Ultem PEI (Polyetherimide)

Complete property data sheet for unfilled Ultem PEI — mechanical, thermal, electrical and physical values, each with its ASTM test method — plus why an amorphous polymer has a glass transition temperature rather than a melting point.

Data verified 2026-09-29 · based on ASTM D5205-17

Mechanical Properties

Thermal Properties

Electrical & Physical Properties

Weight & Volume Calculator

Machining Ultem PEI

Polyetherimide machines more like a stiff, somewhat brittle metal than like nylon or acetal. It is hard (Rockwell M109) and stiff (3.0 GPa), so it takes a clean cut and holds a good finish, but the same stiffness means it chips at edges and corners where a softer plastic would simply deform. Use sharp, positive-rake carbide, keep the feed per tooth high enough to cut rather than rub, and support unsupported corners.

Because water absorption is only 0.25% in 24 hours, dimensions are stable — much closer to acetal than to nylon. There is no moisture-conditioning step and no significant swell in service. Thermal expansion at 56 µm/m·°C is the lowest of the unfilled engineering plastics on this site, which makes Ultem a reasonable choice for parts that must hold tolerance across a wide temperature range.

The two cautions are abrasiveness and stress cracking. Ultem wears tooling faster than nylon or acetal, so expect shorter edge life. And it is susceptible to stress cracking in contact with partially halogenated solvents and some ketones — a machined part left with residual cutting fluid in a stressed area can crack days later. Clean the part after machining, and avoid solvent-based coolants.

Where Ultem PEI Is the Right Choice

Ultem PEI occupies a specific niche: high strength and stiffness, retained to 170 °C continuously, combined with inherent flame retardance to UL94 V-0 without additives, a high limiting oxygen index of 47%, and low smoke generation. That combination is why it is specified for aircraft interior components, medical trays that see repeated sterilisation, and electrical components that must meet flammability requirements without halogenated additives.

Being amorphous rather than semi-crystalline, it has a glass transition temperature instead of a melting point — 217 °C. That is a real distinction rather than a labelling detail: below Tg the material is stiff and glassy, and approach it and properties fall away steeply rather than gradually. The 170 °C continuous rating leaves roughly 50 °C of margin to Tg, which is why the rating is what it is.

Against the alternatives: PEEK is semi-crystalline, chemically far more resistant, and rated to 250 °C, at slightly higher cost; PPS is cheaper, similarly flame retardant and rated to 220 °C, but much more brittle; PES and PSU sit below PEI in temperature. PEI is the usual choice when transparency, flame retardance and hot strength are needed together.

How to Read This Data Sheet

Values are typical published figures for unfilled, natural-grade Ultem PEI, measured on standard test specimens. Every row names the test method it came from, because a polymer property without its test method is not a comparable number — tensile strength measured to ASTM D638 and to ISO 527 differ systematically, and impact values depend heavily on specimen geometry.

Two caveats apply to every figure here. First, temperature: polymer properties are far more temperature-sensitive than metal properties, and a tensile strength quoted at 23 °C tells you little about the same material at 100 °C. Second, conditioning: hygroscopic plastics such as nylon absorb moisture from the air, and absorbed water acts as a plasticiser — the same grade can be stiff and strong when dry and significantly tougher but weaker when conditioned. Where a property is strongly affected by moisture, the row notes it.

Filled, reinforced, lubricated and impact-modified grades of the same base polymer differ substantially from these figures. Use this page to compare materials and to shortlist; use the specific grade's data sheet for design calculations.

Frequently Asked Questions

What is the density of Ultem PEI?

1.27 g/cm³, which is heavier than water and about 11% denser than nylon. Density matters for two practical reasons: it sets the weight of a finished part through the material's cost per kilogram, and it is the number this page's weight calculator uses to convert a volume into a mass.

Can Ultem PEI be machined?

Ultem machines like a hard, brittle metal — it is stiff enough to chip rather than tear, so sharp edges and controlled feeds matter more than with softer plastics. It absorbs little moisture, so dimensions are stable; it is abrasive on tooling and can stress-crack in the presence of some solvents.

How do I choose between Ultem PEI and another engineering plastic?

Choose Ultem PEI when you need high strength at high temperature, or inherent flame retardance, from a transparent-capable plastic — aircraft interiors, sterilisable medical trays, electrical insulators, hot-air ducting.

Why does Ultem have no melting point?

Because it is an amorphous polymer. Its molecular chains are randomly arranged rather than folded into crystalline regions, so there is no crystal lattice to melt. Instead the material softens progressively as it passes its glass transition temperature of 217 °C. Semi-crystalline plastics such as PEEK, nylon and acetal have both a Tg and a sharp melting point; amorphous ones such as PEI, PC, acrylic and PVC have only a Tg.

Can Ultem be autoclaved?

Yes, and it is one of the reasons it is used in medical equipment. Ultem withstands repeated steam autoclave cycles at 134 °C without the hydrolysis damage that destroys acetal, and it tolerates most common disinfectants. It is attacked by partially halogenated solvents and some ketones, so the cleaning chemistry needs checking — and because PEI stress-cracks, the part should not be left under load in contact with an aggressive solvent.

Is Ultem the same as PEEK?

No, and they are frequently confused because both are amber-coloured high-temperature engineering plastics. PEI (Ultem) is amorphous, rated to 170 °C continuously, and considerably cheaper. PEEK is semi-crystalline, rated to 250 °C, mechanically stronger and far more chemically resistant. PEEK also costs roughly 3–5× as much. Choose PEI for hot strength and flame retardance at moderate cost; choose PEEK for chemical exposure or the highest temperatures.

Related

• PEEK Properties
• PTFE Properties
• Delrin (POM-H) Properties
• Plastic Service Temperature Chart
• Plastic Chemical Compatibility
• Plastic Machining Tolerance Chart

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

Data Sources

Cross-checked against:

• Published PEI stock-shape data sheets from two independent suppliers
• Continuous service temperature checked against the UL 746B relative thermal index rather than a supplier claim
• Amorphous structure confirmed by the absence of a melting endotherm in the published DSC data

Values are typical published figures for unfilled, natural-grade resin at 23 °C, dry as molded unless the row states otherwise. They are not specification minima. Design to the specific grade's data sheet, and derate for the service temperature and moisture condition the part will actually see.

Cite This Page

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Each row in the tables above also has a permanent link — hover a row and use the # link to cite a single value rather than the whole page.