Causes and Prevention of Dirty Print Backs (I)

When printing a dark product in a color product and printing a large area on a spot with a spot color, due to the thick ink layer, the back stain is an extremely prone to failure, ink viscosity, pH value, ink layer thickness, and roughness and penetration of the paper surface. There are problems with sex that can lead to dirty backs.

In the offset printing process, the ink of the previous sheet is not dry, and the next sheet is superimposed on it. The ink of the previous sheet may be partially transferred to the back of the next sheet, which is referred to as back scumming. Dirty backside will not only make ink on the back of the printed product, but will also lead to the appearance of streaky images, seriously affecting product quality, and even lead to the entire batch of products scrapped. Especially in the dark part of printing color products and printing large-area real-time with spot colors, due to the thick ink layer, the back blemishes are extremely prone to failure, so care must be taken to prevent them.

In essence, the back of the printed product is caused by the adhesion of the ink on the surface of the previous ink sheet to the back of the next sheet. This adhesion is the intermolecular relationship between the ink on the surface of the previous ink sheet and the next sheet. Secondary force Fb. The surface ink is also affected by the intermolecular secondary force Fa on the front side of the previous sheet and the cohesive force Fc of the ink itself. Only when Fb≥Fa+Fc is the backside blemishes appear.

Since the intermolecular secondary force decreases drastically with distance, Fa is mainly related to the thickness of the ink film, and it will drastically decrease as the thickness of the ink layer increases. Fb is related to the distance between the surface of the ink film and the next printed sheet. When the thickness of the ink layer is determined, Fb is only related to the distance between two adjacent printed sheets, and decreases rapidly as the distance increases. Fc depends on the nature of the ink and the drying of the ink. Because with the dry solidification of the ink, the cohesive force will rise sharply. Therefore, the bottom line to solve the problem is to reduce the thickness of the ink layer, increase the distance between the printed sheets, improve the performance of the ink and speed up the ink drying speed on the paper, the following from these aspects to discuss how to prevent the printing of the back of printed dirt.

First, the performance of ink

The inks of different types and colors have different ingredients, different proportions of pigments and pigments, different drying methods, and different drying speeds. In addition, the thixotropy of the ink, in the printing process due to the shear between the ink roller, the viscosity will drop, and after the completion of the transfer, the viscosity has increased rapidly. Since viscosity is the external manifestation of cohesion, an increase in viscosity indicates that the cohesive force rises, which is beneficial to prevent the back of the printed matter from picking up. Therefore, in the case of printing color prints that have not been subjected to under color removal and large-area on-site printing, inks that are quick-drying and thixotropic should be used as much as possible.

In order to adjust the printability of the ink, it is sometimes necessary to add a detackifier to reduce the viscosity of the ink. Reduced viscosity means that the cohesive force is reduced, and it is easy to cause dirt on the back. Therefore, it is necessary to strictly control the amount of the de-adhesive agent and control the viscosity of the ink.

Appropriate addition of driers can significantly accelerate the drying of the ink, but the driers are also emulsifiers that promote ink emulsification. Excessive addition of driers can retard ink drying.

Second, the fountain solution pH and dosage

When printing is liable to occur on a large area on the backside, the thickness of the ink layer tends to cause the ink to expand toward the blank portion. In this case, lowering the pH of the fountain solution is advantageous in suppressing the expansion of the ink. However, when the pH is too low, the dampening solution reacts with the driers to retard the drying of the ink. Therefore when printing this type of product, the pH of the fountain solution needs to be determined according to the layout and color of the layout.

If the printing ink layer has a thick spot color in the field, and contains a small line in the middle, or a dark picture, the pH of the fountain solution will be lower because of easy paste.

When printing red ink, the pH of the fountain solution is lower; when printing other colors of ink, the pH should be slightly higher.

The more severe the ink is emulsified, the more water that is mixed into the ink layer, the less likely the ink film will be dried. The use of alcohol fountain solution can reduce the amount of fountain solution, reduce emulsification, is conducive to ink drying.

With ordinary fountain solution, first pay attention to the proportion of the ingredients in the formula, and then keep the ink and water balance with as little water as possible. At the same time, controlling the distribution of moisture on the plate is also important for preventing back-scratching. For example: A printing machine axial width of 1020mm, printed paper width of 700mm, and a large area of ​​the text is almost full of paper. In this case, since the ink layer is thick and the amount of water supplied is large, and the printing machine supplies water evenly in the axial direction, neither the ink nor the water emulsifies the left and right ends of the printing plate, and the area other than the paper. The blank part of the paper absorbs moisture. As a result, excessive moisture is infiltrated into the left and right ends of the paper, resulting in excessive emulsification of the printed text and the ink becomes lighter. In order to maintain the evenness of the ink on the layout, the amount of ink must be increased. As a result, the left and right ends of the paper are so large that they are not easy to dry. Therefore, when printing a large area on the ground, the back stains are most likely to appear on the left and right sides of the paper.

It is reasonable to try to use a water-jet blower or stick a note on the fountain roller to reduce the amount of water at both ends of the plate. If the number of prints is not large, you can also consider cutting the paper slightly larger, and let some parts of both ends of the paper absorb some moisture.