| Spec / Behavior | PEEK Filament (PEEK Polymer Baseline)[a] | ULTEM Filament (PEI Resin Baseline)[b] |
|---|---|---|
| Polymer Family | PolyEtherEtherKetone (PEEK), semi-crystalline | Polyetherimide (PEI), amorphous (ULTEM is a trade name) |
| Density | 1.30 g/cm³ (crystalline) | 1.27 g/cm³ |
| Glass Transition Temperature (Tg) | 143°C (onset) | 217°C (Tg) |
| Melting Behavior | Melting temperature 343°C | No true melt point (amorphous); softens above Tg |
| Heat Deflection (HDT) | 156°C (1.8 MPa, unannealed) / 167°C (annealed) | 192°C (1.8 MPa) / 209°C (0.45 MPa) |
| Relative Temperature Index (RTI) | Elec 260°C / Str 240°C / Imp 180°C | Elec 170°C / Mech 170°C / Mech w/o impact 170°C |
| Tensile Modulus | 4100 MPa (23°C) | 3200 MPa |
| Tensile Stress at Yield | 105 MPa (23°C) | 110 MPa |
| Tensile Strain | 25% (break) | ~50% (break) |
| Water Absorption | 0.45% (saturation, 23°C) | 0.25% (24h, 23°C) / 1.25% (saturated) |
| Drying Guidance | 120–150°C for 3–5 h; suggested max moisture 0.020% | 150°C for 4–6 h; max moisture content 0.02% |
| Typical Melt / Nozzle Range (Processing) | Barrel zones ~350–360°C (processing guidance) | Melt 350–410°C; nozzle 345–405°C (processing guidance) |
| Fire Behavior (Grade-Dependent) | Electrical safety ratings are commonly documented via UL methods (grade specific) | Commonly listed as flame-retardant by formulation; UL94 V-0 / 5VA ratings are shown for specific thicknesses and conditions |
| Typical Selection Reason | High temperature capability and long-life performance targets | High heat performance with dimensional stability and flame-retardant options |
PEEK and ULTEM/PEI are high-performance thermoplastics used when temperature, chemical resistance, dimensional stability, or documented material performance matters. They do not behave the same during printing: PEEK is semi-crystalline, while PEI is amorphous. PEKK belongs beside PEEK in the broader PAEK family and adds another crystallization and processing profile that matters in FFF. Material selection depends on the exact grade, printer thermal system, moisture control, part geometry, and service conditions.
Material Snapshot
PEEK is a semi-crystalline polymer: it has a true melting point and develops crystallinity as it cools. That crystallization contributes to high-temperature performance and also increases the need for process control. The baseline data below comes from a PEEK polymer datasheet with explicit Tg, melt temperature, RTI, and drying guidance.[a]
ULTEM (PEI) is amorphous: it does not have a true melting point, and its “high heat” identity centers on a high Tg. The baseline data here comes from a PEI resin listing that includes Tg, HDT, RTI, moisture/drying, and flame performance statements.[b]
PEEK vs PEKK vs PEI/ULTEM
PEEK and PEKK are members of the PAEK (polyaryletherketone) family, while PEI/ULTEM is a different high-temperature polymer family.[g] That family relationship does not make PEEK and PEKK interchangeable. PEKK grades can be tuned across different melting points and crystallization rates, which changes FFF processability, warping, dimensional control, and whether the printed part is left amorphous or driven toward a semi-crystalline state.[d]
| Material | Polymer Position | Thermal / Crystallization Behavior | FFF Implication |
|---|---|---|---|
| PEEK | PAEK family; polyetheretherketone | Semi-crystalline with a 343°C melt temperature in the PEEK baseline used on this page.[a] | Fast crystallization and shrinkage make chamber control, cooling history, geometry, and interlayer bonding especially sensitive. |
| PEKK | PAEK family; polyetherketoneketone | Crystallization rate and melt temperature can be tuned by grade. Arkema lists Kepstan PEKK series with melt temperatures from about 305°C to 358°C and different crystallization speeds.[d] | Some PEKK grades have a wider FFF processing window than traditional PEEK and can be printed in amorphous or semi-crystalline states, but the exact grade still determines the thermal process. |
| PEI / ULTEM | Polyetherimide; not a PAEK | Amorphous, with no crystalline melt point; the listed ULTEM 1000 baseline has a 217°C Tg.[b] | No crystallization step to manage, but high melt temperature, drying, chamber stability, bed adhesion, and geometry remain demanding. |
PEKK is not simply “easier PEEK.” Arkema’s PEKK range shows that melting point and crystallization speed vary by grade, and semi-crystalline PEKK can still require high build temperatures or post-processing to achieve the intended structure.[d]
PEEK vs ULTEM (PEI) Selection Notes
PEEK and ULTEM/PEI solve different thermal and mechanical problems. PEEK is selected when semi-crystalline PAEK behavior, chemical resistance, wear performance, and higher long-term thermal ratings match the application. ULTEM/PEI is selected when an amorphous high-Tg material, dimensional control, electrical insulation, and grade-specific flame documentation fit the requirement.
