Heat-Resistant 3D Printing Materials: How to Read HDT, Vicat and Tg

The Short Answer

No single number tells you whether a printed part will survive a temperature. Heat deflection temperature (HDT), Vicat softening temperature and glass transition temperature (Tg) come from short laboratory tests, and the standards behind HDT warn against reading it as a maximum service temperature. Use them to rank materials, then check the part at its real temperature, load and duration. In the manufacturers' datasheets below, PETG sits lowest, ABS, ASA and polycarbonate sit in the middle, and carbon-fiber nylon, PPS-CF and high-temperature resins reach highest, but how high depends on the test load and on annealing or cure.

Three Numbers, Three Different Tests

Datasheets usually print three heat numbers side by side. They come from different tests and answer different questions.

HDT: Why the Load Matters

ISO 75-1 loads a bar in three-point bending, raises the temperature at a set rate and records the temperature at which the bar deflects a set amount. ISO 75-2 defines three loads: method A at 1.80 MPa, method B at 0.45 MPa and method C at 8.00 MPa. Datasheets that use the American method, ASTM D648, quote 0.455 MPa and 1.82 MPa, and ASTM states that D648 and ISO 75 results shall not be compared. The lower load gives the higher number, and the gap between the two says a lot about the material:

• Polymaker's PolyLite PETG lists 75 °C at 1.8 MPa and 78 °C at 0.45 MPa. In the PolyLite PETG, PolyLite ABS, Polymaker ASA and PolyLite PC datasheets, the two values sit within 4 °C of each other.
• Polymaker's PolyMide CoPA, a nylon 6 and 6,6 copolymer, lists 70 °C and 111 °C. Its headline heat resistance of up to 180 °C matches its Vicat value, not either HDT.
• Polymaker's Fiberon PPS-CF10 lists 133 °C and 252.5 °C.
• Siraya Tech's Sculpt Ultra White resin lists one figure, 250 °C at 0.455 MPa (ASTM D648). A datasheet that quotes only the lower load gives you the higher number; ask the maker for the other.
• For a part that carries load while hot, compare the 1.8 MPa values. The 0.45 MPa value says more about lightly loaded parts. Neither is a rating.

Vicat Softening Temperature

Vicat (ISO 306, ASTM D1525) records the temperature at which a loaded needle penetrates a specimen a specified depth. ISO 306 defines four methods, with a 10 N or 50 N force and a heating rate of 50 °C or 120 °C per hour, and calls the result the temperature at which a thermoplastic starts to soften rapidly. It applies only to thermoplastics, so the resin datasheets cited here list none. The method changes the result: Forward AM's Ultrafuse PA lists 160 °C with 50 N and 189 °C with 10 N, measured on printed specimens. In every filament datasheet cited here, Vicat reads at or above HDT at 1.8 MPa, so treat it as where softening starts, not as a working limit.

Glass Transition: Amorphous vs Semi-Crystalline

Tg is where the amorphous regions of a polymer turn from glassy to rubbery. The University of Cambridge's DoITPoMS notes a large fall in stiffness around Tg, and that its value depends on the measurement method and heating rate, so there is no single exact Tg. Polymaker notes that Tg usually marks the highest working temperature of an amorphous polymer such as PETG, ABS, ASA or PC. Semi-crystalline polymers such as nylon and PPS also contain crystals, and the Polymer Science Learning Center explains that only the amorphous portion goes through the glass transition while the crystals melt at a higher temperature. That is why their HDT can sit far above their Tg:

Why None of Them Is a Service Temperature

The standards say so directly. ISO 75-1 states that its results do not necessarily represent maximum applicable temperatures, because time, loading conditions and surface stress in practice differ from the test. ASTM D648 says its data are not intended for use in design or for predicting endurance at elevated temperatures, and ASTM D5592 calls the single-point values on datasheets inadequate for engineering design. Polymaker's datasheets describe their values as reference and comparison figures that can vary significantly with printing conditions, and Polymaker prints its test specimens fully solid. A long-term rating looks different: UL defines its relative thermal index (RTI), from hot-air aging under UL 746B, as the maximum service temperature at which a critical electrical or mechanical property will not be unacceptably compromised by thermal degradation over a reasonable product lifetime. None of the datasheets cited here lists an RTI, and an RTI describes aging of the material, not deformation of a part under load.

Datasheet Values for Printed Thermoplastics

Each row is one manufacturer's datasheet value for one named grade, tested to ISO 75 and ISO 306. Siraya Tech lists HDT as method A (1.80 MPa) and method B (0.45 MPa). These are examples of how far apart materials sit, not values for the material on your quote.

Reading the Table

• Polymaker recommends annealing Fiberon parts (100 °C for 16 hours for the nylons, 125 °C for 16 hours for PPS-CF10) and annealed its mechanical test specimens the same way.
• Grades differ even within one maker: Polymaker's PolyLite PC and PolyMax PC list 107 °C and 99 °C at 1.8 MPa.
• Nylon changes with moisture. In the Fiberon PA6-CF20 datasheet, X-Y Young's modulus falls from 8636.5 MPa dry to 2508.1 MPa after a 48-hour soak in 60 °C water (nylon moisture conditioning).
• Every family in the table is in our FDM catalog: PETG, ABS, ASA, polycarbonate, PA6, PA12, PA6-CF, PA12-CF, PPS-CF and PPA-CF. For the trade-offs beyond heat, see polycarbonate, nylon grades and PETG vs ASA outdoors.

