PEEK vs ULTEM (PEI) for Aerospace Parts

Updated October 13, 2026

PEEK and ULTEM side by side

The two materials overlap in stiffness and strength at room temperature. They separate on heat, fluids and moisture. The table uses the producers' own data sheets for the unfilled reference grades, VICTREX PEEK 450G (Victrex data sheet, last updated 17 March 2026) and SABIC ULTEM 1000 (Americas technical data sheet, revision 20200108). All values are typical, not specification minimums, and the two producers test to different methods, so treat small differences as noise.

Two rows deserve a second look. PEI holds its heat deflection temperature higher than unannealed PEEK because PEEK's amorphous fraction softens near 143 C, yet PEEK keeps useful stiffness well above that point because its crystals do not melt until 343 C. And the UL thermal indices, which are the closest thing either producer publishes to a "continuous use temperature", favor PEEK by 70 to 90 C on the electrical and unimpacted mechanical ratings.

Semi-crystalline versus amorphous: the root of every difference

PEEK is semi-crystalline: ordered crystalline regions sit in an amorphous matrix. PEI is fully amorphous. Almost every practical difference between them follows from that one fact.

• Behavior above Tg. An amorphous polymer loses most of its stiffness at its glass transition and has no crystals to carry load above it, so 217 C is a hard ceiling for unfilled PEI. PEEK's crystals keep carrying load between 143 C and its melt. Victrex lists flexural stress at 3.5 percent strain of 125 MPa at 23 C, 85 MPa at 125 C, 19 MPa at 175 C and 12.5 MPa at 275 C for 450G, which shows both the drop at Tg and the residual strength well above it.
• Fluid resistance. Crystalline regions resist solvent penetration. That is why semi-crystalline PEEK tolerates a broader range of fluids than any amorphous polymer, including PEI.
• Dimensional behavior. Amorphous PEI shrinks uniformly and machines to crisp edges with little warp. PEEK's crystallinity can still change with heat history, so stock and parts that see temperature for the first time in service can move unless they were annealed.
• Appearance. Unfilled PEI is transparent. Unfilled PEEK is opaque and beige. Color is a quick receiving check that the right polymer arrived, though not a substitute for the cert.

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Temperature: reading Tg, HDT and the UL thermal index correctly

For long-term exposure, compare the UL 746B relative thermal index, not the heat deflection temperature. HDT is a short test under a fixed load; the RTI is the temperature at which a property holds up over long aging. On that basis PEEK 450G is rated 240 to 260 C for unimpacted mechanical and electrical properties, and ULTEM 1000 is rated 170 C. SABIC states the ULTEM family is available with indices up to 180 C.

Practical consequences for hardware:

• A bracket, standoff or insulator that lives at 150 to 170 C is inside both materials' ratings, and PEI's higher Tg gives it better stiffness retention in that band.
• A part that sees 200 C or more for long periods, such as a seal backup ring near a hot valve, a pump component or an insulator on a heater circuit, belongs in PEEK or a higher-temperature polyimide.
• Short excursions above an RTI are not automatically disqualifying, but they need test data at the actual temperature, load and duration. Get the creep or flexural data for the exact grade from the producer.
• Both expand far more than the metals they bolt to. Victrex lists an average CLTE of 55 ppm/K below Tg and 140 ppm/K above it for 450G; SABIC lists about 54 to 56 ppm/C for ULTEM 1000 from minus 20 to 150 C. Design clearances for the full thermal range.

Chemical, fluid and steam resistance

PEEK wins on chemical breadth. Victrex states PEEK is not attacked by prolonged exposure to water, sea water or steam, and publishes tensile retention after 28 days immersion across a range of chemicals; its current chemical resistance list is downloadable from Victrex. PEEK is also a strong gas and liquid barrier: Victrex reports water vapor permeation of 4 cm3 per m2 per day through 100 micron crystalline film.

PEI is good for an amorphous polymer, and SABIC states ULTEM resins resist stress cracking when exposed to automotive and aircraft fluids, aliphatic hydrocarbons, alcohols, acids and weak aqueous solutions. The gap shows with aggressive solvents, strong bases and long hot-water or steam exposure, where a semi-crystalline polymer has the advantage. If a part will be cleaned with a specific solvent, put that solvent and its temperature in the RFQ so the supplier and the material producer can confirm compatibility for the actual grade.

