PEEK vs Ultem (PEI): Which High-Performance Plastic?

PEEK and Ultem PEI are the two most common choices when an application pushes past the temperature ceiling of standard engineering plastics. Both materials clear FDA and USP Class VI compliance, both achieve UL 94 V-0 flame rating without additives, and both machine cleanly to tight tolerances — yet they differ enough that selecting the wrong one is a real engineering mistake. PEEK runs continuous service at 480°F (250°C) and is semicrystalline; Ultem tops out around 340°F (171°C) and is amorphous amber-transparent. Cost separates them by roughly 2–3×.

TL;DR

• Temperature: PEEK handles 480°F continuous; Ultem caps at 340°F — a 140°F gap that matters in autoclave and structural applications.
• Structure: PEEK is semicrystalline (excellent chemical resistance, fatigue strength); Ultem is amorphous (better dimensional stability under heat, optically transparent amber).
• Strength: PEEK tensile strength ~14,500 psi; Ultem 1010 ~15,200 psi (slightly higher) but PEEK holds its strength closer to its service limit.
• Flame/FDA: Both UL 94 V-0 inherently; both FDA 21 CFR and USP Class VI compliant in standard grades.
• Cost: Ultem sheet and rod typically run 40–60% of equivalent PEEK pricing — a meaningful savings on large-volume or large-section parts.
• Transparency: Ultem is amber-transparent in thin sections; PEEK is opaque tan/beige.
• Chemical resistance: PEEK resists virtually all organic solvents; Ultem is attacked by halogenated solvents and strong acids — a critical distinction.

When to Choose PEEK

Sustained High-Temperature Loads

If your part sees continuous operating temperatures above 340°F — exhaust system components, semiconductor process tooling, or downhole oil-and-gas hardware — PEEK is the only unfilled thermoplastic in this class that stays structurally sound. At 400°F, PEEK retains roughly 70% of its room-temperature flexural modulus; Ultem would be approaching or past its glass transition temperature and losing structural integrity rapidly.

Aggressive Chemical Environments

PEEK's semicrystalline structure is the key differentiator for chemical resistance. It withstands concentrated sulfuric acid (cold), chlorinated hydrocarbons, hydraulic fluids, and most organic solvents. If your application contacts methylene chloride, concentrated chlorine, or strong hydrobromic acid at elevated temperatures, PEEK holds where Ultem does not. Review the PEEK chemical resistance data before finalizing grade selection.

Fatigue-Critical or Cyclic-Load Applications

The crystalline morphology of PEEK gives it markedly better fatigue resistance — a key metric for bearing cages, pump impellers, valve seats, and rotating components that see millions of load cycles. Amorphous polymers like Ultem creep more under sustained stress and are less suited to cyclic loading at elevated temperatures.

Wear and Tribology

Unfilled PEEK has a lower coefficient of friction than Ultem, and PEEK grades filled with PTFE, graphite, or carbon fiber are industry standards for dry-running bearings and bushings. If the part must slide against a mating surface under load, PEEK-based compounds outperform PEI-based options. See the PEEK grades overview for filled options.

When to Choose Ultem (PEI)

Cost-Sensitive High-Performance Applications

When your operating temperature lands between 250°F and 330°F — sterilizable surgical instruments, aerospace interior panels, food-processing fixtures — Ultem delivers equivalent or better room-temperature mechanical properties at 40–60% of PEEK's material cost. For parts machined in volume or requiring substantial stock removal, that delta adds up quickly.

Dimensional Stability Over a Wide Range

Amorphous polymers like Ultem have lower mold shrinkage and more predictable dimensional behavior than semicrystalline PEEK, particularly in complex geometries. If you're machining a precision housing that must hold ±0.001″ in all directions without anisotropy from crystalline orientation, Ultem is more forgiving.

Optical and Aesthetic Requirements

Ultem's amber transparency is useful in sight glasses, flow indicators, and light-pipe-adjacent medical device components where at least partial light transmission is needed. PEEK cannot provide transparency in any standard grade.

Repeated Steam Autoclave Sterilization

Both materials handle steam autoclave (270°F/134°C cycles) reliably, but Ultem 1010 has been extensively validated in the medical industry for this application and often carries formal sterilization documentation from material suppliers. For surgical trays and reusable instrument handles, Ultem 1010 is frequently the first call. Review the Ultem material hub for medical-grade documentation guidance.

