Comparisons / PEI
PEEK vs PEI (Ultem)
PEI is the substitution people reach for when a PEEK quote comes back high, and it works right up until steam, solvents or 170 °C arrives. Knowing where that line sits is worth real money.
| Property | Unit | PEEK | PEI (Ultem) |
|---|---|---|---|
| Continuous service temperature | °C | 250 | 170 |
| Tensile strength | MPa | 90 - 100 | 100 - 110 |
| Tensile modulus | GPa | 3.5 - 4.0 | 3.0 - 3.3 |
| Thermal expansion | µm/m·K | 47 | 50 - 56 |
| Coefficient of friction | µ, dry vs steel | 0.30 - 0.40 | 0.20 - 0.40 |
| Water absorption | % at 24 h | 0.1 | 0.25 |
| Relative stock price | acetal = 1 | 10 | 4 |
On the room temperature rows the two look interchangeable. They are not. The differences that matter do not appear in this table: hydrolysis, solvent resistance and stress cracking.
Choose PEI when
- Continuous service stays below about 170 °C with margin
- You need a transparent or tinted grade, which PEEK has no answer for
- The environment is dry and chemically mild
- Cost matters and the part is large, since the gap is roughly two and a half times
- Electrical properties matter and the part is an insulator
Choose PEEK when
- The part is steam sterilised or washed hot, repeatedly
- Solvents are present, especially chlorinated ones, where PEI crazes under stress
- Continuous service passes 170 °C
- The part slides or wears, where PEI has no bearing grade equivalent
- Fatigue and repeated assembly are involved, since PEI is notch sensitive
The failure nobody budgets for
PEI parts crack. Not from load, but from residual machining stress meeting a solvent, a cleaning agent or a threadlocker. It shows up weeks later, in the field, on parts that passed every inspection.
Shops that run PEI regularly anneal after machining for exactly this reason. If you are substituting PEI to save money, put the annealing step back into the quote and compare the totals honestly, because the naive comparison flatters PEI.
- Medical instrumentsPEEK, if it gets autoclaved. PEI is fine for single use or gas sterilised parts.
- Electrical insulatorsEither. PEI often wins on cost, PEEK on temperature and on steam cleaning.
- Optical or sight partsPEI, because transparency settles it.
- Bushings and wear partsPEEK bearing grade. PEI has no equivalent and will gall.
Questions people ask
Can PEI be autoclaved like PEEK?
Not for long. PEI loses strength through repeated steam cycles because it hydrolyses, which is exactly why reusable instrument parts end up in PEEK even though PEI is cheaper and easier to machine.
Why is PEI cheaper if the temperature rating is close?
It is amorphous and simpler to produce, and the ratings are not as close as they look. PEI runs to about 170 °C continuous against 250 °C for PEEK, and it is far more sensitive to solvents and to stress cracking.
Is Ultem the same as PEI?
Ultem is a brand of PEI. Specifying the brand narrows your supply; specifying PEI with the grade requirements keeps more suppliers available at the same performance.