Last updated: 2026-09-05 · 5 min read
PEI/ULTEM Injection Molding — What Makes This High-Temperature Resin Worth the Cost and How Do You Mold It Right?
PEI (polyetherimide, trade name ULTEM) is an amorphous high-performance resin with continuous use at 170°C, Tg of 217°C and inherent UL94 V-0 at 0.8mm without additives. It requires mold temps of 150–180°C, melt temps of 340–400°C and moisture below 0.05% before processing. The resin costs 5–10× more than PC or nylon, but replaces metal in applications where weight, corrosion resistance and net-shape molding matter more than material cost.
Key Facts
- PEI is amorphous — it has no crystalline melt point, only a glass transition (Tg = 217°C). This means near-zero mold shrinkage (0.005–0.007 mm/mm isotropic), excellent dimensional stability and no post-mold crystallization. The trade-off: the resin must be dried to below 0.05% moisture before processing, because amorphous resins absorb moisture into the free volume between chains — see the resin drying guide for how moisture targets vary by resin class — and that moisture causes splay, hydrolysis and molecular weight loss at 360°C melt temperature.
- Mold temperature is the single most important processing parameter. At 150–180°C mold temp, the part surface replicates the mold finish (SPI A2 or better), residual stress stays below 5 MPa and the part does not craze during ejection. At mold temps below 120°C, the part develops frozen-in stress that causes cracking within hours — especially in the presence of solvents, oils or assembly adhesives.
- PEI requires a dedicated screw and barrel — the resin’s processing temperature (340–400°C) and its aromatic molecular structure cause carbon buildup on standard screws within 200–500 shots. The barrel must be cleaned with a high-temp purge compound between runs. Stainless tool steel screws (C-276 or equivalent) resist the corrosion from halogen-free flame retardant byproducts.
How Do PEI Molding Parameters Compare to Standard Engineering Resins?
| Parameter | PEI (ULTEM) | PC | PA66 GF30 |
|---|---|---|---|
| Melt temperature | 340–400°C | 280–320°C | 270–290°C |
| Mold temperature | 150–180°C | 80–120°C | 60–80°C |
| Drying temperature | 150–170°C / 4h | 120°C / 4h | 80°C / 4h |
| Shrinkage (flow) | 0.005 mm/mm | 0.006 mm/mm | 0.003 mm/mm |
| Tg | 217°C | 147°C | N/A (semi-crystalline) |
| Resin cost (approx.) | $15–25/kg | $3–5/kg | $4–7/kg |
Amorphous resin — a polymer with no ordered crystalline structure in the solid state. Chains are frozen in random positions below Tg. Amorphous resins (PEI, PC, PS, PMMA) have lower mold shrinkage, are transparent in thin sections and cannot be remelted and re-crystallized the way semi-crystalline resins (PP, nylon, POM) can. See the amorphous vs semi-crystalline comparison for how resin class affects every downstream decision.
The 3× higher mold temperature is what drives the cycle time difference. A PC part at 100°C mold temp cools to ejection in 20–30 seconds. A PEI part at 170°C mold temp must cool from 370°C to below 180°C (the Tg) before it can be ejected without distortion — and the smaller temperature delta (370–180 = 190°C vs. 300–100 = 200°C for PC) combined with the higher mold temp means the cooling rate is slower and the cycle is 10–30% longer.
What Three Design Rules Govern PEI Parts That Differ from Standard Resins?
- Wall thickness: 1.0–3.0mm, uniform within 0.5mm transitions. PEI’s high viscosity at processing temperature means it does not flow as easily as PC or nylon into thin sections. Walls below 0.8mm require injection pressures above 120 MPa and may not fill without flow marks. Walls above 4.0mm cause internal voids because PEI’s high Tg prevents the skin from sealing before the core shrinks. Step transitions must be gradual (3:1 ratio) to prevent stress concentration at the step.
- Gate design: fan gates or tab gates, not pin-point gates. PEI’s high melt viscosity and high processing temperature cause pin-point gates to freeze prematurely or erode from the abrasive melt. A fan gate (width 2–3× the wall, depth equal to the wall) provides uniform flow into the cavity and reduces jetting risk. Tab gates are used for cosmetic parts where the gate vestige must be hidden — the tab is broken off after molding and the vestige is on the tab, not the part. The gate type selection guide covers when each gate geometry is appropriate across all resin classes.
- Post-mold annealing for parts above 2mm wall or with inserts. Even at 170°C mold temperature, PEI parts above 2mm wall retain frozen-in molecular orientation. Annealing at 10–20°C below the Tg (197–207°C) for 1–2 hours relieves the residual stress. Parts with metal inserts must be annealed with the insert in place — otherwise the differential shrinkage between the metal (CTE 12 ppm/°C) and the PEI (CTE 30–47 ppm/°C) creates cracks around the insert during service.
