Our company's printing machine is primarily designed for thick paper with a basis weight ranging from 250 to 350g/m². However, when we attempted to print thin paper with a large format and a basis weight of only 40g/m², we encountered an issue during the paper transfer between the delivery gripper row and the impression cylinder. The paper would drop at the moment of handover, and in some cases, the grippers on the paper were slightly bent. Interestingly, this problem did not occur when using thick paper. Why was that?
To investigate, I first tested the machine using thick paper. I carefully observed the entire handover process between the delivery gripper row and the impression cylinder. Using a method of forward and reverse clicks, I made the paper move back and forth between the two gripper rows for a two-way transfer. During this test, no paper dropped, which suggested that the system worked correctly with thick paper.
I then repeated the same test with a thin sheet of 40g/m² paper. This time, the problem occurred: as the delivery gripper passed the paper to the impression cylinder, the grippers on the delivery side loosened before the impression cylinder’s grippers had fully closed. As a result, the paper fell. It wasn’t until the machine moved about 3 to 4 degrees that the impression cylinder grippers fully engaged.
As is well known, during the paper transfer between the impression cylinder and the delivery unit, both the delivery and impression gripper rows must grip the paper simultaneously along the same straight line. Otherwise, the paper may fall. From my inspection, it was clear that the two parts of the machine were not properly synchronized in this regard.
The reason why this problem didn’t occur with thick paper is likely due to its greater thickness, stiffness, and strength. These properties allow the paper to remain in place even if the impression cylinder grippers haven’t fully closed. However, with thin paper, the lack of rigidity makes it more prone to falling during the transfer process.
To resolve this, I adjusted the timing of the impression cylinder’s gripper opening so that it aligned better with the handover process from the delivery row. After making this adjustment and testing again by clicking the car forward, I found that the paper no longer dropped. This change significantly improved the reliability of the paper transfer, especially when working with lighter papers.
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