High-Performance Polymers: The Best Alternatives to PEEK
With over 40 years of history, PEEK (polyether ether ketone) is one of the most established players in the high-performance material category (see thermoplastic pyramid). It is common to see PEEK plastic parts manufactured through injection molding and even CNC machining.
The 3D printing of PEEK is comparatively much more recent, with a small but growing number of PEEK-compatible 3D printers on the market.
PEEK is often sought after (and marketed) for any and all 3D printing applications. But, while it is an excellent material, PEEK presents significant entry barriers like pricing and printability. Additive manufacturing (AM) users are therefore increasingly looking for substitutes.
This article will take a look at the main reasons behind this and introduce some of the best alternatives to PEEK.
Why Look for Alternatives to PEEK?
PEEK sits in the high-performance polymer group at the top of the thermoplastic pyramid. It indeed presents outstanding mechanical and thermal properties, unrivaled by common engineering materials. PEEK also offers high chemical and radiation resistance, and inherent flame retardancy characteristics (UL-94 V-0 FST).
Average PEEK properties:
|Filament price /kg||$600-$700|
|Tg (glass transition temp.)||~143°C|
|Tm (melting temp.)||~340°C|
|Required nozzle temp.||360-410°C|
|Required bed temp.||120-170°C|
|Tensile strength||~100 MPa|
|Tensile modulus||~3700 MPa|
This premium thermoplastic is mainly chartered for high-value applications seen in demanding industries such as aerospace, energy, and medical. In numerous cases, PEEK is even considered an alternative to metal for the production of strong, lighter-weight parts.
However, PEEK– especially in 3D printable form, i.e., filament or powder– is substantially pricier than most plastics and much harder to process.
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PEEK is Expensive
High-performance materials are, for obvious reasons, already more expensive than standard materials. However, PEEK is particularly costly, and even more so for 3D printing.
The issue mainly lies in the filament format. To oversimplify, since PEEK must be heated up to high temperatures to melt, it’s expensive to transform PEEK pellets into printer-friendly PEEK filament.
Note: Some 3D printers feature pellet extruders instead of filament extruders. Only a rare few pellet 3D printers are able to heat up to PEEK-compatible temperatures.
PEEK is the Hardest to Print
On the one hand, PEEK’s semi-crystalline behavior is its greatest strength, but on the other, its greatest weakness. This material’s crystallization rate is extremely high, meaning it crystallizes too quickly after even the slightest temperature drop.
Three major issues stem from this:
- Significant shrinkage: As polymer chains reorganize into compact crystalline structures, the object inevitably loses dimensional accuracy and tends to warp. Although this issue is common to high-performance plastics (and even some standard or engineering plastics), PEEK warping is particularly difficult to manage. It is challenging to the point that parts with wide, flat areas are considered almost impossible to print.
- Extremely high processing temperatures: Unlike amorphous materials, which can be processed just above Tg, semi-crystalline materials must reach a much higher threshold, Tm. Undoubtedly, working with PEEK’s extreme temperatures requires very robust equipment and poses issues regarding heat regulation.
- Bad layer adhesion: As the nozzle deposits a layer of PEEK, the material must flow freely to enable optimal layer fusion. That’s not possible if the cooling happens too fast, and this problem becomes even more profound as crystalline structures impede proper diffusion. In the end, PEEK’s superior mechanical qualities become irrelevant if layers can’t properly adhere to each other.
Fortunately, today’s market offers a solid set of cost-effective and more printable alternatives to PEEK. The following materials are some of the best PEEK alternatives.
More on material crystallinity:
Thermoplastics for AM: Semi-Crystalline vs Amorphous
In terms of performance, PEKK (Polyether Ketone Ketone) is the closest to PEEK in this list since it belongs to the same PAEK family. Molecularly, these two materials differ only slightly in the way they order their monomer sequence.
Average PEKK properties:
|Filament price /kg||$700-$800|
|Crystallinity||Semi-crystalline and amorphous|
|Required nozzle temp.||350-380°C|
|Required bed temp.||120-140°C|
|Tensile strength||110 MPa|
|Tensile modulus||2750-3200 MPa|
However, the difference is vast in practical terms. PEKK can offer slightly better properties overall, but what really marks the difference is its tunable crystallization rates.
This means that material providers can modify their PEKK’s crystallinity and thus enhance its printability. As such, different kinds of PEKK are available (e.g., PEKK-A, PEKK-C), from entirely amorphous to highly crystalline.
You can print an amorphous PEKK at lower temperatures, at ease, without all of the issues of PEEK. After printing, PEKK parts can be annealed to harness the advantages of semi-crystalline properties.