High-Temperature Resin Values

A resin's HDT is tied to the post-cure listed beside it, so the cure is part of the number.

What the Resin Values Show

• Cure moves the number. Liqcreate's longer UV cure adds 8 °C at 1.8 MPa and 20 °C at 0.45 MPa, and Henkel states that Loctite 3D 3955 needs a staged oven cure, including 6 hours at 190 °C, to reach its listed properties.
• Heat resistance can cost toughness. Liqcreate lists a notched Izod impact of 13 J/m for Flame Retardant HDT against 31 J/m for its Clear Impact, and notes discoloration above 200 °C.
• A clear resin is not a heat resin. Liqcreate notes that Clear Impact parts soften above their HDT and show cracks and yellowing above 100 °C (clear 3D printed parts).
• Our High-Temperature Resin is printed on SLA; for tough and flexible resins, see engineering SLA resins.

Annealing and Post-Cure: Gains and Costs

Annealing holds a finished part at an elevated temperature for hours. For semi-crystalline materials it raises heat resistance: Polymaker says annealing brings its nylons to their highest crystallinity and best thermal and mechanical properties, and that PPS-CF10 does not reach its full heat resistance without it. The costs, from the same makers:

• Toughness can fall. Siraya Tech's Fibreheart PPA-CF datasheet shows annealing raising HDT at 1.80 MPa from 82.5 °C to 119 °C while Charpy impact strength drops from 11 to 6.3 kJ/m².
• Dimensions change. In Polymaker's shrinkage test on a 40 mm Fiberon PA12-CF10 model, the Z dimension measured 39.79 mm after printing and 39.48 mm after annealing, and Polymaker warns that annealing other semi-crystalline materials risks deforming the part.
• Nylon leaves the oven dry and then absorbs moisture again.
• Amorphous plastics gain little. Polymaker says annealing ABS, ASA and PETG mainly improves dimensional stability and reduces residual stress.
• Annealing is not one of the post-processing options we list, so for a part ordered as printed, compare against the as-printed value where a datasheet gives both.

Creep: Heat, Load and Time Together

An HDT bar only has to hold its shape while the temperature ramps up. A real part may carry a load at temperature for months, and creep is the slow, continuing deformation of a plastic under a constant load. Creep has its own tests, ISO 899-1 (tension) and ISO 899-2 (bending), which ISO describes as data for engineering design, tested over a broad range of stresses, times and environmental conditions. None of the filament datasheets cited here reports creep. In practice:

• Stiffness falls as a plastic approaches its glass transition, so a part that is stiff enough at room temperature can sag slowly when warm, even below its HDT.
• Compare the stress in your part with the test stress. A load that is light for the part as a whole can be far above 1.8 MPa in a thin rib, a snap hook or around a screw boss.
• Layer direction matters. Fiberon PA6-CF20 lists 109.3 MPa tensile strength along layers and 54.0 MPa across them, so plan a hot, loaded part with print orientation in mind.
• If failure of a hot, loaded part could injure someone (overhead loads, people, vehicles, pressure), it needs your own engineering and testing at temperature. We do not provide load or temperature ratings.

Worked Example: Screening a Shortlist

Hypothetical example: a clip that holds a cable inside an enclosure running at 90 °C. Screening on HDT at 1.8 MPa from the table, PolyLite PETG (75 °C), PolyMide CoPA (70 °C) and as-printed Fibreheart PPA-CF (82.5 °C) drop out. PolyLite ABS, Polymaker ASA, Fiberon PA12-CF10 and PolyLite PC clear 90 °C by 8 to 17 °C, and Fiberon PA6-CF20 and PPS-CF10 by more, though Polymaker recommends annealing both. A margin of a few degrees on a short ramp test is not a margin for months of service, so the shortlist is where testing starts: print the clip in two or three candidates and hold it at temperature, under its real load, for as long as you can.

Checklist: Specifying a Hot Part

• Temperature: ____ °C peak, ____ °C while running, for ____ hours at a time.
• Load: what the part carries while hot, and where the stress concentrates.
• Allowable change: how much sag, creep or dimensional change still works.
• Environment: humidity (nylon absorbs moisture), sunlight, oils or cleaners, and heat cycling.
• Basis of every number you compare: ISO 75 or ASTM D648, 1.8 or 0.45 MPa, annealed or as printed, and the resin's post-cure.
• Grade: datasheet values belong to one manufacturer's grade, not to the polymer.
• Proof: a prototype tested at temperature under its real load before you commit to a batch (validation testing).
• When you upload to the quote page, tick Engineering / DFM review, and send the temperature, load and duration through the contact page. An engineer checks printability and material choice at no cost and with no obligation to order. It is a printability review, not engineering validation.