A plastic material’s temperature resistance is broadly characterized by both its ‘temperature of deflection under load’ or ‘Heat Deflection Temperature (HDT)’ and its ‘Continuous Service Temperature’.
The ‘temperature of deflection under load’ or ‘Heat Deflection Temperature (HDT)’, is the temperature at which a standard test bar, loaded to a specified bending stress, deflects by a given value as the temperature is raised at a uniform specified rate. It is related to a certain level of stiffness at elevated temperature.
- The ‘temperature of deflection under load’ according to ISO 75 is the temperature at which a test bar (4 x 10 x 80 mm), loaded to the specified bending stress (method A: 1.8 MPa), deflects by 0.34 mm (tested in the flatwise position and 64 mm span).
- The ‘Heat Deflection Temperature (HDT)’ according to ASTM D 648 is the temperature at which a test bar (127 x 13 x 3 to 13 mm / 5 in. x ½ in. x ⅛ to ½ in.) loaded to the specified bending stress (0.455 or 1.82 MPa / 66 or 264 psi), deflects by 0.25 mm / 0.010 in. (tested in the flatwise position and either 100 or 101.6 mm / 3.937 or 4 in. span).
The ‘temperature of deflection under load’ or ‘Heat Deflection Temperature (HDT)’, is quite often misunderstood. This temperature value does not represent the maximum allowable service temperature of a material, although it is often considered as the maximum temperature limit for moderately to highly stressed, unconstrained components.
In the case of plastic components subjected to very little or no load at higher temperature, one should consider the continuous service temperature as a key characteristic (rather than the HDT) to make a proper material selection.
The ‘Continuous Service Temperature’ is generally reported as the temperature above which significant, permanent physical property degradation occurs after long term exposure. This value is most commonly defined as the maximum ambient service temperature (in air) that a material can withstand and retain at least 50% of its initial physical properties after long term service (approximately 10 years).
Most thermoplastics can withstand short-term exposure to higher temperatures without significant deterioration but the maximum temperature should not exceed the glass transition temperature of amorphous thermoplastics or the melting temperature of semi-crystalline thermoplastics.
Note, however, that the Continuous Service Temperature depends in many cases essentially on the duration and the magnitude of the mechanical stresses to which the material is subjected or in other words on the maximum deformation one can allow in a given application. When selecting materials for components operating at higher temperatures and subjected to load, both the Heat Deflection Temperature (HDT) and Continuous Service Temperature need to be considered.
Please consult Quadrant’s technical literature or product data sheets for temperature resistance ‘ratings’ [Heat Deflection Temperature (HDT) and Continuous Service Temperature] or reach out to Quadrant’s Technical Team for support.