Metal Injection Molding Process

Metal injection molding (MIM) is a manufacturing process, wherein the strength and integrity of pressed, machined or manufactured, small and complex metal parts and the versatility of plastic injection molding is blended. MIM combines the material flexibility of powder metallurgy, and the design flexibility of plastic molding.

Fine metal powders and plastic binders are injected in a mold in standard plastic molding machines. The binders are removed using a solvent, and through thermal processes. The temperature of sintering of the resultant metal part is such that the particles are bound, but the metal does not melt.

The methods used to compact the powder are as follows:

• Forging
• Hot isostatic pressing
• Cold isostatic pressing
• Extrusion
• Rolling
• Injection Molding
• Pressureless compaction

Complex shapes can be created by reassembling powder in a solid part. Compounding metal powders with binders creates a feedstock. This process is used for complex shaped, high performance parts when cost is an important factor.

This process has five steps, the first of which is mixing. Metallic powders are selected based on their strength, impact strength, wear resistance characteristics, hardness, machinability, high and low temperature characteristics, and other inherent abilities. A binding agent is then added to the mix.

The aim of this mixture is to have the strengths and benefits of the metals, and compensate for their weaknesses. When the powders are mixed, a feedstock is created. This mixture is then injected into molds, where they solidify to form green colored parts.

In the debinding stage that follows, the green part is immersed in a water bath to remove the binder. During cross-linking, the debound green part is then exposed to ultraviolet light, which thermosets the metal powders and the binding agents.

During sintering, the component is placed in a furnace and heated to more than 2000 degrees Fahrenheit. Here the metal parts are fused in a solid shape. In the final stage, all surface imperfections are removed, and the component is ready to be shipped.

To avoid distortion or warpage, the wall has a maximum thickness of 5 mm. Sharp thickness transitions are avoided. Sharp corners and edges may lead to sink marks. One side has to be designed flat to place the part, while the draft angles are designed to enhance the mold release.

MIM has applications in aerospace, defense, automotive, dental, hermetic packages, electronic heat sinks, industrial tools, fiber optics connectors, hard disk drives, power hand-tools, electrical connector hardware, fluid spray systems, pharmaceutical devices, surgical instruments, and sporting equipment.

• excellent surface finish
• close porosity
• high performance
• complex geometries
• high final density

The process offers better design freedom, and flexibility. This is suitable for intricate shapes, where the weight of the finished part is up to 100 grams. Depending on the economics of the product, large parts up to 453 grams are also possible.

When compared with other methods, this process aids for a smoother surface finish right out of the mold cavity. Just like wrought alloys, these components can be plated and heat-treated.

The typical materials used in MIM are Copper, Copper-Molybdenum, Tungsten heavy metal, Kovar, 316L Stainless Steel, 304L Stainless Steel and 17-4 PH Stainless Steel.