The Metal Injection Molding Process
The metal injection molding process is made up of the following steps:
The injection moulding process starts with the preparation of the feedstock – a powdered mixture of both metal and polymer. The powdered metal used in the process is extremely fine – usually under the 20 microns mark.
Once the fine metal powder is ready, it is mixed with a hot thermoplastic binder and then cooled. After the cooling process is complete, the mixture is granulated into a homogeneous feedstock, in the form of pellets.
The feedstock produced will typically be between 60% metal and 40% polymer.
Using the same equipment used in plastic injection molding (PIM), the feedstock is now molded. Note however that in the injection moulding process, the mold cavities are designed to be an estimated 20% larger. This larger cavity accounts for the part shrinkage that occurs during sintering processes.
During the metal injection molding process, the feedstock is melted, after which it is injected into the cavity of the mold. The feedstock is then left to cool and solidify into the shape of the desired part.
After cooling and solidification is complete, the polymer has to be removed from the metal. Some cases involves the ‘green molded’ part to be placed in a chemical or water bath, dissolving most of the binder. After this step (or instead of this step), pre-sintering or thermal debinding is done.
This pre-sintering or thermal debinding involves the ‘green molded part’ to be heated in a low temperature oven. This allows for the polymer binder’s removal through evaporation.
The result is the remaining ‘brown’ metal part, which contains an estimated 40% empty space by volume.
The final step in the injection moulding process is to sinter the ‘brown’ part in a high temperature furnace – at temperatures of up to 2 500°F. This step allows for the reduction of the empty spaces to between 1% and 5%. The result will be a high density metal part – between 95% and 99% metal.
The high temperature furnace: This furnace uses an atmosphere of inert gasses and reaches a temperature of about 85% of the metal’s (used in the process) melting point. This step in the process serves to remove pores from the material, which in turn leads to the shrinkage of the part. This part now shrinks to between 75% and 85% of the original molded size.
Note however that shrinkage that does occur, happens uniformly and manufacturers are capable of predicting this shrinkage.
The result of this step in the process is the part, retaining its original molded shape, with a high tolerance. This part also now has a greater density.
After the sintering step in the metal injection molding process, there is no requirement for secondary operations to improve either the part’s tolerance or surface finish.
In order to add beneficial features to the part, secondary processes can be performed. This secondary processes can serve to improve the part’s material properties or for assembling purposes. Parts manufactured using this process can be heat treated, machined or welded.
This process is used in order to manufacture metal components for a wide variety of uses, including components for surgical instruments, consumer products and electronics. Some examples of parts manufactured using the injection moulding process includes:
Fuel injectors, actuators, seating mechanisms, ignition lock components and solenoids.
Switches and connectors.
Medical and Dental:
Endoscopic surgical instruments.
Computer and Electronics:
Components for disk drives.
Seer blocks, triggers, safeties and sights.
Buccal tubes and brackets.
Bolts, lock cylinders and sidebars.
Watch our Injection Moulding Process Video
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Further Suggested Reading:
- Materials used in Injection Moulding
- Metal Injection Molding Tolerances
- Metal Injection Molding Surface Finish
- Metal Injection Molding Wall Thickness