Materials
Heat deflection: choosing a plastic for under-bonnet parts
Heat deflection temperature (HDT) is where a loaded plastic bar bends by a fixed amount. It is a comparison number. Under constant load, allow a margin below it.
HenleyPrint3D engineering2 min readUpdated Published
Key takeaways
- PLA and PETG suit the cabin and cool areas. The engine bay needs ABS, ASA, PA12 or PEI.
- ABS and ASA suit cool corners of the bay, with margin.
- PA12 and PEI handle warmer zones; check fluids.
- Test the part at temperature. HDT is a ranking number and creep needs its own check.
Suits
Engineers choosing a printed plastic for parts near an engine, exhaust or heater.
Worth knowing
HDT ranks materials. Keep working temperature well below it, and test under load at temperature.
Typical HDT by material
| Material | HDT at 0.45 MPa | HDT at 1.82 MPa | Under-bonnet fit |
|---|---|---|---|
| PLA | about 55 °C | about 50 °C | Cabin at room temperature only |
| PETG | about 70 °C | about 65 °C | Cabin or cool areas only |
| ABS | about 95 to 105 °C | about 85 to 95 °C | Cool corners, with margin |
| ASA | about 95 to 100 °C | about 90 to 95 °C | As ABS, better UV resistance |
| PA12 (SLS) | about 160 to 175 °C | about 85 to 100 °C | Light loads; check fluids |
| PEI (ULTEM) | about 160 °C | about 150 °C | Warm zones, short exposure near hot parts |
ULTEM 9085 and 1010 are available on request. See the materials guide.
See the FDM materials and our automotive work. For reinforced options read carbon-fibre nylon and Kevlar 3D printing and PA12 nylon SLS.
Frequently asked questions
What is heat deflection temperature?
It is the temperature at which a standard test bar deflects a set amount under a fixed bending load, measured to ISO 75 or ASTM D648. It ranks materials. Continuous operating temperature sits well below it.
Why do datasheets give two HDT values?
They are measured at two loads, 0.45 MPa and 1.82 MPa. The lighter load gives a higher temperature. Use the value closest to the stress in your part.
Can I use ABS under the bonnet?
Only in cool areas with margin. With an HDT at 1.82 MPa of about 85 to 95 °C, ABS leaves little room in most engine bays. Polycarbonate, PEI or a fibre-filled nylon is safer.
Does annealing make a printed part more heat resistant?
It can raise heat resistance in some polymers, but it also changes dimensions, often by 0.5 to 2 percent. Test on a sample part before annealing production parts.
How do I measure my engine bay temperature?
Fix a thermocouple at the part's location and log it through a normal drive and a hot soak after shutdown. Design to the peak value.
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