Material guide
PEEK vs Ultem (PEI): How to Choose
PEEK and Ultem are both high-performance plastics, but they're not interchangeable. Here's how they compare on heat, chemicals, strength, machining and cost, and when to pick each.
Choose PEEK when the part runs hot (above about 340°F continuously), sees harsh chemicals or steam long-term, or needs the best wear and fatigue life. Choose Ultem when you need high strength, flame resistance and good heat performance at a lower price, especially for electrical, aerospace interior and medical parts.
The short version
PEEK (polyetheretherketone) and Ultem (polyetherimide, PEI) both sit near the top of the plastics pyramid. Both hold their strength far above where acetal, nylon or UHMW give up, both carry flame ratings without additives, and both machine well. The difference is margin: PEEK goes further on temperature, chemicals and wear, and you pay for it.
| PEEK (unfilled) | Ultem 1000 (unfilled PEI) | |
|---|---|---|
| Continuous use temp | ~480°F (250°C) | ~340°F (170°C) |
| Glass transition | ~289°F (143°C) | ~423°F (217°C) |
| Tensile strength | ~14,000–16,000 psi | ~16,500 psi |
| Chemical resistance | Excellent, including steam and hot water | Good; attacked by some chlorinated solvents and strong bases |
| Wear and fatigue | Excellent | Good |
| Flame rating | UL94 V-0 | UL94 V-0, low smoke |
| Color | Tan/beige (black grades common) | Translucent amber (black available) |
| Relative cost | $$$$ | $$$ |
Typical values for unfilled stock shapes. Grades and suppliers vary; confirm with the datasheet for your exact grade.
Temperature: PEEK's biggest advantage
PEEK is semi-crystalline, so it keeps useful strength well above its glass transition and is generally rated for continuous use around 480°F. Ultem is amorphous: it's very stiff right up to its glass transition near 423°F, but its long-term continuous rating is lower, around 340°F.
In practice: if the part lives below about 300°F, both work and Ultem is usually the better value. If it sees sustained heat above that, or hot steam and water for long periods, PEEK is the safer choice.
Chemical resistance
PEEK resists nearly everything a part normally sees: fuels, oils, hydraulic fluids, most acids and bases, and long-term hot water and steam. That's why it shows up in oil and gas, semiconductor and chemical processing equipment.
Ultem holds up well to hydrocarbons, alcohols and many cleaners, but some chlorinated solvents and strong alkaline solutions can attack it or cause stress cracking. If your part will be cleaned or exposed to aggressive chemicals, check the specific chemical against the datasheet.
Strength, wear and dimensional stability
At room temperature the two are close, and unfilled Ultem is slightly stronger on paper. PEEK wins on wear, fatigue and creep, especially when warm, which makes it the go-to for bearings, seals, thrust washers and parts that slide or flex millions of times. Bearing-grade PEEK (filled with carbon, graphite and PTFE) goes further still.
Both absorb very little moisture and hold tolerances well. Ultem's amorphous structure gives it slightly more predictable shrinkage and very good dimensional stability across temperature.
Machining
Both machine cleanly with sharp carbide tooling. A few practical notes your machinist will appreciate:
- PEEK: thick rod and plate can hold internal stress. For tight tolerances, buy stress-relieved (annealed) stock or anneal between roughing and finishing.
- Ultem: amorphous plastics can stress-crack from the wrong coolant or cleaners. Use a plastics-compatible coolant (or air) and avoid aggressive solvents after machining.
- Filled grades (glass or carbon) of either are abrasive, so expect faster tool wear.
Need parts made? See CNC machine shops that list PEEK and Ultem below.
When to choose each
Choose PEEK for
- Continuous heat above ~340°F
- Long-term steam, hot water or harsh chemicals
- Bearings, seals, thrust washers and wear parts
- Oil and gas, semiconductor and chemical processing
- Implant-grade and demanding medical parts
Choose Ultem for
- High strength and stiffness at a lower price
- Electrical insulators, connectors and test sockets
- Aircraft interior parts (flame, smoke and toxicity)
- Autoclavable medical and dental instruments and trays
- Parts where the amber translucent look is useful
Tip from the counter: if a print just says "high-temp plastic," ask what temperature the part actually sees and for how long. A lot of PEEK gets specified where Ultem, or even a glass-filled nylon, would do the job for a fraction of the cost.
Common questions
Are PEI and Ultem the same thing?
Yes. Ultem is SABIC's trade name for polyetherimide (PEI). Ultem 1000 is the standard unfilled grade; Ultem 2300 is 30% glass-filled. Other suppliers sell PEI under their own names, and "Ultem" is often used generically, the way people say Delrin for acetal.
Why is PEEK so expensive?
PEEK's raw resin is costly to produce, it's made by fewer manufacturers, and stock shapes take longer to process. Expect PEEK to cost noticeably more than Ultem for the same size rod or plate, and several times more than acetal or nylon. If Ultem meets your temperature and chemical requirements, it usually saves money.
Is PEEK stronger than Ultem?
Not by much at room temperature. Unfilled Ultem 1000 actually has slightly higher tensile strength and similar stiffness. PEEK pulls ahead at elevated temperatures, in long-term fatigue, and in wear. Glass- or carbon-filled grades of either are much stiffer than the unfilled versions.
What are the disadvantages of PEEK?
Cost is the big one. PEEK also has a lower glass transition temperature (about 289°F) than Ultem, so it can lose stiffness sooner in a narrow band of temperatures under load, and thick stock may need annealing before tight-tolerance machining. It isn't transparent and is attacked by a few strong acids such as concentrated sulfuric acid.
Can Ultem be autoclaved?
Yes. Ultem handles repeated steam sterilization well, which is why it's common in medical and dental instruments and trays. PEEK handles even longer steam and hot-water exposure, so it's often chosen for parts that see continuous hot water or steam.
Values in this guide are typical figures for common unfilled grades, for general comparison only. Properties vary by manufacturer, grade, filler, and shape. Before you specify, buy or machine a material for an application, check the manufacturer's datasheet and the material certifications for the exact grade you're getting, and confirm suitability with your supplier.
Who stocks and machines it
Distributors that stock these materials and shops that machine or fabricate them, from the Stock & Shape directory.
PEEK
- Interstate Advanced Materials – Portland Portland, OR
- Piedmont Plastics – Knoxville Knoxville, TN
- Port Plastics – Albuquerque Albuquerque, NM
- Total Plastics – Chicago Elk Grove Village, IL
- AIP Precision Machining Daytona Beach, FL
- Avion USA Sanborn, NY
- Carlson Tool & Manufacturing Cedarburg, WI
- No Limit Swiss Mentor, OH
Ultem (PEI)
- Piedmont Plastics – Pittsburgh Cranberry Township, PA
- Port Plastics – Seattle Seattle, WA
- Pro Plastics Linden, NJ
- Professional Plastics – Portland Tualatin, OR
- Faro Industries Rochester, NY
- Machine Specialty & Manufacturing Youngsville, LA
- Remarc Manufacturing Reno, NV
- Sure-Fab Two Harbors, MN