While titanium and other refractory metal alloys present problems in PIM due to alloy impurities directly attributable to the injection molding process, a new naphthalene binder based Ti-PIM process offers an injection molding system with high powder loadings, alternative metals and alloys and reduced binder burnout for sintering.
Powder injection molding (PIM) is a well-established, cost-effective method to fabricate small-to-moderate size metal components. The process, derived from plastic injection molding, comprises four main steps of mixing of metal powder with a polymer binder, injection molding the mixture, binder removal (via heat treatment or chemical processes) and sintering or densification. Pellets or granules of the metal powder/polymer binder mixture are heated in a cylinder and the resulting melt is forced under pressure into a split-mold cavity where it quickly cools before the mold is opened and the part is ejected onto a conveyor belt or storage bin. In addition to rapid production, by redesigning and replacing the mold, the shape of the component can be readily changed, offering flexibility in part design. Thus, PIM is an economical, net-shape process for manufacturing large volumes of complex-shaped parts.
PIM is used to manufacture a wide variety of products made of stainless steel, nickel-base superalloys and copper alloys. However, titanium and other refractory metal alloys present problems due to alloy impurities directly attributable to the injection molding process. A unique blend of PIM constituents has been developed where only a small volume fraction of binder (~10-15 wt%) is required for injection molding; the remainder of the mixture consists of the metal powder and binder solvent (patent pending). Because of the nature of the decomposition in the binder system and the relatively small amount used, the binder is eliminated almost completely from the presintered component during the initial stage of a two-step heat treatment process. The use of easily removed fugitive phases in the powder mixture and control of the debinding and sintering heat treatments allows optimizing the porosity of the component, which is advantageous in a number of specialized applications including the design of self-lubricating parts and biomedical implants.