Abstract

Polymer powder bed fusion (PBF/P) is one of many additive manufacturing (AM) processes utilized for producing polymer parts from digital 3D models. AM is preferred over traditional manufacturing methods in many applications due to advantages including tool-less manufacturing, high geometric complexity, short lead times, and reduced material waste. However, many industries that stand to benefit the most from AM are limited in their ability to use AM parts in large part due to low confidence in AM part quality. Among the polymer AM processes, PBF/P processes show significant promise for these applications due to their comparatively high isotropy and mechanical properties. Due to the many process variables present in PBF/P, printing conditions can vary from print to print resulting in poor repeatability of physical properties in printed parts. Many approaches have been studied for addressing this issue such as modeling of print dynamics, print parameter optimization, and process control. However, PBF/P remains largely unutilized in applications where quality control and assurance are high priorities. This work presents a novel approach for in-situ process monitoring and control in PBF/P and is demonstrated for the large area projection sintering (LAPS) process. The method proposed in this study monitors the powder bed surface via visible light cameras and identifies critical steps in the melting process defined as optical melting states (OMSs). The relationship between print parameters, process signatures, and resulting physical properties are studied. This thesis shows that during melting, the changing surface geometry and optical properties of the powder bed can be effectively monitored with optical cameras and are strongly correlated with the final density and ultimate tensile strength (UTS) of the printed part. By implementing closed-loop OMS control, consistent physical properties can be obtained despite different processing conditions. While established methods of identifying the property plateau for other PBF/P processes are not effective for the LAPS process, such as energy density methods, OMS control has been shown to effectively achieve full density and UTS in LAPS parts while optimizing print time. However, OMS methods are limited in their ability to evaluate ductility and percent crystallinity.

Degree

College and Department

Mechanical Engineering

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