PEEK vs PPS vs PEI: which high-temperature plastic fits your part?

Buyers rarely arrive asking for PEEK, PPS or PEI by name. They arrive with a part that failed, a temperature they cannot exceed, or a cleaning cycle that ate the last batch. The three polymers overlap enough on paper that a datasheet comparison can make them look interchangeable — continuous use above 150 °C, good electricals, machinable, available in sheet and rod. They are not interchangeable, and the difference shows up in the second year of service rather than the first week.

What each polymer actually is

PEEK is a semi-crystalline polyetherketone with a glass transition around 143 °C and a melting point near 343 °C. Unfilled it is tough and fatigue resistant; with 30 % glass fibre or carbon fibre it becomes stiff enough to replace metal in structural parts. It is the most expensive of the three by a wide margin, and that price buys temperature headroom, wear performance and tolerance of repeated steam and chemical sterilisation.

PPS is also semi-crystalline, but with a much lower glass transition — roughly 90 °C — and a melting point near 280 °C. Because it crystallises fast and almost always ships glass-filled, it holds tight tolerances and has one of the lowest coefficients of thermal expansion of any engineering plastic. It is inherently flame retardant and resists almost every solvent below 200 °C. Its weakness is brittleness: unfilled PPS cracks under impact, which is why 40 % glass is the default grade.

PEI, better known as Ultem, is amorphous. Its glass transition is 217 °C and it has no melting point, which means it keeps its stiffness right up to the point where it softens suddenly. It is transparent amber, dimensionally stable, and machines to a clean finish. Amorphous also means notch sensitive and vulnerable to chlorinated solvents and strong bases.

The numbers side by side

Temperature is a range, not a number

Continuous use temperature assumes a part under load for thousands of hours. A PPS bracket rated at 220 °C will survive a 260 °C reflow oven for ninety seconds and a PEI housing rated at 170 °C will survive a 200 °C bake-out cycle, because neither is a long-term load case. What kills parts is the combination of heat and sustained stress — creep. Above its glass transition an amorphous polymer creeps steadily; PEI at 180 °C under a bolted preload will lose that preload in months. PPS, despite the lower number, holds preload better because the crystalline phase carries load past the glass transition.

If the service temperature sits above 200 °C continuously, PPS and PEI are both out and PEEK is the answer. If it sits between 150 and 200 °C with no load, all three work and price decides. The dangerous zone is 120 to 180 °C with a preload or a press fit, where datasheet temperature ratings say yes and creep says no.

Chemical resistance decides more cases than temperature

PPS is close to inert. It has no known solvent below 200 °C, resists strong acids and bases, and is unaffected by the degreasers and hydraulic fluids found in a plant. PEEK is equally inert in practice, with the caveat that concentrated sulphuric acid and some halogenated compounds will attack it. PEI is the one to watch: it stress-cracks in chlorinated solvents, in strong bases, and in some ketones, and a molded-in or machined-in stress makes that happen faster.

Machining, annealing and dimensional stability

All three machine with ordinary carbide tooling, but they demand different habits. PEEK and PPS are semi-crystalline, so a machined part carries residual stress that will relax when the part later sees temperature; rough machine, anneal, then finish. PPS glass-filled is abrasive and wears tooling quickly, and its brittleness means sharp edges chip rather than deform. PEI needs drying before any hot work and is notch sensitive, so avoid sharp internal corners and thread roots.

For a part that has to hold a tolerance over a temperature swing, PPS wins on paper: 40 % glass gives a coefficient of thermal expansion close to aluminium, and the low water absorption means humidity does not move the dimensions. PEEK is close behind and far tougher. PEI moves more with humidity and much more above 150 °C.

How to decide in four questions

• Does the part see more than 200 °C continuously, or repeated steam and chemical sterilisation? If yes, PEEK.
• Is there a preload, press fit or sustained load above 120 °C? If yes, PPS for stiffness, PEEK if impact is also present.
• Is the part exposed to chlorinated solvents, strong bases or ketones? If yes, PPS or PEEK — not PEI.
• Is the budget the binding constraint and the temperature below 150 °C? PPS at a tenth of the PEEK price usually wins.

In practice most of our enquiries resolve to PPS. It is the grade that quietly replaces aluminium brackets, jigs and housings because it is stiff, stable, flame retardant and cheap enough to justify. PEEK goes where the failure mode is wear, impact or sterilisation, and PEI goes where the part must be transparent or where its electrical properties at high frequency matter.

Questions we are asked most

Can PPS replace PEEK to save cost?

Only below about 150 °C with modest impact. PPS is stiffer and cheaper, but it is brittle and its glass transition is low, so a PPS part that sees impact or a hot preload will fail where PEEK would not.

Why does my PEI part crack after cleaning?

Almost always a chlorinated solvent or a strong base combined with moulded-in or machined-in stress. Annealing before service and switching the cleaning agent solves it more often than changing material.

Do these materials need annealing after machining?

PEEK and PPS do whenever the part will see temperature above 100 °C or must hold a tight tolerance. PEI benefits from a drying cycle before machining rather than a post-machining anneal.