When Marco’s team in Modena prototyped a new EV battery pack connector in early 2025, they specified unfilled PA66 because the spec sheet listed a melting point of 265 °C. The first thermal-shock validation ran at 130 °C for 1,000 hours, and the connector body warped enough to break the seal. They re-specified PBT-GF30, kept everything else identical, and passed the same test without a single failure. The melting point had nothing to do with it.
That story plays out in procurement and design offices every week. Buyers compare polyamide vs polyester heat resistance using one number on a datasheet, and they’ve often picked the wrong resin before the first part ships. The reality is that “heat resistance” is at least four numbers, and polyamide and polyester each win in different operating windows.
This guide breaks down polyamide vs polyester heat resistance the way an experienced materials engineer actually evaluates it. You’ll see direct comparisons across PA6, PA66, PBT, and PET on melting point, glass transition, heat deflection temperature, and UL RTI. You’ll also learn where moisture, glass fiber, and humidity quietly change the answer, plus a procurement checklist you can hand to a supplier today.
What Heat Resistance Really Means for Polyamide and Polyester
Most datasheets list four thermal numbers, and each one answers a different engineering question. Treating them as interchangeable is the single most common mistake in polyamide vs polyester heat resistance comparisons.
Melting Point Is the Ceiling, Not the Service Limit
Melting point (Tm) tells you the temperature at which the crystalline phase of a semi-crystalline resin flows. For polyamide, PA6 melts near 220 °C and PA66 near 260–265 °C. For polyester, PBT melts near 220–225 °C and engineering-grade PET near 250–260 °C.
These numbers matter for processing, not service. You shouldn’t run a part anywhere near its melt temperature in continuous use. Treat Tm as a processing window, not a performance ceiling.
Heat Deflection Temperature Is the Practical Benchmark
Heat deflection temperature (HDT, also called DTUL) is measured by ASTM D648 or ISO 75. The test loads a bar with 0.45 MPa or 1.82 MPa of bending stress, heats it, and records the temperature at which the bar deflects 0.25 mm. The 1.82 MPa value is the one engineers usually quote.
For unfilled grades at 1.82 MPa, PA66 sits around 70–90 °C, PA6 around 55–70 °C, PBT around 50–65 °C, and PET around 65–75 °C. Glass fiber changes everything. We’ll get to that in a moment.
UL RTI and Continuous Use Temperature Are the Long-Term Reality
UL 746B Relative Thermal Index (RTI) is the temperature at which a resin can operate continuously for the equivalent of 100,000 hours without losing more than 50% of a critical property. It’s the closest thing the industry has to a service-life rating, and it’s published in the UL 746B Polymeric Materials evaluation program.
Typical RTI Electrical values run roughly 120–130 °C for unfilled PA66, 130–140 °C for PBT, and 140–150 °C for properly stabilized PET. Notice that polyester families often beat polyamide on long-term continuous service, even though PA66 has the higher melting point. That counterintuitive result is exactly why this comparison matters.
Need help matching the right thermal class to your part? Our polymer specialists can walk you through HDT and RTI requirements before you cut a single mold. Talk with a Yifuhui engineer →
Polyamide Heat Resistance: PA6 and PA66 in Engineering Service
Polyamide, commonly called nylon, is one of the workhorses of automotive, electrical, and industrial molding. Within the family, PA66 carries higher thermal numbers than PA6 because of its more symmetric crystalline structure. For deeper context on that structural difference, see our companion article on the molecular backbone of polyamide and polyester.
Unfilled PA66 vs PA6 in Hot Service
Unfilled PA66 typically holds tensile strength reasonably well up to about 100–120 °C in short-term exposure. Unfilled PA6 starts to lose stiffness sooner, often near 80–100 °C. Both are semi-crystalline, so service performance depends heavily on crystallinity, which depends on mold temperature.
The dry-as-molded HDT of PA66 at 1.82 MPa is commonly listed near 70–90 °C. PA6 sits 10–20 °C lower. Those numbers are honest only for parts that stay dry.
How Moisture Quietly Drops Polyamide HDT
Here’s a detail competitors usually miss. Polyamide absorbs water, and that water plasticizes the amorphous regions of the polymer. A PA66 part conditioned to 50% relative humidity can see its HDT drop 25–40% compared to dry-as-molded. PA6 absorbs even more moisture and shifts further.
PA66 picks up roughly 1.0–1.5% water in 24 hours at 23 °C, while PA6 picks up 1.5–2.0%. By contrast, PBT absorbs about 0.08% and PET about 0.1%. The polyester families are essentially indifferent to humid air. Polyamide isn’t.
For under-the-hood or dishwasher parts, you must specify and test polyamide in its conditioned state, not dry-as-molded. Procurement teams who only review the dry HDT number frequently overestimate real-world performance.
Glass-Filled PA66 Is the Real Workhorse
Add 30% glass fiber to PA66 and the HDT at 1.82 MPa jumps from roughly 75 °C to 245–250 °C. The number is so dramatic because the fibers carry the bending load above the matrix Tg. PA66-GF30, PA66-GF35, and heat-stabilized PA66-GF30 HS variants dominate automotive intake manifolds, oil pans, and structural brackets.
