The basic process steps of metal injection molding are as follows: firstly, metal powder and binder meeting the requirements of MIM are selected, then the powder and binder are mixed into uniform feed at a certain temperature by an appropriate method, and injection molding is carried out after granulation, and the obtained blank is degreased and sintered to become the final product.
1.MIM powder and its milling technology
MIM requires higher raw material powder, and the choice of powder should be conducive to mixing, injection molding, degreasing and sintering, which is often contradictory. The research of MIM raw powder includes: powder shape, particle size and particle size composition, specific surface area, etc. Table 1 lists the characteristics of the most suitable raw material powder for MIM.
Due to the fine requirements of MIM raw powder, the price of MIM raw powder is generally high, and some even reach 10 times the price of traditional powder metallurgy powder, which is the key factor limiting the wide application of MIM technology at present. At present, the main methods to produce MIM raw powder are carbonyl method, ultra-high pressure water atomization method and high pressure gas atomization method.
2. Adhesives
Binder is the core of metal injection molding technology. In metal injection molding, the binder has two basic functions: enhancing fluidity, being suitable for injection molding and keeping the shape of green block. In addition, it should be easy to remove, pollution-free, non-toxic and reasonable in cost. For this reason, various adhesives have appeared. In recent years, it has gradually developed from relying solely on experience to designing binder system according to the requirements of degreasing method and binder function.
Adhesives are generally composed of low molecular components and high molecular components plus some necessary additives. Low molecular weight components have low viscosity, good fluidity and easy removal; The polymer component has high viscosity and high strength, and maintains the strength of the formed blank. A proper proportion of the two can obtain a higher powder loading, and finally a product with high precision and high uniformity can be obtained.
Step 3: Mix.
Mixing is the process of mixing metal powder with binder to get uniform feed. The mixing process is very important because the nature of the feed determines the performance of the final injection molded product. This involves many factors such as the way and order of adding binder and powder, mixing temperature, characteristics of mixing device and so on. At present, this process step is still at the level of relying on experience, and an important index to evaluate the quality of mixing process is the uniformity and consistency of the obtained feed.
The mixing of MIM feed is completed under the combined action of thermal effect and shear force. The mixing temperature should not be too high, otherwise the binder may decompose or two-phase separation of powder and binder may occur due to too low viscosity. As for the shearing force, it varies according to different mixing methods. The mixing equipment commonly used in MIM includes twin-screw extruder, Z-type impeller mixer, single-screw extruder, plunger extruder, double planetary mixer, double cam mixer and so on. These mixing devices are suitable for preparing mixtures with viscosity in the range of1-1000 pa s.
Generally, the mixing method is to add high melting point components to melt, then cool, add low melting point components, and then add metal powder in batches. This can prevent the gasification or decomposition of low melting point components, and adding metal powder in batches can prevent the torque from increasing sharply due to too fast cooling and reduce the equipment loss.
As for the feeding methods of powders with different particle sizes, Japanese patent introduces that coarse15-40μ m water atomized powder is first added to the binder, then 5-15μ m powder is added, and finally powder with fineness ≤ 5μ m is added, so that the shrinkage change of the final product is very small. In order to evenly coat a layer of binder around the powder, metal powder can also be directly added to the high melting point component, then the low melting point component is added, and finally the air is removed. For example, Anwar directly mixed PMMA suspension with stainless steel powder, then added PEG aqueous solution, dried, and then removed air while stirring. O 'Connor uses solvent mixing, first dry mixing SA with powder, then adding tetrahydrofuran solvent, then adding polymer, after heating, tetrahydrofuran escapes, and then adding powder to mix, so as to obtain uniform feeding.
4. Spray molding method
The purpose of injection molding is to obtain the required shape of MIM blank without defects and uniform particles. As shown in figure 1, firstly, the granular material is heated to a certain high temperature to make it have fluidity, then it is injected into the mold cavity, and after cooling, a blank with a certain rigidity in the required shape is obtained, and then it is taken out from the mold to obtain the MIM formed blank. This process is consistent with the traditional plastic injection molding process, but due to the high content of powder in MIM feed, there are great differences in process parameters, and improper control may easily lead to various defects.
degrease
Since the emergence of MIM technology, with the different binder systems, a variety of MIM process paths have been formed, and degreasing methods are also diverse. The degreasing time has been shortened from the first few days to several hours now. All degreasing methods can be roughly divided into two categories from degreasing steps: one is two-step degreasing method. The two-step degreasing method includes solvent degreasing+thermal degreasing, siphon degreasing-thermal degreasing and so on. One-step degreasing method is mainly one-step thermal degreasing method, and amaetamold method is the most advanced one at present. Here are several representative MIM degreasing methods.
6. Sintering
Sintering is the last step of MIM process, which eliminates the pores between powder particles and makes MIM products completely dense or nearly completely dense. Because a large number of binders are used in metal injection molding technology, the shrinkage during sintering is very large, and its linear shrinkage generally reaches 13%-25%, so there is a problem of deformation control and dimensional accuracy control. Especially because most MIM products are special-shaped parts with complex shapes, this problem has become more and more prominent. Uniform feeding is the key factor for the dimensional accuracy and deformation control of the final sintered product. The high tap density of powder can reduce sintering shrinkage, and is also beneficial to the control of sintering process and dimensional accuracy. For iron-based and stainless steel products, there are still carbon potential control problems in sintering. Due to the high price of fine powder at present, it is an important way to reduce the production cost of powder injection molding and an important research direction of metal powder injection molding to study the intensified sintering technology of coarse green compact.
Due to the complex shape and large sintering shrinkage of MIM products, most products still need post-sintering treatment after sintering, including molding, heat treatment (carburizing, nitriding, carbonitriding, etc. ) and surface treatment (fine grinding, ion nitriding, electroplating, shot peening, etc.). ).