Flammability and FAR 25.853: what each producer states

Both polymers are inherently flame resistant without added flame retardants, and both are used in aircraft interiors. Claims below are the producers' own; a part's compliance is shown by testing the part or a representative specimen at its thickness, not by a resin data sheet.

• PEEK. Victrex states unfilled 450G achieves UL 94 V-0 at 1.5 mm and that glass and carbon filled grades reach V-0 at 0.5 mm over a wide range of filler levels. Victrex also states its smoke levels are over 95 percent lower than limits in aviation smoke standards, giving Boeing BSS 7238 as the example, and that unfilled PEEK reaches a glow wire flammability index of 960 C.
• ULTEM 1000. SABIC lists UL 94 V-0, V-2 and 5VA recognitions and an LOI of 47 percent, and states the family meets UL 94 V-0 in sections as thin as 0.41 mm with very low smoke in the NBS smoke test.
• ULTEM 9085. SABIC's 9085 data sheet states the resin meets FAR 25.853 and OSU heat release requirements for aircraft cabin interiors. Stratasys states its ULTEM 9085 filament meets multiple aerospace and rail flame, smoke and toxicity standards.

Note that UL 94 measures self-extinguishing after ignition. Victrex says plainly that it is not a measure of resistance to ignition. For oxygen-enriched systems the relevant tests are the NASA and ASTM oxygen compatibility tests, covered on our engineering plastics page; neither PEEK nor PEI should be assumed oxygen compatible from a UL rating.

Outgassing for vacuum and spacecraft parts

Both materials are used inside spacecraft, but check the exact grade. Victrex publishes ASTM E595 results for 450G of 0.26 percent TML, 0.00 percent CVCM and 0.12 percent WVR, and similar results for its 30 percent glass (450GL30) and 30 percent carbon (450CA30) grades. Those are producer data, not an independent listing. For PEI, and for any colored, filled or printed grade of either polymer, look up the exact product in the NASA Goddard outgassing database, and if it is not listed, test a sample from the production lot. Printed ULTEM 9085 parts in particular should be judged on printed-part data, because porosity and support material can trap volatiles that a molded plaque does not.

Grades: unfilled, filled and printed

For printed flight parts, the grade name is only the start. Stratasys states that its certified 9085 grade comes with traceability documentation and meets more stringent test criteria than standard filament, and that the material is qualified for flight parts when printed on the F900 in accordance with the published NCAMP allowables. Stratasys also offers ULTEM 1010 filament, which it describes as more rigid than 9085. If you are buying printed parts, the printer, build orientation and process control matter as much as the filament; see our aerospace additive manufacturing service page. Filled grades of either polymer cost stiffness in electrical properties (carbon) or add abrasive wear to cutting tools (glass), so call the filler out explicitly.

Specifications that control these materials

Neither polymer has a widely used AMS or MIL material specification for stock shapes that we could confirm, so most drawings call a producer grade or an ASTM classification. The ASTM line callout system works the same way it does for other plastics: ASTM D4000 sets the general format, and a polymer-specific standard supplies the group, class and grade codes.

ASTM states that D5205 is a means of calling out material for parts and is not intended for material selection, which is the right way to think about all of these classification standards: pick the material on data, then use the standard to lock down what the supplier may deliver. For material paperwork, see material certs and traceability.

Machining and annealing

Both machine well, but they fail in different ways. PEI is the easier of the two to hold flat and true, because it is amorphous and stable. PEEK is tougher and more abrasion resistant, so it is harder on tools, and its residual stress from extrusion or molding relaxes as material is removed.

• PEEK annealing. Boedeker's post-machining air annealing guideline for PEEK is 2 hours to 300 F, then 2 hours to 375 F, holding 60 minutes per 1/4 inch of thickness at each step, then cooling at 50 F per hour.
• PEI annealing. Boedeker's guideline for unfilled ULTEM is 4 hours to 390 F, holding 30 minutes per 1/4 inch of thickness, then cooling at 50 F per hour; glass filled grades go to 400 F with the same hold and cooling rate.
• Boedeker notes that critical dimensions should be finish machined after annealing and that parts should be fixtured to prevent distortion. For tight PEEK parts, a rough, anneal, finish sequence is the usual way to hold flatness.
• PEI's 1.25 percent equilibrium water absorption is roughly three times PEEK's saturation value, so very tight PEI parts should be dried or conditioned before final inspection.

