What PEI is
Polyetherimide is an amorphous engineering thermoplastic with a glass transition of 217 °C, a continuous service temperature around 170 °C, and excellent dimensional stability. It is amber and transparent in thin sections, opaque in thick ones.
Its defining properties are heat resistance, inherent flame retardancy without additives, and very low smoke emission. Those three make it the default for aircraft interior parts, electrical components in high-temperature environments and medical devices that must be repeatedly sterilised.
It is also notch-sensitive and expensive. A sharp corner in PEI is a crack, and the material cost is five to ten times that of ABS. Both facts mean the part design and the tool deserve more attention than a commodity job.
| Własność | PEI | PPSU | PEEK |
|---|---|---|---|
| Glass transition | 217 °C | 220 °C | 143 °C |
| Continuous service | 170 °C | 150 °C | 250 °C |
| Flame retardancy | Inherent V-0 | Needs additive | Inherent V-0 |
| Notched impact | 50 J/m | 70 J/m | 80 J/m |
| Temperatura topnienia | 350 – 410 °C | 350 – 390 °C | 360 – 400 °C |
| Relative cost | Wysoki | Wysoki | Very high |
Table 1 — PEI against the two high-temperature materials it is usually weighed against.
Drying: four hours at 150 °C
PEI absorbs about 0.25 percent moisture and must be dried to 0.02 percent. At 150 °C in a desiccant dryer that takes four hours; at 120 °C it takes eight and does not reach the target reliably.
The consequence of under-drying is not a surface defect. Moisture in PEI at 370 °C produces hydrolysis of the imide linkage, which reduces molecular weight and impact strength. The part looks fine and is brittle.
The dryer must hold a dew point below −40 °C. A dryer that is merely warm will not dry PEI at all; it will equilibrate the resin with the ambient humidity and the part will be as wet as the workshop.
The melt window and the residence time limit
PEI processes between 350 and 410 °C. Below 350 the melt is too viscous to fill a thin section and the granules may not fully melt. Above 410 the polymer begins to degrade and the molecular weight falls.
Residence time is the more dangerous variable. PEI at 380 °C held for 15 minutes loses measurable molecular weight; held for 30 minutes it loses enough to embrittle the part. The rule on most shop floors is that the shot size should be at least 25 percent of the barrel capacity, which keeps residence under 8 minutes at normal cycles.
This is the opposite of the instinct to run a small part on a big machine for scheduling convenience. A PEI job on an oversized press will produce parts that pass inspection and fail a drop test, and the cause will not be visible.
| Parametr | Value | Note |
|---|---|---|
| Drying | 150 °C, 4 h, −40 °C dew point | To 0.02% |
| Temperatura topnienia | 350 – 410 °C | 370 – 390 °C typical |
| Mould temperature | 120 – 160 °C | Hot oil circuit |
| Injection pressure | 90 – 130 MPa | High viscosity |
| Back pressure | Niski | High back pressure adds heat |
| Residence time | Under 8 min at 380 °C | Shot size ≥ 25% of barrel |
| Regrind | Up to 20%, dried | Each pass costs molecular weight |
Table 2 — PEI process parameters. The residence time limit is the one that is most often broken and least often noticed.
Tooling for a 370 °C melt
The tool must be hot-work steel, typically H13 hardened to 48 to 52 HRC. P20 will soften and lose its polish. The mould runs at 120 to 160 °C, which needs an oil heater rather than a water circuit.
Gates must be generous. PEI at 370 °C is viscous, and a small gate produces high shear and a stress concentration. A gate that is adequate for PC will cause cracking in PEI, and the crack will appear after the part has been in service at temperature.
Draft is 1.5 to 2 degrees on polished surfaces. PEI grips a hot polished tool, and a part that is dragged out will have surface marks that cannot be removed.
Venting is critical. PEI fills fast at high pressure, and trapped gas produces a burn mark that is indistinguishable from a material defect. Vent depth of 0.015 to 0.02 mm at the parting line and at the last point to fill is the minimum.
Notch sensitivity and stress
PEI's notched impact strength is only 50 J/m. A sharp corner, a moulded thread root or a gate scar is enough to start a crack, and the crack propagates without the stress whitening that would warn you in a tougher material.
The design rules follow: internal radii of at least 0.5 mm and preferably 1 mm, no sharp gate scars, generous fillets at every section change, and no moulded threads where a machined thread would do.
Annealing at 200 °C for two hours relieves moulded-in stress and is common for parts that will be machined or that must hold dimension at temperature. It also improves the part's resistance to solvent stress cracking, which matters because PEI is attacked by chlorinated solvents and some amines.
Defects
| Wada | Przyczyna | Fix |
|---|---|---|
| Brittle parts | Moisture or long residence | Dry to 0.02%, reduce shot size |
| Dark streaks | Degradation in the barrel | Purge, reduce temperature, check residence |
| Bubbles | Moisture | Extend drying, verify dew point |
| Cracks at the gate | Small gate, high shear | Enlarge the gate, slow injection |
| Short shots | Melt too cold or pressure low | Raise 10 °C, increase pressure |
| Burn marks | Trapped gas | Add vents at the last point to fill |
| Warp | Differential cooling | Uniform mould temperature, anneal |
| Poor surface at the gate | Cold slug | Add a cold slug well |
Table 3 — PEI defects. The first two are thermal history; the third is drying. All three are invisible in the finished part.
At what temperature does PEI injection mould?
350 to 410 °C melt temperature, with 370 to 390 °C typical. Mould temperature is 120 to 160 °C and needs an oil heater.
How long does PEI need to dry?
Four hours at 150 °C in a dryer holding −40 °C dew point or better. The target is 0.02 percent moisture; under-dried PEI hydrolyses and loses impact strength without any visible defect.
What is the residence time limit for PEI?
About 8 minutes at 380 °C. Keep the shot size at 25 percent or more of the barrel capacity. A small part on an oversized press will produce brittle parts that pass inspection.
Can PEI be reground?
Up to about 20 percent, dried, and with the understanding that each pass reduces molecular weight. For a structural or high-temperature part, virgin material is safer.
Why does my PEI part crack at the gate?
Notch sensitivity plus shear orientation at a small gate. PEI has a notched impact of only 50 J/m, so a gate scar is a crack initiator. Enlarge the gate and slow the injection.
Is PEI the same as Ultem?
Ultem is SABIC's trade name for PEI. The material is polyetherimide; Ultem is one brand of it.
Moulding PEI parts and fighting brittleness?
Send us the drawing, the service temperature and the machine you have. We will tell you whether the shot size is right and what the tool needs for a 370 °C melt.
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