| 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) |
| Inherent Fire Behavior (Grade-Dependent) | Electrical safety ratings are commonly documented via UL methods (grade specific) | Commonly listed as inherently flame retardant; UL94 V-0 / 5VA ratings are shown for specific thicknesses and conditions |
| Practical “Why People Pick It” | Extreme temperature capability and long-life performance targets | High heat performance with strong dimensional stability and flame-retardant options |
PEEK and ULTEM sit in the “serious industrial” end of the filament world. They’re both high-performance thermoplastics, but they behave very differently during printing and in finished parts. The trick is to match the polymer’s thermal personality (semi-crystalline vs amorphous), moisture habits, and compliance needs to the real job your part must do.
Table of Contents
🧪 Material Snapshot
PEEK is a semi-crystalline polymer: it has a true melting point and develops crystallinity as it cools. That crystallization is a big reason it can deliver very high temperature capability, but it also raises the bar 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]
🧭 Choosing Between Them
If you’re choosing strictly by “maximum heat,” it’s tempting to jump straight to PEEK. If you’re choosing by a balanced mix of heat, stability, and compliance options, ULTEM/PEI often sits in a sweet spot. Here’s a clean way to think about it, without the marketing fog.
PEEK Is Usually the Pick 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.
Selection signal: you’re replacing metal in a hot, chemically aggressive, or electrically demanding environment and you’re okay investing in process discipline.
ULTEM (PEI) Is Usually the Pick When
- You want a high-Tg polymer that stays dimensionally stable and predictable (amorphous behavior).
- Inherent flame retardancy 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).
Selection signal: you need high heat performance plus strong compliance documentation, and you want fewer crystallization-driven surprises.
🧬 Polymer Behavior
Understanding these two 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, shrink, and internal stress—so temperature control is not a “nice to have,” it’s the whole game.[a]
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 nature often makes it feel more “predictable” for large enclosures and fixtures, even at high temperatures.
- Both are hygroscopic enough that moisture control matters; drying guidance and max moisture targets are actually documented, not guesswork.[a]
🧱 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]
Practical translation: PEEK often leans a bit stiffer, PEI often leans a bit more forgiving—then printing parameters decide how much of that potential survives layer-by-layer.
🔎 A smart way to compare printed parts is to test in two orientations: one that loads along the bead direction and one that loads across layers. That’s where you see whether your process is building real interlayer strength or just stacking hot spaghetti.
🌡️ 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).
Real-world meaning: PEEK is the one you reach for when sustained heat is non-negotiable and you can support the manufacturing controls that let it shine.[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.
Real-world meaning: PEI stays confident deep into the 100–200°C zone and is often selected where documentation for flame behavior and dimensional stability is part of the spec language.[b]
🧴 Chemical and Wear
Both materials are used in harsh environments, but they arrive there by different routes.
PEEK: “Chemical Confidence” Plus 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, “print settings” are not a casual thing. The printer is part of the material system: hotend capability, chamber stability, bed planarity, and airflow control are what separate a clean industrial part from a stressed, delaminated one.
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 show up as surface bubbling, poor bead consistency, and weaker interlayer bonding—especially at the high melt temperatures these materials live in.
Thermal Hardware Expectations
- High-temperature melt path with verified thermal stability (thermistor/thermocouple accuracy matters more than people think).
- Enclosed build environment with stable internal temperature—drafts are the enemy of layer adhesion.
- 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).
One more detail that gets overlooked: both materials are commonly processed with melt temperatures deep in the high-300°C range in published processing guidance. That’s a reminder that you’re operating in a zone where sensor calibration, heater headroom, and filament dryness become non-negotiable engineering variables—not “tuning.”[b]
📏 Quality and Compliance
For industrial filaments, it’s rarely just “does it print?” The real question is whether you can prove consistency: dimensional drift, moisture control, traceability, and the ratings your industry requires.
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]
Clean approach: document the specific grade, the print parameters, post-processing (like annealing), and the test method. That’s how you turn material reputation into a defendable spec.
❓ 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.
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