HDT at 0.45 MPa vs 1.8 MPa explained

Heat deflection temperature, or HDT, is the most quoted “how hot can it go?” number on a plastics datasheet. It is also one of the most misunderstood. The same material can show two very different HDT values, and neither is a safe operating temperature. Here is what the number really means.

What HDT measures

A rectangular test bar is supported near its ends and loaded in the middle, so it is in bending. It sits in a heated bath or chamber, and the temperature rises at a steady rate. The HDT is the temperature at which the bar bends by a set small amount.

In other words, HDT tells you when a material gets soft enough to deflect under one specific stress, during one specific short heating test. It is a comparison tool, not a design limit.

The two common loads

Both ISO 75 and ASTM D648 test at one of two bending stresses:

ISO 75 also defines a much higher 8 MPa load (Method C), which you will rarely see for printing materials. Be careful with the letters: ASTM D648 also uses “Method A” and “Method B”, but there they describe test procedure, not the load. So always read the stress value, not the letter.

Why 0.45 MPa gives a higher number

A lighter load needs the material to be softer before the bar bends by the set amount. So the bar survives to a higher temperature. The same material always shows a higher HDT at 0.45 MPa than at 1.8 MPa.

How big the gap is depends on the polymer:

• Amorphous polymers such as ABS, PC and many resins soften fairly sharply near their glass transition. Their two HDT values tend to sit closer together.
• Semi-crystalline polymers such as nylons, PP and annealed PLA keep some stiffness above their glass transition because the crystals hold the structure together. The gap between their two values can be large.
• Fibre-filled grades often raise the 1.8 MPa value a lot, because the fibres add stiffness at temperature.

This is why mixing the two loads leads to wrong conclusions. PLA materials in our catalog report HDT values of 48–137 °C (median 55 °C, 34 materials). That range pools values at both loads, plus annealed and standard grades, which is part of why it is wide. Open each material page to see which load applies.

1. PLA 3148–137
2. PETG 1558–78
3. ABS 1770–118.1
4. PA12 1153–165
5. PC 1299–163
6. PEI 13152–215.12
7. PEEK 11140–305

Bar = lowest to highest, dot = median. 110 materials, live from our database.

HDT is not a maximum service temperature

HDT is a short-term test on a small bar with a fixed stress. Your part may be:

• Loaded for hours, days or years. Plastics creep, so sustained loads cause deformation at temperatures well below HDT.
• Loaded more heavily, or with stress concentrated at a hole or corner.
• Printed with layer lines and infill, which are not in the test bar.
• Exposed to moisture, chemicals or UV at the same time.

Annealing and post-cure

Post-processing can change HDT a lot, so check the sheet’s footnotes.

• Annealing (heating a printed part below its melting point and letting it crystallise) can raise HDT substantially for semi-crystalline materials such as PLA and nylons. It can also cause shrinkage and warping, so allow for it in your design.
• Thermal post-cure of resins, after UV curing, can increase crosslinking and push HDT up. Many high-temperature resins only reach their published HDT after a specific heat cycle.
• Green or under-cured resin parts will fall well short of the published value.

HDT, Tg and Vicat: how they relate

Datasheets often list three thermal numbers. They measure different things:

For amorphous materials, HDT at 1.8 MPa usually sits somewhat below Tg. For semi-crystalline materials, HDT at 0.45 MPa can sit well above Tg. Vicat values tend to be higher than HDT for the same material. None of the three is a safe working temperature on its own. Together they give a clearer picture of how a material softens. See glass transition temperature and Vicat softening temperature.

Highest HDT resins in our catalog

Here are the resins with the highest reported HDT. Loads and post-cure conditions vary, so check each material page.

Reading HDT on the hub

Each HDT value on the hub is shown with its test method and load, exactly as the manufacturer printed it, whether that is written in MPa or psi. When a sheet gives both loads, we note which one is shown. Some sheets also state the heating rate or a post-cure condition; where they do, that detail appears with the value. Ranges and medians across a family pool all of these together, so use them as a rough guide and open the material pages before comparing two grades directly.

Quick checklist

1. Compare HDT values only at the same load.
2. Note the post-processing state: as printed, annealed or post-cured.
3. Leave a generous margin below HDT for any loaded part.
4. Test at temperature for anything that matters.

For more context, see how to read a TDS and heat deflection temperature.

Find materials with HDT over 150 °C