Saturday, November 21, 2009

Polycarbonate sheet - stresses and shrinkage explained

To understand the concept of shrinkage it is important to know how extruded Polycarbonate sheet is produced.

Polycarbonate resin is melted in an extruder and then is passed through a die to form the shape of a sheet.  At this stage the sheet is molten because it is above its glass transition temperature of 150 C / 302 F.  The molten sheet is then pulled though chrome polishing rolls to give it a high quality surface. The sheet is then cooled below its glass transition temperature to give it a rigid structure.

The process of pulling the sheet through the chrome rolls stretches the sheet in the extrusion direction and puts stresses in the sheet.  When the sheet drops below the glass transition temperature these stresses remain in the sheet.

Pulling the sheet through the rolls is only one way in which stresses are put into the sheet. Another is uneven cooling of the sheet.  Usually there are more stresses in the direction of extrusion than in the perpendicular direction.

These stresses in the sheet can cause problems for customers.  When the sheet is heated above the glass transition temperature of 150 C / 302F, these stresses are able to relax.  During this process the sheet behaves a bit like a rubber band and the sheet "shrinks" in the extrusion direction as the stresses are released.  One standard test to measure shrinkage is to take a 12"x12" piece of sheet and draw a 10" diameter circle on the sheet with a marker pen.  The sheet is then placed in an oven that is just above the glass transition temperature for a few hours.  When the sheet is removed and cooled, it is easy to see that the sheet has shrunk in the extrusion direction (as seen in the above diagram).  The percentage change in the diameter of the circle in the extrusion direction is know as the "percentage shrinkage".

The percentage shrinkage is particularly important for customers that thermoform sheet because if the sheet is cut too small, the shrinkage could cause the sheet not to fill the mold after processing.  Using a larger sheet than required is one method of solving this problem, but this leads to increased scrap and increased costs.
For thermoformers it is important not only to have low shrinkage, but also consistent shrinkage from one batch to the next.  With modern sheet extrusion lines, regular monitoring and an experienced production team it is easily possible to control shrinkage to below 1.0%, in fact shrinkage levels of 0.5% should be the target.
Unfortunately not all Polycarbonate sheet producers regularly control shrinkage and some often advise that thermoformers should use a 5% overage on the sheet.  To resolve this issue, thermoformers should ask their suppliers to report the shrinkage test results and ask for them to supply sheet with less than 2.0% shrinkage which can be easily achieved without additional cost.

One of the other problems that can be caused by the stresses in the sheet and high shrinkage is warping of the sheet.  This is often observed as the sheet not remaining flat on a pallet, but hills and valleys forming along the length of the sheet.  Often high humidity can cause the stresses in the sheet to relax and as a result, the sheet will often warp.  One way to mitigate this problem is to keep all pallets of Polycarbonate sheet wrapped until they are used to prevent moisture reaching the sheet.  If a sheet is taken from the pallet, the pallet should then be re-sealed. However, a much better way to resolve the problem is to ask the supplier to supply sheet with low shrinkage. 

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