Generally, there are four commonly used thermal properties of plastics: Heat Deflection Temperature, Continuous Service Temperature, Melting Point or for some, Glass Transition Temperature (Tg) and Coefficient of Thermal Expansion.
Heat Deflection Temperature (ASTM D648): Heat Deflection Temperature or heat distortion temperature (HDT) is the temperature at which a polymer deforms to heating or cooling under a specified load. ASTM stands for the American Society for Testing and Materials which is an international society that develops and publishes voluntary technical standards for materials among other things. The standard test method for deflection temperature of plastics under load is ASTM D648.
Continuous Service Temperature: This is the maximum temperature at which a material can perform reliably in a long-term application. Continuous Service Temperature ensures the stability and integrity of the material for the expected life of the part in its intended application. There is no ASTM D test for this.
Melting Point (ASTM D3418) or Glass Transition Temperature Tg (ASTM D3418): The melting point refers to the temperature at which crystalline polymers become a disordered liquid. Crystalline polymers are those that have a regular and defined pattern to their molecular structure. Crystalline resins include PEEK, PEK, PPS and PFA. Although they have a melting temperature, they do not have a glass transition temperature. Polymers with regular chain structures are the most likely to form crystalline regions. The more crystalline a polymer is, the stronger and less flexible it becomes. These types of polymers generally allow less light to pass through them. Crystallinity creates the benefits of strength, stiffness, chemical resistance and stability.
Amorphous plastics do not have a melting point but rather a glass transition temperature. Instead of melting, these polymers soften over a broad range of temperatures. Amorphous materials are made up of polymers whose chains are not arranged in ordered crystals, but are strewn around randomly even though they are in a solid state. Transition Temperature (Tg) is the temperature below which a polymer becomes hard and brittle. Generally speaking, amorphous polymers are transparent and used to make things like plastic wrap, contact lenses and plastic windows.
Polymer molecules are often partially crystalline (semicrystalline), with crystalline regions dispersed within amorphous material. The crystalline molecules have a melting temperature while the amorphous regions have a glass transition temperature.
Coefficient of Linear Thermal Expansion: (CLTE) is the relationship between a material’s dimensional response to heating and cooling. Linear thermal expansion means the product will expand in all directions, and this needs to be allowed for in design calculations. The calculation is: (given factor) x 10-6 x length x change in temperature C°. (ASTM D E-831 TMA)
The Coefficient of Linear Thermal Expansion is often shown as a factor (10-4 m/mK) in many tables. All materials expand with changes in temperature. Thermoplastics expand considerably more than metals, e.g. Carbon Steel 10.8 (10)-6 compared to UHMWPE 200 (10)-6 is approximately 18 times more.
Table of Thermal Properties of Common Plastics
HDT Continuous Service Temp. Melting Point CLTE
Nylon, Crystalline 200°F 210°F 500°F 5.5 x.10-5
PPS, Semi-Crystalline 250°F 425°F 426°F 2.8 x 10-5
PEI, Amorphous 410°F 340°F 410°F (Tg) 3.1 x 10-5