The short answer: choose PEEK when nothing else survives the heat, chemicals and load; PPS for chemical resistance at a lower price; PSU for medical and sterilisation; PEI for strong, flame-retardant structural parts. If you are not sure, start from the temperature and the chemicals — everything else follows.
The four plastics at a glance
| Plastic | Continuous temp | What it is best at | Relative cost |
|---|---|---|---|
| PEI (Ultem) | about 170°C | Strength, flame retardancy, dimensional stability | $$$ |
| PSU | about 150°C | Sterilisation, hydrolysis resistance, transparency | $$$ |
| PEEK | about 250°C | The metal-replacement king — heat, chemicals, wear | $$$$ |
| PPS | about 200°C | Chemical resistance, precision molding | $$$ |
PEI — the all-rounder for hot, structural parts
PEI (often known by the trade name Ultem) is the workhorse of high-temp engineering. It stays strong and dimensionally stable up to about 170°C continuous, is inherently flame-retardant, and machines cleanly.
Pick PEI when: you need a stiff, precise, flame-rated structural part that does not face aggressive chemicals — housings, insulators, brackets, and parts that must hold shape when things get hot.
PSU — the one for medical and fluids
PSU stands out for surviving repeated steam sterilisation and hot water without degrading — which is why you find it everywhere in medical devices and fluid-handling. It is nearly transparent with only a faint yellowish tint — unlike PEI's deep amber — so you can see through it clearly.
Pick PSU when: the part will be autoclaved, sits in hot water or humid environments, or you need transparency — surgical instrument handles, fluid manifolds, sight glasses, fittings.
PEEK — when it has to be better than metal
PEEK is the top of the engineering-plastics ladder: continuous service around 250°C, outstanding chemical and wear resistance, and strength approaching metals. It is the material you reach for when aluminium or stainless was the safe choice but you want to cut weight and corrosion.
Pick PEEK when: the part faces heat, chemicals and load at the same time — valve seats and seals, pump and compressor components, bushings and wear parts, aerospace and oil-and-gas fittings. Yes, it is the most expensive plastic — but it is still cheaper than machining the same part in metal.
PPS — chemical resistance without the PEEK price
PPS combines strong chemical resistance with excellent dimensional stability and a lower cost than PEEK. It molds to tight tolerances and holds them in harsh chemical environments.
Pick PPS when: the part sits in aggressive chemicals or high heat (to about 200°C) but does not need PEEK's ultimate mechanical performance — chemical-processing valve components, pump housings, electrical connectors, precision fittings.
The decision in one flow
- Above about 200°C, or heat plus chemicals plus load together? — PEEK
- Aggressive chemicals, precise part, cost matters? — PPS
- Steam or hot water, sterilisation, or transparency? — PSU
- Strong, stiff, flame-rated structural part? — PEI
The mistakes we see most
- Defaulting to PEEK to be safe — it is overkill for many parts, and you pay several times more than PEI (and far more than PPS) for properties you do not need
- Picking PEI for chemical service — PEI can crack in some solvents; PPS or PEEK is the right call there
- Ignoring water — hot water and steam destroy many plastics; for wet-and-hot service, PEI, PSU or PEEK are the safe calls
The bottom line
The right high-temp plastic is the one that matches your part's temperature, chemicals and load — not the most expensive one. We mold all four (plus standard engineering resins) in-house, so we will recommend the honest choice for your application, not the one that pads our margin.
Send us your drawing or 3D file, and we will confirm material, temperature rating and tolerances — with a quote within 24 hours.
Continuous-use temperature figures reflect publicly available technical data from material suppliers (SABIC, BASF, Solvay).