PEEK Fits When
- You need a polymer with a real melt point and semi-crystalline strength retention at elevated temperatures.
- Long-term thermal ratings matter (RTI values are explicitly documented in polymer datasheets).
- Low moisture saturation is helpful for dimensional control in humid environments.
- Your workflow can support controlled cooling, annealing, and tight thermal management.
Typical selection: you’re replacing metal in a hot, chemically aggressive, or electrically demanding environment and you’re okay investing in process discipline.
ULTEM (PEI) Fits When
- You want a high-Tg polymer that stays dimensionally stable and predictable (amorphous behavior).
- Built-in flame-retardant behavior and documented UL94 ratings are a requirement for your material spec.
- HDT around the ~200°C range covers your real operating temperature needs.
- Electrical insulation performance is part of the job (dielectric properties are often well-characterized in datasheets).
Typical selection: you need high heat performance plus strong compliance documentation, and you want fewer crystallization-driven surprises.
Polymer Behavior
Understanding these materials starts with one question: does the polymer crystallize?
PEEK (semi-crystalline): It has a documented Tg (143°C onset) and a true melting temperature (343°C). Cooling history influences crystallinity, shrinkage, and internal stress, so temperature control directly affects part quality.[a]
PEKK (grade-dependent PAEK behavior): PEKK can be formulated with different crystallization rates and melt temperatures. Arkema lists 3D-printing-oriented Kepstan 6000-series grades around a 305°C melt temperature with slow crystallization, while other series melt higher and crystallize faster.[d] That tunability is why some PEKK grades can be easier to process by FFF than traditional PEEK while still requiring controlled thermal conditions.
ULTEM/PEI (amorphous): It’s characterized by a high Tg (217°C) and softening behavior rather than a crystalline melt point. This tends to translate into stable, repeatable thermal movement across heating/cooling cycles, especially for big parts with long print times.[b]
Why This Matters in Filament Form
- Warp and stress are often about thermal gradients, but PEEK adds crystallization into the mix—cooling rate and part geometry can shift outcomes.
- ULTEM’s amorphous structure avoids crystallization-driven shrinkage, which can simplify dimensional control in large high-temperature prints.
- PEEK, PEKK, and PEI grades used for high-temperature FFF require material-specific drying and storage. Moisture targets and drying cycles should come from the exact resin or filament documentation rather than a family-level assumption.
Mechanical Performance
Mechanical numbers on datasheets are usually measured on standardized specimens made by traditional processing. Filament-printed parts can land above or below those values depending on layer bonding, raster direction, porosity, annealing, and thermal control. Still, the datasheet numbers are a useful baseline for how the polymer “wants” to behave.
Baseline Strength and Stiffness (Unfilled Grades)
- PEEK baseline shows a tensile modulus of 4100 MPa and tensile yield stress of 105 MPa at 23°C (with high ductility shown by 25% strain at break).[a]
- ULTEM/PEI baseline lists tensile modulus around 3200 MPa and tensile yield stress around 110 MPa (and very high elongation at break depending on test method).[b]
In these unfilled resin baselines, PEEK is somewhat stiffer while PEI shows higher elongation at break. Printed results still depend on layer bonding, orientation, porosity, and thermal control.
For printed-part comparisons, test at least two orientations: one loading mainly along deposited roads and one loading across layers. This separates bulk polymer behavior from interlayer bonding limits.
Thermal Limits
Thermal performance is not one number. Tg tells you when the polymer matrix starts getting rubbery. HDT tells you when a loaded part starts to deflect. RTI is about long-term thermal aging under defined conditions. With PEEK and ULTEM, you get all three kinds of signals right in the published data.
PEEK Thermal Notes
- Tg: 143°C (onset) — start of major stiffness drop.
- Melting: 343°C — true melt point for flow processing.
- HDT: 156°C (unannealed) and 167°C (annealed) at 1.8 MPa.
- RTI: Elec 260°C / Str 240°C / Imp 180°C (UL method documented).
