3D printing ultra-high performance polymers: why PEKK is the ultimate choice
More accurate, more efficient and even faster, 3D printing is changing the game in the industry, including in the most demanding areas such as aeronautics or automotive. Material innovation is at the heart of this "revolution", because they directly affect the performance of printed parts. PAEKs, ultra-high performance polymers can both meet the needs of the most demanding markets while being printable. Focus on PEKK and PEEK - extreme polymers and their use for 3D printing.
The strengths of PEEK
PEEK (Poly-ether-ether-ketone) is an ultra-high performance polymer near the very top of the performance pyramid. PEEK and its cousins (PAEKs, poly-aryl-ether-ketones, including PEKK), are known for their ultra high performance in aerospace, automotive, and other industries where strength and high-temperature resistance are required. PEEK is well-suited to stock-shape extrusion and injection molding.
PEEK remains challenging to use
PEEK is probably the most widely known PAEK. It is a semi crystalline material with very fast crystallization kinetics that make it well suited for many conventional processing methods. Like most semi-crystalline materials, it undergoes a significant dimensional change (shrinkage) when it crystallizes. In traditional processing techniques, this dimensional change is easily accounted for and is widely understood, but for additive manufacturing processes, this shrinkage can result in warping and failed prints.
These rapidly formed crystalline domains are almost completely resistant to further chain diffusion and entanglements, which dramatically reduces layer to layer adhesion. To overcome these challenges, many 3D printer manufacturers have taken extreme measures to achieve reasonable part quality and print reliability. These process requirements limit the overall size and real world mechanical properties of parts printed form PEEK.
What is PEKK?
Arkema has developed a range of PEKK (poly-ether-ketone-ketone) grades, Kepstan® PEKK, offering the excellent properties of PAEKs, with a much wider and more flexible processing window.
PEKK and PEEK are very similar in structure, but with two key differences:
- PEKK has an additional ketone group replacing one ether group in the backbone. This ketone group is a stiffer bond than the ether linkage, increasing the material’s glass transition temperature. (The temperature where the polymer first begins to soften.).
- One of these ketone bonds can either be in the para (straight) or meta (kinked) position.
By controlling the ratio of straight to kinked units in the polymer backbone, it is possible to precisely control the melt temperature, crystallinity, and crystallization rate. This control enables PEKK to be processed in ways that are difficult or impossible with many similar polymers such as PEEK. This means it can be much more easily printed using FFF."
This flexibility to tune PEKK according to a user’s needs means that this one material can serve many different purposes, and it also provides manufacturers with increased printing reliability. Fewer failed prints + less need for fine tuning of settings gives users a significant reduction in both manufacturing time and cost.
Along with this flexibility, PEKK provides users with even more advantages over PEEK:
- More strength at high temperatures due to a higher glass transition temperature (Tg)
- Higher compression strength
- Improved barrier performance
- Better wear and friction properties
Kepstan® PEKK polymers can be further tailored to suit specific needs with molecular weight control or the addition of carefully selected additives such as glass fibers, carbon fibers, or carbon nanotubes. Lightweight, strong, and resistant to all but the most aggressive chemicals, it can be used to replace metal components in highly demanding environments to reduce weight. PEKK’s unique properties make it the ideal high performance polymer choice for either FFF or LS processes.
These advantages mean that PEKK can be used to 3D print parts for use in highly-demanding industrial applications, whereas for PEEK the manufacturing of these has been restricted to more conventional methods. PEKK’s strength and resistance make it useful in high heat and pressure environments such as engines, and its low outgassing means it can be used in satellites and submarines. It has already been used to 3D print several components of the commercial spacecraft for crew transportation.
Looking to the future
PEKK allows greater manufacturing flexibility than other PAEK polymers. This means that the potential of 3D printed PEKK for numerous highly-demanding industrial applications is huge. As the field of 3D printing moves rapidly from simple prototyping toward industrial uses, PEKK will open up countless new manufacturing possibilities.
To accelerate application-specific use of PEKK, Arkema has developed partnerships with 3D printer developer miniFactory and filament producer Kimya. Together, they have developed a reliable supply chain and controlled process using PEKK filament to create parts for the most demanding applications. PEKK is among the strongest and stiffest polymers possible to process by FFF, including in the “Z” direction.
In summary, when choosing the material for creating components intended for highly demanding environments, the choice among high performance polymers is clear: first, PEKK offers among the best physical and mechanical properties of any material that can be processed in additive manufacturing. Second, and more importantly, PEKK allows users to choose the manufacturing method that best suits their infrastructure and end purposes.
