Over the past few years, there has been lots of buzz over 3D printers that can process PEEK (polyetheretherketone), one of the most widely known ultra-high performance polymers.
The desire to print PEEK is understandable, as it is often the go-to high performance material for stock shape machining or injection molding.
However, many of the properties that make it so appealing for these processing techniques make it incredibly difficult to process in FFF (fused filament fabrication).
Amorphous and semi-crystalline thermoplastics: what’s the difference?
Plastic materials can be divided into two categories:
- Amorphous : amorphous polymers are simply tangled messes of long molecules. The analogy often used is a bowl of cooked spaghetti.
- Semi-crystalline : semi-crystalline polymers contain both disordered, amorphous regions as well as crystalline domains where the chains arrange into ordered patterns.
Almost all polymers used in FFF fall into the category of amorphous materials as they are far easier to process than semicrystalline materials.
Indeed, amorphous polymers undergo less dimension change when cooling, and the disordered structure allows some diffusion and entanglement between polymer chains in adjacent layers. This results in better dimensional accuracy and layer adhesion for FFF objects.
PEEK, a highly crystalline material
PEEK is a highly crystalline material with very fast crystallization kinetics. Like most semi-crystalline materials, it undergoes a significant dimensional change (shrinkage) when it crystalizes.
Its crystalline segments are almost completely resistant to further chain diffusion and entanglements. In conventional processing techniques, this dimension change is easily accounted for and is widely understood.
To overcome this, many 3D printer manufacturers have taken extreme measures to increase bed adhesion. They are also often extruding at very high temperatures to partially melt the previous layer in an attempt to make a part with acceptably low warping and decent layer adhesion.
The PAEK family of ultra-high performance materials
PEKK (Poly-Ether-Ketone-Ketone) and PEEK (Poly-Ether-Ether-Ketone) are both part of the PAEK (Poly-Aryl-Ether-Ketone) family of ultra-high performance thermoplastic polymers. This family of polymers is known for their:
- excellent strength,
- chemical resistance,
- high use temperatures,
- and low flammability.
Hence, these materials are commonly used in the most demanding applications.
PEKK vs. PEEK
PEKK and PEEK have very similar chemical structures, except for two key differences.
1. PEKK replaces one of the flexible Ether linkages with a more rigid Ketone group. This increases the glass transition temperature (Tg)– where the material first begins to soften– by about 15°C over PEEK.
2. The second Ketone group is selectively ortho (straight) or para (kinked) substituted. By varying the number of straight and kinked sections, it is possible to control the melting point and crystallization rate.
For example, a PEKK polymer with 60% straight and 40% kinked segments will melt at about 305°C and be so slow to crystallize that it is often called “pseudo-amorphous”.
The same polymer with 80% straight and 20% kinked segments will have a melting point of about 360°C, and a crystallization rate similar to PEEK.
The tunable crystallization rate of PEKK allows an FFF user to take advantage of both the extreme performance of PAEKs and the easier processing of amorphous materials. Put simply, PEKK is easier to 3D print than PEEK (i.e. better layer adhesion), all while offering similar strength and resistance properties (i.e. better dimensional accuracy).
Most users of filament made with Kepstan ® PEKK have reported successful prints on their first try, with many commenting that it is as easy to print as ABS.
Filaments made with Arkema’s Kepstan ® PEKK are available from several independent filament converters. The material is also available in carbon fiber reinforced and electrostatic dissipative (ESD) grades.
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