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The decentralized production associated with material extrusion additive manufacture (MEX) has been proposed as an ideal path to increase the circularity of plastics through direct recycling. Although multiple studies have reported on the 3D printing of various recycled plastics, variability in recycled materials, in particular post-consumer waste, challenges the direct extension of these results into production through MEX. Here, we demonstrate filament fabrication and printing of post-consumer polypropylene (PP), where the PP is sourced from clear, cold drink cups from three large international food service and beverage retail chains to provide well defined plastic waste that is perfectly sorted for recycling. These sources for the recycled PP were selected due to their ready availability to enable the results to be directly applied for hobbyist printing, blow molded products to provide good mechanical performance, and the clarity of the PP that suggests formulation design to minimize the PP crystal size. Despite the similarities in the end use product and their physical appearance, the source for the PP impacted the mechanical properties and the visual appearance of the printed objects. These differences can be directly traced to the rheological properties and oxidative stability of the PP at conditions relevant with the print process. These results clearly illustrate differences in initial formulation design and branding details, even when the product is for the same application, impacts the performance of recycled plastics in AM. The high viscosity associated with the PP from blow molding leads to requirements for higher extrusion temperatures for printing. The combination of high temperature and shear during extrusion process of 3D printing degrades the recycled PP. For circularity with MEX with recycled PP, one needs to consider the evolution in the properties of the polymer. Rheological details of recycled plastics are critical to selection of processing conditions and performance of MEX parts. Reporting of rheological data of recycled plastics and these properties after printing is critical to enable translation towards circular 3D printing of recycled plastics. 

Non-destructive characterization of 3D printed parts is critical for quality control and adoption of additive manufacturing (AM). The low-cost driver for AM of thermoplastics, typically through material extrusion AM (MEAM), challenges the integration of real-time, operando characterization and control schemes that have been developed for metals. Here, we demonstrate that the surface topology determined from optical profilometry provides information about the mechanical response of the printed part using commercial ABS filaments through calibration based correlations. The influence of layer thickness was examined on the tensile properties of MEAM ABS. Surface topology was converted into amplitude spectra using fast Fourier transforms. The scatter in the tensile strength of the replicate samples was well represented by the differences in the amplitude of the two fundamental waves that describe the periodicity of the printed roads. These results suggest that information about previously printed layers is transferred to subsequent layers that can be resolved from optical profilometry and offers the potential of a rapid, nondestructive post-print characterization for improved quality control. 

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. 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. 

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. 

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. 

Acoustic/elastic metamaterials that rely on engineered microstructures instead of chemical composition enable a rich variety of extraordinary effective properties that are suited for various applications including vibration/noise isolation, high-resolution medical imaging, and energy harvesting and mitigation. However, the static nature of these elastic wave guides limits their potential for active elastic-wave guiding, as microstructure transformation remains a challenge to effectively apply in traditional elastic metamaterials due to the interplay of polarization and structural sensitivity. Here, a tunable, locally resonant structural waveguide is proposed and demonstrated for active vibration bandgap switching and elastic-wave manipulation between 1000–4000 Hz based on 3D printed building blocks of zinc-neutralized poly(ethylene- co -methacrylic acid) ionomer (Surlyn 9910). The ionomer exhibits shape memory behavior to enable rearrangement into new shape patterns through application of thermal stimuli that tunes mechanical performance in both space and time dimensions (4D metamaterial). The thermally induced shape-reorganization is programed to flip a series of frequency bands from passbands to bandgaps and vice versa . The continuously switched bandwidth can exceed 500 Hz. Consequently, altering the bandgap from “on” to “off” produces programmable elastic-wave propagation paths to achieve active wave guiding phenomena. An anisotropic cantilever-in-mass model is demonstrated to predict the self-adaptive dynamic responses of the printed structures with good agreement between the analytical work and experimental results. The tunable metamaterial-based waveguides illustrate the potential of 4D printed shape memory polymers in the designing and manufacturing of intelligent devices for elastic-wave control and vibration isolation.