HDT is an abbreviation for “Heat Distortion Temperature” or “Heat Deflection Temperature” as it relates to these types of materials. It measures a material’s ability to resist a force (flexing or a weight pressing down) on a swatch of material when exposed to different temperatures.
For example, the HDT of a 40A durometer urethane rubber is below 32°F / 0°C, as a swatch of the rubber that measures 6” x 1” 0.25” (15.24cm x 2.54cm x .64 cm) will have some flexibility. A general purpose liquid plastic with the same dimensions that has a durometer of 85D (such as Smooth-Cast™ 385) will be rigid and unbendable at 32°F / 0°C.
Know that different materials have different HDT’s and that choosing a material that has an insufficient HDT for your specific application can result in material failure Recommended; if the cured material you will be using for your application will be exposed to elevated temperatures, either direct or indirect, know the HDT of the material you are considering before you purchase.