PEEK vs Ultem

Both high-temperature.
Very different uses.
Cost matters.

PEEK and Ultem (polyetherimide) are both high-performance engineering plastics for aerospace, medical, and demanding electrical applications. PEEK is stronger, higher-temperature, more expensive. Ultem is transparent, cheaper, lower performance. Here's the decision guide.

Get a Quote DFM guide

01 · At a glance

Side-by-side summary.

Option A

PEEK (Polyetheretherketone)

Highest-performance thermoplastic. 260 °C continuous service, excellent chemical resistance, biocompatible medical grades. The premium choice for demanding applications.

Option B

Ultem (Polyetherimide, PEI)

High-performance polyetherimide. 170 °C continuous service, inherently flame-retardant UL 94 V-0, transparent amber color, lower cost than PEEK. Good middle-ground high-temperature plastic.

02 · Detailed comparison

Feature-by-feature breakdown.

Attribute	PEEK	Ultem (PEI)
Continuous service temp	260 °C	170 °C
Peak temp (short)	315 °C	200 °C
Tensile strength	96 MPa	110 MPa
Flexural modulus	4.0 GPa	3.4 GPa
Transparency	Opaque (natural)	Transparent amber
Flame retardant	UL 94 V-0	UL 94 V-0
Chemical resistance	Excellent (approaches PTFE)	Moderate
Biocompatibility	USP VI, ISO 10993 (medical grades)	USP VI
Implantable medical	PEEK-OPTIMA available	Not typical
Machinability	Good	Good
Cost (per kg)	$120–180 unfilled, $300+ medical	$50–90 unfilled
Sterilization	Autoclave, gamma, ETO	Autoclave, gamma, ETO
Density	1.32 g/cc	1.27 g/cc
FDA food contact	FDA 21 CFR 177.2415	FDA 21 CFR 177.1595

03 · Decision guide

When to choose each.

Choose PEEK (Polyetheretherketone) when:

Continuous service above 170 °C (Ultem's limit)
Implantable medical devices (PEEK-OPTIMA)
Aggressive chemical environments
Critical wear applications (self-lubricating grades)
Aerospace hot section plastic components
Applications where PEEK's superior performance justifies premium cost

Choose Ultem (Polyetherimide, PEI) when:

Service temperature 100–170 °C
Transparent parts needing high-temperature capability
Medical instruments (autoclavable) where PEEK is overkill
Aerospace interior per FAR 25.853 (inherently FR)
Electrical insulators needing dielectric + temperature
Cost-sensitive high-temperature applications

FAQ

Common questions.

PEEK — significantly. PEEK resists essentially all common solvents, acids, and bases at moderate temperatures. Approaches PTFE in chemical inertness while retaining mechanical strength. Ultem has moderate chemical resistance — attacked by many common solvents (acetone, DMSO, DMF) and prolonged exposure to strong alkalis. For chemical processing applications, PEEK is the default. For electronics and medical where chemical exposure is limited, Ultem is often adequate.

Substantial. PEEK unfilled: $120–180/kg billet. Ultem unfilled: $50–90/kg. PEEK medical-grade (Invibio PEEK-OPTIMA): $300–400+/kg. Ultem 2300 medical: $80–120/kg. For equivalent part, PEEK version typically 2–3× cost of Ultem. Justify PEEK premium only when application genuinely requires its superior temperature or chemical performance.

PEEK is semi-crystalline — polymer chains pack into crystalline regions that scatter light, producing opaque appearance. Ultem is amorphous (chains randomly oriented) — no crystalline regions to scatter light, so inherently transparent amber. This crystallinity difference also explains why PEEK has higher temperature performance (crystalline regions don't soften until higher temperature) and better chemical resistance (crystals less permeable).

Almost always, yes. Invibio PEEK-OPTIMA is the standard implantable polymer for spine cages, orthopedic hardware, dental abutments. ISO 10993 compliant, FDA 510(k) cleared implant-grade materials available. Ultem is USP Class VI biocompatible but rarely used for permanent implants — PEEK's superior long-term biostability wins. For surgical instruments (non-implantable), Ultem works fine and is much cheaper.

Both machine similarly to mid-difficulty engineering plastics. PEEK has higher glass transition (143 °C) so cuts cooler without softening — slightly forgiving for feeds/speeds. Ultem (Tg 217 °C) is also quite machining-stable. Both benefit from: sharp tools, moderate cutting speeds (150–200 m/min), light air or water coolant, periodic chip evacuation. Glass-filled variants of both are harder on tools (PCD tooling preferred).

Both available via FDM with appropriate high-temperature printers (Stratasys Fortus, Roboze ARGO). PEEK printing requires specialized machines (chamber temperature 160+ °C, bed 200 °C, nozzle 400+ °C). Ultem more accessible — prints on machines with 140 °C chamber. For prototypes, FDM Ultem is common; FDM PEEK requires specialist service. SLS/MJF of PEEK and Ultem emerging but not mature. For CNC-machined parts in these materials, we offer both.