What Is Heat Deflection Temperature (HDT)?

Heat deflection temperature (HDT) is the temperature at which a standard plastic test bar, loaded in bending, deflects by a set amount as it is steadily heated. Measured to ASTM D648 or ISO 75, it is the datasheet's quick indicator of how hot a plastic can get before it starts to soften under load.

In short: HDT tells you roughly when a plastic part stops holding its shape under load as the temperature rises — always read it together with the load it was measured at.

What Is Heat Deflection Temperature in Plastics?

HDT is also called heat distortion temperature, or deflection temperature under load (DTUL), after the ASTM title. Under ASTM D648, a rectangular bar is supported edgewise as a simple beam and loaded at its center to a maximum fiber stress of either 0.455 MPa (66 psi) or 1.82 MPa (264 psi). The loaded bar sits in a heat-transfer bath whose temperature rises at 2 ± 0.2 °C (3.6 ± 0.4 °F) per minute. The HDT is the bath temperature when the bar has deflected 0.25 mm (0.010 in).

Because the load is part of the result, an HDT value always needs its stress attached — for example "HDT 95 °C at 1.82 MPa." The same material reads higher at the lighter 0.455 MPa load, so comparing values measured at different loads is misleading.

ASTM D648 applies to rigid or semirigid materials at least 3 mm (1/8 in) thick. ISO 75 covers the same property, but ASTM states the two methods differ in technical content and their results should not be compared.

Reinforcement has a large effect. Glass fibers make a plastic stiffer and can raise its HDT substantially, even though they barely change the polymer's glass transition temperature. That is why a glass-filled grade often carries a much higher HDT than the unfilled base resin.

HDT vs Vicat Softening Point vs Glass Transition Temperature

	Heat deflection temperature (HDT)	Vicat softening point	Glass transition temperature (Tg)
What is measured	Temperature at which a loaded bar bends 0.25 mm	Temperature at which a flat needle penetrates 1 mm into the surface	Temperature at which polymer chain segments freeze or unfreeze
Load	0.455 or 1.82 MPa bending stress (ASTM D648)	10 N or 50 N on a 1 mm² needle	None — a material transition
Heating rate	2 °C/min	50 or 120 °C per hour	Depends on the test method
Changed by glass fiber?	Yes, often greatly	—	Not noticeably
Best used for	Comparing short-term heat resistance of rigid parts	Softening point of materials with no sharp melting point	Understanding the polymer itself

How to Use HDT When Choosing a Material

Treat it as a comparison tool, not a rating. ASTM D648 states the data are not intended for design or for predicting endurance at elevated temperatures.
Match the load. Compare grades only at the same stress, and use the 1.82 MPa value when the part carries significant load when hot.
Leave a margin. A part that must hold tight tolerances near its HDT will already be softening; choose a material whose HDT sits comfortably above the service temperature.
Test the real part. Wall thickness, ribbing and molded-in stress all change how a part behaves in heat, so validate at temperature before production.

At NICE Rapid, heat-exposed prototypes can be built in SLA high-temperature resin (HDT above 180 °C / 356 °F) or cast in high-temperature PU (HDT above 120 °C / 248 °F) through vacuum casting.

Frequently Asked Questions