From Feedstock to Sintering: An In-depth Look at the Core Processes of Metal Injection Molding

As a core preparation technology for high-precision complex metal components, Metal Injection Molding (MIM) has occupied an important position in aerospace, electronic communication, medical device and other fields by virtue of its **near-net-shape forming** advantage. Its entire process centers on a core sequence: feedstock preparation, injection molding, debinding, and sintering. The process control of each link directly determines the precision, density and mechanical properties of the components. Below is an in-depth breakdown of the core logic and key points of each stage. **Feedstock preparation** is the foundation of MIM, with the core goal of obtaining a homogeneous, stable "metal powder-binder" mixture with excellent fluidity. First, metal powders need to be precisely screened.

 Commonly used powders include stainless steel, titanium alloy and tungsten alloy, with particle size generally controlled within 10–20 micrometers. Spherical powder morphology is preferred, as it can enhance fluidity and molding density. The binder acts as a "carrier", formulated from polymer materials such as paraffin, polyethylene and polypropylene, together with additives, accounting for approximately 15%–25% of the mixture. During preparation, the powder and binder are fully stirred and fused in high-temperature mixing equipment at 150–200℃, ensuring that the powder is uniformly coated by the binder. The final product is granular feedstock ready for injection molding, whose homogeneity directly affects the defect rate of subsequent molded components. The **injection molding** stage realizes "green part forming", following a principle similar to plastic injection molding but tailored to the characteristics of metal feedstock. The feedstock is fed into an injection molding machine, heated and melted at 130–200℃, then injected into high-precision mold cavities under a pressure of 50–200 MPa. Mold design must reserve shrinkage allowance (usually 5%–20%) for subsequent debinding and sintering processes, while ensuring smooth cavity surfaces to prevent defects such as burrs and incomplete filling on the green parts. Injection parameters require precise matching: excessively high temperature may cause binder decomposition, while insufficient temperature results in poor fluidity; insufficient pressure leads to incomplete filling, whereas excessive pressure may damage the mold. Parameter combinations need to be optimized through multiple tests. **Debinding** is the critical link connecting molding and sintering, with the core objective of removing the binder from green parts to reserve pore channels for subsequent sintering. Residual binder will decompose and generate gas during sintering, causing defects such as porosity, deformation and even cracking of components. The debinding method is selected based on binder composition, with the combined process of **solvent debinding + thermal debinding** being widely adopted. First, the green parts are immersed in organic solvents to dissolve and remove soluble components (accounting for 60%–80% of the total binder content), reducing volume shrinkage in the subsequent thermal debinding step. Then, the parts are placed in a debinding furnace and heated gradually to 400–600℃ under inert gas protection to remove the remaining insoluble binder. The entire process requires strict control of heating rate (typically 5–10℃ per hour) to avoid stress cracking of green parts caused by excessive temperature differences between internal and external layers. **Sintering** is the core stage determining the final properties of components, through which porous debound parts are densified under high-temperature conditions. The debound parts are placed in a sintering furnace and heated to 70%–90% of the metal melting point (e.g., approximately 1350–1450℃ for stainless steel) in a vacuum or inert gas atmosphere. At high temperatures, atoms on the surface of metal powder particles become active, forming neck connections through diffusion and fusion, gradually eliminating pores and shrinking the parts to the designed dimensions. The sintering process requires precise control of heating rate, holding time and cooling rate: excessively fast heating may cause part deformation, insufficient holding time leads to incomplete densification, and rapid cooling can generate internal stress. Finally, the sintered components achieve a density of over 95% of the theoretical density, with mechanical properties close to those of forged parts. Some high-precision components also require post-processing, such as sizing, heat treatment and surface treatment, to further improve dimensional accuracy and surface quality. From feedstock preparation to sintering, the core of the entire MIM process lies in **precision control**. Deviations in any detail—including powder characteristics, binder ratio, injection parameters, debinding rate and sintering atmosphere—may affect component quality. It is this relentless pursuit of process details that makes MIM technology the preferred solution for mass production of complex metal components, continuously driving innovation and development in the field of high-end manufacturing.