Metal Injection Molding (MIM) Technology for Tiny Gears

1 Micro gear MIM production process and parameter selection
Figure 1 is a schematic diagram of the MIM process, and Figure 2 is the experimental selection method of the process parameters and the main parameters of the MIM technology for mass production of a small gear.

2 Selection of metal powder and binder
The particle size of the metal powder used in the MIM process is generally 0.5-20 μm. In theory, the finer the particles, the larger the specific surface area, which is easier to shape and sinter. At present, the main methods of producing powder for MIM are: water atomization method, gas atomization method, and base method. Each method has its own advantages and disadvantages: the water atomization method is the main powdering process. It has high efficiency and economical large-scale production. It can make the powder finer, but the shape is irregular, which is conducive to shape retention. There are more binding agents, which affect the accuracy. In addition, the oxide film formed by the high-temperature reaction of water and metal hinders sintering. The gas atomization method is the main method for producing powder for MIM. The powder produced is spherical, with low oxidation degree, less binder required, good formability, but high price and poor shape retention. The powder produced by the base method has high purity and extremely fine particle size. It is most suitable for MIM, but it is limited to Fe, Ni and other powders, which cannot meet the requirements of a variety of materials. In order to meet the requirements of MIM powder, many milling companies have improved the above methods, and have also developed micro-atomization, laminar atomization and other milling methods. The selection of powder should be comprehensively considered from the aspects of MIM technology, product shape, performance, and price. Nowadays, water atomized powder and gas atomized powder are usually mixed. The former increases the tap density and the latter maintains shape retention. Because the gear is used in a corrosive environment, water atomized 316L stainless steel powder is used. Its chemical composition (mass fraction) is Cr: 17.0%, N: 11.5%, Mo: 2.2%, C: no more than 0.3%, Fe: about 69%. Its physical properties are shown in Table 1.

In the process of MIM, the binder plays a very important role, it directly affects the mixing, injection molding, degreasing and other processes, and has a great impact on the quality of the injection molded blank, degreasing and dimensional accuracy, and alloy composition. The adhesives used in MIM include thermoplastic systems, thermosetting systems, water-soluble systems, gel systems, and special systems. They each have their own advantages and disadvantages. The thermoplastic adhesive system is the mainstream and leader of MIM adhesives. Binders are used less frequently. Although this type of binder has good shape retention, it is difficult to remove. Here, the binder adopts a thermoplastic binder with a formula of 70% paraffin wax and 30% high-density polyethylene.

3 Mixing, granulation and injection molding
After the powder and binder are determined, they must be mixed. Mixing is a complicated process to improve powder fluidity and complete dispersion. Commonly used mixing devices include twin-screw extruders, Z-shaped impeller mixers, double planetary mixers, etc. The continuous mixing process is currently being developed. The feeding rate, mixing temperature, rotation speed, etc. during mixing will affect the mixing effect. Here, the powder and binder are mixed on a dual planetary mixer for 1.5 h at a 63:37 loading (volume fraction), and the mixing temperature is 130±10°C. The powder and binder are fully mixed and then mixed in the single Pelletizing on the screw extrusion device, the pelletizing temperature is 130℃-150℃, and the screw rotation speed is 40 r/min. Use TMC60EV injection molding machine for injection molding. One of the key issues of injection molding is the design of molding, including product design and mold design. Although the products currently produced can range from 0.003 g to 200 g, and important progress has been made in improving accuracy, most designs, especially mold design, are based on experience and lack reliable design knowledge. CAD systems are difficult to apply well. MIM. The principle of plastic molds has been used to gradually standardize MIM molds. With the accumulation of experience, the time for mold design and production will be greatly reduced, and multi-cavity molds are used as much as possible to improve injection efficiency.

The purpose of injection molding is to obtain a defect-free formed blank of the desired shape. The injection defect cannot be eliminated in the subsequent process, so this step must be strictly controlled. Ultrasonic testing technology can detect the internal defects of the injection molded blank. The defect control in the injection stage is currently mainly operated by experience. With the advancement of science and technology, the use of computer to simulate the injection molding process of feeding, and to link it with the feeding performance, optimize the injection condition parameters and eliminate injection defects is the current advanced experimental method and the future development trend. There are reports abroad on the application of moldflow to the analysis of MIM injection process, and good results have been obtained. We also tried to apply this technology, but found that the simulation results did not match the experimental results very well. This aspect needs further research.

4Degreasing and pre-sintering
The degreasing method adopts thermal degreasing. The thermal degreasing process should be reasonably determined according to the thermal decomposition characteristics of the binder components. At the same time, it is necessary to prevent defects such as bubbling and cracking of the degreased body due to the fast degreasing speed. As stainless steel powder is very sensitive to carbon content, it is necessary to choose a reducing atmosphere to prevent residual carbon from the decomposition of the binder. In the temperature range from room temperature to 200°C, the decomposition of paraffin is mainly the binder. Medium paraffin is the most important component, so in order to successfully remove paraffin, the heating speed is generally lower than 1℃/min. The degreasing furnace of this process is in a hydrogen atmosphere. The degreasing temperature is below 200℃ and the heating rate is 0.8℃/min. When the temperature reaches 200℃, the temperature is kept for 1.5 hours, and then the temperature is increased to 450℃ for h at a rate of 1.5℃/min. , To remove the binder of the polymer component high-density polyethylene, and form a continuous hole. After 450℃, use 4℃/min to quickly raise the temperature to 800℃ and keep it for 45 minutes to completely decompose the polymer component in the binder, and complete the degreasing and pre-sintering of the blank.

5 sintering
Sintering is carried out in a vacuum sintering furnace with a vacuum degree of 0.1 Pa,

The sintering process is as follows: the temperature rises to 1000°C at a heating rate of 4°C/min, the temperature is kept for 45 minutes, and the sintering temperature is quickly raised to 1 380 ±10(°C) at 6°C/min, and the temperature is kept for 45 minutes. Room temperature. The sintering temperature should be as stable as possible. The sintering temperature fluctuates by tens of degrees Celsius, which can cause the sintering density to fluctuate by 10% and the shrinkage rate to change by 3%.

The dimensional accuracy and mechanical properties of the final product:

For finished parts (as shown in Figure 3), metallographic analysis and mechanical performance tests were carried out on the standard specimens prepared with the parts. The metallographic structure of the part is pure austenite, and its mechanical performance test results: the yield strength is 220 MPa, the tensile strength is 510 MPa, and the elongation is 45%.

Take any 10 pieces and measure the average density to be 98.8% of the theoretical density. Basically reached the theoretical performance index, meet the use requirements. The structure and size meet the accuracy requirements, and no processing is required.