PEEK vs Ultem
PEEK and Ultem are the two high-performance plastics that survive where acetal and nylon melt. PEEK is a semi-crystalline polyketone: 250 °C continuous, steam and solvent resistant, excellent for bearings and seals. Ultem (polyetherimide, PEI) is an amorphous polymer: 170 °C continuous, transparent amber, strong and stiff, with excellent dielectric properties and much better dimensional stability at about half the price of PEEK.
Choose PEEK for the hottest, wettest and most chemically hostile jobs and for parts that wear. Choose Ultem for electrical insulators, test sockets, medical parts that see repeated autoclaving at moderate temperature, and structural parts where PEEK cost is hard to justify. Ultem is also the easier one to machine flat and hold tolerance on.
| Property | PEEK | Ultem (PEI) |
|---|---|---|
| Polymer | Polyetheretherketone, semi-crystalline | Polyetherimide, amorphous |
| Density | 1.30 g/cm³ | 1.27 g/cm³ |
| Tensile strength | ~100 MPa (14.5 ksi) | ~110 MPa (16 ksi) |
| Flexural modulus | ~3.7 GPa (540 ksi) | ~3.3 GPa (480 ksi) |
| Elongation | 20-40% | ~60% |
| Glass transition | 143 °C (289 °F) | 217 °C (423 °F) |
| Melting point | 343 °C (649 °F) | n/a (amorphous) |
| Max continuous service | ~250 °C (480 °F) | ~170 °C (340 °F) |
| Chemical resistance | Excellent; steam, fuels, most solvents | Good; attacked by some chlorinated and polar solvents |
| Dielectric strength | Good | Excellent, stable over temperature |
| Relative cost | Baseline (very high) | About 0.4-0.6x of PEEK |
Typical values for the stated conditions. Confirm against the mill certificate and your spec.
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Decision matrix
A check marks the side with the edge on that dimension.
| Dimension | PEEK | Ultem (PEI) |
|---|---|---|
| Machining | Good; needs stress relief, internal stresses cause movement | ✓ Good; stable, holds flatness, prone to notch cracking |
| Welding | Fair; laser and ultrasonic possible, rarely done on stock shapes | ✓ Fair; solvent and adhesive bonding easier |
| Forming | Poor; machined or molded | Poor; machined or molded |
| Strength | ✓ Keeps strength past 200 °C; excellent wear and fatigue | Slightly higher at room temperature; drops off above ~170 °C |
| Corrosion resistance | ✓ Excellent; steam, hydraulic fluids, most solvents | Good; not for chlorinated solvents or strong bases |
| Finishing | Machined finish; bonds poorly without treatment | ✓ Machined finish; bonds and paints acceptably |
| Material cost | Among the most expensive stock plastics | ✓ Roughly half of PEEK |
| Availability | ✓ Rod and plate; large sections on lead time | Rod and plate; fewer sizes than PEEK |
Choose PEEK when
- Continuous service above about 170 °C or repeated steam sterilization
- Bearings, seals, bushings and wear rings in hot or chemically aggressive service
- Aggressive solvents, fuels or hydraulic fluids
- Semiconductor, aerospace or oil-and-gas parts where PEEK is already the specified material
Choose Ultem (PEI) when
- Electrical insulators, test sockets, connectors and probe fixtures
- Flat, stable machined parts with tight tolerances
- Service up to about 170 °C where the cost of PEEK is not justified
- Medical or lab parts needing a transparent, autoclavable material
Frequently asked questions
- Is PEEK better than Ultem?
- For temperature, chemical resistance and wear, yes. Ultem matches or beats PEEK on room-temperature strength, dielectric performance, dimensional stability and cost. Neither is better in general; PEEK is the escalation when Ultem is not hot or chemically tough enough.
- What temperature can PEEK and Ultem handle?
- PEEK is rated around 250 °C continuous and melts at 343 °C. Ultem is rated around 170 °C continuous and, being amorphous, softens rather than melts above its 217 °C glass transition. Both take repeated autoclave cycles; PEEK tolerates far more of them.
- Which is easier to machine?
- Ultem for flatness and tolerance; it is stable and does not move after roughing. PEEK machines cleanly but carries internal stress from extrusion, so thick or asymmetric parts should be stress relieved and roughed before finishing. Ultem is more notch sensitive, so avoid sharp internal corners.
- Why is PEEK so expensive?
- The monomer chemistry and the high processing temperatures make the resin costly, and stock shapes are extruded slowly. Ultem is cheaper to make and process, which is why it costs roughly half as much for the same size of rod or plate.
