Heat transfer during thermoforming can be conceptually modeled in three distinct stages. In the first stage, the polymer sheet is heated to a temperature above its glass transition temperature in preparation for forming. Heating during this stage is usually accomplished using radiant heaters and causes sagging of the plastic sheet. Once the sheet has attained the necessary forming temperature, the second stage of heat transfer occurs when the polymer sheet is subjected to large stretching during rapid inflation. Heat transfer during this inflation stage is strongly influenced by the large increase in surface area that accompanies stretching of the plastic. Finally, in the third conceptual heat transfer stage, the plastic contacts the metal mold surface and heat is conducted from the hot plastic to the cooler metal surface. Of particular interest in this paper, is the calculation of the cooling that occurs during the rapid inflation phase of polymer processing. Sample calculations are presented for various thermoforming scenarios that illustrate the nature of this cooling mechanism.

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