Curvature in sheet pressed lexan

[Bester2]

I am having trouble with curvature from pressing (with heat) Lexan (by GE). The sheets that I am pressing the Lexan between are curved in one direction, they are made of AL. When I take the Lexan sheets out of the press the Lexan sheets are curved the opposite way. It would be logical to me for the sheets to bend the same way as the AL not the opposite way. (I do not want the sheets to bend at all but before I buy new AL plates I want to make certain that these new plates are going to solve the problem) Any ideas of why this is happening? Am I inducing stresses, releaving stresses, or maybe a compination of both?

 

[patprimmer]

Polycarbonate has poor thermal conductivity and about 10 times the thermal expansion of steel.

This causes it to bend away from the side that is heated.
If you wish to thermoform it, heat it in an oven, slowly at first to dry it, then to it-s softening temp, then place it over a cool former.

If you heat it to high to quick, it will outgas and bubble as steam is formed within the polymer.

Regards

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[Bester2]

I am theoretically heating the sheet evenly on both sides, but if it is curving one way, one side must be hotter than the other. I will put thermocouples on both sides tomorrow to see if that is the case. Thanks for the help.