Metal injection molding (MIM) and die casting are two common processes used to economically produce high volumes of small metal parts. Metal injection molding (MIM) adapts plastic injection molding techniques to produce metal parts by injecting metal powder bound to a thermoplastic binder. Unlike plastic injection molded parts, however, MIM parts require additional processing steps to remove the binder and sinter the powder. In the die casting process, molten metal is injected into a mold and then allowed to cool. Some post-processing clean-up is needed to remove casting flash. Die casting is used almost exclusively for non-ferrous metals.
MIM can produce small parts with complex features, whereas die casting is better suited to larger parts. Product designers have access to a wider range of materials using MIM than with die casting. Die casting is generally cheaper than MIM because it involves fewer production steps and cheaper raw materials. Production of the green MIM part is very fast, but the subsequent binder removal and densification processes reduce this advantage. Taking into account both the molding process and post-processing, the cycle time to produce a finished part by die casting is generally quicker than the MIM cycle time. This article will compare the processes, costs, materials, and applications of metal injection molding vs. die casting.
What Is the Metal Injection Molding Process?
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.
the Advantages of Metal Injection Molding:
Listed below are some advantages of metal injection molding compared to die casting:
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- 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.
- 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.
What Is the Die Casting Process?
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.
the Advantages of Die Casting:
Listed below are some advantages of die casting compared to metal injection molding:
- Die casting molds last longer than MIM molds. In some cases, a single die-casting mold can be used to make a million parts, whereas a MIM mold can only produce a couple hundred thousand.
- Diecast parts do not shrink the way MIM parts do. This means that molds can be more easily manufactured to the required size.