Investment casting is one of the oldest metal working techniques, and refers to the process in which molten metal is poured into a mold formed by using a wax pattern, which is then melted away. Metal Injection Molding (MIM) is a more recent metal working process, and uses molds and injection molding equipment to create metal parts.
Both processes result in strong, intricately shaped metal parts that are difficult to produce by traditional machining processes such as forging. However, there are certain differences between the two processes, and the suitability of one process over the other depends mainly on your business and product engineering needs.
A wide range of materials such as stainless and carbon steels, and aluminum alloys can be used in in the investment casting process. Since ceramic molds are used in this process, they can be heated to a temperature above the melting point of the metal, and will not cool down when the metal is being poured. This makes investment casting the best process for high quality steel parts.
MIM uses a ‘feedstock’ that comprises finely-powdered metal mixed with a measured amount of binder material. Materials suitable for MIM are alloys with a higher melt temperature than copper; metals like zinc and aluminum that have a lower melt temperature, and alloys (like titanium) that form strong oxides are not suitable for the MIM process.
MIM allows for more design complexity – for example, thinner wall sections, sharper cutting points and tighter tolerances of up to +0.005″ per linear inch – as compared to the investment casting process. In addition, MIM is better suited for the production of smaller parts that weight less than 20 grams, and that are less than 100 mm long.
MIM can achieve a surface finish of 1 µm, whereas the surface roughness of an investment cast part is usually around 3.2 µm. In other words, MIM produces a better surface finish than investment casting, and does not usually require post-production machining.
Tooling is a major cost factor in MIM, and therefore MIM is considered an ‘economy of scale’ technology, ideal mainly for large scale production of runs of millions per year. In general, for production volumes of less than 1000 parts per run, investment casting is the more economical process.
There is obviously a place for both the MIM and the investment casting processes – each has its own strengths and weaknesses. In general, however, MIM is the process of choice to produce large volumes of small, highly intricate parts made of high melt alloys, where finish and tolerance are critical. Investment casting is more suitable for smaller production runs of larger parts made of lower melt temperature alloys.