Open almost any thermoplastics datasheet and you will find two numbers sitting next to each other under “thermal properties.” One is heat deflection temperature, HDT for short. The other is Vicat softening temperature, usually abbreviated VST. They are printed in adjacent cells, both in degrees Celsius, and people read them as two flavors of the same thing. They are not the same thing, and treating them as synonyms is how parts end up sagging in service. HDT asks: how hot can this loaded part get before it bends too far? Vicat asks: how hot can this surface get before something starts sinking into it? One is about structure holding load.…