Slurry deposition of high quality layers for 3D printing

Abstract

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1999.Includes bibliographical references (leaf 108).Slurry-based three dimensional printing is being used to create ceramic parts directly from CAD files. Layers of slurry are deposited by using an x-y positioning system to raster a slurry nozzle over a powder bed. A binder material is selectively printed into each layer to define a slice of the part. This process is repeated until the last slice of the powder bed is defined. Afterwards, the powder bed is re dispersed in water, leaving behind the printed green part. The green part is then sintered to full density. This thesis focuses on new ways to make slurry layers to minimize defects and improve surface quality. A technique called line merging has been shown to rapidly create slurry layers containing minimal defects. This technique produces layers with better surf ace finish than other proven techniques. Line merging occurs when adjacent lines of slurry are deposited in rapid succession such that they merge together prior to slip casting. Line merging differs from nozzle rastering in two ways: the cycle time between deposited lines is reduced from approximately I second to as little as 0.1 second, and lines are deposited in one direction only. The very short cycle times are achieved by using springs rather than electromechanical means to reverse the direction of the nozzle assembly. Initial line merging exploration was conducted with a rotary device that rapidly deposited arcs of slurry onto a moving substrate. Subsequently, an apparatus consisting of an oscillating wheel with a nozzle at the bottom was constructed to examine the operating space for successful line merging. Three boundaries to this operating space were discovered: bubbling, stitching, and perpendicular migration. Experimental results and physical equations were combined to produce a mathematical model predicting the behavior of the system. This model was useful in predicting the occurrence of bubbling and stitching, but was not helpful in predicting the occurrence of perpendicular migratior.. Using this model, a five layer powder bed was made that was free of line stitching defects and interlayer defects. Future machines will adapt the line merging technique to a linear system.by Bjørn N. DeBear.S.M