Microwave Nondestructive Evaluation of Moisture Content of Polymeric Filaments Used in Additive Manufacturing

In recent decades, the utilization of polymer-based additive manufacturing (AM) products has grown significantly across industries such as medical, aerospace, and textiles. This is driven by rapid and cost-effective production capabilities of AM. However, real-time inspection of polymeric filaments used in the printing process presents considerable challenges for quality control. Studies have demonstrated that even small amounts of moisture present in the polymeric feedstock can adversely affect the quality and mechanical properties (i.e., strength) of the final printed part. The primary focus of this study is to investigate and compare the performance of three near-field millimeter-wave probes for detecting absorbed moisture by polylactic acid (PLA) filaments commonly used in polymeric AM. The probe set includes a standard open-ended rectangular waveguide (OERW) and two variations that employ the same probe but utilize optimized dielectric inserts, namely, an extended dielectric-loaded waveguide (EDLW) and a dielectric-loaded waveguide (DLW) probe. These dielectric inserts help concentrate the electric field within a smaller "probing" region compared to the OERW. Furthermore, this study investigates the impact of the dielectric constants of the material used for loading the waveguide probes (with respect to the same for the inspected filament) on moisture detection sensitivity. Numerical electromagnetic (EM) simulations were performed at Ka-band (26.5-40 GHz), using CST Studio Suite, followed by relevant experimental validations. The results demonstrate that the EDLW probe exhibits the highest sensitivity for detecting moisture when loaded with a material possessing a dielectric constant similar to that of the inspected filament.