MIM Design Guide

Metal Injection Molding is a net-shape process for producing solid metal parts that combines the design freedom of plastic injection molding with material properties near that of wrought metals. With its inherent design flexibility, MIM is capable of producing an almost limitless array of geometries in many different alloys.

An emphasis on plastic part design flexibility should be applied to metal part geometries developed with the MIM process in mind. Traditional metalworking technology limitations should be ignored. The MIM process can allow significant shape sophistication, the combination of multiple parts, multiple feature/functions within a single component, product assembly enhancement features, miniaturization of mechanical assemblies, mass reduction, and custom tailored physical properties for the intended end use are all possibilities with MIM. So MIM offers you more design freedom and material options than many other manufacturing processes.

The MIM design guide is intended to serve as a reference for applying MIM design principles to new components and evaluating existing components for possible conversion to this manufacturing technology. Properly designed MIM parts maximize the economic benefits of the process by ensuring that net shape results and targeted dimensional Cpk’s are attained.

MIM Design Guidelines for Single MIM Component

A very effective way of utilizing MIM’s inherent design freedom is to combine multiple components in an assembly into a single MIM component.

MIM Design Guidelines for Uniform Wall Thickness

Maintaining a uniform wall thickness throughout a component reduces the likelihood of molding process flaws, thus improving the overall part quality, cosmetics, and the resulting dimensional tolerances.

MIM Design Guidelines for Gating

In general, the gate is placed at the thickest cross-section to allow the material to flow from thick to thin cross-sections. Additionally, the location of the gate(s) should be placed to allow uniform filling of the mold cavity.

MIM Design Guidelines for Ejection

Once the molded part has cooled it must be ejected, the ejector pins push the part out of the cavity. The location and number of ejector pins depend on the component size, binder strength, and tooling complexity. Knock-out ejector pins are usually required for removing parts from the mold, and good design of these pins is critical to minimise flash marking of the parts.

MIM Design Guidelines for Parting Lines

The parting line location is one important decision in MIM tooling design. The parting line is the trace left on the molded component surface where the mold sections meet. Cosmetic and functional requirements may help guide parting line location.

MIM Design Guidelines for Threads

Both internal and external threads can be formed in MIM process, internal thread is more precise and cost-effective than unscrewing cores. The optimum location of external threads is on a parting line. To hold thread tolerance on thread diameter, narrow flat is typically 0.005”.

Internal threads can be molded directly into the component using unscrewing cores.
External threads can be molded directly onto the component thus eliminating the need for secondary thread-forming operations.

MIM Design Guidelines for Undercuts

External undercuts are easy to form on a parting line by split mold. While internal undercuts only can be produced by slides, or collapsible cores. In MIM design, we should avoid internal undercuts because of additional cost and potential flashing problems.
Internal undercuts that can be produced with a mechanical or hydraulic actuated slide can be readily produced.

MIM Design Guidelines for Coring Holes

Coring holes are normally used to reduce MIM parts cross-section, uniform wall thickness, reduce metal material consumption, reduce or eliminate secondary machining operation. Its preferred direction is parallel to mold opening and perpendicular to parting plane. When length/diameter ratio is greater than 4:1, the through hole is preferred because both ends support the core pin, otherwise, blind holes will be used with the cantilevered pin.

MIM Design Guidelines for Holes and Slots

Hole and Slots can provide functional features in MIM parts without additional cost, besides reduce part mass and uniform wall thickness. Holes are perpendicular to parting line is easy to mold with the least cost, these are parallel require mechanical sliders or hydraulic cylinders. For internal connected holes, we need to consider carefully of potential sealing-off problems and flashing issues.

MIM Design Guidelines for Decorative Features

MIM can easily mold features as logo, knurl, part number and identification marks in place without added cost, all these feature can be raised or sub-surface. We also can pride high level feature detail like diamond knurling in MIM process.

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MIM Design Guidelines for Thickness Transition

Although MIM part wall thickness is uniformed desirably, coring for thickness uniformity is not a perfect option, and it is difficult to avoid variations. We need to design a gradual transition between different thickness.

In summary, the MIM Design Guide serves as an invaluable resource for leveraging the unique capabilities of the Metal Injection Molding (MIM) process. By understanding and applying its principles, designers can fully exploit MIM’s design freedom, which allows for the creation of complex geometries, combination of multiple parts into one, and customization of physical properties.

When designing MIM parts, aspects like uniform wall thickness, proper gating, ejection, and parting line location are crucial. Attention to these details helps to enhance part quality, minimize manufacturing flaws, and ensure dimensional accuracy. Additionally, considerations for threads, undercuts, coring holes, holes and slots, decorative features, and thickness transition are essential to optimize the manufacturing process and the final product.

Whether creating new components or converting existing ones to MIM, adhering to the MIM Design Guide is key to maximizing the economic benefits of the process, achieving net – shape results, and meeting targeted dimensional Cpk requirements. It paves the way for the production of high – quality, cost – effective metal parts that can meet the diverse needs of various industries.