Glossary · Value
Heat deflection temperature (HDT)
The temperature at which a plastic bends under a specified load. It shows roughly how warm a part may get.
Explained in more detail
HDT stands for heat deflection temperature. According to ISO 75, a standardised test bar rests on two supports, is loaded in the middle and heated evenly, usually at 120 K per hour. The value is reached when the bar deflects by a defined amount. There are three load levels: method A with 1.8 MPa, B with 0.45 MPa and C with 8 MPa, and the result depends strongly on it. Fiberlogy gives 64 °C at 1.8 MPa and 76 °C at 0.45 MPa for its PCTG. A related value is the Vicat softening temperature according to ISO 306, at which a loaded needle penetrates 1 mm deep.
Where does the term come from?
Heat deflection temperature literally means the temperature at which a part starts to bend under heat.
In practice
- Compare HDT values only at the same load: a value at 0.45 MPa is considerably higher than one at 1.8 MPa.
- Datasheet values usually come from standardised test bars. Infill, walls and print orientation affect how a printed part behaves.
- For parts in a car or near a heat source, keep a margin from the HDT instead of going right up to the limit.
- If the data sheet gives only a Vicat value, it is not directly comparable with an HDT.
Not to be confused
Heat deflection temperature is not the glass transition temperature: HDT is a test value under load, the glass transition a material property without load.
Frequently asked questions
What do A and B mean for HDT?
They give the test load according to ISO 75: A stands for 1.8 MPa, B for 0.45 MPa. The lower the load, the higher the measured value.
How hot can a printed part get?
As a rough guide, use the HDT from the datasheet. The load the part carries, how thick it is and how it was printed shift the limit. For critical parts, the only option is a test under real conditions.