Metal powder injection molding

Powder injection molding is a new part forming technology in powder metallurgy industry, which is a combination of traditional powder metallurgy and plastic forming. Whether it is metal or ceramic, it can be used to produce high density, high performance and complex near-final shape products in large quantities by injection molding. It has been widely used in computer hardware, office machinery, medical equipment, aerospace, military and other industrial fields.

The basic process of powder injection molding process is to mix small special powder (about 10μm) and polymer bonding agent accounting for about 35%~50% of the volume content into a rheological slurry, and then like plastic injection molding machine injection molding, and degreasing steps to remove the bonding agent. Finally, after sintering at 1200~1370℃ for 30~120min, the relative density of 90%~100% products are obtained. During sintering, the size shrinkage of the product can reach 30~50%.

 

Compared with powder molding technology, powder injection molding has the following characteristics: ① the forming is not limited by axial pressing, and can form precision parts with complex geometric shapes, and the dimensional accuracy can be as high as ±0.1%; ② High density, excellent mechanical properties; ③ Wide application range, can be used in metals, ceramics, intermetallic compounds and other materials. The disadvantage is that the removal of the adhesive is difficult and the cost is high.

 

In the powder injection molding process, powder injection molding is the most critical step, because the possible defects in the injection molding products are basically formed in this step, such as cracks, pores, welding lines, delamination, uneven density, powder and adhesive separation phenomenon. This requires the correct selection of injection molding parameters in the production (including injection temperature, injection speed, injection pressure, pressure holding pressure, pressure holding time and mold temperature, etc.) and reasonable mold design. Previous process parameters are obtained by experiment, high cost, long time, and because of the influence of random factors in the experiment process, it is difficult to effectively analyze the main causes of forming blank defects, and it is difficult to realize the optimal combination of injection molding process parameters and control the formation of forming blank defects. Computer simulation has a wide application prospect in this respect, which can greatly reduce the process design time, reduce the scrap rate of molds and products and reduce the cost. Determining the rheological behavior of polymer binder powder mixture during injection molding is the key to popularize the powder injection process by using computer simulation technology. The rheological behavior of powder mixture is very complicated, which is affected by bonding agent, powder content, temperature, shear force, wetting effect between powder and bonding agent, etc.

 

The constitutive equation of viscous band flow can solve the conservation relationship between temperature, mass and momentum equations in fluid flow. For example, the finite difference method can solve the powder flow during mold filling and the continuous temperature gradient change during cooling.