Metal injection molding (MIM) is a form of powder metallurgy process that is ideally suited for the production of small complex parts. Its main advantage lies in its ability to produce finished parts to net-shape, requiring little or no expensive secondary operations.
Hence, it is extremely attractive for hard metals and high precision or high performance parts.
Although the MIM process has great potential and numerous advantages, much of the process technology has been kept in commercial secrecy, licensees and guarded by numerous patents.
Despite the increase in scientific research into this area in the recent years, in particular by institutions like Rensselaer Polytechnic Institute, Penn State and Brunei University, there still remains much to be investigated in detail.
One of the major technical challenges faced by the researchers lies in the formulation of a right binder system for optimizing the molding and debinding characteristics of the MIM process.
This chapter focuses on the application of polymers in the formulation of binder blends for MIM. A brief summary of the different classifications of binder blends is presented. The rheological, thermal and mechanical properties of a feedstock with EVA/beeswax binder is discussed to highlight the influence of binder-powder interactions on these properties.
MIM Metal Injection Molding Process
The metal injection molding process consists essentially of the following steps:
- Preparation of powders
- Design and preparation of binder system
- Mixing to form a homogeneous feedstock
- Pelletising or granulation of feedstock
- Injection molding in a closed die
- Binder removal (De-binding)
The MIM process, illustrated in Figure, involves incorporating a binder into fine metallic powders. The binder serves a dual role of a transport medium allowing flow and packing of the powder in the mold cavity, as well as a medium to retain the shape of the molded part. The binder and powder is mixed in a mixer until a homogeneous mixture is obtained. The mixture is then pelletised to form a feedstock and subsequently fed into an injection molding machine. Next, the binder in the molded parts is removed in the de binding step. Debinding is achieved either by solvent extraction or thermal decomposition. Once the binder is entirely removed, the molded parts are sintered in a controlled atmosphere furnace or vacuum furnace to full density.
In view of the present trend of miniaturization, commercial and domestic products are progressively designed to be smaller, lighter and more compact. As a result, there is a growing need for smaller and more complex shaped parts. Moreover, factory automation requires products to be designed with ease of assembly. This inevitably requires the number of parts per product to be reduced and hence, the deliberate redesign of parts with multiple functions. Individual components become more complex in shape.
Due to these various reasons, it is evident that a cost effective method to produce small and complex precision metal components in high run volumes for custom parts markets is very much in demand. MIM, which possess such properties, offers a very attractive alternative over the conventional methods, such as investment casting, discrete machining and other conventional powder metallurgy process where secondary machining would be needed to create additional geometry.
Figure shows the basic attributes of MIM, a combination of low cost, high performance and shape complexity. MIM would achieve low cost through ease of processing, low cost of capital equipment and elimination of secondary operations. It is capable of producing high performance parts through high final density and even the possibility of composites (e.g., metal matrix composites). Finally, it is capable of producing net shape parts of complex geometry with excellent surface finish. In addition, the process can be automated and is open to modeling via modification of modeling methods already established in thermoplastics molding . As MIM’s potential is being recognized, it has found applications in a variety of areas, including firearms, orthodontic devices, business machines/printers, computer disk drives, dental and medical instruments, household appliances, watches, jewelry and electronic packages. Billiet reported that the number of organizations engaged in MIM research or production had risen from less than half a dozen in 1980 to well over 100 worldwide in the 1990s.
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