Application note: PEEK fits applications that need high sustained-temperature capability when the manufacturing process can control crystallization, shrinkage, and interlayer bonding.[a]
ULTEM (PEI) Thermal Notes
- Tg: 217°C — a very high glass transition for an amorphous polymer.
- HDT: 192°C at 1.8 MPa (and 209°C at 0.45 MPa).
- RTI: 170°C ratings listed across electrical and mechanical categories.
- Softening: since it’s amorphous, behavior centers on Tg/HDT rather than a melt point.
Application note: PEI retains useful thermal performance deep into the 100–200°C range and is often selected where flame-behavior documentation and dimensional stability are part of the material specification.[b]
Chemical and Wear
PEEK and ULTEM/PEI are both used in demanding chemical and wear environments, but their moisture behavior and polymer structure differ.
PEEK: Chemical Resistance and Low Moisture Saturation
PEEK polymer datasheets often highlight chemical resistance in aggressive environments and suitability for steam sterilization. Its published water absorption at saturation (23°C) is 0.45%, which is comparatively restrained for a high-performance polymer family member.[a]
- Moisture angle: lower saturation can help keep dimensions predictable in humid service, especially for tight-fit parts.
- Wear angle: PEEK is frequently chosen for low-friction, wear-exposed components when paired with the right design and surface finish (grade dependent).
ULTEM (PEI): Strong Chemical Resistance for an Amorphous Polymer
The PEI listing notes that the material may offer very good chemical resistance for an amorphous polymer, and it documents water absorption at 0.25% (24h, 23°C) and 1.25% at saturation. That’s a bigger spread than PEEK’s saturation number, which is why drying, storage, and service humidity deserve real attention.[b]
- Moisture angle: PEI can absorb more over time; for precision parts, keep drying and storage consistent.
- Stability angle: amorphous structure often supports predictable shrink and low warpage when process conditions are right.
Printing Requirements
With these filaments, the printer is part of the material system. Hotend capability, chamber temperature, bed temperature and surface, airflow, drying, and cooling history all affect warping, interlayer bonding, crystallization, and dimensional stability.
Drying and Moisture Targets
- PEEK baseline guidance: drying temperature 120–150°C for 3–5 hours; suggested max moisture 0.020%.[a]
- ULTEM/PEI baseline guidance: drying temperature 150°C for 4–6 hours; max moisture 0.02%.[b]
Why this matters: moisture can cause surface bubbling, poor bead consistency, and weaker interlayer bonding, especially at high processing temperatures.
Thermal Hardware Expectations
- High-temperature melt path with verified sensor accuracy and thermal stability.
- Enclosed build environment with stable internal temperature to reduce thermal gradients and layer separation.
- Bed surface and adhesion strategy appropriate for high-heat polymers (release behavior must be controlled too).
- Cooling management that avoids sharp thermal gradients across the part (especially at corners and thin ribs).
A 400°C Hotend Is Not the Same as a PEEK/PEKK-Capable Process
Current desktop and prosumer hardware makes this distinction easier to miss. Prusa’s HT Hotend for the CORE One family raises the nozzle ceiling to 400°C and its hardware table includes materials such as PPS-CF, PPA-GF, PEKK-CF, PPSU, and PEEK when the specified HT hotend and idler are used.[c] That confirms hotend compatibility; it does not make nozzle temperature the only requirement for every PEEK or PEKK grade.
Check the complete thermal process. Prusa’s separate PEKK-CF guidance for the HT90 specifies a 380–440°C nozzle, 135–150°C bed, drying, and an actively heated chamber of at least 85°C.[e] Its PEEK-CF guidance lists 380–440°C nozzle, 140–155°C bed, drying, and the same minimum 85°C actively heated chamber requirement.[f] A printer that can reach 400°C at the nozzle may still fall outside the required bed, chamber, or upper nozzle range for a specific material.
- Nozzle range: verify the full manufacturer range, not only whether the hotend can touch its lower end.
- Bed temperature and surface: high-temperature polymers can require hotter beds and specialized adhesion systems.
- Chamber temperature: active chamber heat can be required to reduce warping and maintain interlayer bonding.
- Crystallization: PEEK and semi-crystalline PEKK need cooling history and thermal gradients controlled around the intended material state.
- Geometry: thick sections, sharp corners, thin ribs, and large flat parts respond differently to thermal contraction.
- Validated profile: use the exact filament maker’s nozzle, bed, chamber, drying, cooling, and post-processing instructions for that grade.