Specs Head-to-Head

Thermal Performance

PEEK's glass transition temperature is approximately 289°F (143°C), but because it is semicrystalline, it retains significant mechanical properties well above Tg. Its melting point sits at roughly 644°F (340°C). Ultem PEI's Tg is ~419°F (215°C) — higher than PEEK's Tg — but since Ultem is amorphous, the material softens progressively as it approaches Tg without the structural backbone of a crystalline phase. Practical continuous-service ratings reflect this: PEEK at 480°F, Ultem at 340°F.

Mechanical Properties

At room temperature, Ultem 1010 edges out natural PEEK in tensile strength (15,200 psi vs. 14,500 psi) and is comparable in impact resistance. Both materials have similar notched Izod values near 1.0 ft·lb/in. PEEK's flexural modulus (600,000 psi) exceeds Ultem's (480,000 psi), meaning PEEK is stiffer — a consideration for thin-wall structures under bending loads.

At elevated temperature, the gap reverses sharply. A PEEK component at 400°F retains structural utility; an Ultem component at the same temperature is approaching its practical limit.

Electrical Properties

Both materials are excellent electrical insulators with dielectric strength exceeding 400 V/mil. Ultem has a slightly better dielectric constant (3.15 vs. PEEK's ~3.3 at 1 MHz), making it preferred for high-frequency electrical applications. PEEK's arc resistance (180 seconds) exceeds Ultem's for applications where arcing is a failure mode.

Chemical Resistance Summary

PEEK resists: virtually all organic solvents, dilute and concentrated mineral acids (except concentrated sulfuric above 150°F), hydrocarbons, hydraulic fluids, and chlorinated solvents. Its semicrystalline structure is fundamentally more resistant to solvent attack than amorphous polymers because crystalline domains act as barriers to solvent diffusion.

Ultem is attacked by: methylene chloride, trichloroethylene, concentrated acids, and halogenated solvents. Ultem performs adequately against alcohols, water, dilute aqueous solutions, and mild process chemicals. For a complete fluid-by-fluid comparison, request resistance tables from your material supplier before finalizing specifications in chemical-contact applications.

Machinability and Fabrication

Both PEEK and Ultem machine cleanly with carbide tooling. PEEK is the more commonly machined of the two — it has well-documented tooling recommendations, good chip clearance behavior, and predictable dimensional behavior. Ultem machines similarly but tends to be slightly more notch-sensitive in thin sections; avoid sharp internal corners in Ultem components under load. Both materials accept standard surface finishes and can be tapped and threaded without special tooling. Neither material requires post-processing after machining.

Cost & Availability

PEEK sheet and rod in standard natural grades typically price 2–3× higher than equivalent Ultem 1010 stock. On a machined-part basis, the gap may be compressed somewhat by similar machine times, but material cost remains the dominant factor for most stock-removal parts. The premium reflects PEEK's more demanding synthesis and processing requirements. Both materials are stocked by major plastic distributors in sheet (typically 0.125″–4″ thick) and rod (0.25″–6″ diameter); tube is available but less commonly stocked.

Lead times for standard sizes are generally short — days, not weeks — from distributors. Custom profiles and large-diameter sections may require mill orders with 4–8 week lead times for either material.

Filled grades (carbon fiber, glass fiber, PTFE, graphite) carry additional premiums and longer lead times but are widely available for both PEEK and Ultem.

Common Alternatives

If neither PEEK nor Ultem fits your requirements, consider:

• PEEK vs Torlon (PAI) — Torlon exceeds both in tensile strength and wear resistance, though post-cure adds processing complexity.
• Ultem vs Polysulfone — Polysulfone offers similar autoclave performance at a lower cost; PPSU pushes service temperature to 360°F.
• PEEK vs PTFE — If chemical resistance and non-stick properties matter more than structural strength, PTFE enters the conversation.

More related guides

Explore related material decisions and where these plastics show up in real production:

Applications

• Bushings Bearings
• Gears
• Manifolds

Industries

• Aerospace
• Medical

Other comparisons

• Peek Vs. Pai Torlon
• Peek Vs. Acetal Delrin
• Peek Vs. Ptfe

Spec sheets

• PEEK hub
• Ultem / PEI hub