Shape Contraction in Sintering of 3D Objects Fabricated via Metal Material Extrusion in Additive Manufacturing

Additive manufacturing (AM) using metal materials (metal AM) is useful in the fabrication of metal parts with complex shapes, which are difficult to manufacture via subtractive processing. Metal AM is employed in the manufacture of final products as well as in prototyping. Recently, certain metal-AM machines have been commercialized. Powder-bed fusion and direct energy deposition are the main types of metal AM; they require the use of a high-power laser or electron beam and most of them are highly expensive. On the other hand, AM machines of the material-extrusion (ME) type can fabricate metal parts at a low cost. ME is the method of extruding materials from a nozzle and fabricating thin layers. By mixing a metal filler with a base material, it is possible to impart various mechanical properties to the extruded material, such as electrical or thermal conductivity. If the extruded material is baked in a furnace after fabrication, the object can be sintered. During the sintering process, the fabricated objects always shrink and dimensional errors occur. One of the reasons for the shrinkage is that voids are generated inside the object after the degreasing process and collapse during the sintering process. Because the void is generated as a space by replacing a binder that becomes vaporized during the degreasing process, the shrinkage may be controlled by decreasing the content in polymers. In this study, the effect of the metal filler density on the shrinkage in shape was investigated through experiments using two types of metal ME AM. One type is the fused filament fabrication (FFF), in which a material that consists of a metal filler and fused plastics is extruded; the other type is the ultrasonic vibration-assisted ME (UVAME) device, in which a metal powder suspension with a small amount of thickening polymer is extruded. In the latter method, materials with an extremely high density in metal fillers were used; it was considered that degreasing was not required. Two types of specimens were fabricated using AM devices; they were then degreased and sintered. The resulting shapes of the objects were measured with a 3D scanner and were compared. The experimental results showed that the shrinkage of the material with a high density of metal fillers was less than that of the material with a low density of metal fillers.