There’s not much to say about PEKK’s downsides, aside from the fact that it is even more expensive than PEEK. It’s also relatively new, with a shorter track record and fewer certifications than other high-end plastics. Though a 1-kg spool of PEKK can set you back an additional $200 compared to PEEK, it provides more reliability and repeatability, ultimately reducing costs.
PEI (Polyetherimide), better known by its SABIC-trademarked name ULTEM, is another popular high-performance plastic. Although it lacks the semi-crystalline benefits of PAEKs and provides less performance overall, PEI has been one of the favorites in high-end industries for decades due to its extraordinary flame retardancy properties.
PEI, available for a third of PEEK’s price, also offers excellent layer adhesion and is much easier to print (with the right equipment). However, PEI has an extremely high Tg, so it’s not as easy to print as other materials such as PEKK.
For AM, PEI is available in two versions, ULTEM 1010 and ULTEM 9085. The difference between them is quite subtle. While ULTEM 1010 is stiffer, ULTEM 9085 has better impact resistance properties.
Despite ULTEM 1010’s proven record in the medical and food industries, ULTEM 9085 is more popular due to its longstanding use for aerospace interior components through additive manufacturing means. ULTEM 9085 is well-known for its FAR 25.853 certification, emitted by the FAA for aircraft interiors.
Average PEI properties:
|Filament price /kg||$220-$250|
|Required nozzle temp.||350-390°C|
|Required bed temp.||120-160°C|
|Tensile strength||~55 MPa|
|Tensile modulus||2000-2500 MPa|
Polysulfones (PPSU, PSU, PES)
Polysulfones or PAEs are often seen as an upgraded replacement for PC (polycarbonate) parts. They boast an amorphous structure, translucent appearance, and toughness, as well as excellent chemical, UV, and gamma resistance.
However, the aspect in which they excel the most is hydrolysis resistance, making them ideal for food and medical applications where autoclave sterilization is necessary.
This family of materials is often compared to PEI given their similar performances, wide adoption, and certifications (compliant with FDA, USDA, USP Class VI). Polysulfones tend to be more affordable than PEI but are harder to print, partly because they have the highest Tgs on this list.
Average Polysulfone material family properties:
|Filament price /kg||$150-$220|
|Required nozzle temp.||360-390°C|
|Required bed temp.||140-160°C|
|Tensile strength||~55 MPa|
|Tensile modulus||2100-2300 MPa|
PPS (Polyphenylene sulfide; not to be confused with PPSU or polyphenyl sulfone) is a semi-crystalline thermoplastic. This material is at the boundary between engineering and high-performance plastics.
While it has the lowest Tg and isn’t the toughest material on this list, PPS has the best chemical resistance among all existing thermoplastics. Thanks to its unique crystalline structure, this material is unsolvable in any known solvent under 200°C and is among the hardest polymers. For this reason, PPS is highly demanded in chemical, oil & gas, and fuel tank applications.
Similar to PEEK, PPS has the benefits of semi-crystalline thermoplastics. One could say that PPS is the “Lite” version of PEEK. Although its layer adhesion is not as good as other options on this list, PPS is relatively easy to print (partly thanks to its low Tg and crystallization rates).
Average PPS properties:
|Filament price /kg||~$200|
|Required nozzle temp.||315-345°C|
|Required bed temp.||120-160°C|
|Tensile strength||~50 MPa|
|Tensile modulus||2650 MPa|
Carbon Fiber PEEK (CF-PEEK, Carbon-fiber-filled PEEK)
This blend combines the best of PEEK and carbon fiber into one material. As it usually happens with any other CF (carbon fiber) blend, CF PEEK can be around four times stiffer (though less ductile) than standard PEEK, depending on the specific CF percentage (usually between 10 to 30%).
Additionally, carbon fiber increases thermal conductivity, enhancing the matrix material’s stability under higher temperatures.
One of the main benefits of CF-PEEK blends is their printability. CF strands hold the material in place, enabling optimal dimensional accuracy, strong layer adhesion, and sleek surface finishes. Given its benefits, many would expect it to be more expensive than PEEK, but pricing for the two is similar.
The main disadvantages of this material are, firstly, its lack of certification needed for strict compliance regulations. Secondly, its high abrasiveness can be a problem in high-wear applications and means the 3D printer must be equipped with hardened components (nozzle, extruder gears, etc.).
We did not include a table for CF blend properties as their prices and printing temperatures can vary greatly from one matrix material to another. We can however say that by incorporating CF strands into high-performance materials, their tensile strength and modulus can be significantly enhanced:
- Tensile Strength: 100 – 200 MPa
- Tensile Modulus: 8000 – 17500 MPa
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Carbon fiber 3D printers: 2022 guide to continuous fiber 3D printing
There are many alternatives to PEEK, and each material has its own strengths and weaknesses. Choosing the best PEEK alternative for you will depend on your use case, budget, and 3D printer if you already own one.