MIM Process

MIM process is a metal parts manufacturing process for complex structure with high precision. This molding process combines metal powder, injection molding and sintering technologies together, in order to guarantee MIM parts can be complex with tight tolerance as well as excellent surfaces. In addition, select metal powder with various size and shape and complement additives will create different MIM parts chemical and physical properties, binder component for every powder particle will determine final form rigidity.

Our main step of MIM process including: Feedstock Mixing and Granulating, Injection Molding, Debinding, Sintering.

Feedstock Mixing and Granulating

Feedstock production is the first step in MIM manufacturing process, extraordinary fine metal powder (usually smaller than 15 microns) are mixed with binder component, which consist of primary paraffin material, secondary thermoplastic polymer and other usage materials. In this step, all very fine elemental or prealloyed metal powders will get extensive characterization by uniformly coated with binder, in order to achieve the flow character required in the next injection molding process.
Unlike standard powder metalurgy, which can achieve only 80-90% of theoretical density, MIM results in 95-100%. This means we can achieve close tolerances and reduce costs by producing small, complex parts over high production runs.

Injection Molding

This step is MIM parts molding in conventional molding machine,

Feed pellets are fed into the molding machine barrel through a hopper by gravity.
Then binder in feedstock will be heated by heaters in the barrel, this will make feedstock melted into toothpaste consistency.
Reciprocating screw will force molten material into a two-part mold through opening gates.
Raw parts with fully formed geometry will be ejected from mold once cooled

Normally, we call raw part as green part, which is still composed of same proportion of metal powder and binder as feedstock. But because of binder expanding, its dimension will be 20% (binder volume dependent) larger than the finished MIM part.

We also can easily add special design features like undercut or cross holes during this step, which is not feasible in molding process, but convenient by machining or secondary operation way.

Debinding

Debinding or binder removal step is a controlled process to remove most of the binder and leave enough binder for backbone, which will hold parts size and geometry of completely intact. This process is performed chemically (catalytic debinding) or thermally, in some case solvent or water bath will be the initial step. The debinding method depends on processed material type, physical and metallurgical properties and chemical composition. The parts after debinding process is referred as brown part.

Sintering

Sintering is commonly processed slightly below the liquids/solidus temperature, this also called solid-state sintering. In this temperature, metal powder will be atomic motive for metal particle bonding, to form final natural density parts. As we know, metal or alloy sintering temperature is below melting temperature. In sintering process, brown parts are stage on ceramic setters, and then placed into bath or continuous furnaces with protective or vacuum atmosphere. A precise temperature profile will be controlled and monitored to increase gradually to a specific temperature.

Secondary Operation

Secondary operation is used to improve final parts material properties, achieve tight tolerances, enhance a cosmetic surface, or assemble additional components. MIM parts can use machining, tapping, drilling, broach, grounding or welding operation same as wrought counterparts. Heat isostatic and heat treating will improve MIM parts strength, hardness and wear resistance. Standard coloring and plating can be applied without surface preparation, because of MIM part’s low interconnected porosity.

Post-sintering Operation

Zhuorui post-sintering operation including:

Coining

Force sintered components to conform rigid mandrel, substrate to straighten, in order to guarantee desired MIM parts flatness and dimensions. This process will reduce spread dimensions for proper size features.

Machining

All common machining methods can apply to sintered MIM parts, like add threads, undercuts, grooves, ultra-tight tolerances, or special features.

Heat Treating

Sintering process leave MIM parts in an annealed state, heat treatment can adjust high carbon ferrous alloys hardness and other properties. Precipitation hardened stainless steels need cycles of heat to optimize its mechanical properties. Sometimes, heat treatment are incorporated into sintering cooling cycle.

Hot Deformation

Sintered parts will be heated and deformed by rapid forging stoke, in order to ensure proper size and density. Steel strength will increase from 500Mpa with sintering to 720Mpa with hot deformation.

Surface Carburization

Carbon is an important element to attain high strength steel, utilize heating cycle with methane atmosphere will attain high surface hardness on MIM parts. Surface carburization results in dimensional precision loss, so we need to trade off MIM parts surface hardness and dimensional accuracy.

Joining

Sintered parts can be joined together by welding, brazing or adhesive techniques as other metallic components. MIM parts metal materials behave same as standard metals, laser welding is very effective for MIM stainless steels.

Surface Treatments

MIM parts can be applied by various surface treatment as polishing, coating, painting, cleaning, anodizing, plating, sealing, and laser glazing. Electroplating is applied to improve MIM parts aesthetics and corrosion resistance. Nickel electroplate is perfect for instrumentation, firearm and magnetic components.

Hot Isostatic Press (HIP)

The MIM parts are typically 96-98% density after sintering process. HIP treatment will increase density to 100%. The sintered component will be loaded into a sealed chamber, then heat the chamber and introduce argon gas. The heated gas create pressure to collapse internal porosity and compress MIM parts.

In summary, the MIM process is a highly sophisticated and versatile manufacturing method that offers numerous advantages for producing complex, high – precision metal parts. It combines the precision of injection molding with the material integrity of sintering, starting from the crucial feedstock mixing and granulating stage where fine metal powder is uniformly coated with binder to ensure proper flow in the subsequent injection molding step.

The injection molding phase enables the creation of parts with complex geometries, and the subsequent debinding process carefully removes the binder while maintaining the part’s shape. Sintering then densifies the parts, achieving a high level of density and mechanical properties.

After sintering, a variety of secondary and post – sintering operations are available. These operations, such as coining, machining, heat treating, hot deformation, surface carburization, joining, surface treatments, and Hot Isostatic Press (HIP), further enhance the part’s properties, dimensional accuracy, and surface finish.

In conclusion, the MIM process provides manufacturers with the flexibility to produce parts tailored to specific requirements across different industries. Understanding each stage of the MIM process is essential for optimizing production, ensuring high – quality output, and fully leveraging the potential of this advanced manufacturing technology.