Material selection
PEEK vs PEI
Both are high-performance polymers specified when a metal part needs to be lighter or non-conductive and nothing cheaper will survive the environment. The question between them is rarely price, since both sit well above every commodity and engineering plastic on this site; it is whether the part actually needs PEEK's higher ceiling or whether PEI covers the requirement at a lower cost and an easier mold. The numbers below are the same published datasheet values used on each material's own page, set side by side.
| PEEK | PEI (Ultem) | |
|---|---|---|
| Structure | Semi-crystalline | Amorphous |
| Density | 1.30 g/cm³ | 1.27 g/cm³ |
| Tensile strength | 95–100 MPa | 105 MPa |
| Flexural modulus | 3.9 GPa | 3.3 GPa |
| Continuous service temperature | 250 °C | 170 °C |
| Optical | Not offered in a transparent grade | Transparent, amber tint |
| Relative cost | Very high | High |
Published values for unfilled grades at 23 °C unless stated, taken from each material's own datasheet page. Confirm against the supplier datasheet for the grade you specify.
What PEEK is
PEEK is a semi-crystalline aromatic polymer that keeps useful mechanical properties to 250 °C continuously, resists almost all chemicals except concentrated sulphuric acid, and survives repeated steam sterilization without degrading. It is the polymer specified when a metal part needs to be lighter, non-conductive, or radiolucent and the environment is too hot or too aggressive for anything else on this site's material list. Material cost is roughly a hundred times ABS, so the case has to be made on requirement rather than preference.
What PEI is
PEI is an amorphous polymer with inherent flame retardance and very low smoke emission, which is why it dominates aircraft interiors. It holds dimensional stability to 170 °C and is transparent in an amber tint. Being amorphous rather than semi-crystalline, it machines and molds more predictably than PEEK, with less shrinkage variation to manage across a wall-thickness change, and costs substantially less, at the price of lower chemical and wear resistance.
PEEK against PEI
| PEEK | PEI (Ultem) | |
|---|---|---|
| Heat ceiling | 250 °C continuous, semi-crystalline melt at 343 °C | 170 °C continuous, no melt point, softens rather than melting |
| Chemical resistance | Resists almost all chemicals except concentrated sulphuric acid | Good, but lower than PEEK; not the choice for aggressive solvent or acid exposure |
| Sterilization | Survives repeated steam sterilization without degrading | Tolerates it occasionally; not the first choice for repeated autoclave cycles |
| Optical clarity | Not offered transparent | Transparent, amber tint, useful when the part must be seen through |
| Flame performance | Self-extinguishing but not published to a UL 94 V-0 rating in the unfilled grade used on this site's own datasheet page | Inherent UL 94 V-0, no flame-retardant additive needed |
| Molding behaviour | Semi-crystalline shrinkage, needs a hot tool and tighter process control | Amorphous, more predictable shrinkage and easier to hold tolerance across wall sections |
| Relative cost | Very high | High, substantially less than PEEK |
Which one, for which part
- Continuous exposure above 170 °C, or contact with anything other than concentrated sulphuric acid is PEEK's case: its chemical resistance and heat ceiling are both higher.
- Repeated steam sterilization or autoclave cycles favours PEEK, which is documented to survive them without degrading.
- A housing or window that must stay transparent is PEI's case: PEEK has no clear grade at all.
- An enclosure or connector that needs inherent flame retardance without an additive fits PEI's UL 94 V-0 rating.
- A requirement that tops out at 170 °C where budget and mold predictability matter favours PEI: it costs substantially less than PEEK and its amorphous shrinkage is easier to hold to tolerance.
Our status
Trumould molds both PEEK and PEI in-house through injection molding, both on our high-temperature-capable presses with a heated tool; CNC machining and 3D printing are available for both materials too. Send the part, its continuous service temperature, chemical or optical requirement, and target volume, and the quote states which one is actually required, not just a price for whichever one you asked about first.
Specifying this material?
Send your CAD and target volumes. We come back with a price, a lead time, and any material or design notes that would reduce either.