Heat-stabilized PA66 (often labeled “HS” or “HR”) adds copper-iodide or amine antioxidant packages that extend retained tensile strength after 1,000 hours at 180–210 °C. That’s the difference between a 5-year warranty part and a 15-year warranty part.
Polyester Heat Resistance: PBT and PET for Engineering Use
Polyester in engineering plastics almost always means polybutylene terephthalate (PBT) or polyethylene terephthalate (PET). They share a backbone family with textile polyester but are formulated and processed for very different applications.
PBT Is the Connector Standard
PBT crystallizes quickly, has excellent dimensional stability, and absorbs very little water. Those three properties make it the default resin for surface-mount electrical connectors, automotive sensors, and switch housings. PBT-GF30 has an HDT at 1.82 MPa near 205–215 °C, and its RTI Electrical typically sits at 130–140 °C.
PBT also tolerates the 240–260 °C reflow soldering profile used in lead-free SMT assembly. If you specify polyamide for an SMT board, you’re usually fighting your material. PBT was designed for that environment.
PET Has Higher Melt Temperature but Trickier Crystallization
Engineering-grade PET, often supplied as PET-GF30 or PET-GF45, has a higher melting point than PBT and a higher unfilled HDT once it has crystallized fully. PET-GF30 can reach 225–235 °C HDT and RTI Electrical near 140–150 °C with proper stabilization.
The catch is that PET crystallizes slowly. Mold temperatures of 140–160 °C are common to develop enough crystallinity for thermal performance. Cold molds give you amorphous PET that distorts at low temperatures. That’s why PBT, even with lower melting point, often wins on the production floor.
Hydrolysis Is Polyester’s Achilles Heel
Polyester chain links are ester bonds, and ester bonds hydrolyze in hot, humid environments. Long-term exposure to steam, glycol, or wet under-hood air at temperatures above 100 °C can break polyester chains, drop molecular weight, and reduce tensile strength.
Modern PBT grades use carbodiimide or epoxy chain extenders to slow hydrolysis. Even with those packages, in saturated steam at 120 °C polyester degrades faster than polyamide. If your part lives in a dishwasher, an autoclave, or a steam-cleaned food line, polyamide often outlasts polyester despite the lower melting point.
A real procurement story: When Anita’s appliance team in Düsseldorf moved a steam-iron base plate from PBT-GF30 to PA66-GF30 in late 2024, they did it because the PBT version was failing field returns after 18 months of daily use. The PA66 version is now at 30 months with no returns. Polyester won the spec sheet. Polyamide won the field.
Comparing PBT and PA66 grades for your next part? Yifuhui ships both families with full datasheets, batch certificates, and 24-hour quote response. Request a comparative quote →
Side by Side: Polyamide vs Polyester Heat Resistance Data Table
The table below summarizes typical published values for the four resins. Always confirm against the specific grade datasheet from your supplier, because values shift with stabilization package, glass loading, and impact modifier.
| Property (typical, 1.82 MPa where applicable) | PA6 | PA66 | PBT | PET (engineering) |
|---|---|---|---|---|
| Melting point Tm (°C) | 215–225 | 255–265 | 220–225 | 250–260 |
| Glass transition Tg, dry (°C) | 50–55 | 50–55 | 45–55 | 70–80 |
| HDT unfilled (°C) | 55–70 | 70–90 | 50–65 | 65–75 |
| HDT 30% glass fiber (°C) | 195–215 | 245–250 | 205–215 | 225–235 |
| UL RTI Electrical, GF30 (°C) | 110–125 | 120–130 | 130–140 | 140–150 |
| 24 h water absorption (%) | 1.5–2.0 | 1.0–1.5 | 0.07–0.09 | 0.08–0.12 |
| Hydrolysis resistance | Good | Good | Moderate | Moderate |
| SMT reflow capable | No (most) | No (most) | Yes | Yes |
Two patterns jump out. First, glass-fiber reinforcement adds 130–170 °C of HDT to every resin, which often matters more than the family choice. Second, polyester wins on long-term electrical RTI and SMT compatibility, while polyamide wins on water resistance and on retained toughness in humid heat. To gain a deeper understanding of Polyamide vs Polyester Injection Molding and Polyamide vs Polyester Mechanical Properties, please refer to our accompanying guide.
When to Choose Polyamide and When to Choose Polyester
The choice between polyamide vs polyester heat resistance comes down to the operating envelope, not a single number. Use the following decision logic as a starting point.
Under-the-Hood Automotive
Air-intake manifolds, oil pans, and engine covers see 130–180 °C continuous, dry-ish air, and significant mechanical load. PA66-GF30 HS and PA66-GF35 dominate here because they combine high HDT, fatigue resistance, and good chemical compatibility with engine oils and coolants.
For sensor housings near very hot exhaust paths, specialty grades such as PPA (polyphthalamide) or high-temperature PA blends extend the envelope. PBT can serve quieter under-hood locations, especially connectors and switch bodies, but it loses to PA66 on structural parts.
