Metal Injection Molding vs. Die Casting: A Comparison

Metal injection molding was first invented in the 1970s by Raymond Welch. It is used to create metal parts with a process similar to that of plastic injection molding. The key difference is that with MIM, the raw material is metal powder coated with a thermoplastic binder which will later be burned out, whereas, with conventional plastic injection molding, additive materials such as glass or ceramic fibers are incorporated permanently into the part to enhance its mechanical properties.

The granulated metal/binder mix is fed into an injection molding barrel. The barrel contains a screw with a shaft whose diameter increases from the material inlet to the outlet to the mold. As the screw rotates, the powdered feedstock is forced into incrementally smaller volumes. 

The compression generated by the screw is the primary mechanism responsible for heating and melting the plastic binder carrying the metal powder. The barrel is also heated to supply additional energy. Once enough material is melted, the screw retracts and then forces the metal powder suspended in the plastic binder into a two-part mold that is clamped shut during the injection, the plastic binder then transports the metal powder into the mold. Once cooled, the green part is ejected from the mold. To achieve its final properties, the binder must be removed from the structure. The empty spaces left by the binder must be filled by sintering to create a fully dense, structurally capable part. This green part is then exposed to solvents or a catalyst, and elevated temperatures help remove the thermoplastic binder, which is called the brown part. Finally, the part is placed in a furnace to sinter the metal powder particles together. During this process, the part can shrink from 15-30% (depending on the material used). After sintering, the part will have its final mechanical properties. Figure 1 below shows examples of metal injection molded parts:

Listed below are some advantages of metal injection molding compared to die casting:

1. MIM can manufacture a wide variety of small, complex parts with fine features using the same techniques in standard plastic injection molding. Die casting, on the other hand, struggles to produce fine feature parts.
2. The MIM process only makes use of high temperatures during the sintering process.  A molten metal feedstock is not required like with die casting. This also means that materials with very high melting temperatures can be used without the challenges of processing and handling these metals in molten form. 

Listed below are some disadvantages of metal injection molding compared to die casting:

1. Due to the complex high wear resistant tooling required for metal injection molded parts, metal injection molding (MIM) equipment can be expensive. MIM molds do not last as long as diecast molds because of the abrasive nature of the metal powder feedstock.
2. MIM requires multiple post-molding steps to produce the final part. This adds cost to the final parts compared to those produced by die casting.  
3. Metal injection molded parts shrink significantly during the densification process. Mold design requires an advanced understanding of the properties of the binder, the metal, and the interaction between the two during post-processing to produce final parts that can meet dimensional requirements.

Die casting is a process for manufacturing metal parts by injecting molten metal into a mold. This process was first invented in 1838 and was patented in 1849. The first die casting materials used were lead and tin. In 1914, the process was further developed to also accommodate the use of aluminum and zinc. Today magnesium, copper, and silicon are also used. Die casting is limited to non-ferrous materials. Die casting ferrous metals is possible, but is an uncommon practice.  

The molten metal to be cast can be injected under high pressure or simply flow in by gravity feed. Once the part has been allowed to cool, it can be ejected from the mold. This can take up to a minute, depending on part size and wall thicknesses. Excess material due to gates, runners, and parting line flash must be removed either using a manual process or a press die. To learn more, see our guide on the Process of Die Casting.

Table 1 below compares some common properties of metal injection molding and die casting:

• Cost comparison: Due to the cost of mold manufacture for MIM and die casting, both technologies require relatively large production volumes to justify the mold costs. However, at medium to large volumes, both technologies produce very low-cost parts. Die casting is up to 30% less expensive than MIM due to there being no need for multiple post-processing steps as is the case with MIM.
• Speed comparison: Comparing only the speed of the actual molding process, metal injection molding (MIM) is faster than die casting. However, MIM requires post-molding processes that make its overall cycle time to produce a finished part longer than for a finished diecast part.
• Volume comparison: Metal injection molding and die casting are both high production-volume technologies. Typical production runs on a mold for both can easily reach hundreds of thousands or even millions of parts. MIM is ideally suited to high-volume production of intricate parts, whereas die casting is better suited to high-volume runs of larger, simpler parts. 
• Materials comparison: Die casting typically only makes use of non-ferrous metals like aluminum, copper, zinc, magnesium, and lead. MIM can make use of ferrous metals as well as more advanced materials like titanium and nickel alloys. One of the most commonly used MIM materials is stainless steel. MIM can essentially be used with any metal that can be converted into powder form.

A mutual alternative to MIM and die casting is Selective Laser Melting (SLM). SLM is a 3D printing technique used to produce complex metal parts. It consists of a laser selectively melting metal powder in the shape of the part cross-section. Once a layer is complete, another layer of metal powder is deposited on top and the process repeats until the part is complete. SLM can produce fine features like MIM.