Millimeter Wave Near-Field Evaluation of Moisture Content of Polymeric Filaments Used in Additive Manufacturing

In recent decades, polymer-based additively manufactured (AM) products have seen a significant increase in utility in medical, aerospace, and textile industries, to name a few. AM has the capability of rapidly producing low-cost and customized products. However, on-line inspection of the polymeric filaments used to print a part, for quality control purposes, remains a challenge. It has been shown that the presence of a small amounts of moisture in the polymeric feedstock has the potential to significantly degrade the quality and strength associated with the final printed product. This paper investigates and compares the efficacy of three near-field millimeter wave probes for detecting absorbed moisture by the polylactic acid (PLA) filaments used in polymeric AM. The probes are designed to concentrate their respective electric fields in such a way to spatially produce high resolution and detection sensitivities. These probes include a standard open-ended rectangular waveguide probe and two others using the same probe but with dielectric inserts that tend to concentrate the electric field distribution to a smaller region than the waveguide aperture. Numerical electromagnetic (EM) simulations were performed at Ka-band (26.5-40 GHz) using CST Studio Suite (R), and the results showed that the open-ended rectangular waveguide is sufficiently sensitive to detect similar to 0.52% moisture level (by weight) absorbed by the PLA filaments, and the modified probes did not help in improving the detection sensitivity, even though they produced a higher spatial resolution. The efficacy of the open-ended rectangular waveguide probe for detecting a slight amount of moisture present in PLA filaments was also corroborated by measurements.