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Question
#38
Subject: We have a need to control curl in many of our constructions.
We have a need to control curl in many of our constructions, and do not understand the interaction of PVDC, PE, moisture addition, drying and other processing to curl.
Answers (links jump to full text of answer below)
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#39
Author: Eldridge Mount, AIMCAL
Email: emmount@msn.com
Subject: Curl can be caused by many aspects of the process and the materials you are using.
Curl of course can be caused by many aspects of the process and the materials you are using to make your structures. Because your structure is asymmetric there are several possible causes for the curl.
The curl may be caused by moisture variations as you suggest and the direction of the curl would vary depending on moisture pick up or moisture loss from the paper layer. If you can make the curl change direction with humidity changes this would indicate that the structure is unbalanced. The curl can be caused by poor tension control during the lamination and coating process, which would cause the frozen in strains in the polymer layers to recover and create a force imbalance in the structure causing the curl. Additional bending forces can be induced by density changes in the polymer layers which will depend on the heating history as would be found in the drying oven for the coating and the ink. Crystallization of the PVDC and the PE will drive these. The PE crystallinity should be fairly uniform but the PVDC crystallinity should change with time.
I hope this gives you some direction as where to look for you problem.
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