The following takes the French ATN company's gravure production line as an example to analyze the electrical principle of overprinting.
In each color printing unit, each time the plate cylinder makes one revolution, it generates a pulse, which enters the encoding circuit board together with 500 pulses generated by the encoder on the printing unit. After processing through the encoding circuit board, 4 pulses are formed, which are sent to the longitudinal (or horizontal) register circuit board. Here, these 4 pulse signals are compared with 1 pulse signal obtained by detecting the cursor on the printed matter. If there is a deviation in the overprint at this time, the registration circuit board will output a voltage of about 5 volts to enter the PLC. After being processed by the PLC, it then goes to the stepper motor driver, thereby pushing the stepper motor to achieve the purpose of register adjustment.
To check the overprint fluctuations caused by electrical faults, generally follow the method of stumbling around. On the basis of understanding the electrical principles of the overprinting system, check step by step whether the related electrical originals are normal and the circuit is smooth. However, after having considerable experience in adjusting the registration of a specific type of gravure printing machine, you can also adopt a leaping way of thinking, which can find out the fault point more quickly.
2. Overprint fluctuation caused by mechanical failure
From the practical experience, there is no fixed method for the mechanical inspection of the overprinting system. Only after understanding and mastering the mechanical part of the gravure printing machine, and knowing which mechanical structures will affect the overprint, can we find the crux as soon as possible. Of course, since the "appearance" of mechanical faults is generally much more obvious than the "appearance" of electrical faults, it is relatively easy to find the resulting overprint fluctuations.
For example, after long-term use of the impression cylinder, there will be obvious wear marks on the surface, which will affect the accuracy of overprinting. The thinner the substrate, the greater the impact on overprint accuracy. In addition, if the impression cylinder is observed to vibrate up and down during the printing process, it is likely that the impression cylinder bearing is damaged, which will affect the longitudinal registration of the printed product.
As another example, if the plate cylinder is not locked, there will be slight movement, which often causes sudden jumps in lateral overprinting. In addition, the lateral movement of the paper guide roller and problems with the transmission part of the stepper motor will also cause overprint fluctuations. Moreover, this type of overprinting failure is basically manifested in large-scale and sudden overprinting jumps from time to time during the production process.
3. Overprint fluctuations caused by other factors
In addition to electrical and mechanical reasons, overprint fluctuations caused by other aspects can not be ignored.
First of all, improper operation will cause overprint fluctuation. For example, if the tension is not set properly, the paper tape will suddenly loosen and tighten during the high-speed operation of the device, which will affect the longitudinal registration. As another example, if the oven temperature is set too high, it may cause the paper to deform too much after passing through the oven, affecting the next color overprint.
Secondly, raw materials and production environment also have an impact on overprinting. If the paper is too loose, the actual tension will change frequently and increase the difficulty of operation. The overprinting caused by the paper is mostly in the longitudinal direction; the uneven edge of the paper will cause the paper guide system to stop moving and affect the horizontal overprinting. The temperature and humidity of the production environment will have a certain effect on the deformation of the paper during the printing process.
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