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.
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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
- NSF-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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Additive manufacturing (AM) is often used to create designs inspired by topology optimization and biological structures, yielding unique cross-sectional geometries spanning across scales. However, manufacturing defects intrinsic to AM can affect material properties, limiting the applicability of a uniform material model across diverse cross-sections. To examine this phenomenon, this paper explores the influence of specimen size and layer height on the compressive modulus of polycarbonate (PC) and thermoplastic polyurethane (TPU) specimens fabricated using fused filament fabrication (FFF). Micro-computed tomography imaging and compression testing were conducted on the printed samples. The results indicate that while variations in the modulus were statistically significant due to both layer height and size of the specimen in TPU, variations in PC were only statistically significant due to layer height. The highest elastic modulus was observed at a 0.2 mm layer height for both materials across different sizes. These findings offer valuable insights into design components for FFF, emphasizing the importance of considering mechanical property variations due to feature size, especially in TPU. Furthermore, locations with a higher probability of failure are recommended to be printed closer to the print bed, especially for TPU, because of the lower void volume fraction observed near the heated print bed.more » « less
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null (Ed.)Purpose The purpose of this study is to understand how printing parameters and subsequent annealing impacts porosity and crystallinity of 3D printed polylactic acid (PLA) and how these structural characteristics impact the printed material’s tensile strength in various build directions. Design/methodology/approach Two experimental studies were used, and samples with a flat vs upright print orientation were compared. The first experiment investigates a scan of printing parameters and annealing times and temperatures above the cold crystallization temperature ( T cc ) for PLA. The second experiment investigates annealing above and below T cc at multiple points over 12 h. Findings Annealing above T cc does not significantly impact the porosity but it does increase crystallinity. The increase in crystallinity does not contribute to an increase in strength, suggesting that co-crystallization across the weld does not occur. Atomic force microscopy (AFM) images show that weld interfaces between printed fibers are still visible after annealing above T cc , confirming the lack of co-crystallization. Annealing below T cc does not significantly impact porosity or crystallinity. However, there is an increase in tensile strength. AFM images show that annealing below T cc reduces thermal stresses that form at the interfaces during printing and slightly “heals” the as-printed interface resulting in an increase in tensile strength. Originality/value While annealing has been explored in the literature, it is unclear how it affects porosity, crystallinity and thermal stresses in fused filament fabrication PLA and how those factors contribute to mechanical properties. This study explains how co-crystallization across weld interfaces is necessary for crystallinity to increase strength and uses AFM as a technique to observe morphology at the weld.more » « less
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Abstract 3D printing of thermoplastics through local melting and deposition via material extrusion additive manufacturing provides a simple route to the near net‐shape manufacture of complex objects. However, the mechanical properties resulting from these 3D printed structures tend to be inferior when compared to traditionally manufactured thermoplastics. These unfavorable characteristics are generally attributed to the structure of the interface between printed roads. Here, we illustrate how the molecular mass distribution for a model thermoplastic, poly(methyl methacrylate) (PMMA), can be tuned to enhance the Young's modulus of 3D printed plastics. Engineering the molecular mass distribution alters the entanglement density, which controls the strength of the PMMA in the solid state and the chain diffusion in the melt. Increasing the low molecular mass tail increases Young's modulus and ultimate tensile strength of the printed parts. These changes in mechanical properties are comparable to more complex routes previously reported involving new chemistry or nanoparticles to enhance the mechanical performance of 3D printed thermoplastics. Controlling the molecular mass distribution provides a simple route to improve the performance in 3D printing of thermoplastics that can be as effective as more complex approaches.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1007/s40964-022-00275-w
