Reasons for the poor fixation of offset printing

3. Effect of ink viscosity on the fixation fastness of imprinting

Ink viscosity refers to the size of the ink resisting the ink film splitting force. The transfer condition of the ink in the printing process is related to the viscosity of the ink. Since the ink is a fluid substance, it has fluid properties. The viscosity of the ink used in offset printing not only has a plastic viscosity but also has a structural viscosity.

Insufficient adhesion of the ink itself causes the ink film to be firmly fixed, and it is better to replace other types of inks for printing. The printing materials of different printing methods and the inks, diluting agents and retreating inks that are compatible with the printing materials and suitability of the ink are selected. Agents, bright oils, desiccants and other auxiliary materials, the amount can not be too large, improper use of ink thinner will also appear ineffective fixed ink film, in the choice of dilution solvent should take into account the nature of the ink in order to avoid ink with printing Inadequate bonding occurs.

Third, the printing pressure and imprint ink firmness

In the entire offset printing process, it is the process of resolving the contradiction between ink and paper. To transfer the ink-coated graphic on the printing plate to the printed paper, it must be applied during the graphic transfer process. The printing pressure.

People often say that "there is no printing without pressure." This sentence fully illustrates the importance of printing pressure. Since printing pressure is so important, isn't the pressure greater? The better, of course. In general, the appropriateness of printing pressure is often manifested by the print quality of printed products. The printing pressure should be based on the premise that the printed products have strong dots, clear graphics, bright colors, and adequate shade; and the smaller the application, the better.

When printing pressure is too small it will cause:

1. The transfer of printing graphics is not complete;
2. The network is not strong and the color is gray;
3, imprinting is not firmly fixed, the picture is made false, network points are not true, the color is pale.
4. Make original defects such as printing plates, linings, paper and printing machines more obvious.

Therefore, the ideal printing pressure should be: Under the condition that the certain printing surface is deformed, the minimum pressure is evenly used on the basis that the printing is sufficiently strong. The printing pressure is suitable for fixing the print firmly. Printing pressure is too large or too small is not conducive to print quality, and it is a great harm to the correct transmission of the print.

The ideal pressure is not a fixed concept, but it is based on the premise that it is in favor of product quality, based on a variety of objective printing conditions, appropriate adjustment and adaptation, in practice, the ideal pressure is generally controlled at 0.15-0 .25mm or so, and the pressure between the platen and the blanket cylinder is greater than the pressure between the plate cylinder and the blanket cylinder by about 0.5-0.10mm.

Fourth, the impact of printing speed on imprinted fastness

The printing speed of the offset press has an effect on the printing pressure and the imprinted fastness. Many examples show that as the printing speed increases, the printing pressure increases. This is due to the increase in the printing speed, which inevitably results in a corresponding decrease in the contact time between the corresponding points of the platen blanket. This reduction in time means that the degree of incomplete contact between printed surfaces increases, which is very dangerous. In order to effectively avoid the incomplete transfer of images and texts caused by incomplete contact between printed surfaces, inaccurate mesh and light gray inks, and incomplete fixation of blotting ink, a large printing pressure must be used. In this way, even if the ink can be quickly transferred from the ink-supply surface to the ink-accepting surface, the ink can be made to complete its ideal image transfer process in a relatively short period of time.

Now, the speed of the offset press has increased rapidly, with a maximum printing speed of 15,000 sheets/hour. The speed of the machine is high, and its printing pressure must be increased accordingly. The reasons are:

1. In order to increase the pressure of printing, it is incompetent to make up for the shortcomings of imprints that are immature and empty due to imprinting time, in order to improve the robustness of imprinting.

2. The machine speed keeps increasing, and the drum radius must also decrease as the drum utilization factor increases. The small diameter of the roller, the compression width in the rolling process is also reduced accordingly, the compression width in the rolling process is also reduced accordingly, if not increase the printing pressure, it will also affect the transfer of ink.

(Author: Wang Dong Road, New City, Hubei Province Yun County Yun County printing plant No. 53)