Hot and Cold: The Role of Heat Transfer in Thermoforming

With thermoforming, a plastic sheet is heated to its forming temperature, then shaped by a mold, but just what is “forming temperature?”

Most materials have a range of forming temperatures. For example, polystyrene has a lower forming temperature—the temperature at which it can be bent from its flat shape—of 260 degrees Fahrenheit and an upper forming temperature— the highest temperature at which the plastic remains a sheet—of 360 degrees Fahrenheit. However, practically speaking, there’s a normal forming temperature, which in the case of polystyrene, is 300 degrees Fahrenheit. When striving for the right temperature, remember that it’s the temperature of the sheet that matters, not the heater temperature.

Once the sheet is heated, it’s placed into a mold and formed by cooling the mold surface. Production molds are typically actively cooled by a medium such as water in channels to maintain a uniform temperature across the entire mold. When the plastic sheet contacts the mold, the energy is removed through conduction, the rate of which depends on factors like the thickness of the mold material.

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