Typical automotive choices:
- Structural parts (manifolds, brackets): PA66-GF30 HS or PA66-GF35
- Sensors and connectors: PBT-GF30 FR or PA66-GF30 FR
- Very hot exhaust zones: PPA or high-heat PA blends
Electrical and Electronic Connectors
For SMT-compatible connectors, motor controllers, and sensor housings on PCB assemblies, PBT-GF30 with FR additives is the standard. Its low water absorption keeps dielectric properties stable, and its quick crystallization gives the dimensional stability that fine-pitch connectors need.
PA66 can be specified with FR packages too, but it tends to need rigorous moisture conditioning before assembly. If your supply chain doesn’t have a reliable dry-bag process, polyester is the lower-risk choice.
Key points for connector selection:
- SMT reflow lines: PBT-GF30 FR is the default choice
- Dielectric stability: PBT and PET resist moisture, so their dielectric constant stays consistent
- PA66 on connectors: Possible, but requires strict dry-bag handling and moisture conditioning
- Fine-pitch geometry: PBT’s dimensional stability wins at tight tolerances
Appliances, Industrial, and Steam-Cleaned Parts
For dishwasher manifolds, washing machine motor end caps, coffee machine boilers, and steam-cleaned food contact surfaces, polyamide tends to win. PA66 tolerates hot wet exposure longer than PBT, and PA6 is widely used where cost matters more than peak temperature.
A second real-world example: David’s design team at a North American appliance maker had a coffee-machine steam-valve housing in PBT-GF30 that started cracking after 24 months. The team switched to PA66-GF30 with copper-iodide heat stabilization, kept the geometry identical, and the part now passes their 5-year accelerated wet-heat aging protocol. The total resin cost increase was around 8% per kilogram. The warranty cost dropped to near zero.
5G and Industrial Electronics Housings
Low-warpage, dielectric-stable enclosures for 5G base stations or industrial sensors often use specialty PBT, modified PET, or PPS. PA66 plays a smaller role here because its dielectric constant is moisture-sensitive.
How to Source Heat-Resistant PA and Polyester Pellets with Confidence
Once you’ve selected the right family and grade, the next risk is supplier consistency. A perfectly specified PA66-GF30 from one batch can underperform a different batch from a less rigorous supplier. Procurement teams should ask for the following with every quote.
What to Request from Your Resin Supplier
- Full datasheet with HDT at both 0.45 MPa and 1.82 MPa, dry and conditioned
- UL Yellow Card confirming RTI Electrical, RTI Mechanical, and flammability rating
- Batch certificate of analysis for moisture content, melt flow, and tensile properties
- Heat-aging data at the target service temperature for at least 1,000 hours
- Hydrolysis data for polyester grades intended for wet service
- Country-of-origin and lead-time visibility for your production calendar
Common Branded Grades Worth Knowing
For polyamide: PA66 grades from Envalior (Stanyl, Akulon), BASF (Ultramid), Lanxess (Durethan), Ascend (Vydyne), and Asahi Kasei (Leona). For polyester: PBT grades from Celanese (Celanex), SABIC (Valox), BASF (Ultradur), and DuPont (Crastin), with PET engineering grades from Indorama (Ramapet), Toray, and Polyplastics. Yifuhui supplies both equivalent and branded grades across these polymer families.
How Yifuhui Supports Heat-Critical Sourcing
At Suzhou Yifuhui New Material Co., Ltd. we stock a comprehensive range of PA66 plastic pellets and PBT engineering pellets, including heat-stabilized, glass-filled, and flame-retardant variants. Every shipment carries a full certificate of analysis, and our technical team responds to quote and spec requests within 24 hours, seven days a week.
We also support direct material consultation. If your team is weighing polyamide vs polyester heat resistance for a specific part, our engineers can review the operating profile and recommend the grade families and stabilization packages that match your service envelope.
Ready to specify with confidence? Request a custom quote from our polymer experts → and a Yifuhui specialist will return a detailed grade recommendation, current pricing, and lead time within one business day.
Conclusion: Three Rules for Polyamide vs Polyester Heat Resistance
Comparing polyamide vs polyester heat resistance is not a one-number exercise. Three principles cut through most confusion and lead to better material decisions:
- Melting point is for processing, not for service. Use HDT for short-term peaks and UL RTI for long-term continuous use.
- Polyamide wins in dry heat and humid heat with steam exposure; polyester wins on long-term electrical service, SMT reflow, and dimensional stability. Pick by operating envelope, not by family reputation.
- Glass fiber changes everything. Reinforcement adds 130–170 °C of HDT and often matters more than choosing PA66 over PBT or vice versa.
The next time your team is choosing between PA66 and PBT, or between PA6 and PET, take the four-number view. Specify HDT conditioned, RTI Electrical at the right glass loading, hydrolysis exposure for wet service, and verified batch consistency from your supplier.
Yifuhui ships heat-resistant polyamide and polyester pellets to manufacturers across automotive, electrical, appliance, and industrial sectors, with transparent pricing, 24-hour quote response, and reliable global logistics. Contact our polymer team to secure the right grade for your next heat-critical part.
