Infiltration stop, epoxy tooling development, and shrinkage studies in three dimensional printing

Abstract

Three Dimensional Printing (3DP) is an advanced rapid prototyping process which can not only fabricate prototypes but functional tooling as well. In the process, a thin layer of powder is spread onto a powder bed to which a printhead delivers the binder that selectively joins the particles. Successive layers are printed and thus a 3-D object is constructed. In the case of metal tooling fabrication, the part will be sintered, followed by infiltration to make a fully dense tool. To minimize the shrinkage and reduce the cycle time in 3DP metal powder postprocessing, the idea of infiltration stop was developed. In the process, a shell, which defined the negative of the part to be created, was printed with a ceramic binder. The shell, containing the loose powder, was then directly transferred to the furnace where the infiltration would take place. Molten infiltrant would stop spreading at the interface between the shell and the loose powder. After infiltration, the shell would be removed when the ceramic binder was etched away. Two fundamental factors limited this development: controlling the infiltration stop interface and loose powder infiltration. However, the concept of infiltration stop could be extended to fabricate porous tooling which has many potential applications in manufacturing processes. The major structure of a porous tool would still be printed with a polymer binder. Only the region that was designed to be porous would be printed with infiltration stop. The tool would be sintered and infiltrated subsequently. The region where infiltration stop was printed would remain porous, as in sintered state. A demonstration unit was successfully created. The procedure of epoxy infiltration into as-printed green metal parts using a vacuum setup was established. This process was to create a soft prototype tool with much faster cycle time and lower costs since high temperature furnace operation was not required. Successful infiltration was carried out for 316L / Rophlex slurry cast samples by using a specially formulated unfilled epoxy. The average modulus of rupture value for infiltrated samples was found to be 50 MPa. In addition, bond strength between metal particles and epoxy substrate could be reinforced by adding chemical coupling agents or using irregularly shaped particles. The shrinkage of printed metal parts was studied by measuring the shrinkage of rectangular bars at each of the post-processing stages. The total shrinkage after infiltration for the fast axis, slow axis and Z axis were measured as 1.45%, 1.48% and 1.7%, respectively.