outlets are not real and the color is gray.
3. Make the original defects and problems of printing plates, linings, papers and printing presses more obvious.
The size of the printing pressure can be adjusted, and the adjustment means is
1. Roller center distance adjustment method: By adjusting the center distance between the two rollers to increase or decrease the printing pressure, this is the most common and most important adjustment method.
2. Lining thickness adjustment method: 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 absolutely necessary.
It should be noted that it is easy to increase or decrease the thickness of the blanket of the blanket cylinder, which often causes the diameter of the drum to change, which causes the circumferential speed to be different. The result will inevitably lead to the deformation of the dot and the inaccuracy of the overprint, which may result in "ink". The phenomenon of "bars". There are two main ways to adjust the pressure between the rollers: adjusting the center distance between the rollers, thickening and thinning the printing paper and the backing paper under the blanket. As far as China's current situation is concerned, most printers and operators are willing to use the latter to meet the requirements of imprinting force.
Print 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 is expressed by line pressure, kg/cm.
The unit length is expressed by the line pressure, which is the total pressure of the two rollers in contact with the pressure, that is, the pressure acting on the bearing is divided by the effective length of the drum, that is, the average pressure per unit length is obtained. This method has limitations and does not correctly represent the pressure distribution across the width of the contact zone, and is difficult to measure and actually use.
3 The pressure of printing is usually expressed by the "compression amount".
The amount of compression is the sum of the radii of the two-phase roller, plus the difference between the thickness of the pad 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, really can
The ideal basis for fully expressing the printing pressure is the amount of compression deformation of the blanket cylinder liner. That is, the amount of compression deformation of the blanket cylinder liner is proportional to the pressure it receives.
The ideal printing pressure, there is currently no quantitative. It is not a fixed concept, but should be properly 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.
The pressure between the blanket cylinders is about 0.5-0.10 mm, and the ideal pressure is generally controlled to be about 0.15-0.25 mm in production practice. have to
Emphasize that the ideal printing pressure is in the case of compression deformation of a certain printing surface,