The Causes of the Double-Effects in Offset Printing and the Ways to Eliminate them

Ghosting problems are more common in offset printing, and there are many reasons for this. As long as the operator can find out the cause of the fault according to different fault phenomena, it can be easily eliminated. In the printing process, the author once encountered a special ghosting fault, which is described below and is for reference only to the peers.

Fault phenomenon

A five-drum two-color machine starts printing after the school edition is finished, or when the printer stops printing after shutting down, the ink of the first 20 or so colors is darkened, after which the ink color gradually becomes stable, and the ink darkens irregularly in the normal printing process. phenomenon. This phenomenon is not obvious when printing live jobs; in the case of printed screen printing, the ink of the whole print sheet shows irregular lumps, or occasionally one of the inks is particularly dark. Observed with a magnifying glass, there are virtual points around the actual dot, which is a typical ghosting fault, this phenomenon occurs in the last color version. In order to verify if there are any problems with the machine, we changed the color sequence of the top and bottom. Although the situation was slightly improved, there was still a phenomenon in which the ink of a certain print sheet suddenly increased.

According to the routine, check the pressure, lining and blanket, etc., all within the scope of technical requirements. At first, it was suspected that the bearing of the impression cylinder was damaged, but the two-color group sharing one impression cylinder did not have a ghost phenomenon at the same time. In the case of unresolved problems, it is only necessary to continue to use the machine to focus on printing live jobs, and the screen will be given to other machines.

In the next printing process, the machine suddenly issued a "bang" noise. Emergency stop check, move the machine slowly at the position of the console, listen to the sound, and carefully observe the operation of the drum. As a result, it was found that the rubber drum of the previous color group had a significant axial turbulence phenomenon, and the hub of the drum collided with the wall portion and an abnormal noise was emitted. Open the operating side shield and found that the shaft end thrust nut is loose. Center the roller to the right position, lock the thrust nut, and turn it on again. The abnormal noise disappears. The darkening of the ink will disappear when the printer starts printing. The troubleshooting will be eliminated.

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