Vector drop-on-demand production of tungsten carbide-cobalt tooling inserts by three dimensional printing

Abstract

Three Dimensional Printing (3DP) is a solid freeform fabrication process used to generate solid parts directly from three-dimensional computer models. A part geometry is created by selectively depositing binder into sequentially spread layers of powder. In slurry-based 3DP, a suspension of powder in a solvent is used to form the powderbed layer. This slurry-based powderbed yields higher green density and part resolution than dry powder-based 3DP because of smaller particle size. Vector printing requires that the printhead trace and define the external geometries of a part before raster filling the interior, a new approach in comparison to conventional, raster-only printing. Drop-on-demand (DOD) printheads allow binder droplets to be ejected when needed rather than relying upon charge-and-deflect mechanisms used in continuous jet printheads. Integrating these concepts for vector, DOD printing has the potential to enhance the 3DP process by providing greater part resolution and surface finish. The 3DP slurry-based process and vector, drop-on-demand printing are examined as potential methods to produce Tungsten Carbide-Cobalt (WC-Co) tooling inserts. The research focuses on three fundamental process steps: (1) development of a stable slurry, (2) determination of jetting parameter values for optimal powderbed deposition, and (3) implementation of vector, DOD printing for the binder. Due to unforeseen circumstances, the first two objectives are only briefly introduced in Chapter 1 and summarized in Chapter 3. Further details may be found in the Diplomarbeit document of Olaf Dambon. Two approaches are explored to develop a stable, jettable slurry. One method involves using a water-based Tungsten Carbide slurry and a

(cont.) Cobalt Acetate binder; the other method utilizes an alcohol-based Tungsten Carbide-Cobalt slurry and an organic binder. Various suspension properties, such as sedimentation density and viscosity, are measured to assess the degree of slurry stability. After adequate slurry formulations are developed, an investigation of powderbed formation is conducted. Due to the low solubility limit of the Cobalt salt in water and the persistent defects in water-based slurry powderbeds, the alcohol-based approach is pursued and, because of its greater efficacy, is used for optimizing powderbed jetting parameters. An effective combination of line spacing, flow rate, and drying time is determined for producing powderbeds with minimal surface roughness and high packing density. Experiments are subsequently conducted in vector DOD printing of various geometries using a piezo-actuated, drop-on-demand printhead and Bridgeport three-axis milling machine. A Hewlett-Packard inkjet cartridge is initially used for vector testing of the milling machine; a Siemens PT-88S printhead is used to assess and optimize binder droplet formation parameters, such as voltage waveform and fluid properties. Functional conditions for vector printing and DOD droplet generation are developed and deliver acceptable performance. Successfully printed geometries with high-definition lines (140-170 [mu]m line width) and smooth surface finish are produced using sanded, jetted alumina slurry powderbeds. Following necessary refinements in slurry redispersion and slurry-binder compatibility, the same vector process can be repeated with jetted WC-Co slurry powderbeds.