Radio frequency additive manufacturing : a volumetric approach to polymer powder bed fusion
Polymer powder bed fusion (PBF) additive manufacturing offers a number of advantages over conventional manufacturing techniques, particularly in the areas of reduced tooling costs and the added geometric complexity available to designers. However, existing methods require heat to be applied to the powder bed at each layer to fuse the powders and form parts. The layer-wise heating strategies used in current PBF processes contribute to a reduction in the mechanical performance of the parts and increase the time required to fabricate them. To address these issues, a volumetric heating strategy is implemented through a novel radio frequency additive manufacturing (RFAM) process. Radio frequency (RF) radiation is a heating mechanism that is capable of penetrating into materials to cause a simultaneous temperature rise throughout the material volume. Given the insulating nature of most polymers, electrically conductive dopants can be added to a polymer powder bed such that the effective composite properties are suitable for RF heating. By selectively patterning the dopant in the powder bed and applying RF radiation, heat generation can be contained to the RF-absorbing doped region with little effect to the surrounding powder bed. With powder mixtures of nylon 12 as the polymer and graphite as the dopant, it is possible to fuse the host polymer using RF radiation as the sole energy source. One of the consequences of creating parts with this method is the complex interaction between the part geometry and the applied RF field that can cause non-uniform heating to develop within the part. Aided by computational design approaches, methods for improving the heating uniformity are proposed including a functional grading scheme to vary the dopant concentration throughout the powder bed. To validate the computational models and further develop the RFAM process, the design of a prototype machine capable of three dimensional dopant patterning is presented. The prototype system is used to create RFAM parts and evaluate the effectiveness of the different strategies aimed at improving the heating uniformity within the doped powder beds. As a result of this work, the feasibility of a volumetric, RF-assisted additive manufacturing process is demonstrated.