DefinitionHeat 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.
Explained
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.
Key takeaways
- HDT measures the temperature at which a loaded plastic bar deflects 0.25 mm while heating at 2 °C/min.
- Always compare HDT values measured at the same stress (0.455 MPa or 1.82 MPa).
- ASTM D648 and ISO 75 test the same property but use different technical content; their results should not be compared.
- Glass fiber reinforcement can greatly raise HDT without noticeably changing the glass transition temperature.
- Use HDT as a comparison tool, not a design rating or continuous-use temperature.
Example
Related terms
References
- ASTM D648-18, Standard Test Method for Deflection Temperature of Plastics Under Flexural Load in the Edgewise Position — ASTM International
- Plastics Science & Engineering, Module 3 — Birla Institute of Technology, Mesra
- Effect of 20 wt% Glass Fiber Reinforcement on the Mechanical Properties and Microstructure of Injection-Molded PA6 and PA66 — Polymers (Basel) 2026;18(3):357