Process Intermittent Measurement for Powder-Bed Based Additive Manufacturing | NIST

Abstract

Process intermittent measurements of parts fabricated by additive manufacturing (AM) can enable both process improvement and characterization of internal part geometries. The planar, layer-upon-layer nature of most AM processes allows two-dimensional geometric measurements with a vision system, because the part’s current layer is continually in focus. Proof of this concept has been shown through measurement of parts made using a three-dimensional (3D) printer. Process intermittent measurements were compared to contact and non-contact measurements of the finished parts to characterize deviations in printed layer positions and changes in part dimensions resulting from post-process treatments. Introduction Powder-bed based AM methods include powder bed fusion processes, during which thermal energy selectively fuses regions of a powder bed, and binder-jetting processes, during which a liquid bonding agent is selectively deposited to join powder materials in a bed. 3D printing is a binder-jetting process that is the topic of this paper. It is a method in which primarily starch-based materials can be joined with clear or colored binder to produce 3D parts. As with all powder bed based processes, layers of powder are spread to cover a build platform to allow single cross sections (or layers) of the part to be produced. This planar, layerwise nature of the process allows two dimensional (2D) optical measurements of the bonded layer geometry to be made intermittently for all layers. Calibration, which includes determination of measurement uncertainty of the optical system, enables quantitative comparisons to be made between geometries and measurements. This method of in-situ part inspection is a form of real-time nondestructive analysis (NDA), which was identified as a research opportunity by the 2009 Roadmap for Additive Manufacturing [1]. Previous research has incorporated an optical system to perform real-time NDA of the 3D printing process in order to map the internal structures of the 3D part being built [2]. This current study is a first step in developing a closed-loop control system for a metal-based AM machine, which will monitor the part being made and communicate necessary information to the machine’s controller to alter process parameters to 1 This work is an official contribution of the National Institute of Standards and Technology and is not subject to copyright in the United States. Commercial equipment and materials are identified in order to adequately specify certain procedures. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply the materials or equipment are necessarily the best available for the purpose.