Abstract: This article analyzes the reasons and solutions for inaccurate overprinting in gravure printing from aspects of printing materials, tension control, printing press performance, operating methods, printing environment, and gravure suitability.
Inaccurate overprinting is one of the problems often encountered in gravure printing, and it is also a very tricky thing. Because there are many factors that affect it, it is sometimes difficult to find the cause. Generally speaking, the main factors affecting the inaccuracy of gravure overprinting include: printing materials, unstable tension, printing machine performance, operating methods, printing environment, and gravure suitability.
I. Impact of printed materials on overprinting inaccuracy
Due to the difference in temperature and humidity of the printing environment, the printing material will inevitably be deformed. Therefore, it is recommended that the printing material be placed in the workshop three days before printing to improve its temperature and humidity suitability and prevent the occurrence of material sweating. , Printing suitability of different brand substrates and inks.
Second, the unstable tension is the main factor affecting the overprinting inaccuracy
It is well known that instability of tension will definitely lead to a reduction in overprint accuracy. But the tension instability is divided into the following types:
(a) In the process of unwinding, the diameter of the unwinding constantly changes, and the detection of the winding diameter is not accurate, which will inevitably cause the change of the tension of the tape.
(b) The tension detection unit, the detected signal is distorted, causing tension instability. For example, if the swing roller detection device is distorted due to the loosening of the tensioning sleeve, it will inevitably give a wrong signal to the PLC, so it will also send a wrong command, which will eventually cause overprinting.
(c) Poor manufacturing accuracy and assembly accuracy of the main components of the gravure printing machine are also important factors that cause tension to float. For example, the processing accuracy of the guide roller in the gravure printing machine is poor, so the tension of the material cannot be guaranteed, which affects the printing effect. The overall requirements for the guide rollers are: ① The guide rollers must be parallel to each other and the parallelism does not exceed 0.05mm. It is required that the round runout of the guide roller cannot be greater than 0.02mm. If the runout is too large, the tension of the material will change randomly with the rotation of the guide roller. ② The guide roller is balanced. If the guide roller is unbalanced, the rotation speed of the guide roller changes within a week. Generally, the partial weight of the outer diameter of the guide roller does not exceed 5g. Turn the guide roller by hand. After the hand is loosened, the guide roller can be rotated for a long time, stopped by hand, and stopped at any position. ③ The guide roller should be as light as possible, and the moment of inertia should be as small as possible to keep consistent with the speed of the host.
(d) In the automatic cutting process of the gravure printing machine, the material receiving and material cutting will cause the original machine's original stable tension to change suddenly.
(e) The effect of the quality of the printing material. The continuous change of the dynamic tension of the roll during the printing process can cause tension to promote the inherent non-uniformity of the strip, which will cause the fluctuation of the tension. The heavier the roll, the faster the mechanical pull starts The greater the applied tension, the heavier the roll, the faster the speed changes, and the faster the tension changes, the greater the printing overprint error. In addition, changes in the temperature and humidity of the production environment will cause subtle changes to the tension fluctuations of the entire machine.
Performance of printing machine
Printing machine performance affects overprinting more seriously, such as imprint rubber roller hardness is not suitable, or the outer diameter of the impression rubber roller is not round, it will affect the overprinting; the stability of the whole machine frame also has a greater impact on overprinting.
Fourth, the operation process
In operation, the causes of overprint deviation are:
1. The squeegee blade is not adjusted properly, which causes the ink on the photoelectric signal end to be dirty, and the electric eye to receive the signal is distorted.
2. Ink accumulates at both ends or in the middle of the gravure cylinder, which affects the change of pressure and feeding tension.
3. The gear box of the printing plate is tightened. After the components are worn out in the horizontal adjustment, there is a gap, which causes the radial runout of the top plate to be severe or the shaft to penetrate. (Or the plate roller is not tightened, a. The single-sided key transmits power. B. The air pressure of the top plate cylinder is insufficient)
4. The axis of the printing plate is not parallel to the axis of the printing rubber roller. Mainly because the impression rubber roller is not parallel and horizontal to the plate roller.
5. The gravure printing route is long. The printing material is affected by printing pressure, tension and drying temperature during the movement. With the volatilization of the liquid in the printing material, the fiber will deform after shrinking. If it exceeds a certain amount, it will affect the printing. Overprint accuracy. If the drying time of the ink is different (that is, the printing speed is different), the drying temperature is different, and the color sequence position of the fiber shrinkage and deformation is also different. This is one reason why the printing offset of the same set is different each time. This requires printers to be proficient in the characteristics of each printing material during operation, the drying temperature and printing speed of various materials during printing; static printing substrates can produce burrs, wood grains or flying ink, etc. It will also affect the smoothness of printing feed. The substrate in motion undergoes electrostatic adsorption, which will affect the stability of tension and cause overprint deviation. This requires appropriate increase in the humidity of the printing environment or activation of a static elimination mechanism.
Five, printing environment, gravure suitability
Due to the deformation of the paper caused by the ambient temperature and humidity, the overprinting is ultimately inaccurate; the suitability of the gravure is mainly due to the deviation of the patterns on the two sides of the printing plate, which can not be registered at the same time, or the increment between the plate rollers. The amount is not suitable, can not make up the amount of deformation of the printing material, and is also the direct cause of overprinting.