ULTEM shows up in aircraft interiors, semiconductor wafer carriers, and under-hood automotive brackets that need to survive heat most plastics cannot touch. It is the amber-colored engineering resin shops reach for when nylon and polycarbonate fall short but a full PEEK budget is not available. Among the materials in FirstLayerGuide’s filament types overview, PEI sits near the top of the engineering filament tier, one step below PEEK on both price and peak service temperature. This guide covers what PEI actually is, why the name ULTEM is on every data sheet, and what it takes to print it.
What PEI actually is
PEI, short for polyetherimide, is an amorphous engineering thermoplastic that in unfilled form is naturally transparent and amber colored, often enough to identify a spool before anyone reads the label. Almost nobody says “PEI” out loud; the trade name ULTEM is what shows up on every data sheet. GE’s Plastics division developed the resin in the 1980s and marketed it as ULTEM, then sold the business, patents included, to Saudi Basic Industries Corporation (SABIC) in 2007, which has produced ULTEM since, per 3Dnatives. SABIC still owns it: a 2026 sale of its lower-margin engineering thermoplastics business to the private equity firm Mutares excluded ULTEM, which stayed in SABIC’s specialties division with Noryl and LNP, per Plastics News.
Two grades matter here. ULTEM 1010 is the unfilled, general-purpose grade with the highest glass transition temperature in the line, plus NSF 51 food-contact certification and steam-sterilization tolerance. ULTEM 9085 trades some of that ceiling for toughness and meets flame, smoke, and toxicity (FST) requirements, the grade behind Stratasys’s aerospace FDM line. Both are amorphous rather than semi-crystalline like PEEK, a distinction that matters more in the printer than on the spec sheet.
The properties that justify the cost
SABIC lists ULTEM 1010 resin with a glass transition temperature (Tg) of 217°C and a heat deflection temperature up to 201°C to 210°C depending on test load, figures measured on molded resin rather than printed parts, per SABIC’s material finder. ULTEM 9085 runs cooler: Stratasys lists a Tg of 180°C and a heat deflection temperature of 173°C to 177°C for parts printed on its own Fortus and F900 systems, per the official data sheet.
ULTEM 1010’s Tg of 217°C is actually higher than PEEK’s 143°C, per 3DXTech’s own PEEK data sheet. PEEK still wins on continuous service temperature, 240°C to 260°C by that source, because it is semi-crystalline: its crystalline regions stay dimensionally stable well above their Tg, letting the material keep working past the point where an amorphous polymer would soften. PEI has no such backup, so its practical ceiling tracks close to its Tg, the real, data-backed version of “PEI sits one tier below PEEK.” FLG’s own PEEK filament guide covers that upper tier in full.
Printed ULTEM 9085 runs 40 to 70 MPa in tensile strength depending on print orientation, typical FDM anisotropy. Both grades carry a UL94 V0 flame rating with no added retardants, and 9085 adds full FST compliance under 14 CFR 25.853, the aircraft interior materials standard. SABIC describes the resin’s chemical resistance as very good “for an amorphous material,” a fair qualifier: PEEK’s semi-crystalline structure gives it a real edge here.
Why it is hard to print
PEEK’s difficulty comes largely from managing crystallization: cool it too fast and the part stays weak, cool it too slowly or unevenly and thermal stress warps it anyway. PEI skips that step. As an amorphous polymer, its chains bond across layers without organizing into a crystalline structure, part of why PEI has a reputation for printing more predictably than PEEK, a comparison 3DXTech makes directly, calling ULTEM the option for “a more manageable processing window.”
That reputation undersells PEI’s own problem. Its Tg is so high that a part is still shrinking after it leaves the nozzle; if the surrounding air runs much cooler than that Tg, the outer layers lock in place while the inner ones keep contracting, producing warping, delamination, or cracking. 3Dnatives puts the rule simply: chamber temperature should sit just below the glass transition temperature, close enough that the part stays above its rigid point until the print finishes, which is why a heated chamber is not optional for either grade. 3DXTech recommends one for ULTEM 1010 and holds its own ULTEM 9085 chamber near 190°C for the best layer adhesion. Moisture is the other quiet failure point: both grades need drying before use, 150°C for four hours by 3DXTech’s spec for 1010, and filament printed wet tends to blister and bond weakly in ways easy to mistake for a temperature problem.
