Metal Injection Molding Process

Metal injection molding (MIM) is a manufacturing process. The strength and integrity of pressed, machined or manufactured, small and complex metal parts and the versatility of plastic injection molding is blended. MIM mixes 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 by the solvent and thermal processes. The temperature of sintering of resultant metal part is such that the particles are bound, but the metal is not melted.

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 these metal powders with binders creates a feedstock. This process is used for complex shaped, high performance parts when cost is an important factor.

Procedure of Metal injection molding

This process has five steps. The first step is mixing. Metallic powders are selected for their strengths, impact strength, wear resistance characteristics, hardness, machinability, high and low temperature characteristics and other inherent abilities. These powders are mixed with a binding agent. The aim of this mixture is to have the strengths and benefits of the metals and compensate for the weaknesses. When the powders are mixed, a feedstock is created. This is injected into the molds. The injection molding stage gives rise to a green part. In the debinding stage, the green part is immersed in a water bath so that the binder is removed. In cross-linking, the debound green part exposed to ultraviolet light. Due to this, the binding agents used along with the metal powders are thermoset. 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 finishing stage, burrs and surface imperfections are removed. Now, the component can be shipped.

Design guidelines

Change in a wall thickness in a part may cause distortion or warpage. To overcome distortion, the wall thickness may be maximum 5 mm. Sharp thickness transitions have to be avoided. Sharp corners and edges may lead to sink marks. One side has to be designed flat to place the part. The draft angles have to be designed to enhance the mold release.

Applications

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. In the medical field, biopsy jaws, dental brackets, laparoscopic cutting tips and surgical instruments are manufactured. In the electronic field, microwave packaging, fiber optic transmitter, disc drive components and RF connectors are done.

Advantages

The advantages of MIM are:

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

There is a better design freedom and flexibility. This is suitable for intricate shapes where the weight of the finished part is upto 100 grams. Depending on the economics of the product, large parts upto 453 grams are also possible.

When compared with other methods, a smoother surface finish can be produced right from the mold cavity.

The components can be plated and heat-treated, similar to wrought alloys.

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.