Experimental and Numerical Study on the Flow of Fine Powders from Small-Scale Hoppers Applied to SLS Multi-Material Deposition-Part I

We present experimental guidelines for the delivery of powders under 100μm through hopper-nozzle orifice diameters on the order of 1mm. Small-scale hoppers will be incorporated into an SLS powder deposition system for creating thin layers of multiple powdered materials in a patterned bed. This is a preliminary investigation on the flow behavior for selected orifice diameters and particle sizes under gravity or low pressure-assisted flow conditions. A method for numerically modeling the gas-particle behavior in hopper-nozzles is presented and conditions for achieving continuous mass flow rates are demonstrated. INTRODUCTION The development of SFF techniques for producing a new class of artifacts with spatiallyvarying structure and multi-functional characteristics is in part dependent upon the ability to deposit and consolidate multiple materials. In particular, the selective laser sintering (SLS) process is well-suited to the incorporation of multiple powdered materials. The SLS process presently uses a roller device to sweep thin layers of a single powdered material across the build area. It has been proposed that this roller device be replaced by an array of hopper-nozzles that discretely deposit lines, dots and regions of multiple powdered material in a patterned bed. The designated name “hopper-nozzle” refers to the design of experimental nozzles based on existing hopper theory. In the chemical and process industries, hoppers have been inexpensively designed to store, discharge bulk solids, and eliminate undesirable flow instabilities (i.e. arching, rat-holing, and oscillatory flow). Unfortunately, difficult hopper and powder sizes are avoided due to the lack of fundamental understanding of flow phenomena. This can be attributed to the complex interaction of granular solid and interstitial fluid that plays a large role in delivery through small orifice diameters. BACKGROUND Granular bulk solids, like powder, exhibit characteristics unique from any other state. The granular mass is generally an amorphous, random-packing of particles influenced by the interstitial fluid occupying its voids. Unlike a fluid, the pressure under a vertical column of granular material is independent of its height, which makes a constant flow rate of material possible. Most flow theory has targeted particles above 500μm that are cohesionless (fair to freeflowing in nature) and insensitive to drag. Generally, particles below 50μm are small enough to be subject to cohesive effects. Those used with SLS are typically 0.1 to 100μm in diameter, and are more susceptible to air resistance when passing through the hopper orifice.