PEEK vs Ultem comes up when a part needs more heat, strength or chemical resistance than acetal or nylon can offer. PEEK (polyetheretherketone) is a semi-crystalline plastic with outstanding temperature capability, chemical resistance and wear. Ultem is SABIC's trade name for polyetherimide (PEI), an amorphous plastic with high strength, inherent flame resistance and good dimensional stability at a lower price. PEEK typically costs about two to three times as much as Ultem in stock shapes.
PEEK vs Ultem: Comparison Table
Values are typical for unfilled PEEK and Ultem 1000. Cost is a relative per-part index including material, where one Ultem part at quantity 1 equals 100.
| Factor | PEEK | Ultem 1000 (PEI) |
|---|---|---|
| Relative cost, qty 1 | 150 to 220 | 100 |
| Relative cost, qty 10 | 75 to 110 | 45 |
| Relative cost, qty 100 | 45 to 70 | 24 |
| Relative cost, qty 1,000 | 35 to 55 | 17 |
| Structure | Semi-crystalline | Amorphous |
| Continuous service temperature | About 480°F (250°C) | About 340°F (170°C) |
| Glass transition | About 290°F (143°C) | About 420°F (217°C) |
| Tensile strength | About 14 to 16 ksi | About 15 to 16 ksi |
| Chemical resistance | Excellent, including most solvents and hydrocarbons | Good; can stress crack in some solvents |
| Wear and friction | Very good, especially bearing grades | Fair |
| Flame rating | Inherently flame resistant | Inherently flame resistant, low smoke |
| Typical machined tolerance | ±0.001 to ±0.003 in | ±0.001 to ±0.003 in |
| Surface finish | Ra 32 to 63 µin | Ra 32 to 63 µin |
| Typical use case | Seals, bearings, valve seats, oil and gas, semiconductor, medical | Electrical insulators, test sockets, manifolds, aircraft interiors, sterilizable housings |
When PEEK Wins
- High continuous temperature. PEEK keeps useful properties well above Ultem's limits.
- Aggressive chemicals. PEEK resists most solvents, fuels and hydraulic fluids, and handles steam well.
- Wear and sliding. Bearing grades with carbon, PTFE and graphite fillers are used for seals, thrust washers and bushings.
- Fatigue. The semi-crystalline structure handles cyclic loads well.
When Ultem Wins
- Cost-sensitive high-performance parts. Ultem offers high strength and heat resistance at a much lower price.
- Dimensional stability to moderate temperatures. Its high glass transition keeps stiffness steady up to roughly 400°F.
- Electrical insulation. Stable dielectric properties make it common for sockets and insulators.
- Flame and smoke requirements. Ultem is widely used where low smoke is required.
Common Mistakes
- Specifying PEEK for a part that runs below 300°F in a benign environment, where Ultem would perform at lower cost.
- Exposing Ultem to incompatible solvents, some cleaners or certain cutting fluids, which can cause stress cracking.
- Skipping annealing on tight-tolerance parts with heavy material removal. Annealing stock or parts reduces movement after machining.
- Calling out "PEEK" without a grade. Unfilled, glass-filled, carbon-filled and bearing grades have very different properties.
Machining Notes
Both materials machine well with sharp carbide tools. Heat control matters, so light finishing passes and good chip evacuation help. For Ultem, coolant choice is important to avoid stress cracking, and for both materials, parts with thin walls or tight flatness benefit from roughing, stress relief and finishing. Material certifications are especially important for these plastics because counterfeit and mislabeled stock exists.
How This Applies at Wexmar
Wexmar machines PEEK, Ultem and other engineered plastics on our CNC lathes, horizontal machining centers and Maier Swiss-type machines, with material certifications available. Small PEEK seals, spacers and insulators to 32 mm diameter run efficiently on our Swiss machines. See materials, industries, and the guide on Delrin vs nylon.
Getting a Quote
Include the grade, any filler and certification requirements. You can upload a drawing for an instant estimate, confirmed by a human estimator, or call (315) 400-2654.