What do we mean by the heat resistance of thermoplastics?

In some applications heat resistance can mean the material’s ability to carry out its intended function at elevated temperatures for short periods.  For others it may imply surviving high temperatures for long periods or coping with very high temperatures for short periods during processing.

For short-term exposure, the main concern is often the softening as the thermoplastic approaches its glass transition temperature (in the case of amorphous thermoplastics) or its melt transition temperature (for semi-crystalline thermoplastics). Material datasheets will provide data on ‘Deflection Temperature Under Load’ but designers are reminded that this applies to a narrow test, involving specific loads, geometry, heating rates and acceptable deflection, which may or may not coincide with the service conditions and criteria for failure.  At least the test does give 

data for two levels of loading.

For long term ageing, designers should be aware that ‘Continuous Use Temperature’ usually refers to the temperature at which the mechanical properties decrease by 50 percent after a period of 5000 or 20,000 hours, which again may or may not cover all scenarios.

A useful rule of thumb for processors is that, raising the processing temperature by 10°C will roughly double the extent of thermal degradation in the same time frame. 

When processors are tempted to push processing temperatures five degrees beyond the recommended limits, in order to reduce melt viscosity and make processing easier, they could inflict damage, which, although not immediately apparent, may eventually affect the properties of the product.