What Is Metal Injection Molding?

Metal Injection Molding (MIM for short) is a new type of powder metallurgy near net forming technology that is extended from the plastic injection molding industry. The strength of the product is not high. In order to improve its performance, metal or ceramic powder can be added to the plastic to obtain a product with higher strength and good wear resistance. In recent years, this idea has evolved to maximize the solid particle content and completely remove the binder and densify the shaped body during the subsequent sintering process. This new powder metallurgy forming method is called metal injection molding.

Metal injection molding

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Metal Injection Molding (MIM for short) is a new type of powder metallurgy near net forming technology that is extended from the plastic injection molding industry. The strength of the product is not high. In order to improve its performance, metal or ceramic powder can be added to the plastic to obtain a product with higher strength and good wear resistance. In recent years, this idea has evolved to maximize the solid particle content and completely remove the binder and densify the shaped body during the subsequent sintering process. This new powder metallurgy forming method is called metal injection molding.

Chinese name

Metal injection molding

Foreign name

Metal Injection Molding

Field

Plastic injection molding industry

Molding method

Powder metallurgy

The basic process steps of metal injection molding are: first, select the metal powder and binder that meet the MIM requirements, and then use a suitable method to mix the powder and binder into a uniform feed at a certain temperature. After injection molding, the obtained preform is sintered and densified into the final product after degreasing.

1.MIM powder and milling technology

MIM has high requirements on raw material powders, and the choice of powders is conducive to mixing, injection molding, degreasing and sintering, which are often contradictory. Research on MIM raw material powders includes: powder shape, particle size and particle size composition, specific surface Etc. Table 1 shows the properties of the raw material powders most suitable for MIM.

Due to the very fine requirements of MIM raw material powder, the price of MIM raw material powder is generally high, and some even reach 10 times the price of traditional PM powder. This is a key factor that limits the widespread application of MIM technology. At present, the main methods of producing raw material powder for MIM are There are carbonyl method, ultra-high pressure water atomization method, high-pressure gas atomization method and so on.

2. Binder

Binder is the core of MIM technology. In MIM, the binder has the two most basic functions of enhancing fluidity to be suitable for injection molding and maintaining the shape of the green block. In addition, it should have easy removal, no pollution, and no toxicity. And reasonable cost, a variety of binders have appeared for this purpose. In recent years, from the experience alone to the design of binders based on the requirements of degreasing methods and the function of the binder, The direction of the system.

The binder is generally composed of a low molecular component and a high molecular component plus some necessary additives. Low-molecular components have low viscosity, good fluidity, and are easy to be removed; high-molecular components have high viscosity, high strength, and maintain the strength of the formed billet. The two are appropriately proportioned to obtain a high powder loading capacity, and finally to obtain a product with high accuracy and high uniformity.

3. Mixing

Mixing is the process of mixing metal powder and binder to obtain a uniform feed. Since the nature of the feed determines the performance of the final injection molded product, this process step of mixing is very important. This involves various factors such as the way and order of adding the binder and powder, the mixing temperature, the characteristics of the mixing device, and the like. This process step has been at a level based on experience, and an important indicator of the final evaluation of the mixing process is the uniformity and consistency of the obtained feed.

Mixing of MIM feeds is done under a combination of thermal effects and shear forces. The mixing temperature should not be too high, otherwise the binder may decompose or the two-phase separation of the powder and the binder may occur due to the too low viscosity. As for the magnitude of the shear force, it will vary depending on the mixing method. MIM commonly used mixing devices include twin screw extruder, Z-shaped impeller mixer, single screw extruder, plunger extruder, double planetary mixer, dual cam mixer, etc. These mixing devices Both are suitable for preparing mixtures with a viscosity in the range of 1-1000Pa · s.

The mixing method is generally to first add a high melting point component to melt, then lower the temperature, add a low melting point component, and then add the metal powder in batches. This can prevent the vaporization or decomposition of low-melting-point components, and the addition of metal powder in batches can prevent the rapid increase in torque caused by the rapid temperature drop and reduce equipment loss.

For the method of adding powder with different particle sizes, the Japanese patent introduces: firstly add the coarser 15-40um water atomized powder into the binder, then add 5-15um powder, and finally add the powder 5um powder. There is little change in shrinkage of the final product. In order to uniformly coat a layer of binder around the powder, metal powder can also be directly added to the high melting point component, and then the low melting point component is added, and finally air can be removed. For example, Anwar directly added the PMMA suspension to the stainless steel powder and mixed it, then added the PEG aqueous solution, dried, and then removed the air while stirring. O'connor uses solvent mixing, first dry-mix SA with powder, then add tetrahydrofuran solvent, and then add the polymer. After the tetrahydrofuran escapes under heat, add powder and mix to obtain a uniform feed.

4. Injection molding

The purpose of injection molding is to obtain a defect-free MIM shaped body with a uniform shape and a uniform flow of particles. As shown in Figure 1, the granular feed is first heated to a certain high temperature to make it fluid, and then it is poured into the mold cavity and cooled down to obtain a desired shape of a certain rigid body, and then it is removed from the mold. Take out the MIM formed blank. This process is consistent with the traditional plastic injection molding process, but due to the high powder content of the MIM feed, the injection molding process has great differences in process parameters and other aspects, and improper control is prone to various defects.

5. skim

Since the emergence of MIM technology, with the different binder systems, a variety of MIM process paths have been formed, and degreasing methods have also varied. The degreasing time has been shortened from the first few days to the current hours. From the degreasing step, all degreasing methods can be roughly divided into two categories: one is the two-step degreasing method. The two-step degreasing method includes solvent degreasing + thermal degreasing, siphon degreasing-thermal degreasing, and the like. The one-step degreasing method is mainly a one-step thermal degreasing method. At present, the most advanced is the amaetamold method. The following describes several representative MIM degreasing methods.

6 sintering

Sintering is the last step in the MIM process. Sintering eliminates the pores between the powder particles. Make MIM products reach full density or near full density. Due to the large amount of binder used in metal injection molding technology, the shrinkage during sintering is very large, and the linear shrinkage rate is generally 13% -25%, so there is a problem of deformation control and dimensional accuracy control. Especially because most of MIM products are complex shaped parts, this problem becomes more prominent. Uniform feeding is a key factor for the dimensional accuracy and deformation control of the final sintered product. High powder compaction density can reduce sintering shrinkage, and it is also conducive to the sintering process and dimensional accuracy control. For iron-based and stainless steel products, there is also a carbon potential control problem in sintering. Due to the current high price of fine powders, research on the sintering technology of coarse powder compacts is an important way to reduce the production cost of powder injection molding. This technology is an important research aspect of current metal powder injection molding research.

Due to the complex shape of the MIM product and the large sintering shrinkage, most of the products still require post-sintering treatment after sintering, including shaping, heat treatment (carburizing, nitriding, carbon-nitriding, etc.), surface treatment (finishing, ion nitrogen Chemical, electroplating, shot peening, etc.).