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In a paper titled “Flexural Properties and Fracture Behavior of CF/PEEK in Orthogonal Building Orientation by FDM: Microstructure and Mechanism,” (link) Qiushi Li’s team reveals their findings on the effects of adding short carbon fibers (SCFs) into the filament when 3D printing with PEEK.

PEEK filament has experienced a sharp spike in popularity in recent years as industrial FDM 3D printers that can operate at higher temperatures have become more available. The polymer has high tensile strength and temperature resistance, making it desirable in aerospace, medical, and other industrial applications. Still, parts 3D printed with PEEK suffer from the same problem as all other parts 3D printed on an FDM (fused deposition modeling) machine: weakness in the vertical layers due to poor layer bonding.

The Chinese researchers wanted to investigate whether adding SCFs into the PEEK would improve its strength. A Funmat HT FDM 3D printer was used to print the four test pieces; two were printed vertically with SCFs and two horizontally without SCFs. Pellets of the plastic were also used to make identical injection molded parts to compare the printed parts against.

Their results are intriguing. The strength differences between the vertically 3D printed parts and molded parts were negligible, whether or not SCFs were added, though the vertical CF/PEEK print was slightly stronger than the standard vertical PEEK print. The horizontal CF/PEEK print, however, was actually weaker than its standard PEEK counterpart.

“With the addition of CFs, the horizontally printed CF/PEEK composites exhibited a 7.1% lower value of than that of PEEK, which agrees with a previous report on the addition of carbon nanotubes (CNTs). Surprisingly, the vertically printed PEEK and CF/PEEK composites displayed a value of 146 MPa, which was similar to the value of the molded samples. Such a similar value between printed samples and molded samples is rare, but quite highly desired, for 3D printing,” stated the authors.

The benefits of adding CFs to molded parts was also noted. “The incorporation of CF resulted in an 8.30% higher modulus for the CF/PEEK molded samples relative to those of the PEEK samples. A statistically significant increase in the modulus was also observed for the vertically printed samples.”

Adding SCFs “raises the uniform nucleation process of PEEK during 3D printing, decreases the layer-to-layer bonding strength, and greatly changes the fracture mode,” and the results of the study indicate that print orientation is sometimes more important than the composite used.

“The design of a printing route along the stress orientation that cooperates with the incorporation of a reinforced phase into the matrix provides an effective method to enhance the mechanical properties of composites and enlarges the application of 3D printing in lightweight design fields,” stated the researchers. “This study will be helpful to designers to investigate the influence of microstructures on printed composites during the printing process.”

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