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One critical challenge for commercial products manufactured via material extrusion 3D printing is their inferior mechanical properties in comparison to injection molding; in particular, 3D printing leads to weaker properties perpendicular to the plane of the printed roads (z-direction). Here, rapid (≤20 s) post-processing of 3D printed carbon- poly(ether ether ketone) (PEEK) with microwaves is demonstrated to dramatically increase the modulus, such that the z-direction after microwave processing (2.7–3.8 GPa) exhibits a higher elastic modulus than the maximum in any direction for the as-printed part (2.3 GPa). Additionally, the stress at break in the z-orientation is increased by an order of magnitude by microwaves to slign with the stress for other print orientations in the as-printed state. The rapid heating and cooling by coupling of the microwave energy with the carbon filler in the PEEK does not increase the crystallinity of the PEEK, so the increased mechanical properties are attributed to improved interfaces between printed roads. This simple microwave post-processing enables large increases in the elastic modulus of the printed parts and can be tuned by the microwave power. As PEEK is generally difficult to print, these concepts can likely be applied to other commercial engineering plastic filaments that contain carbon or other fillers that are microwave active to rapidly post process 3D printed thermoplastics without requiring modification of the filament with selective placement of microwave absorbers. Additionally, these results demonstrate that the average crystallinity does not necessarily correlate with the strength of 3D printed semicrystalline plastics due to the importance of the details of the interface between adjacent printed roads.
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Size and print path effects on mechanical properties of material extrusion 3D printed plastics
Print conditions for thermoplastics by filament-based material extrusion (MatEx) are commonly optimized to maximize the elastic modulus. However, these optimizations tend to ignore the impact of thermal history that depends on the specimen size and print path selection. Here, we investigate the effect of size print path (raster angle and build orientation) and print sequence on the mechanical properties of polycarbonate (PC) and polypropylene (PP). Examination of parallel and series printing of flat (XY) and stand-on (YZ) orientation of Type V specimens demonstrated that to observe statistical differences in the mechanical response that the interlayer time between printed roads should be approximately 5 s or less. The print time for a single layer in XY orientation is much longer than that for a single layer in YZ orientation, so print sequence only impacts the mechanical response in the YZ orientation. However, the specimen size and raster angle did influence the mechanical properties in XY orientation due to the differences in thermal history associated with intralayer time between adjacent roads. Moreover, all of these effects are significantly larger when printing PC than PP. These differences between PP and PC are mostly attributed to the mechanism of interface consolidation (crystallization vs. glass formation), which changes the requirements for a strong interface between roads (crystals vs. entanglements). These results illustrate how the print times dictated by the print path layout impact observed mechanical properties. This work also demonstrated that the options available in some standards developed for traditional manufacturing will change the quantitative results when applied to 3D printed parts.
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- Award ID(s):
- 2011289
- PAR ID:
- 10344333
- Date Published:
- Journal Name:
- Progress in Additive Manufacturing
- ISSN:
- 2363-9512
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Composites printed using material extrusion additive manufacturing (AM) typically exhibit alignment of high- aspect-ratio reinforcements parallel to the print direction. This alignment leads to highly anisotropic stiffness, strength, and transport properties. In many cases, it would be desirable to increase mechanical and transport properties transverse to the print direction, for example, in 3D-printed heat sinks or heat exchangers where heat must be moved efficiently between printed roads or layers. Rotational direct ink writing (RDIW), where the deposition nozzle simultaneously rotates and translates during deposition, provides a method to reorient fibers transverse to the print direction during the printing process. In the present work, carbon fiber-reinforced epoxy composites were printed by RDIW with a range of nozzle rotation rates and the in-plane and through-thickness thermal conductivity was measured. In addition, the orientation of carbon fiber (CF) in the composites was measured using optical microscopy and image analysis, from which second-order fiber orientation tensors were calculated. These results showed that the orientation of CF became less anisotropic as nozzle rotation rate increased, leading to increased through-thickness thermal conductivity, which increased by 40% at the highest rotation rate. The orientation tensors also showed that RDIW was more effective at reorienting fibers within the in-plane transverse direction compared to the through-thickness transverse direction. The results presented here demonstrate that a current weakness of material extrusion AM composites—poor thermal conductivity in the through-thickness direction—can be significantly improved with RDIW.more » « less
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Understanding the mechanical properties of three-dimensional (3D)-printed ceramics while keeping the parts intact is crucial for advancing their application in high-performance and biocompatible fields, such as biomedical and aerospace engineering. This study uses non-destructive nanoindentation techniques to investigate the mechanical performance of 3D-printed zirconia across pre-conditioned and sintered states. Vat photopolymerization-based additive manufacturing (AM) was employed to fabricate zirconia samples. The structural and mechanical properties of the printed zirconia samples were explored, focusing on hardness and elastic modulus variations influenced by printing orientation and post-processing conditions. Nanoindentation data, analyzed using the Oliver and Pharr method, provided insights into the elastic and plastic responses of the material, showing the highest hardness and elastic modulus in the 0° print orientation. The microstructural analysis, conducted via scanning electron microscopy (SEM), illustrated notable changes in grain size and porosity, emphasizing the influencing of the printing orientation and thermal treatment on material properties. This research uniquely investigates zirconia’s mechanical evolution at the nanoscale across different processing stages—pre-conditioned and sintered—using nanoindentation. Unlike prior studies, which have focused on bulk mechanical properties post-sintering, this work elucidates how nano-mechanical behavior develops throughout additive manufacturing, bridging critical knowledge gaps in material performance optimization.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1007/s40964-022-00275-w
