Heating Polystyrene: Highest Temp Without Deformation?

[gkiverm]

I placed some polystyrene dishes in an oven. Now, the melting temperature of this material is 240C. And I was at a temperature of 85C. Although this is lower than the melting temperature, the dishes (35 mm diameter) still became deformed.

Does anyone know the highest temperature this material can be heated without deforming the dishes? I realize this is a very specific question, and I could figure out the answer using trial and error, but in the interest of time and limited materials I was wondering if anyone here knew.

Thanks

 

[UltrafastPED]

"The plastic has a glass transition temperature Tg of ~90 °C."
http://en.wikipedia.org/wiki/PolystyreneOther references:
http://www.matweb.com/reference/deflection-temperature.aspx

And the "Expanded Polystyrene Handbook" is sure to have lots of information:
http://www.texasfoam.com/EPS-Book.pdf