Choosing the right engineering plastic for a machined part is one of those decisions that looks simple on paper and turns out to be surprisingly nuanced. I've seen datasheets lead engineers to the wrong material more times than I can count — not because the datasheet was wrong, but because it didn't capture what happens on a shop floor.
Here's a practical framework for narrowing down your options, based on the materials we machine every day.
Start with temperature
This is the fastest way to eliminate candidates:
- Below 100°C: Almost everything is in play. Acetal, nylon, polycarbonate, and all the engineering plastics.
- 100-150°C: Engineering plastics territory. PPS, PEI, PVDF start becoming relevant.
- 150-200°C: PEEK, PPS (glass-filled), some PI grades. PEI drops out above 170°C.
- 200-260°C: PEEK, PI, PAI. PPS is marginal at the high end.
- Above 260°C: PI and PAI are the only realistic options for machined parts. Ceramic-filled grades extend the range.
Continuous service temperature (UL746B rating) is more useful than short-term HDT. A PPS part rated at 200°C continuous will survive brief excursions to 260°C, but don't design around the excursion unless you've tested it.
Chemical exposure
If the part touches anything other than air or water, check compatibility:
- Aggressive solvents, acids, or steam: PTFE or PEEK. PTFE is essentially immune to everything. PEEK is nearly as good and carries mechanical load. PPS handles most industrial chemicals below 200°C but is more limited with oxidizing acids.
- Hydrocarbons, oils, hydraulic fluid: PEEK and PPS both handle these well. PTFE too, but it can swell slightly in some hydrocarbons.
- Hot water or steam long-term: PEEK. PAI and standard PI can hydrolyze. PTFE doesn't degrade but may creep dimensionally.
- Semiconductor process chemicals: PTFE and PFA for wet chemistry. PEEK for plasma-exposed components. PI for high-temperature dry process.
When in doubt, request chemical compatibility data from the material supplier for your specific chemical, concentration, and temperature. Generic "excellent chemical resistance" claims are not enough.
Mechanical load
High structural load: PEEK. It's the strongest and toughest of the group. PAI if the temperature demands it, but only if you can handle the brittleness and cost.
Moderate load, lower cost: PPS (glass-filled). Good strength, much cheaper than PEEK.
Low load, primarily sealing or insulating: PTFE or unfilled PPS. The soft option that handles chemistry and temperature.
Impact or shock loading: PEEK. PPS and PAI are too brittle. PEI can work for moderate impact below 170°C.
Wear and sliding: Filled PEEK (carbon fiber or PTFE-filled) for most applications. Filled PAI (Torlon 4301) for high-temperature wear. Filled PTFE for low-load, low-friction.
Dimensional stability
If your part has tight tolerances (under ±0.05mm):
- Best: PAI and PI — lowest, most linear CTE. Very stable.
- Good: PPS and PEI — low moisture absorption, predictable expansion.
- OK with care: PEEK — higher CTE but machinable to tight tolerances with thermal management.
- Challenging: PTFE — creeps and has high CTE. ±0.05mm is the practical limit.
For precision parts that span wide temperature ranges, factor thermal expansion into the tolerance budget. A 100mm PEEK part grows about 0.05mm per 1°C.
Cost sensitivity
Rough cost ranking (unfilled grades, relative):
- PTFE (least expensive)
- PPS
- PVDF
- PEI
- PEEK
- PAI
- PI (most expensive)
Glass or carbon fiber fillers add 15-30% to the base material cost. The premium for higher-temp or stronger materials is real, but field failures from choosing the cheaper option always cost more.
A quick decision flow
- What's the max continuous temperature? → Eliminate anything rated below it.
- What chemicals does it touch? → Eliminate anything incompatible. When unsure, PTFE or PEEK.
- What's the mechanical demand? → Structural = PEEK or PPS. Low load seal/gasket = PTFE.
- What tolerances do you need? → Tight = PAI, PI, PPS, or PEEK with thermal management.
- What's the budget? → PPS and PTFE for cost-sensitive. PEEK for demanding. PAI/PI when nothing else works.
We stock the full range and can help you narrow the choice. Send your drawing and tell us what the part needs to do — we'll give you a recommendation based on what actually works, not what looks good on a comparison chart.