Ultem (PEI) vs PEEK
Ultem (PEI) and PEEK are both high-performance, high-temperature engineering thermoplastics, but they sit at different points on the price/performance curve. The choice usually comes down to how much continuous heat, chemical exposure, and mechanical load the part really sees — and whether you can justify PEEK's premium.
The verdict
Choose Ultem (PEI) when you need flame-retardant, dimensionally stable, transparent-amber parts up to roughly 170-200°C at a far lower cost — typical for aerospace interiors, electrical, and sterilizable medical housings. Choose PEEK when service temperatures, continuous mechanical load, wear, or aggressive chemical exposure exceed what PEI tolerates. PEEK is the tougher, hotter, more chemically inert material; Ultem is the cost-effective default until PEI's limits are exceeded.
Side-by-side data
| Property | Ultem (PEI) | PEEK |
|---|---|---|
| Category | Engineering Plastic | Engineering Plastic |
| Density (g/cm³) | 1.27 | 1.32 |
| Tensile strength (MPa) | 105 | 100 |
| Yield strength (MPa) | 105 | 97 |
| Elongation (%) | 60 | 45 |
| Hardness | M109 | R126 |
| Max service temp (°C) | 170 | 250 |
| Machinability | ●●●●● | ●●●●● |
| Corrosion resistance | ●●●●● | ●●●●● |
| Relative cost | ●●●●● | ●●●●● |
| Thermal cond. (W/m·K) | 0.22 | 0.25 |
| Typically used for | High-temp, flame-retardant structural parts | High-temp, chemical & medical-grade parts |
Which should you choose?
Choose Ultem (PEI) if…
- Continuous service stays roughly at or below 170°C and you don't need PEEK's headroom — PEI gives you most of the high-temp benefit for a fraction of the cost.
- Inherent flame, smoke, and toxicity (FST) performance matters, e.g. aircraft interiors and rail — PEI is naturally flame-retardant without additives.
- You need a transparent or tinted amber part, or tight dimensional stability and low creep in a structural housing.
- Cost and machinability drive the decision and you want predictable, low-stress machining without crystallinity/annealing complications.
- Parts are electrically insulating connectors, sterilizable medical components, or semiconductor handling fixtures within PEI's chemical envelope.
Choose PEEK if…
- Continuous operating temperature climbs toward or above 250°C, or you need sustained mechanical load while hot — PEI softens well before PEEK does.
- The part faces aggressive solvents, hot water/steam, hydraulic fluids, or repeated autoclaving — PEEK's semi-crystalline structure resists chemical attack and stress cracking that PEI can suffer.
- You need high wear resistance, fatigue strength, or low friction (especially bearing/seal grades filled with carbon, glass, or PTFE).
- The application is a flight-critical bracket, downhole/oilfield component, or pump/valve seat where toughness and chemical inertness justify the price premium.
- Long-term hydrolytic stability under steam or chemical cycling is required over many service cycles.
Key differences that matter
- Temperature is the primary divider: PEI is amorphous and softens near its glass transition, while semi-crystalline PEEK keeps useful strength much hotter and tolerates continuous heat PEI cannot.
- Chemical and hydrolytic resistance favors PEEK — it shrugs off solvents, steam, and autoclave cycles that can stress-crack or attack PEI, which is more sensitive to certain chlorinated and aromatic solvents.
- Cost is the main reason to pick PEI: it typically runs a large fraction cheaper than PEEK per kg and machines more easily, so it's the sensible default until a real PEI limit is hit.
- Both machine well, but PEEK's crystallinity means stock and finished parts may need annealing to relieve stress and hold tolerance; PEI machines with less of that fuss and offers optical clarity.
- Neither is weldable like a metal, but both can be ultrasonically welded, bonded, or laser/hot-plate welded; PEI generally bonds and solvent-welds more readily, while PEEK's chemical inertness makes adhesive bonding harder.
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Open the Material SelectorGet a Quote →Frequently asked questions
Is Ultem (PEI) stronger than PEEK?
At room temperature their tensile strengths are broadly comparable, and unfilled PEI can even edge out unfilled PEEK in some stiffness/strength metrics. The real difference shows up with heat and fatigue: PEEK retains strength at high temperature and resists cyclic loading far better, so for hot or load-cycled parts PEEK is effectively the stronger choice.
Which is cheaper, Ultem or PEEK?
Ultem (PEI) is significantly cheaper — often roughly a third to half the price of PEEK per kilogram in stock form, with the gap varying by grade and supplier. PEI is also easier and cheaper to machine. If your requirements fall inside PEI's envelope, it's usually the more economical engineering choice.
What is the maximum continuous service temperature for each?
PEI is generally rated for continuous use up to roughly 170-200°C, limited by its glass transition near 217°C. PEEK has a glass transition around 143°C but, being semi-crystalline, holds useful properties to a continuous ~250°C with a much higher melting point. For sustained high heat, PEEK is the clear pick.
Can I machine, weld, or bond these plastics?
Both machine on standard CNC equipment; PEEK parts may need annealing to relieve stress and hold tight tolerances, while PEI machines more predictably and can be transparent. Neither is welded like metal, but both accept ultrasonic, laser, and hot-plate welding. PEI bonds and solvent-welds more easily; PEEK's chemical inertness makes adhesive bonding harder, often requiring surface treatment.
Which resists chemicals and autoclaving better?
PEEK. Its semi-crystalline structure gives excellent resistance to solvents, hot water, steam, and repeated autoclave cycles, with strong hydrolytic stability. PEI is chemically capable and routinely sterilized, but it's more vulnerable to certain chlorinated and aromatic solvents and to stress cracking under aggressive chemical or thermal cycling.
Property values are typical/nominal figures for early-stage guidance only and vary by temper, grade, supplier and heat treatment. Confirm critical specifications against a certified datasheet or with an mfgiq engineer before production.