Heat Deflection Temperature Testing

Have you ever seen an HDT number on a datasheet and thought "I can probably use the material up to that temperature"?

Well, that's not how it actually works. The standard ISO 75 / ASTM D648 tests commonly found in filament datasheets are fast tests that measure when the deflection crosses a threshold. The samples are under a constant load and the temperature increases linearly, and very fast, not allowing the material to fully relax at each step. This results in an overestimation of the usage temp, with the real CUT (Continuous Use Temperature) being much lower than even HDT/A.

I tested Extrudr DuraPro ASA, which has a 96° HDT/B (0.45MPa) rating on its datasheet, and tested it both at 1.8MPa (HDT/A inspired) and 0.45MPa (HDT/B inspired). The test ended when a deflection of 1mm was observed. The sample loaded at 0.45MPa failed at 91°, while the sample loaded at 1.8MPa (to match HDT/A) failed at only 80°. This is in line with the fact that black parts are under IR sources, so they will fail earlier than the lab test results. Both samples started deforming at the same time, and movement can already be seen at 60°, but as expected the more loaded one bent more and failed earlier. The early deformation is typical, as when the temperature increase the material becomes less stiff, but when cooled it can spring back to its original shape. When the deflection becomes too high the deformation will be permanent, and that depends on the load.

This shows that ABS/ASA shouldn't be in chambers reaching 80°, as they're unsuitable for load bearing parts.

For reference, the higher 1.8MPa load is only around 5% of the usual 35-45MPa tensile strength of most ABS filaments.