PEEK, PEKK, and PEI/ULTEM are commonly processed in high-temperature melt ranges, but each grade has its own usable window. Sensor accuracy, heater headroom, chamber control, filament dryness, and the intended crystalline state should be treated as part of one process rather than independent settings.[b][d]
Quality and Compliance
For industrial filaments, printability is only one requirement. Dimensional drift, moisture control, traceability, and the ratings required by the application also need verification.
What to Verify on Every Spool
- Moisture handling: confirm the drying plan and storage method match the resin guidance (both list max moisture targets).[a]
- Diameter tolerance and ovality: high-temp polymers amplify extrusion inconsistencies into flow instability at the nozzle.
- Batch traceability: essential if your parts are validated for regulated or safety-driven environments.
- Thermal ratings that match the job: RTI, HDT, and Tg should be aligned to service temperature, load, and lifetime expectations.
Fire and Heat Documentation (Keep It Specific)
The PEI listing explicitly calls out UL94 V-0 and 5VA ratings and notes aerospace interior compliance claims for the grade—yet those statements are still thickness- and condition-dependent in real qualification work. Treat them as a starting point, then validate against the exact thickness, color variant, and process used for your printed part.[b]
Document the specific grade, print parameters, post-processing such as annealing, and the test method. Material-family reputation does not replace validation of the printed part.
FAQ
Is ULTEM the same thing as PEI filament?
ULTEM is a trade name commonly used for polyetherimide (PEI) resins and related grades. Many “ULTEM filament” products are PEI-based, but properties can vary by specific grade, additives, and how the filament is produced.
Why does PEEK have a melting temperature but ULTEM doesn’t?
PEEK is semi-crystalline, so it has a defined melting temperature where crystalline regions melt. ULTEM/PEI is amorphous, so it transitions through softening behavior centered on Tg instead of having a true crystalline melt point.
Do I need to dry both PEEK and ULTEM filaments?
Yes. Published guidance includes explicit drying temperatures, times, and maximum moisture targets for both polymer families. Consistent drying and sealed storage help protect surface quality and interlayer bonding, especially at high processing temperatures.
Which one is better for parts that see continuous heat?
It depends on the exact service temperature, load, and lifetime target. Datasheets provide different types of thermal indicators—Tg, HDT, and RTI. PEEK’s RTI values are listed higher in the baseline polymer data, while ULTEM/PEI is known for a very high Tg and strong HDT near the ~200°C range for the listed grade.
Can I compare datasheet tensile numbers directly to 3D printed parts?
Use datasheet numbers as a baseline, not a guarantee. Printed properties depend heavily on bead placement, layer bonding, print temperature stability, and orientation. For a fair comparison, test printed coupons in at least two orientations to capture anisotropy.
Where does PEKK fit compared with PEEK and ULTEM/PEI?
PEEK and PEKK are both members of the PAEK family, while PEI/ULTEM is a different amorphous polymer family. PEKK can be tuned across different melting points and crystallization rates, so its FFF behavior depends strongly on the exact grade. Some PEKK grades offer a wider processing window than traditional PEEK, but they still need material-specific thermal control.
Does a 400°C hotend mean a printer can print PEEK or PEKK?
No. A 400°C nozzle ceiling only describes one part of the printer. The exact filament may also require a hotter bed, an actively heated chamber, a specific build surface, controlled cooling or crystallization, strict drying, and a validated material profile. Some PEEK-CF and PEKK-CF guidance extends to 440°C nozzle temperatures and requires an 85°C-or-hotter active chamber.[e][f]
Sources
- Victrex — VICTREX™ PEEK Polymer 150G Datasheet — material properties, Tg, melting temperature, RTI, and drying guidance.
- SABIC — ULTEM™ Resin 1000 (PEI) — Tg, HDT, RTI, moisture/drying guidance, processing ranges, and grade-specific flame characteristics.
- Prusa Knowledge Base — Using the HT Hotend — 400°C nozzle capability, compatible CORE One family printers, required nozzle/idler hardware, and the current high-temperature material table.
- Arkema — Kepstan® PEKK Polymer Range — PEKK grade families, melt temperatures, Tg values, crystallization speeds, and 3D-printing positioning within the PAEK family.
- Prusa Knowledge Base — PEKK-CF — 380–440°C nozzle, 135–150°C bed, drying guidance, and minimum 85°C actively heated chamber requirement for the documented HT90 setup.
- Prusa Knowledge Base — PEEK-CF — 380–440°C nozzle, 140–155°C bed, drying guidance, and minimum 85°C actively heated chamber requirement for the documented HT90 setup.
- Victrex — PEEK & PAEK Polymer Filaments — PEEK and PAEK family positioning and additive-manufacturing notes on crystallinity, warping, layer adhesion, and dimensional stability.