Heat Resistance

Heat resistance is not a single material specification, it is a sourcing decision that selects an entire material family. A connector that survives a 5-second 260 C reflow oven, a turbocharger duct that lives at 220 C continuous for five years, and a sterilization tray autoclaved 1,000 cycles at 134 C are three different problems with three different answers. Pretending one resin solves all of them is the most expensive mistake we see on incoming RFQs.

This guide is a comparative shopping list for engineers who already know they need heat tolerance and want to land on the cheapest material that meets the duty cycle. We cover thermoplastics from PC at 115 C continuous use up to PEEK at 250 C, and metals from aluminum 6061 to Inconel 718. The cost ladder is real: PC sits at roughly USD 4 per kilogram, PEEK at USD 90 to 120 per kilogram, Inconel at USD 35 per kilogram of stock plus heavy machining cost. Picking one tier higher than you need can multiply the program cost by ten.

We use HDT (heat deflection temperature, measured at 0.45 or 1.82 MPa load), CUT (continuous use temperature, the temperature at which the material survives long-term oxidation and creep), and short-term excursion limits as the three numbers that should appear on every drawing. Spec one without the others and you will build a part that passes incoming inspection and fails in the field.

HDT at 0.45 MPa is the marketing number, almost always 30 to 60 C higher than the same resin at 1.82 MPa. Quote at 1.82 MPa unless your part is genuinely unloaded. CUT is from UL 746B and represents 100,000-hour service with under 50 percent property loss; this is the number you commit to in a service-life calculation. Glass transition Tg is informative for amorphous resins (PEI, PC, PSU) because mechanical properties drop sharply through Tg; for semi-crystalline resins (PEEK, PPS) the melt temperature Tm sets the upper bound and stiffness drops gradually.

Watch for oxidation behaviour. PEEK in air loses 50 percent tensile strength after 5,000 hours at 260 C, but in nitrogen it survives 20,000 hours. PPS yellows but holds modulus; PEI hydrolyzes if it sees steam continuously. Aluminum loses 50 percent yield strength at 200 C; stainless 316 holds full strength to 500 C; Inconel 718 holds to 700 C and is used to 870 C with strength derating. None of those numbers transfer between data sources without checking the test atmosphere.

The benchmark below is an injection-molded 80 mm electrical housing, 5,000 units annually, FOB Taiwan, tooling amortized. The cost ladder is steep. Move up only when the duty cycle truly requires it.

Use this ladder to sanity-check your candidate before deep diligence. Each step roughly doubles material cost, so the right answer is almost always the lowest step that meets the worst hour of your duty cycle plus a 20 C margin.

Numbers below are typical injection-grade or wrought-stock values. Always pull the supplier datasheet for your specific lot, especially for filled grades where Tg, HDT and CUT shift with filler content.

High-temperature resins demand more from the process than commodity resins. PEEK injection requires barrel temperatures of 380 to 400 C, hot mold at 180 to 200 C, and dried resin under 50 ppm moisture. PPS and PEI tolerate slightly less, but if your molder is not equipped for hot-runner systems above 350 C and high-flow drying, you will see splay, voids, and crystallinity defects.

Map duty cycle to material with this matrix. The third column is the cheapest material we would actually quote, not the safest one.

Sterilization is the second axis after raw temperature. A material can hold 134 C for 1,000 cycles dry but fail in 100 cycles of saturated steam. The matrix below covers the four common sterilization regimes plus repeated chemical wipe-down.

CTE mismatch will kill your assembly faster than any single material limit. PC grows 0.065 mm per meter per degree Celsius, aluminum 0.024, steel 0.012. Design a polymer housing screwed to a steel frame on a 200 mm span and a 100 C swing produces 1.0 mm of differential growth, enough to either crack the boss or strip the screw. Slot one set of holes, use compliant brass inserts, or pick a glass-filled grade with CTE near 0.020.

For loaded parts above 100 C, design for creep, not yield. Long-term stress should stay below 30 percent of the room-temperature yield, and you should derate by another 30 percent at the upper service temperature. Walls in PEI and PEEK should be at least 1.5 mm; below that, weld lines become hot crack initiation sites. Avoid sharp internal corners (R less than 0.5 mm) on filled grades, because residual stress concentrates there and oxidation cracks initiate after a few hundred thermal cycles.

When metal inserts are unavoidable, choose brass over steel for thermoset and PEEK overmolds: brass CTE around 0.019 sits between steel and most filled polymers, reducing residual hoop stress. For aerospace and medical, specify the post-mold annealing schedule (PEI: 4 hours at 200 C; PEEK: 6 hours at 200 C) on the drawing, otherwise field parts will distort during their first thermal cycle.

A Penang EMS asked us in 2025 to replace their FR4 carrier trays after warpage caused 0.3 mm BGA misalignment on a high-mix line. Profile peaks at 260 C for 90 seconds, full cycle 6 minutes. We CNC machined carriers from 6 mm Ultem 1000 plate with a milled BGA pocket array and four steel locating pins. Per-carrier cost USD 142 in a 60-piece run, lead time 9 business days.

After 8,000 cycles flatness was rechecked at 0.04 mm across 350 mm, well inside the 0.10 mm tolerance. Yield on the BGA placement step recovered from 96.2 percent to 99.4 percent. The customer ordered another 240 carriers over the next year. We did consider PEEK, but the dwell at peak was short enough that PEI's USD 60 cost saving per carrier paid for itself; a PEEK quote came in at USD 205 per carrier.

A US-listed surgical OEM needed a tray validated for 1,000 autoclave cycles at 134 C with a 5-year service life. We injection molded the tray in PPSU (Radel R 5500) on a 350-ton press with hot mold at 160 C and dried resin to 30 ppm. Tooling USD 42,000 amortized over 8,000 trays, per-part landed at USD 18.50 including bagging.

The first article validation ran 1,200 sterilization cycles in 14 weeks with no measurable hydrolysis or color shift. We chose PPSU over PEI because PEI's 200-cycle steam limit would have failed the 5-year life requirement. Annual run rate is 6,000 trays. The OEM expanded the program to two more tray geometries the following year, total program revenue USD 184,000.

A Tier 1 automotive supplier needed an oil-temperature sensor housing for a 2026 hybrid powertrain. Worst-case continuous temperature 150 C, peak 180 C, oil immersion, 15-year service life. We molded the housing in PPS-GF40 (Fortron 1140L4) at USD 2.80 per part in a 25,000-unit annual run. Tooling USD 78,000 amortized over three model years.

We initially proposed PA66-GF50, but the long-term oil-aging test at 150 C showed 18 percent tensile loss after 3,000 hours, outside the OEM spec. PPS dropped only 4 percent at the same point. PEEK was overspec for this temperature window and would have raised the part cost to USD 11.40, killing the sourcing target. The program is now in series production at 25,000 housings per year for three years.

Pick the cheapest material that survives the worst hour of your duty cycle plus 20 C of margin, then derate strength for long-term creep and oxidation. Most heat-resistance failures come from spec-shopping the wrong axis: people pay for PEEK when they needed steam compatibility, or use PEI when they needed long-term steam life. Identify the binding constraint (peak temperature, total cycles, atmosphere, CTE match) and let it drive the family selection.

Process readiness matters as much as resin choice. PEEK and PEI need molders with hot runners above 350 C, dehumidifying dryers, and mold heating to 180 C. If the molder is two tiers below that capability, the resin you specified does not matter; the parts will fail incoming inspection. Validate molder capability with a first-article report on a representative section before you write the production purchase order.