PAI (polyamide-imide), best known by the trade name Torlon, is the material you reach for when the temperature is off the charts and the load won't quit. It's expensive, it's hard to machine, and it's the right answer for a specific set of extreme applications.
We don't machine PAI every day — the volume is lower than PEEK or PTFE — but when we do, it's usually for aerospace, oil and gas downhole tools, or semiconductor equipment. These are applications where failure isn't an option and cost is secondary.
What PAI is
PAI combines aromatic imide rings (like polyimide) with amide linkages in the polymer chain. The imide groups provide thermal stability. The amide groups add toughness and processability that pure polyimide lacks.
The result is a material with the highest Tg (275°C) and the highest strength at extreme temperature of any melt-processable thermoplastic. Continuous service is rated at 260°C, same as PEEK, but PAI retains more of its room-temperature mechanical properties as you approach that limit.
Compressive strength is exceptional: 220 MPa for unfilled PAI versus 150 MPa for unfilled PEEK. The stiffness is similar (modulus ~4.0 GPa unfilled), but PAI's thermal performance — particularly its ability to hold dimensions at extreme temperature — is what you pay for.
The grades that matter
Torlon comes in several standard grades. The ones we see most:
- Torlon 4203 (unfilled): The baseline. Best electrical properties, good wear resistance for an unfilled material, lowest cost.
- Torlon 4301 (PTFE/graphite filled): The standard wear grade. Self-lubricating, excellent in dry-running bearings and bushings to 260°C.
- Torlon 4275 (carbon fiber filled): Higher modulus and strength. For structural applications at temperature.
- Torlon 5030 (30% glass fiber): Highest strength and stiffness. Most abrasive to tooling.
Where PAI earns its premium
Extreme heat with load. Above 230°C, PAI is simply stronger than PEEK. Aerospace engine hardware, downhole tooling components, high-temperature test fixtures.
Wear at high temperature. PAI's wear resistance holds up better than PEEK's as the temperature climbs. Graphite-filled grades (4301) are genuinely impressive in dry-running applications at 200°C+.
Dimensional stability. PAI has a very low, very linear CTE. Parts that must hold micron-level tolerances across wide temperature swings (aerospace instrumentation, semiconductor metrology fixtures) favor PAI over any other machinable polymer.
Strength-to-weight at temperature. PAI is slightly denser than PEEK (1.41 vs 1.32 g/cm³), but its higher specific strength at elevated temperature makes it the choice for weight-critical hot applications.
The downsides
Cost. PAI stock shapes cost 30-50% more than equivalent PEEK shapes. Between the material premium and the harder machining, PAI parts typically cost 40-70% more than PEEK equivalents.
Machining difficulty. PAI is harder, more abrasive, and more notch-sensitive than PEEK. It demands sharper tools, lighter cuts, and more careful thermal management. Edge chipping and micro-cracking are real risks if you push too hard. Mist coolant recommended — avoid thermal shock.
Brittleness. Elongation is 6-12% versus PEEK's 30-50%. PAI parts crack under impact or sharp corner stress that PEEK parts survive. Don't design snap-fits in PAI.
Hydrolysis sensitivity. PAI degrades in prolonged hot water or steam exposure. For autoclave-sterilized medical devices, PEEK wins. For downhole tools below the water table, check your conditions carefully.
Machining PAI
- Carbide tooling, polished flutes, dead sharp. Dull tools cause chipping.
- Lower feeds than PEEK: PAI is harder and the cutting forces are higher.
- Mist coolant for thermal control. Avoid flood coolant — thermal shock causes micro-cracks.
- PAI is abrasive, especially filled grades. Tool life is shorter than with PEEK. Budget accordingly for production runs.
- Annealing before machining helps. PAI stock shapes carry residual stress from the manufacturing process. A stress-relief cycle before roughing reduces warpage.
We stock Torlon 4203 and 4301. If your part runs above 230°C under structural or wear load, PAI is almost certainly the right technical answer despite the cost. Send us your drawing and operating conditions — we'll confirm whether PEEK would do the job or whether you really need PAI.