CHARACTERIZATION AND OPTIMIZATION OF GEOMETRICAL ACCURACY AND MECHANICAL PROPERTIES OF SPECIMENS PREPARED BY FUSED FILAMENT FABRICATION
Embargo until
2021-08-01
Date
2020-04-21
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Publisher
Johns Hopkins University
Abstract
Fused filament fabrication (FFF), also known as fused deposition modeling (FDM), is one of the most popular additive manufacturing processes. This low-cost technology gives more flexibility in designing complex three-dimensional (3D) structures and allows users to easily transform digital designs into physical items. However, advanced applications of FFF are still limited by the large variability of mechanical property and structural geometry of printed parts. It has been known that those printing qualities are highly dependent on the processing parameters, including layer height, print speed, nozzle temperature as well as environmental factors like chamber temperature and relative humidity. Still, there is a lack of fundamental understanding of those effects.
To address this knowledge gap, we used X-ray micro-computed tomography (micro-CT) to perform full 3D geometrical characterizations on printed Polycarbonate specimens with different printing parameters. The results showed significant geometry variations depending on different printing conditions. We demonstrated the effects of reducing layer height, increasing nozzle temperature, as well as compensating material extrusion rate to improve geometric precision to a minimum of 0.8 % deviation compared to the original digital design.
In addition to geometry variations, the environmental conditions could introduce multiple printing defects. In-situ infrared imaging analysis revealed the presence of up to 5 °C/mm thermal gradient when printing using an open-chamber printer and a heated build plate. Further experiments showed that this undesirable thermal gradient, as well as the corresponding warping defects, can be mitigated by adding a closed chamber and elevating the chamber (environmental) temperature. Regarding the environmental humidity, analysis of micro-CT scans showed up to 11.7 % porosity, which is caused by polymer water content absorbed from environmental moisture. Meanwhile, tensile tests showed the mechanical strength loss associated with those defects. Further experiments showed that this undesirable porosity could be minimized by drying filaments before printing and printing in a dry environment.
We also conducted numerical studies by using finite element analysis based on material properties and CT-scanned geometries and found that the numerical model can accurately capture the modulus change brought by geometrical variations. Inspired by that, an empirical method is proposed to estimate the effective elastic modulus of the printed specimen.
This study serves as a guideline for future FFF characterization and optimization, demonstrating a method of how to obtain the modulus of FFF specimens and improve both mechanical performance and geometrical accuracy by parameter selections. Meanwhile, the findings about environmental factors can help to enhance printing quality when applied in various locations and weather conditions. Furthermore, our experimental efforts can be integrated with advanced thermo-mechanical computational models to provide insight into the FFF process.
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Keywords
Fused filament fabrication, X-ray computed tomography, polycarbonate