HDT vs. continuous use temperature (RTI) explained

Data sheets list several heat numbers, and the biggest one is rarely the one to design to. For a ranking of resins by heat, see high-temperature plastics.

What HDT measures

ASTM D648 and ISO 75 load a rectangular bar in three-point bending at a fixed stress, put it in a heated bath, and raise the temperature at 2°C (3.6°F) per minute. The HDT is the temperature at which the bar has deflected a set amount, 0.25 mm in ASTM D648.

Two stresses are standard: 0.45 MPa (66 psi) and 1.8 MPa (264 psi). ISO 75 calls them method B and method A, and has a rarely used 8.0 MPa method C. The load matters a lot. Unfilled polypropylene is about 50–60°C (122–140°F) at 1.8 MPa and about 80–105°C (176–221°F) at 0.45 MPa. Always compare grades at the same load.

ASTM D648 tests a 127 × 13 × 3.2 mm bar on edge. ISO 75 usually tests the 80 × 10 × 4 mm ISO bar lying flat. ASTM and ISO values for the same grade are close but not identical. See how to read a resin data sheet.

HDT tells you how stiff a material stays under a modest load for a few minutes as it heats. It is useful for ranking materials and judging short heat events such as a paint bake. It says nothing about years of oxidation or creep under a sustained load.

Why glass-filled semi-crystalline resins show such high HDT

In an amorphous resin such as polycarbonate, stiffness falls off sharply at the glass transition temperature (Tg), so HDT at 1.8 MPa lands roughly 10–25°C (18–45°F) below Tg. Polycarbonate has a Tg of about 145–150°C (293–302°F) and an HDT of 124–132°C (255–270°F). Glass fiber adds little: 30% glass-filled PC is only about 140–150°C (284–302°F).

A semi-crystalline resin has crystals that hold together until the melting point (Tm), plus amorphous regions that soften at Tg. Unfilled, a 1.8 MPa bar bends not far above Tg: about 70–90°C (158–194°F) for nylon 66 and 55–65°C (131–149°F) for PBT. Add 30% glass and the fibers, tied together by the crystals, carry the load until the crystals melt. Nylon 66 GF30 reaches 240–255°C (464–491°F), close to its 260–265°C (500–509°F) melting point. See Tg vs. melting point.

Vicat softening temperature

ASTM D1525 and ISO 306 press a flat-ended needle with a 1 mm² tip into the specimen under 10 N (method A) or 50 N (method B), heating at 50 or 120°C (90 or 216°F) per hour. The Vicat softening temperature is where the needle has sunk 1 mm. It reads higher than HDT at 1.8 MPa for the same grade, and it is most useful for amorphous resins.

Continuous use temperature and UL RTI

Under UL 746B, specimens are aged in hot-air ovens at several temperatures and tested at intervals until a property falls to 50% of its starting value. The results are extrapolated to lower temperatures and compared with a control material whose field performance is known. The RTI is often described as the temperature at which the property should still be at half its starting value after about 100,000 hours, though the correlation times used in the program can be shorter. There are three RTIs:

• Electrical: based on an electrical property such as dielectric strength.
• Mechanical with impact: based on an impact property. It applies to parts that see shock or vibration.
• Mechanical without impact: based on a strength property such as tensile strength.

RTIs are listed by thickness on the grade's UL yellow card. The impact RTI is often the lowest, because toughness goes first as a resin oxidizes. A grade that has not been through the long-term aging program may show only a generic RTI for its polymer family, which can be well below what a tested grade earns. The end-product standard decides which RTI applies to your part. See UL 94 flame ratings explained for how yellow cards work.

A "continuous use temperature" on a data sheet without a UL listing may come from the producer's own aging test or from an IEC 60216 temperature index, which is commonly based on 20,000 hours. Ask what test, property and endpoint are behind it.

HDT vs. RTI for common resins

Typical values. HDT ranges follow our material pages. RTI depends on the grade, thickness, color and which of the three RTIs you read, and many grades carry no tested RTI at all. Check the yellow card for your exact grade.

Why an HDT of 250°C (482°F) can mean only 130–150°C (266–302°F) long term

HDT measures stiffness over minutes. The long-term limit is set by oxidation: in hot air, polymer chains slowly break, and the part loses elongation and impact strength first, then tensile strength. Glass fiber does not stop that. Heat stabilizers slow it, which is why heat-stabilized grades exist. So a heat-stabilized PA66 GF30 with an HDT of 250°C (482°F) is typically good for something like 130–150°C (266–302°F) continuous, depending on the property, thickness and stabilizer package.

Short trips above the RTI are often acceptable, but how far and how long depends on the grade, the stress and the environment. Under sustained load, a part can also creep well below its HDT. Hot water, oil and coolant change the picture again; see plastics for hot water and steam.

The reverse also happens. Unfilled PEEK has an HDT of only 150–160°C (302–320°F), held down by its Tg, but electrical and strength RTIs around 240–260°C (464–500°F) because it resists oxidation so well; the impact RTI of a common grade is lower, about 180°C (356°F). A lightly loaded PEEK part can live above its HDT; a heavily loaded one cannot, which is why glass- or carbon-filled PEEK is used for hot structural parts.

Which number to use

1. Write down the heat profile: the peak temperature and how long it lasts, and the continuous temperature.
2. For short peaks, compare HDT at a stress close to what the part sees.
3. For years of heat, use the RTI for the property that matters: impact for parts that take knocks, electrical for insulators.
4. For load at temperature, ask for creep data or test parts. See parts softening under heat.
5. Confirm the exact grade on its data sheet and yellow card, at your wall thickness.

What to send us for a quote

• Peak and continuous temperatures, and the load on the part
• Any RTI, UL 94 or end-product standard requirement
• The grade you run now, or the part and resin family
• Quantity and ship-to city

We aim to send your quote within one to three business days.

Common questions