Annealing (in the context of PEI and other high-Tg amorphous resins) — a stress-relief heat treatment held at 10–20°C below Tg for a time proportional to wall thickness. It allows frozen-in molecular orientation to relax without distorting the part. Skipping annealing on PEI parts above 2mm wall is the most common cause of delayed solvent-cracking and adhesive-crazing failures in the field.
PEI (ULTEM): Tg 217°C, continuous use 170°C, shrinkage 0.005 mm/mm, UL94 V-0 at 0.8mm (no additives). Molding: melt 340–400°C, mold 150–180°C, dry 150–170°C/4h to <0.05% moisture. Resin cost $15–25/kg (5–10× PC). Requires dedicated screw, high-temp purge, annealing for walls >2mm.
- Resin dried to <0.05% moisture — desiccant dryer at 150–170°C for 4h minimum; verify with a moisture analyzer, not oven-loss time.
- Mold temperature unit stable at 150–180°C — confirm the unit can hold ±2°C under continuous operation; oil-temperature units are standard, steam units for SPI A1–A2 finishes.
- Dedicated screw installed — stainless tool steel (C-276 or equivalent), high-temp purge completed within the last 50 shots.
- Wall thickness uniform within 0.5mm transitions — thick islands above 3mm require core-out or gas-assist; see the wall thickness design rules.
- Annealing capacity confirmed — oven available at 197–207°C, parts/fixtures ready for 1–2h cycle with parts held in fixture to prevent warpage during cool-down.
Frequently Asked Questions
Can PEI be glass-filled or mineral-filled?
Yes — glass-filled PEI (10–30% glass fiber) increases flexural modulus from 3.3 GPa to 10–17 GPa and reduces shrinkage from 0.005 to 0.002 mm/mm. Mineral-filled grades improve dimensional stability and reduce warpage for flat parts like connector housings. The trade-off: filled grades are more abrasive on the screw and barrel (bimetallic barrel recommended) and the surface finish is not as smooth as unfilled PEI.
Filled PEI is specified for structural applications (brackets, supports, load-bearing housings) where stiffness matters more than cosmetic appearance. Unfilled PEI is used for optical, medical and food-contact applications where clarity or surface quality is the priority.
What adhesives bond to PEI?
PEI bonds well with epoxy adhesives, cyanoacrylates (super glue) and solvent-based adhesives using MEK or NMP as the solvent. The bond strength on PEI is typically 15–25 MPa in lap shear — higher than the cohesive strength of the resin itself, meaning the part will fail before the adhesive joint. Ultrasonic welding is also effective at 20 kHz with an energy director of 0.3–0.5mm height and 90° included angle.
Solvent bonding requires care — the solvent must soften the PEI surface without dissolving it. MEK is the most common solvent for PEI bonding; acetone is too aggressive and causes crazing. The bonded assembly should be fixture-held for 24 hours at room temperature before testing.
Why is PEI used in aerospace and medical instead of PEEK?
PEI costs 3–5× less than PEEK ($15–25/kg vs. $80–120/kg), can be injection molded on standard machines (with high-temp mold capability) and is inherently flame resistant without additives. PEEK offers higher continuous use temperature (250°C vs. 170°C), better chemical resistance and semi-crystalline structure — but at 3–5× the material cost and requiring 380–400°C melt temperature with mold temps of 170–200°C.
For aerospace interior panels, medical device housings and sterilizable trays, PEI meets the flame, smoke and toxicity requirements (FAR 25.853) at a fraction of the PEEK cost. PEEK is reserved for applications above 170°C continuous, or where chemical resistance to fuels, hydraulic fluids or strong acids is required.
Key Takeaways
- PEI (ULTEM): Tg 217°C, continuous 170°C, UL94 V-0 at 0.8mm, shrinkage 0.005 mm/mm.
- Molding: melt 340–400°C, mold 150–180°C, dry <0.05% moisture, dedicated screw.
- Design: walls 1.0–3.0mm, fan/tab gates (not pin-point), anneal walls >2mm.
- Cost: $15–25/kg (5–10× PC), but replaces metal in high-temp, flame-critical applications.
Send the part drawing with the operating temperature and flame requirements — the quotation evaluates whether PEI (ULTEM) meets the thermal and regulatory requirements or whether a lower-cost resin like PC or PBT can serve the same function at a fraction of the material cost.
Sources: SABIC — ULTEM Resin Product Data Sheets and Processing Guidelines · Plastics Technology — PEI/ULTEM Injection Molding Parameters and Troubleshooting · Society of Plastics Engineers (SPE) — High-Temperature Resin Processing Guidelines · MatWeb — PEI (Polyetherimide) Material Property Database · AZoM — Polyetherimide Properties, Applications and Processing Methods
— Arthur, Tooling Engineer at aCatMould