Steps and design rules of metal injection molding process

Steps and design rules of metal injection molding process

 

Metal injection molding is the traditional injection molding, which changes the manufacturing of solid metal parts using injection molding technology. In this process, the raw material is a powder mixture of metal and polymer. For this reason, the MIM is sometimes called powder injection molding (PIM). Using a standard injection molding machine, the powder is melted and injected into the mold, where it is cooled and solidified into the shape of the desired part. The subsequent heating process removes the unwanted polymer and produces a high-density metal part. Metal injection molding is suitable for the production of small metal parts in large quantities. Unlike injection molding, these parts may be geometrically complex and have thin walls and details. The use of metal powder enables the use of a variety of iron and non-iron alloys and the material properties (strength, hardness, wear resistance, corrosion resistance, etc.) to be close to those made of forged metal. In addition, because the metal is not melted in the MIM process (including metal casting process), the superalloy can be used for tool life without any negative impact. Metals commonly used in MIM include the following:

Low alloy steel;

stainless steel;

High speed steel;

Iron pole;

Cobalt alloy;

copper alloy;

Nickel alloy;

Tungsten alloy;

Titanium alloy.

Metal parts manufactured from MIM process are found in many industries, including aerospace, automobile, consumer goods, medical dentistry and telecommunications. MIM components can be found in mobile phone accessories, sporting goods, electric tools, surgical instruments, and various electronic and optical devices.

Metal injection molding electronic accessories include the following steps:

MIM raw material preparation

Raw material preparation - the first step is the powder mixture of metal and polymer. The powder metal used here is more fine (usually 20 microns) than those used in traditional powder metallurgy methods. The metal powder is mixed with a hot thermoplastic binder, cooled, and then granulated to form a uniform raw material. The raw materials obtained are typically 60% metal and 40% polymer (volume).

MIM injection molding

Injection molding - The raw powder uses the same equipment and tools that are used for plastic injection molding. Therefore, the design of the mold cavity sintering part is about 20% larger, accounting for shrinkage. In the injection molding cycle, raw materials are melted and injected into the mold cavity, where they are cooled and solidified into the shape of parts. A part of the molded "green" is ejected and then cleaned to remove all flash memory.

MIM degrease

Degreasing - In this step, the metal polymer adhesive is removed. In some cases, the "green" part of the first solvent degrease is placed in a water or chemical bath to dissolve most of the adhesive. After (instead of 1) in this step, thermal degreasing or presintering is performed. A part of "green" is heated in a low-temperature furnace to remove the polymer adhesive by evaporation. As a result, the remaining "brown" metal part will contain about 40% of the empty space through the volume.

MIM sintering

The step after sintering is sintering to reduce the empty space by about 1-5%, so as to produce a metal part with high density in the "brown" part (95-99%) of the high temperature furnace (up to 2500 ° F). The furnace uses an inert gas atmosphere and shows a temperature close to 85% of the melting point of the metal. This process hole is removed from the material, resulting in part reduction to 75-85% of its molding size. However, this shrinkage is uniform and can be accurately predicted. The obtained part retains the high tolerance of the original molded shape, but the density is much higher now.

After the completion of the sintering process, the required secondary operation can improve the tolerance or surface finish. However, like a part of a cast metal, some secondary processes can be carried out to add new functions, improve material properties, or other components. For example, the MIM part can be machined, heat treated, or welded.

Metal injection molding (MIM) design rules:

When the design part is made with MIM, most of the design rules for injection molding still apply. However, there are some exceptions or supplements, such as the following:

Wall thickness - just like plastic injection molding, the wall thickness should not be minimized, and keep the whole uniform part. It is worth noting that in the MIM process, reducing the wall thickness not only reduces the material quantity and cycle time, but also reduces the degreasing and sintering time.

Draft - There is plastic injection molding, and many enterprises of MIM do not need any draft. In the powder material version, it is easier to use polymer adhesives from molds than most injection molding polymers. In addition, MIM parts completely cool and shrink around the molding feature before they are sprayed out, because the mixture of metal powder needs longer time to cool.

Sintering support - During sintering, MIM components must be properly supported or twisted as they may shrink. By designing the flat surface of the parts on the same plane, the standard flat support tray can be used. Otherwise, more expensive customized brackets may be required.