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There are many kinds of defects in the gravure printing of plastic film, and the inaccurate overprinting before and after the graphic is one of the common faults. Material waste due to inaccurate overprinting. Therefore, it has attracted the attention of packaging manufacturers. It is necessary to understand the reasons for the inaccuracy of overprinting and to master the methods of elimination. Bite Packaging focuses on the following aspects of composite packaging for the overprinting, for your reference.
The process of gravure printing of plastic film is: the substrate passes through the discharging device, the conductive roller enters the first unit for printing, and then enters the drying device, and then enters the second unit to be printed by the conductive roller, and so on, and finally enters the collection. Rolling device. In this process, in order to avoid or reduce the loss caused by inaccurate overprinting, it is necessary to ensure that the unwinding and winding tension of the substrate is consistent, and also pay attention to the direction of overprinting, and then take corresponding measures.
There are many reasons for the inaccurate overprinting of plastic film, including the influence of objective factors such as mechanical equipment, ink suitability, and ambient temperature, as well as subjective factors such as the quality awareness of operators.
Let's talk about it one by one.
Effect of roll diameter
The larger the diameter of the coil, the easier the plastic is to be deflected during operation, which brings hidden dangers to overprinting. According to experience, the diameter of the coil is generally 35-40cra. When printing, as the diameter of the roll is gradually reduced, the unwinding tension should be reduced accordingly to ensure that the take-up tension remains stable. This is done by manual gear or magnetic powder brakes. When doing this, you should pay attention to whether the printed color standard is out of shape.
The plate cylinder of the domestic multi-color gravure printing machine is positioned by the U-shaped groove of the drum end and the bearing on the 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, The plate-driven plate cylinder may cause axial turbulence, causing misregistration. When the wear amount exceeds 0.3 mm, the overprinting is inaccurate and the pattern is out of shape. As an operator, oil should be injected into the sliding part of the gravure press, gears, etc., to ensure the accuracy of overprinting and to extend the life of the machine and reduce wear. The wear mechanism should be replaced when significant wear occurs.
When the axis of the plate is axially twitched, the substrate color sample is unstable and the color code is displaced upward.
Guide roller charged
The plastic film is pulled by the conductive roller into the next unit during printing or enters the drying device. However, after the plastic film is rubbed against the conductive roller, the conductive roller will carry static electricity, adsorbing dust in the air, and locally increasing the conductive roller, causing the film to slide axially, causing misregistration. For this reason, the cleaning roller should be cleaned during shutdown to lay a good foundation for the next printing.
Impressed version is incorrect
Imprinting is not allowed to be related to the plate. The plate shaft should be cleaned before loading, and then the plate should be installed at the center of the shaft. The plugs at both ends of the plate (also known as "bulk head") should also be flattened, and the force should be even when the plate is locked, so as to ensure accurate overprinting. In general, the shaft fit clearance is 0.04-0.06mm. When worn to 0.2mm, the color registration is very difficult; when worn to 0.4-0.6mm, it should be replaced.
After a long period of operation, the gears of each printing unit will also be worn to varying degrees. If the plate roller gear and the main transmission gear on the drive shaft are not engaged at the indexing circle, the rotation speed is unstable, resulting in a decrease in accuracy and inaccurate overprinting. It appears that the color scale on the substrate appears up and down, which is very unstable.
Improper selection of rubber rollers
In order to obtain a more accurate registration, the selection of the rubber roller should also be very careful. In our factory, there was an overprint failure, and the plate was re-detected by changing materials. Adjusting the tension did not achieve good results. Later, we found that the diameter of the two ends of a rubber press roller was 0.015 mm, and replacing the new rubber press roller eliminated the fault.
In general, when printing a wide film with a thickness of 12 μm, the glue is
The requirements of the rolls are more stringent. At the same time, it should also ensure that the pressure of the pressure roller and the printing plate is even, otherwise it will cause malfunctions such as overprinting. Unbalanced pressure of the rubber roller and thickness deviation of the plastic film will directly lead to inaccurate overprinting. On the substrate, most of it appears as a lateral shift of the image.
Environmental temperature and humidity effects
In the printing process, the last unit and the first unit will expand and contract with the temperature change, resulting in inaccurate 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, it is necessary to control the temperature of the printing environment. humidity. Generally, the temperature control in the workshop is 25 °C, and the humidity is controlled at 40%. The over- and under-low will affect the overprinting accuracy.
If the viscosity of the ink is too large, the image will be displaced, so the viscosity value should be adjusted according to the actual situation.
In gravure printing, in consideration of the stretching of the plastic film, the printing plate has an increasing amount according to the color order, which is gradually increased. If there is an error in the increment, beyond the allowable range, the color registration accuracy will also decrease. Therefore, before the end of the plate cylinder, it is necessary to carefully check and record it. When printing, the print should be carefully compared with the original.
During the operation of the plastic film, the winding tension is required to be stable, and as the winding diameter is gradually increased, the winding tension should also be increased accordingly. At the same time, attention should be paid to the range of jitter of the winding, because the winding tension is adjusted by rubbing the friction plate on the rolling bearing at the end of the winding end. If the shaking range is large, the film shrinks and the tension value increases. The general control tension value can be achieved by a magnetic powder brake or manual adjustment. We should also note that the longer the coil, the easier it is to stretch, the tension value should change accordingly, and the constant and stable tension is very important.