Flexible pvc end product issues (curved rolls)
Submitted by Brian Cooley on Mon, 04/20/2009 - 12:44pm
We are having a very serious issue with curving or race tracking of are finished sheet. It is straight coming off the line, once it has set up and cured for 72 hrs it starts to curve. The material in ? is a 78 A shore flexible clear PVC strip. Gauge is .060" to .080". The variance on the sheet is .0005" from one side of the sheet to the other. Any help would be greatly apreciated.
Comments
curving
are you talking of flexible PVC edgeband for the joinery?
depending on your application, I may be able to shed some light.
Nas
Stress relaxation in Flex PVC sheet
The sheet has a extruded in tensile stress in it which relaxes over time causing the curve when spread out. The usual cause for this is the dies are not uni formally heated and therefore the material has more stress in the cooler area as it is pulled from the machine. You can use an spot check IR type thermometer or surface contact thermal couple to check for uneven heating. The other thing which you might do is to use less draw down coming out of your extruder which will minimize the stress. Tighten the die lip gap closer to the product specified thickness and slow down you pull off winder.