Views: 0 Author: Site Editor Publish Time: 2025-09-16 Origin: Site
Metal Injection Molding (MIM) is an advanced process that combines plastic injection molding and powder metallurgy. Its core goal is to achieve mass production of complex metal parts through the synergistic effect of "metal powder + binder." It addresses the difficulty of traditional powder metallurgy in forming complex structures while also addressing the shortcomings of plastic injection molding's material properties. Since the 1970s, it has become a key technology for the manufacture of small, precision metal parts.
1. Core Principle: "Carrier-Assisted Molding + High-Temperature Densification"
MIM's fundamental logic is a two-step transformation: first, using a binder as a carrier, the metal powder acquires plastic-like fluidity, enabling easy injection into complex molds. Then, degreasing removes the binder, followed by high-temperature sintering to fuse and densify the metal powder, ultimately yielding high-performance metal parts.
Traditional powder metallurgy relies on die pressing, making it difficult to uniformly fill complex structures with micropores, thin walls, and curved surfaces. While plastic injection molding can achieve complex shapes, the finished product is made of a polymer material and cannot meet the required metal properties, such as strength and high-temperature resistance. MIM, through its "binder-coated metal powder" composite system, combines the molding flexibility of plastics with the ability to activate the material properties of metals through subsequent processing, ultimately achieving both complex structures and high performance.
II. Four Core Processes: Critical Transformations from Raw Materials to Finished Products
The MIM process can be condensed into four key steps, and control at each stage directly determines part quality.
1. Raw Material Preparation: Uniform Feeding is Essential
The first step is to mix the metal powder and binder in appropriate proportions to create the "feedstock." Metal powders must meet high purity, fine particle size (1-20μm), and a narrow particle size distribution. Common materials include stainless steel and titanium alloys. Fine powders enhance subsequent sintering density and reduce internal porosity. The binder, consisting of a high molecular weight polymer (such as polyethylene), a plasticizer, and a lubricant, accounts for 8%-15% of the feedstock weight. Its functions are to coat the powder, prevent agglomeration, impart fluidity, and maintain the shape of the green body after molding. Mixing must be performed at 100-200°C to ensure the binder is completely melted and evenly coats each powder particle. Inhomogeneous mixing can lead to material shortages and bubbles during subsequent injection, and the performance of the sintered part will also be affected.
2. Injection Molding: The mold is shaped and the prototype is created.
This step is similar to the logic of plastic injection molding: the feed material is fed into the injection molding machine barrel, heated and melted, and then the screw, at a high pressure of 50-200 MPa, pushes the melt into the mold cavity, filling micropores, threads, and other detailed structures. Water cooling is then applied to solidify the binder, and the mold is opened to remove the "green body" (undegreased body).
Mold precision must match part requirements (tolerances are typically within ±0.005mm). Temperature and pressure must be adjusted based on the feed material's fluidity. Too low a temperature will result in poor fluidity and chipping of the green body; too high a pressure may damage the mold or generate internal stress. 3. Debinding: Removing the Carrier to Preserve the Frame
The green body contains a large amount of binder. If sintered directly, the binder will rapidly decompose and produce gas, causing cracking. Therefore, debinding is required first to remove most of the binder, forming a "brown body" (debinded body). Common debinding methods include solvent debinding/catalytic debinding (which removes 50%-70%/92-98% of the binder quickly and minimizes deformation) and thermal debinding (using a debinding furnace with a slow temperature increase of 1-5°C/min to decompose the remaining binder and achieve complete debinding). In actual production, a combination of these two methods is often used.
After debinding, only 5%-50% of the binder remains in the brown body, forming tiny pores within the body (to allow for sintering shrinkage). However, the green body shape is still maintained, and the powder particles are initially connected by the residual binder, providing sufficient handling strength. 4. Sintering: High-temperature densification is key.
This is the "qualitative transformation" step in MIM: the brown billet is placed in a sintering furnace and heated to 70%-90% of the metal's melting point (for example, stainless steel sintering temperature is 1300-1450°C) in a protective atmosphere such as hydrogen or nitrogen (to prevent metal oxidation) or in a vacuum. The temperature is then maintained for several hours. At high temperatures, the atoms of the metal powder particles become active, fusing together through "diffusion welding." Simultaneously, the part shrinks by 10%-20% (allowing for this shrinkage during mold design), ultimately resulting in a metal part with a density exceeding 95% (or even approaching 100%).
Sintering parameters require precise control: too low a temperature or insufficient insulation will result in incomplete powder fusion and poor part strength. Too high a temperature or rapid heating will lead to deformation and coarsening of the grains, which in turn reduces performance.
