1999 / Solid Freeform Fabrication Proceedings
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Author Emmanuel Sachs (a), Samuel Allen (a), Costas Hadjiloucas (a), Jeannie Yoo (b), Michael Cima (a), (a) Massachusetts Institute of Technology, (b) Imation Enterprises Corporation
Source Solid Freeform Fabrication Proceedings, 1999, pp 411-426
Abstract Three Dimensional Printing (3DP) is used to create metal parts by spreading a metal powder and printing a binder to define the geometry. In the currently used process a polymeric binder is used to define a green component. The polymer is burned out and the component lightly sintered to produce a skeleton. This sintering operation is causing 1.5 + - 0.2% shrinkage. The 3DP process can compensate for a given amount of predicted shrinkage by beginning with a larger part. However, the + - 0.2% uncertainty in the value of shrinkage translates directly to the loss of dimensional control of the parts. Therefore, there is a need to improve the dimensional control of metal parts produced by 3D Printing. The current work investigates the possibility of decreasing the average shrinkage by eliminating the sintering step. The concept under investigation is an alternative method of forming the skeleton where the metal needed to create the necks between powder particles is provided through the binder - a salt so lution. The metal is obtained from the salt by means of a heat treatment in a reducing atmosphere. It was found that strong metallic bonding can be obtained by melting the metal or the alloy derived from the salt solutions resulting, essentially, in brazing of the powder particles. Injection molding tools were fabricated by printing a copper nitrate solution into a dry mixture of 66 micron molybdenum and l micron silver powder. The metal skeletons were infiltrated with epoxies. The shrinkage caused by the brazing step on the 661 micron Mo powder is approximately 0.15% which compares favorably with the 1.5% shrinkage obtained by the standard sinter-based method. It was hypothesized that the shrinkage of this new binding method is due to the capillary forces induced by the molten metal necks. The traction force due to molten metal necks was modeled analytically and the compressibility of the powder was measured. The shrinkage determined by the equilibrium of these two effects match observed shrinkages well. The model suggests certain approaches to the further reduction of shrinkage, including attaining a higher packing fraction of the powder-bed. (Auth abstract) [References: 12]
Solid Freeform Fabrication Proceedings can be obtained from: The Solid Freeform Fabrication Symposium
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