Exact temperatures and the hardware that gets there
Numbers vary by grade and data sheet. For ULTEM 1010, 3DXTech’s own ThermaX PEI 1010 data sheet lists a nozzle range of 370°C to 390°C, a bed of 120°C to 160°C, and a recommended heated chamber. For ULTEM 9085, vendor guidance puts the nozzle closer to 370°C to 410°C with a bed of 140°C to 180°C. One gap is worth flagging: 3DXTech’s own testing puts ULTEM 9085’s heat deflection temperature around 158°C, well below the 173°C to 177°C Stratasys lists for its own Fortus and F900 parts, likely a hardware and test-orientation difference rather than one correct number.
Printer requirements are consistent even where exact temperatures are not: a hotend that reaches 400°C or higher, a bed holding 130°C to 180°C, and a chamber sustaining 120°C to 200°C depending on part size and grade. That rules out nearly every consumer FDM printer sold unmodified, and it means confirming nearby wiring is rated for sustained exposure, since fires on unmodified high-temperature builds are a documented risk. This is the territory of enclosed industrial systems: INTAMSYS, AON3D, Roboze, and, for aerospace work, Stratasys Fortus and F900 machines running certified ULTEM 9085 CG.
Cost and when it is worth it
Price is where the one-tier-below-PEEK framing holds up cleanly. 3DXTech sells unfilled ThermaX PEI 1010 at $195 for a 1kg spool against $588 for the same size spool of unfilled ThermaX PEEK, close to three times the price one step up. 3Dnatives reports a smaller but directionally identical gap in Europe: roughly 150 euros per kilogram for PEI against at least 300 for PEEK. Genuine Stratasys 9085 cartridges cost more still: a 4,100cc canister lists for $1,770 at GoEngineer, roughly $340 per kilogram by the resin’s specific gravity of 1.27.
PEI earns its cost when a part needs sustained heat, chemical exposure, or an FST rating and PEEK’s full 240°C-plus service window is not required. It is not worth it for parts that only see occasional hot water or a warm engine bay, where PC, nylon, or PPSU cover the same ground for less money and hardware. Downgrading to PEI to save on filament when a design genuinely needs PEEK’s higher service temperature is a false economy the day the part fails.
Real use cases
ULTEM 9085’s FST rating makes it standard for aircraft interior parts (vents, bin dividers, armrests, light panels), per 3DXTech’s ULTEM 9085 guide. Rail transport draws on the same properties: Bombardier has used PEI for tooling, fixtures, and finished train components, per 3Dnatives. ULTEM 1010’s NSF 51 certification and steam-sterilization tolerance put it in medical housings, surgical trays, and food-processing parts that would degrade cheaper plastics under repeated autoclaving. SABIC also lists ULTEM in electronic connectors, semiconductor handling equipment, and automotive under-hood brackets, anywhere dielectric stability or sustained heat would make nylon sag or fail.
PEI/Ultem filament checklist
- Hotend rated for continuous operation at 400°C or higher, not a stock PTFE-lined assembly, with nearby wiring confirmed safe for sustained heat.
- Heated chamber holding at least 120°C for ULTEM 9085, closer to 200°C for ULTEM 1010, since chamber heat near the resin’s Tg prevents warping.
- Heated bed in the 120°C to 180°C range, matched to whichever grade is loaded, plus filament dried at 150°C for four hours before use.
- A build surface built for the job, such as PEI film or glass with a PVC-based adhesive.
- The grade matched to the job: ULTEM 1010 for the highest heat resistance, ULTEM 9085 for FST compliance and toughness.
- A budget that reflects reality: roughly $195 per kilogram and up for unfilled desktop-compatible spools, several times that for OEM Stratasys cartridges.
- A genuine need for PEI’s heat and chemical resistance, rather than PC, nylon, or PPSU, which cost less and print on far more machines.