Investigating Annealing as Fix for Poor CF Adhesion in 3D Prints

After recently publishing a few videos covering research into the poor adhesion between chopped carbon fiber (CCF) and the thermoplastic filaments as used with FDM 3D printing, some of the feedback received by [I built a thing] included the idea that the missing step to make CCF additives work was post-print annealing. Naturally this claim had to be investigated, both through the resulting physical characteristics as well as on a microscopic level in the same scanning electron microscope (SEM) as before.

Theories as to why annealing the parts would help here seem to focus on increased bonding and filling of voids in the printed CCF-infused material, while there are the typical worries with annealing such as parts warping and shrinking to also take into account as potential downsides of this treatment.

For the sample materials PETG and PETG-CF, as well as PLA and PLA-CF filaments are used, with each filament type featuring an annealed and not annealed version. These were then tested for tensile strength, stiffness and failure type, as well as dimensional accuracy and warping, before being examined under the SEM. A total of 160 samples were used, with 20 samples per material and annealing state.

Perhaps the biggest surprise here was how much PETG benefits from annealing, making it much more resilient to breaking, whereas neither PLA nor PLA-CF seemed to see much benefit. Shocking was how much worse PETG-CF performs than PETG, with the former being worse than both PLA and PLA-CF here.

In terms of dimensional accuracy, annealing caused a Z direction expansion while shrinking the samples in the  other directions. The CCF addition here actually prevented much of the shrinking and expansion, showing the first clear benefit of this additive. Yet despite annealing at right above the glass transition temperature as is proper, this would seem to be the limit of this approach in terms of practical benefits.

Compared to the previous research that focused on PLA-CF, PETG-CF would seem to make the case even more strongly that there’s no real purpose to CCF additives, especially since you can already account for parts shrinkage during annealing before printing. That there’s no improvement to the CCF and thermoplastic interface adhesion is also no mystery, considering the science behind how e.g. thermoset materials create bonds with CF.