Heat deflection temperature (HDT)

HDT is the temperature at which a loaded plastic bar starts to sag. Learn how it is tested and why the 0.45 and 1.8 MPa loads differ.

4 min read · Updated October 11, 2026 · Numbers update live from our materials database.

What it means in practice

Heat deflection temperature, also called heat distortion temperature, is the temperature at which a plastic bar under a fixed load bends by a set amount. It is the most common way to compare how well materials hold their shape when hot.

Key takeaways

HDT is the temperature at which a loaded plastic bar bends by a set amount.
It is a comparison tool, not a maximum service temperature.
Always check the test load: 1.8 MPa gives a lower HDT than 0.45 MPa.
Annealing and resin post-cure can change HDT, so check how the sample was made.

Filaments98 °Ctypical · range 40–305 · 214 materials

Resins57 °Ctypical · range 40–250 · 141 materials

Powders87 °Ctypical · range 46–182 · 16 materials

It matters for:

Parts inside cars, which can get very hot when parked in the sun.
Housings near motors, lamps, heaters or power electronics.
Fixtures used with hot processes, such as soldering or curing ovens.
Anything washed in hot water or near a kettle or coffee machine.

Good to know

HDT is a comparison tool, not a maximum service temperature. A part with low load may survive above its HDT. A thin, heavily loaded part may sag below it.

How it’s measured

A bar is supported at both ends and loaded in the middle. It sits in a bath or chamber that heats at a steady 2 °C per minute. The temperature is recorded when the bar has bent by a set amount.

Standards: ISO 75 and ASTM D648.
Loads: 0.45 MPa (ISO Method B) or 1.80 MPa (ISO Method A). ASTM uses 0.455 and 1.82 MPa. ISO also has a rarely used 8 MPa method.
Units: °C (or °F on some US sheets).

0.45 MPa (Method B)

The lighter load. It gives the higher reading for the same material.

1.80 MPa (Method A)

The heavier load. It gives a lower reading, sometimes much lower.

How to read and compare it

Always check the load. The same material gives a lower HDT at 1.8 MPa than at 0.45 MPa, sometimes much lower. Our catalog contains both, so compare only values with the same load. See HDT at 0.45 vs 1.8 MPa.
Annealing and crystallinity. Semi-crystalline materials such as PLA, nylon and PEEK can gain a lot of heat resistance after annealing. Check whether the value is for an as-printed or annealed part.
Resin post-cure. Many resins only reach their quoted HDT after a heated post-cure. A part cured differently may behave differently.
Printed vs moulded. Some values are from moulded bars of the base polymer, not printed ones.
Amorphous vs semi-crystalline. For amorphous plastics such as ABS and PC, HDT sits a little below the glass transition. Filled semi-crystalline plastics can stay rigid well above it.

Range in our catalog

These ranges mix both test loads, so treat them as a rough picture.

All materials: 40–305 °C (median 74 °C, 399 materials)
Filaments: 40–305 °C (median 98 °C, 214 materials)
Resins: 40–250 °C (median 57 °C, 141 materials)
Standard materials: 42–98 °C (median 55.6 °C, 88 materials)
High-performance materials: 90–305 °C (median 179.5 °C, 50 materials)

Heat deflection temperature of common filaments (°C, mixed loads)

PLA 3148–137
PETG 1558–78
ABS 1770–118.1
ASA 887–100
PC 1299–163
PA12 1153–165
PEEK 11140–305
PEI 13152–215.12

Bar = lowest to highest, dot = median. 118 materials, live from our database.

Watch out

Each bar mixes 0.45 MPa and 1.8 MPa results. A long bar may reflect the test load as much as the material.

Materials with an HDT of 100 °C or more carry our high-temp badge: 128 materials so far.

Highest and lowest in our catalog

Highest HDT

Material	Polymer	Heat deflection temperature
3DXTECH CarbonX 20% Carbon Fiber PEEK	PEEK	305 °C
3DXTECH ThermaX PEEK GF20	PEEK	300 °C
3DXTECH CarbonX Carbon Fiber PEKK-A CF15 [AERO]	PEKK	285 °C
LEHVOSS LUVOCOM 3F PEEK CF 9676	PEEK	280 °C
3DXTECH CarbonX Carbon Fiber PEEK	PEEK	265 °C

Highest HDT among resins

Material	Polymer	Heat deflection temperature
Kexcelled MOLEGRID AcryHeat	Photopolymer	>250 °C
Siraya Tech Sculpt Ultra White	Photopolymer	250 °C
Phrozen Ceramic Pro	Photopolymer	>230 °C
Phrozen TR300	Photopolymer	160 °C
Siraya Tech Sculpt	Photopolymer	160 °C

Lowest HDT

Material	Polymer	Heat deflection temperature
Fiberlogy rPP	PP	40 °C
Formlabs BioMed Durable Resin	Photopolymer	40 °C
Formlabs Durable Resin	Photopolymer	41 °C
Liqcreate Tough-X	Photopolymer	41 °C
Markforged Nylon	Nylon	41 °C

Open each material and check the test method before you compare rows.

Glass transition temperature — where a plastic turns from rigid to rubbery.
Vicat softening temperature — another softening test, without bending.
Melting temperature — the upper limit for semi-crystalline plastics.
Flexural modulus — the stiffness that HDT tests under heat.

For design advice, see high-temperature parts.

See materials with an HDT of 100 °C or more 128 materials