PEEK vs Ultem PEI: Which High-Performance Polymer Should You Choose?
Published: 2026-05-27
The choice between PEEK and Ultem PEI represents one of the most common decision points in high-performance polymer selection. Both materials offer exceptional mechanical properties and high-temperature performance, but their fundamental...
The choice between PEEK and Ultem PEI represents one of the most common decision points in high-performance polymer selection. Both materials offer exceptional mechanical properties and high-temperature performance, but their fundamental difference—PEEK is semicrystalline while Ultem PEI is amorphous—drives diverging behavior in chemical resistance, dimensional stability, processing requirements, and cost.
When to choose PEEK: PEEK's semicrystalline structure provides superior chemical resistance (resists most solvents, acids, and bases), higher continuous service temperature (260°C vs 170°C), better fatigue resistance, and excellent wear properties. It is the default choice for oil & gas downhole components, aerospace structural brackets, and applications where the part will contact aggressive chemicals at elevated temperature. The tradeoff: PEEK costs 2-3× more than Ultem, requires mold temperatures of 160-200°C (oil-heated molds mandatory), and its semicrystalline nature means shrinkage and warpage are anisotropic and crystallinity-dependent—requiring more sophisticated mold design and process control.
When to choose Ultem PEI: Ultem's amorphous structure provides inherently lower and more isotropic shrinkage (0.5-0.7% vs 1.2% for PEEK), enabling tighter-tolerance parts without the crystallinity-management complexity. Ultem offers inherent UL94 V-0 flame retardancy without additives, superior dielectric properties (dielectric constant 3.15 at 1 MHz vs 3.3 for PEEK), and natural transparency (amber) useful for visual flow indicators. It is preferred for electrical connectors, medical device housings requiring repeated autoclave sterilization, and semiconductor wafer handling where static dissipation and low outgassing matter. Ultem can be processed on standard injection molding machines with mold temperatures of 120-150°C (pressurized water or oil), reducing tooling and processing cost versus PEEK.
Comparison at a Glance
| Material A | PEEK (Polyetheretherketone) |
|---|---|
| Material B | Ultem PEI (Polyetherimide) |
| Polymer Type | PEEK: Semicrystalline | Ultem: Amorphous |
| Continuous Temp A | 260 °C |
| Continuous Temp B | 170 °C |
| Tensile Strength A | 100 MPa |
| Tensile Strength B | 105 MPa |
| Cost Relative | PEEK 2-3× more expensive than Ultem PEI |
| Best For A | Extreme chemical + thermal environments (oil & gas, aerospace) |
| Best For B | Electrical/electronic + flame resistance (connectors, medical devices) |
Frequently Asked Questions
At what temperature does Ultem PEI become unsuitable and PEEK required?
Ultem PEI's continuous service temperature is 170°C, with its glass transition at 217°C. Above 180°C, Ultem undergoes significant creep and property loss even under moderate load. PEEK maintains full mechanical properties to 260°C continuous. As a rule of thumb: below 160°C continuous, Ultem is usually sufficient and more cost-effective; 160-200°C is the gray zone where Ultem may work with derated loads; above 200°C, PEEK is required. For applications with temperature spikes (e.g., solder reflow at 260°C peak), Ultem 2300 (30% GF, HDT 210°C) can survive brief excursions, but PEEK (HDT 160°C at 1.82 MPa for unfilled, 315°C for 30% CF) provides a wider safety margin.
Which material is easier to injection mold—PEEK or Ultem?
Ultem PEI is generally easier to mold than PEEK for three reasons: (1) Ultem's mold temperature requirement (120-150°C) can be achieved with pressurized water systems, while PEEK requires 160-200°C oil-heated molds. (2) Ultem's amorphous structure produces isotropic, predictable shrinkage; PEEK's crystallinity means shrinkage varies with cooling rate and mold temperature—requiring more iterative mold tuning. (3) Ultem's wider processing window (melt temp 350-400°C) is more forgiving than PEEK (360-400°C, but residence time above 400°C causes rapid degradation). However, both materials require thorough drying (<0.02% moisture), high-temperature barrels, and ceramic heater bands—neither can be processed on a general-purpose polyolefin machine.
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References & Industry Standards
- ASTM International. Standard Specifications for Engineering Plastics & Thermoplastics. astm.org
- UL Prospector. Plastics & Elastomers Material Database. ulprospector.com
- MatWeb. Material Property Data for Engineering Thermoplastics. matweb.com
- ISO 1043. Plastics — Symbols and Abbreviated Terms. iso.org