Fine features in either material are common on Swiss turning and micro machining equipment.

Choose PEEK when, choose ULTEM when

Choose PEEK when:

• Long-term temperature is above about 170 C, or the part must keep some load-carrying ability above 200 C.
• The part sees steam, hot water, hydraulic fluids, fuels or aggressive cleaning solvents for long periods.
• Wear, fatigue or impact toughness matters: bushings, seal backups, clips and snap features.
• You need low moisture uptake for dimensional stability across humidity.

Choose ULTEM when:

• Service stays below about 170 C and you want the higher Tg's stiffness retention between 150 and 200 C.
• You need a dimensionally stable insulator, fixture or manifold that machines cleanly to tight flatness.
• Transparency helps, for example to see fluid or alignment features.
• You are printing interior parts that must meet FAR 25.853, where ULTEM 9085 is the established FDM choice.

Cost and availability drivers

We do not quote material prices here, but the drivers are predictable. Resin grade sets the base: filled and certified grades of either polymer, and producer-specific grades with a single source, cost more than unfilled general purpose stock, and the price gap between the two polymers is worth asking about on every quote rather than assuming. Large diameter rod and thick plate are less common in both, and thick PEEK sections are more prone to internal stress, so the yield from a blank may be lower. Producer-specific callouts narrow the supply base; an "or approved equal" clause, or an ASTM classification callout, widens it. Lot certs to the producer level, oxygen or vacuum cleaning and bake requirements add process steps and lead time. For printed ULTEM 9085, certified filament and process documentation for flight use are the main cost adders.

Callout examples for the drawing or PO

These are formats, not grades to copy blindly. Adjust to your program's requirements.

• MATERIAL: PEEK, UNFILLED, VICTREX PEEK 450G OR APPROVED EQUAL. STOCK STRESS RELIEVED. ANNEAL AFTER ROUGH MACHINING. CERT TO STATE PRODUCER, GRADE AND LOT.
• MATERIAL: PEI, UNFILLED, ULTEM 1000 OR APPROVED EQUAL PER ASTM D5205 (CONFIRM CURRENT REVISION WITH ASTM). NATURAL COLOR. NO REGRIND.
• MATERIAL: PEEK, 30 PERCENT GLASS FILLED, VICTREX 450GL30 OR APPROVED EQUAL. OUTGASSING DATA PER ASTM E595 REQUIRED FOR SUPPLIED LOT.
• MATERIAL: ULTEM 9085, CERTIFIED GRADE FILAMENT WITH LOT TRACEABILITY. BUILD ORIENTATION PER DRAWING NOTE. FLAMMABILITY PER 14 CFR 25.853 SHOWN BY TEST REPORT FOR THE CONFIGURATION.

Add the measurement temperature for tight tolerances, the cleaning and packaging level for clean or vacuum parts, and any prohibition on coolants or marking inks that could add outgassing.

Get PEEK or ULTEM parts quoted

Send the drawing with the grade, filler, annealing and cleaning requirements through a CNC quote request. We flow the grade and certification requirements down to the machining supplier, check that the material cert matches the grade on the drawing, and ship with the cert package. If the part is controlled, read how we handle controlled programs before sending files. For the other side of the plastics family, see PTFE vs PCTFE for seals and seats, and the materials hub.

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Related

• Engineering Plastics for Flight and Ground Hardware
• PTFE vs PCTFE for Seats, Seals and Oxygen Service
• Material Certs, Specialty Metals and Lot Traceability
• Aerospace Materials: Selection, Specs and Paperwork

Sources

• Victrex, VICTREX PEEK 450G data sheet (last updated 17 March 2026)
• Victrex, VICTREX PEEK properties guide (September 2018)
• Victrex, Victrex Pipes brochure (September 2016), UL 94 and LOI
• SABIC, ULTEM resin 1000 Americas technical data sheet (revision 20200108)
• SABIC, ULTEM resin family overview
• SABIC, ULTEM resin 9085 Americas technical data sheet
• Stratasys, ULTEM 9085 filament material page
• ASTM D8033-22, classification system for PEEK molding and extrusion materials
• ASTM D5205-24, classification system for PEI materials
• Boedeker Plastics, plastic annealing guidelines
• eCFR, 14 CFR 25.853 compartment interiors
• NASA Goddard outgassing database