Plastic film gravure overprint problem analysis

There are many kinds of plastic film gravure printing failures, and one of the most common failures is the inaccurate printing before and after graphics and text. Improper overprinting will result in material waste. Therefore, it attracted the attention of people in the industry. It is necessary to understand the reasons for overprinting, and to know how to eliminate them. This article is based on the author's experience in gravure printing for several years. The following analysis of overprinting was made for reference by those in the industry. The process of gravure printing of plastic film is as follows: the printing material passes through the discharge device, the conductive roller enters the first unit for printing, and then enters the drying device, and then it is driven by the conductive roller to enter the printing of the second unit, and so on. Roll device. In this process, in order to avoid or reduce the losses caused by overprinting, it is necessary to ensure that the unwinding and winding tension of the substrate are the same, and the direction of overprinting should be taken into account, and corresponding solutions should be taken.

There are many reasons for inadmissible overprinting of plastic films, including the influence of objective factors such as mechanical equipment, ink suitability, and ambient temperature, as well as subjective factors such as the quality consciousness of operators. The following is a detailed analysis of each one.

Reel diameter effect

The larger the diameter of the roll, the easier it is for the plastic to go in operation, causing hidden trouble for overprinting. According to experience, the coil diameter is generally 35-40cra more appropriate. During printing, as the diameter of the roll decreases gradually, the unwinding tension should be reduced accordingly to ensure that the feeding tension remains stable. This is achieved with manual gears or magnetic powder brakes. When performing this operation, it should be observed whether the printed color code is missing or not.

The axial direction of the multi-color domestic gravure printing machine is based on the U-shaped groove of the drum end and the bearing on the machine frame. The thickness of the bearing is exactly equal to the width of the u-shaped groove. When the width of the U-shaped groove is increased due to wear, The platen-driven platen roller may cause axial turbulence, causing overprinting. When the wear amount exceeds 0.3mm, overprinting is inaccurate and pattern aliasing is very obvious. As an operator, machine oil should be injected into the gravitational machine's sliding parts, gears, and other operating areas from time to time to ensure the accuracy of overprinting and to extend the mechanical life and reduce wear. When significant wear occurs, wear parts should be replaced.
When the axis of the plate shaft is shaken, the substrate is unstable in color and the color plate is displaced upward.

Guide roller charged

During the printing process, the plastic film is pulled by the conductive roller into the next unit to print or enter the drying device. However, after the plastic film is rubbed against the conductive roller, the conductive roller is charged with static electricity, and the dust in the air is adsorbed, so that the conductive roller is locally increased, causing the film to slide axially and causing overprinting. For this reason, in the shutdown, the conductive roller should be cleaned to lay a good foundation for the next printing.

Improper loading

Imprinting is also not related to loading. The plate shaft should be washed clean before loading, and then the plate is installed in the center of the shaft. The plugs at both ends of the plate (also referred to as "boring heads") should also be flattened, and the force should be even when the plates are locked, so as to ensure accurate overprinting. In general, the plate shaft fit clearance is 0.04-0.06mm. When it wears to 0.2mm, the color registration is very difficult. When it wears to 0.4-0.6mm, it should be replaced.

Gear wear

After a long period of operation, the gears of the printing units will also be worn to varying degrees. If the plate roller gear and the main drive gear on the drive shaft are not meshed at the indexing circle, the rotation speed is unstable, which leads to a decrease in accuracy and overprinting. The performance of the color standard on the substrate has shifted up and down, and it is very unstable.
Inappropriate selection of rubber roller

In order to obtain more accurate registration, the selection of the rubber roller should also be very careful. There was a failure of overprinting in our factory. We changed the material and retested the plate. Adjust tension, failed to get good results. Later, we found that the difference between the diameters of two rubber rollers is 0.015mm. The replacement of a new rubber roller eliminates the malfunction.

In general, when printing a wide film with a thickness of 12 μm, the requirements for a rubber roller are more stringent. At the same time, the pressure of the pressure roller and the printing plate should be ensured to be even, otherwise it will cause failures such as overprinting. The uneven pressure of the rubber roller and the thickness deviation of the plastic film will directly lead to overprinting. On the substrate, most of the performance of the image is laterally offset.

Influence of environmental temperature and humidity

In the printing process, compared with the first unit in the last unit, the film will expand and contract with the temperature change, resulting in overprinting. Therefore, in the printing process, as long as the solvent can be effectively volatilized, the lower the drying temperature, the better.

At the same time, we must also control the temperature of the printing environment. humidity. Generally, the temperature in the workshop is controlled at 25°C and the humidity is controlled at 40%. If the temperature is too high or too low, the overprint accuracy will be affected.

Ink

If the viscosity of the ink is too large, the image will be displaced. Therefore, the viscosity value should be adjusted according to the actual situation.

Plate increment

In gravure printing, considering the stretching of the plastic film, the printing plate has an increasing amount according to the color sequence, and it gradually increases. If there is an error in the incremental amount, which exceeds the allowable range, the trapping accuracy will also decrease. Therefore, before the plate cylinder is placed on the machine, it is necessary to inspect it carefully and make a record. When printing, it is necessary to carefully compare the printed parts with the original ones.

Winding tension

During the operation of the plastic film, it is required that the winding tension be stable, and as the winding diameter gradually increases, the winding tension should also increase accordingly. At the same time, attention should be paid to the jitter range of the winding, because the winding tension is adjusted by the friction plate on the rolling bearing at the winding shaft end. If the shaking range is large, the film shrinks and the tension value increases. The general control tension value can be achieved by magnetic brake or manual adjustment. We should also note that the longer the roll, the easier it is to stretch, the tension value should be changed accordingly, and constant and stable tension is very important.

In short, in order to obtain accurate overprinting and printing of fine plastic film printing products, each process must be properly handled, and problems should be carefully analyzed and lessons learned in order to prepare for future reference.

(author/Chen Jindong)

Source: Chinese Art Federation