Plastic film gravure printing has many kinds of faults, one of the common faults is inaccurate printing before and after pictures and texts. Due to overprint will cause material waste. Therefore, it has attracted the attention of the industry. It is necessary to understand the causes of overprint inaccuracy and to master its exclusion methods. Based on the author's years of experience in gravure printing. The overprint is not allowed to make the following analysis, for the reference of the industry. The process of plastic film gravure printing is as follows: the substrate passes through the discharge device, the conduction roller enters the first unit for printing, then enters the drying device, and then enters the second unit printing driven by the conduction roller, and so on, and finally enters the winding device. In this process, in order to avoid or reduce the losses caused by overprint inaccuracy, we should not only ensure that the tension between unwinding and rewinding of substrate is consistent, but also pay attention to the trend of overprint inaccuracy, and then take corresponding solutions.
There are many reasons for inaccuracy of plastic film gravure printing, including the influence of objective factors such as mechanical equipment, ink suitability, environmental temperature, etc., as well as subjective factors such as operators' quality awareness. The following is a detailed analysis one by one.
Influence of drum diameter
The larger the roll diameter is, the easier the plastic deviates in operation, which brings hidden trouble to overprint. According to experience, the coil diameter of 35-40cra is suitable. When printing, the unwinding tension should be reduced with the gradual decrease of the coil diameter to ensure that the feeding tension remains stable. This is achieved by manual gear or magnetic powder brake. In this operation, attention should be paid to observe whether the color code is out of shape.
The axial direction of the plate cylinder of the domestic multi-color gravure press is located by the U-groove at the end of the drum and the bearing on the frame. The thickness of the bearing is exactly equal to the width of the U-groove. When the width increases due to the wear of the U-groove, the plate shaft may drive the plate cylinder to move in the axial direction, resulting in inaccurate overprint. When the wear is more than 0.3mm, the overprint is not accurate and the pattern is out of shape. As an operator, it is necessary to inject oil into the sliding part and gear of gravure press to ensure the accuracy of overprint, and to prolong the mechanical life and reduce wear. When significant wear occurs, the worn parts should be replaced.
When the plate shaft axial movement, the substrate on the color sample instability, color code upward displacement.
Live guide roller
Plastic film in the process of printing by the conduction roller into the next unit of printing or into the drying device. However, after the plastic film is rubbed with the conductive roller, the conductive roll will carry static electricity, absorb the dust in the air, and make the conduction roller locally increase, resulting in the film axial sliding, resulting in inaccurate overprint. Therefore, in the shutdown, the conduction roller should be cleaned to lay a good foundation for the next printing.
Incorrect loading
Overprint is also related to the loading plate. Before mounting the plate, the plate shaft should be cleaned, and then the plate should be installed in the center of the shaft. Both ends of the plate plug (also known as "bulkhead") should be installed flat, and when locking the plate, the force should be even, so as to ensure accurate overprinter. Generally speaking, the fit clearance of the plate shaft is 0.04-0.06mm. When it is worn to 0.2mm, it is difficult to register; when it is worn to 0.4-0.6mm, it should be replaced.
Gear wear
After long-term operation, the gears of each printing unit will also be worn to varying degrees. If the cylinder gear and the main drive gear on the drive shaft do not mesh at the dividing circle, the rotation speed will be unstable, resulting in a decline in accuracy and inaccurate overprint. The color code on the substrate appears to be offset up and down, which is very unstable.
Improper selection of Cots

In order to obtain higher precision registration, the selection of COTS should be very careful. Our factory once had a overprint inaccuracy fault, through changing materials, re testing plate, adjusting tension, failed to achieve good results. Later, we found that the diameter difference between the two ends of a rubber press roller was 0.015mm, and the fault was eliminated by replacing it with a new one.

Generally speaking, when printing 12 μ m wide film, the
The roller requirements are more stringent. At the same time, it should also ensure that the pressure of the pressure roller and plate is even, otherwise, the overprint will be inaccurate. The uneven pressure of cots and the thickness deviation of plastic film will directly lead to inaccurate overprint. On the substrate, most of the images show lateral offset.
Influence of environmental temperature and humidity
In the printing process, compared with the first unit, the film of the last unit will stretch and shrink with the change of temperature, which will lead to inaccurate overprint. Therefore, in the printing process, as long as the solvent can be effectively volatilized, the lower the drying temperature is, the better.
At the same time, it is necessary to control the temperature of printing environment. Humidity. General workshop temperature control in 25 ℃, humidity control in 40%, too high, too low will affect the overprint accuracy.
printing ink
Too much viscosity of the ink will also lead to image displacement, so the viscosity value should be adjusted according to the actual situation.
Plate increment
In gravure printing, considering the stretch of plastic film, according to the color sequence, the plate has a gradual increase, which is gradually increased. If the increment error exceeds the allowable range, the register accuracy will also decrease. Therefore, before the printing plate cylinder end on the machine, it is necessary to carefully check and record, and carefully compare the printed part with the original one when printing.
Winding tension
During the operation of plastic film, the winding tension is required to be stable. With the gradual increase of winding diameter, the winding tension should be increased 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 end of the winding shaft. If the shaking range is large, the film shrinks and the tension value increases. Generally, the control tension value can be realized by magnetic powder brake or manual adjustment. We should also note that the longer the coil is, the easier it is to stretch, and the tension value should change accordingly. Constant and stable tension is very important.
In a word, in order to get accurate overprint and exquisite plastic film printing, we must deal with every process, and we should carefully analyze the problems and summarize the experience for reference in the future.
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