What Are MIM Parts

Metal injection molding (MIM) is a sophisticated manufacturing process that transforms metal powder into complex components, known as MIM parts. This process utilizes specialized tooling and injection molding machinery, similar with the equipment used in plastic injection molding. The level of complexity achievable in MIM parts closely parallels what is attainable in plastic injection molding. However, MIM distinguishes itself by incorporating multiple post-molding stages, most notably debinding and sintering. Consequently, careful attention is essential when contemplating factors such as cross-sectional thickness and geometric attributes during the design of MIM parts. In this comprehensive article, we will delve into all aspects related to MIM parts. This includes the selection of materials, exploration of surface finish options, and a detailed guide on how to effectively design MIM parts for your specific product.

How Your MIM Parts Are Crafted

1. Preparation of the Feedstock

In the MIM process, our primary raw materials are metal powders and a thermoplastic binder. It’s worth noting that the binder functions as an interim processing aid, necessitating its removal post-injection molding. The final properties of the MIM product are inherently tied to the properties of the powder employed.

The process begins with a carefully orchestrated blending of the powder mixture with the thermoplastic binder at an elevated temperature, accomplished via a kneader or a shear roll extruder. This amalgamated product is referred to as the ‘feedstock.’ Typically, it is granulated into particles of several millimeters, mirroring the common practice in the plastic injection molding industry. Producers of MIM parts have the option to either acquire ready-to-mold feedstock from international suppliers or manufacture it in-house provided they possess the requisite expertise.

2. Injection Molding

The production of ‘green’ MIM parts follows an injection molding process akin to that utilized in the manufacture of plastic parts. The versatility of this process permits the creation of an array of part geometries, akin to the diversity achievable with plastic components.

3. Binder Removal

The subsequent phase revolves around the removal of the binder, an essential step in obtaining components endowed with an interconnected pore network while preserving their structural integrity. The nuances of various binder removal techniques will be elucidated in a subsequent section of this discourse.

It is important to acknowledge that post-binder removal, residual binder material may still be present within the components, acting to maintain cohesion among the metal powder particles. Notably, the pore network established in the product expedites the evaporation of this residual binder, coinciding with the onset of sintering neck formation among the metallic particles.

4. Sintering

The sintering process culminates in the elimination of the majority of the pore volume previously occupied by the binder. Consequently, the MIM parts undergo a substantial linear shrinkage, typically in the range of 15 to 20%.

If required, sintered MIM parts can undergo additional processing through conventional metalworking methods, including heat treatments or surface treatments, analogous to the treatment of cast or wrought components. For specific applications, such as those in the automotive, medical, and aerospace sectors, the deployment of Hot Isostatic Pressing (HIP) may be a viable strategy for the comprehensive removal of residual porosity. This approach is notably cost-effective, especially in the context of critical components.

Material Options of MIM Parts

Metal Injection Molding (MIM) offers a wide range of commonly used structural materials suitable for applications in the medical, military, hardware, electronic, and aerospace sectors. If the metal powder is available in the requisite size, typically less than 25μm, and can sinter to achieve a high density without any alteration in alloy chemistry, it becomes feasible to produce the material through the MIM process. Below table shows the materials that are available for your MIM parts.

Surface Finish Options of MIM Parts

Metal Injection Molding (MIM) is renowned for its exceptional surface finish. Typically, it achieves a surface roughness of 0.8μm (32μin) Ra. However, it can reach an impressively smooth surface finish as low as 0.3–0.5μm (12–20μin) Ra under specific conditions.

The quality of the surface finish depends on several factors, including the size and composition of the metal powders used, the precise sintering conditions, and any secondary operations, such as bead blasting or tumbling. Interestingly, while sandblasting and beadblasting can lead to increased surface roughness due to pitting, tumbling tends to decrease surface roughness.

It’s worth noting that the surface finish of the final MIMed component can also be influenced by the quality of the tooling used in the manufacturing process. Any pits resulting from Electrical Discharge Machining (EDM) on the tooling may be translated to the finished MIM component, affecting the overall surface quality.

Electroless Nickel Finish

Electroless nickel plating is a sophisticated surface treatment for MIM parts. It involves a chemical process that deposits a uniform and corrosion-resistant layer of nickel onto the components. This finish enhances both the aesthetics and durability of MIM parts, making it a popular choice in various industries.

Chrome Finish

Chrome plating is a well-known surface finishing method that provides MIM parts with an attractive, lustrous appearance. Beyond aesthetics, chrome finishing offers exceptional corrosion resistance, making it ideal for components exposed to harsh environmental conditions.

Black Oxide Finish

Black oxide coating is a conversion coating that offers both aesthetic appeal and improved wear resistance. This finish is particularly suitable for MIM parts that require a sleek, matte-black appearance and enhanced protection against corrosion and abrasion.

Passivation

Passivation is a chemical treatment used to enhance the corrosion resistance of stainless steel MIM parts. It removes free iron and other contaminants from the part’s surface, promoting a passive oxide layer. This finish is essential in applications where maintaining the material’s corrosion resistance is crucial.

PVD Treatments

Physical Vapor Deposition (PVD) treatments involve the deposition of thin, durable coatings onto MIM parts. These treatments can impart various properties, including enhanced hardness, wear resistance, and improved aesthetic appeal. PVD treatments offer a versatile range of finishes, making them highly sought after in the MIM industry.

Design Consideration of MIM Parts

Avoid components over 12.5mm (0.5in.) thick. This is a function of MIM technology and alloy, for example 4140 and alloys that use carbonyl powder can have thicker wall sections than those that use gas-atomized powders that have larger particles. Also modifications to binder systems can be made to allow thicker sections to debind.

Avoid components over 100 g in mass; however, 300 g are possible for some technologies. l Avoid long pieces without a draft (2 degrees) to allow ejection. l Avoid holes smaller than 0.1mm (0.0039in.) in diameter.

Avoid walls thinner than 0.1mm (0.0039in.), although 0.030mm walls are possible in some cases.

Maintain uniform wall thickness; thin, slender sections attached to thick sections should be avoided to enhance flow during molding, to avoid sinks and voids, and to limit distortion during sintering.

Core out thick areas to avoid sinks, warpage, and debinding defects.

Avoid sharp corners. The desired radius is >0.05mm (0.002in.).

Learn the complete design guide here.