The quick answer
Default to Ultem. It's half the cost, machines well, meets FAA flammability requirements, and handles continuous service temperatures up to 170 °C. It's what's on the inside of most of the aircraft you've flown on.
Step up to PEEK when your application exceeds Ultem on any of these: continuous temperature above 170 °C, repeated steam sterilization cycles above 50, aggressive chemical exposure (especially chlorinated solvents), UHV vacuum cleanliness requirements, or implant-grade biocompatibility specifications.
Temperature — the cleanest decision axis
Temperature is the easiest property to compare. Ultem 1000 maintains mechanical properties to ~170 °C continuous; PEEK 450G maintains properties to ~250 °C continuous. For brief excursions, PEEK tolerates 300 °C+ while Ultem softens and deforms.
If your part sees engine-compartment heat, solder-reflow fixtures, or extended autoclave cycles above 135 °C, PEEK is the right call. For aircraft cabin parts, electrical insulators, and below-the-hood automotive plastics that stay below 150 °C most of the time, Ultem is economically smart.
Side-by-side comparison
| Property | PEEK 450G | Ultem 1000 |
|---|---|---|
| Continuous service temp | 250 °C | 170 °C |
| Tensile strength | 100 MPa | 85 MPa |
| Tensile modulus | 3,600 MPa | 3,000 MPa |
| Density | 1.32 g/cm³ | 1.27 g/cm³ |
| Appearance | Opaque tan | Translucent amber |
| Autoclave cycles | Unlimited | ~100 |
| Chemical resistance | Exceptional | Very good |
| FAA FST rating | Yes | Yes |
| Relative cost | 2.0× | 1.0× |
Transparency — when Ultem wins
The most overlooked difference between these two is appearance. PEEK is opaque tan — you cannot see into the part. Ultem is translucent amber — you can. For medical devices where visual inspection of internal features (fluid channels, electrical routing, fill levels) matters, Ultem wins by default.
A typical use case: a surgical fluid-management manifold with internal channels. The designer wants visual confirmation that the channels are unobstructed. Machined in PEEK, the manifold is opaque; you'd need to rely on flow testing. Machined in Ultem, you can see channel geometry at a glance. The cost difference is genuinely worth it.
Medical autoclave — when PEEK wins
For medical instruments that see unlimited autoclave cycles across a multi-year service life, PEEK is the right call. Over 1,000 steam cycles, unfilled Ultem gradually becomes brittle and can crack at stress concentrations — we've seen customer returns on Ultem parts that were specified for "autoclavable" applications without clarifying cycle count.
PEEK survives effectively unlimited cycles, which is why implant-grade PEEK (Invibio PEEK-OPTIMA®) became the standard for permanent orthopedic hardware rather than Ultem. For dental and surgical instruments that see daily sterilization over 5–10 year service life, PEEK is worth the premium.
Semiconductor and chemical process applications
Semiconductor wafer handling fixtures and chemical process pump components almost always specify PEEK over Ultem. The driver is chemical resistance, not temperature — chlorinated solvents (TCE, methylene chloride) attack Ultem but leave PEEK untouched. If your fixture will be cleaned with aggressive solvents or handle wafer-etch chemistries, PEEK is the default.
Exception: for fixtures handling only aqueous chemistries and seeing temperatures below 150 °C, Ultem is perfectly adequate and saves meaningful cost.
Aerospace interior — typically Ultem
Most aircraft cabin parts that are visible to passengers — tray tables, overhead bin latches, light lens housings, seat back components — are Ultem. The combination of FAA flammability approval, translucent amber aesthetic that works with cabin lighting, and lower cost makes Ultem the default for visible interior parts.
PEEK shows up in aerospace for structural and engine-adjacent parts: cabling insulators near heat sources, fuel system fittings, and interior structural brackets where fire-safety performance must be exceptional rather than merely adequate.
The decision framework
Start with Ultem 1000. Upgrade to PEEK 450G only if your application requires one of:
- Continuous service temperature above 170 °C
- More than 100 steam autoclave cycles over service life
- Chlorinated solvent exposure or other aggressive chemistries
- Implant-grade biocompatibility (Invibio PEEK-OPTIMA®)
- UHV vacuum cleanliness below 10⁻⁸ torr
- Aerospace structural fire-safety applications
If none of these apply, Ultem will do the job at half the cost, and the translucent appearance may be an advantage rather than a compromise. See the PEEK materials page for implant-grade sourcing detail and PPSU as a cheaper third option for medical applications.