Offset printing is indirect printing, using the elastic body of the rubber cylinder to transfer the graphics on the plate cylinder to the printed paper under the action of the printing pressure of the impression cylinder. The printing speed of the offset printing machine is related to the printing pressure. The impact is very large. Many printing examples show that when the printing speed is greatly increased, the printing pressure will also increase. This is because the increase in printing speed inevitably reduces the contact time between the corresponding points between the printing plate and the blanket, and between the blanket and the paper to be printed. This reduction in time means that there is an increased degree of incomplete contact between the printed surfaces, which is very dangerous.
In order to effectively avoid the incomplete transfer of the graphic dots caused by the incomplete contact between the printing surfaces during offset printing, the dots and the graphic are not strong, the version is off, and the ink is gray, etc., you must use a large printing pressure . In this way, not only can the ink be transferred from the ink supply surface to the ink receiving surface faster, but also the ink can complete its ideal transfer process in a shorter time.
The ideal pressure is not static, so when deciding the many factors that affect the pressure, it must be carefully analyzed according to the actual situation in the printing process.
In recent years, the printing speed of offset printing presses has been increased relatively fast. The highest speed sheet-fed offset printing press has reached 12,000-15,000 sheets per hour. The machine speed is high, and its printing pressure must be increased accordingly. The reasons are as follows:
(1) Increase the printing pressure to make up for a series of shortcomings such as incomplete transfer of graphic dots caused by incomplete contact between printed surfaces caused by short imprint time, incomplete imprinting of graphic dots, out-of-print, etc. .
(2) As the printing speed of the offset press continues to increase, the diameter of the cylinder becomes smaller, and the compression width during the rolling process also decreases accordingly. If you do not increase the printing pressure, it will also affect the transfer of ink graphics.
If the printing pressure is too large or too small, it will lead to a reduction in the quality of the printing. The excessive printing pressure brings the following disadvantages:
(1) The picture and text are distorted, the imprint becomes thicker, and the dots are expanded greatly.
(2) The ink is partially "overfilled", resulting in inconsistent color tone of the printed matter.
(3) The paper is fluffed and powdered.
(4) The main pressure-bearing parts and rotating parts of the printing machine are greatly deformed, thereby affecting the working life of parts.
(5) Compared with normal pressure and friction conditions, it accelerates the wear of the printing plate.
(6) The imprinted dots are expanded, the level of the printed product is unclear, and the printing durability of the blanket is affected.
(7) Due to the increased load of the machine, the power consumption is also large.
When the printing pressure is too small, it will cause:
(1) The transfer of printed graphics is incomplete.
(2) The dots are not real and the colors are gray.
(3) Make the original defects and defects of printing plate, wrapping lining, paper and printing machine more obvious.
The ideal pressure should be the minimum amount of ink used evenly under the condition that the printing surface is deformed, and on the basis that the print is sufficiently strong, and is recognized by the vast majority of people and is satisfied by the majority of people. Printing pressure that is properly shifted properly.
In general, the appropriateness of printing pressure is often expressed by the print quality of the printed product. The pressure should be based on the premise that the printed product has strong dots, clear graphics, bright colors and shades, and the more it is applied The smaller the better.
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