Basic steps of metal injection molding

Basic steps of metal injection molding

The basic process steps of metal injection molding are as follows: first, select the metal powder and binder that meet the requirements of MIM, and then mix the powder and binder into a uniform feed at a certain temperature by appropriate methods. After granulation, the injection molding is carried out. The formed blank is sintered and densified into a final product after degreasing treatment.

1. MIM powder and milling technology

MIM has high requirements for raw powder, and the selection of powder should be conducive to mixing, injection molding, degreasing and sintering, which is often contradictory. The study of MIM raw powder includes: powder shape, particle size and particle size composition, specific surface, etc. Table 1 lists the properties of raw powder suitable for MIM.

Due to the fine requirements of MIM raw powder, the price of MIM raw powder is generally high, some of which are even 10 times the price of traditional PM powder. This is a key factor limiting the wide application of MIM technology. At present, the main methods of producing raw powder for MIM include carbonyl method, ultra-high pressure water atomization method, high pressure gas atomization method, etc.

2. Binder

Binder is the core of MIM technology. In MIM, the binder has two basic functions: enhancing fluidity to fit for injection molding and maintaining the shape of the green block. In addition, it should also have the characteristics of easy removal, non-toxicity, and reasonable cost. Therefore, a variety of binders have emerged. In recent years, it is gradually changing from the selection based on experience to the requirements for degreasing methods and functions of the binder, The development direction of the adhesive system should be targeted.

Binder is generally composed of low molecular component and high molecular component plus some necessary additives. Low molecular component has low viscosity, good fluidity and easy removal; The polymer component has high viscosity and strength to maintain the strength of the forming blank. The two are properly proportioned to obtain a high powder loading capacity, and finally a product with high precision and high uniformity will be obtained.

3. Mixing

Mixing is the process of mixing metal powder and binder to obtain uniform feed. Because the nature of the feed determines the performance of the final injection molding product, the mixing process is very important. This involves many factors such as the way and sequence of adding binder and powder, mixing temperature, characteristics of mixing device, etc. At present, this process step has remained at the level of relying on experience. An important indicator for final evaluation of the mixing process is the uniformity and consistency of the 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 the powder and binder may be separated due to the low viscosity. The shear force will vary according to the mixing method. Common mixing devices of MIM include double screw extruder, Z-shaped impeller mixer, single screw extruder, plunger extruder, double planetary mixer, double cam mixer, etc. These mixing devices are suitable for preparing mixtures with viscosity in the range of 1-1000Pa · s.

The mixing method is generally to add high melting point components for melting, then cool down, add low melting point components, and then add metal powder in batches. This can prevent the gasification or decomposition of low melting point components. Adding metal powder in batches can prevent the sudden increase of torque caused by too fast cooling and reduce equipment loss.

For the feeding method of different particle size powder matching, the Japanese patent introduces: first add the thicker 15-40um water atomized powder into the binder, then add the 5-15um powder, and then add the powder with the powder size ≤ 5um, so that the shrinkage of the final product will change little. In order to evenly coat a layer of adhesive around the powder, the metal powder can also be directly added to the high melting point component, and then added to the low melting point component, and then the air can be removed. For example, Anwar directly added PMMA suspension to the stainless steel powder and mixed it, then added PEG aqueous solution, dried it, and removed the air while stirring. O'Connor uses solvent to mix, first dry mix SA with powder, then add tetrahydrofuran solvent, and then add polymer. After tetrahydrofuran escapes in the heat, then add powder to mix, to obtain uniform feed.

4. Injection molding

The purpose of injection molding is to obtain the desired shape of the MIM green body without defects and with uniform particles. As shown in Figure 1, first heat the granular feed to a certain high temperature to make it flowable, then inject it into the mold cavity to cool down to obtain the required shape of a rigid body, and then take it out of the mold to obtain the MIM forming blank. This process is the same as the traditional plastic injection molding process, but because of the high powder content of the MIM feed, there are great differences in the process parameters and other aspects of the injection molding process. Improper control will easily lead to various defects.

5. Degreasing

Since the emergence of MIM technology, with the different binder systems, a variety of MIM process paths and degreasing methods have been formed. The degreasing time has been shortened from a few days in the beginning to a few hours now. In terms of degreasing steps, all degreasing methods can be roughly divided into two categories: one is two-step degreasing method. The two-step degreasing method includes solvent degreasing+thermal degreasing, siphon degreasing - thermal degreasing, etc. The one-step degreasing method is mainly the one-step thermal degreasing method. At present, the advanced method is the amaetamold method. Below are several representative MIM degreasing methods.

6. Sintering

Sintering is the next step in the MIM process. Sintering eliminates the pores between the powder particles and makes the MIM product fully dense or nearly fully dense. Due to the use of a large number of binders in the metal injection molding technology, the shrinkage during sintering is very large, and its linear shrinkage rate generally reaches 13% - 25%, so there is a problem of deformation control and dimensional accuracy control. Especially because MIM products are mostly shaped parts with complex shapes, this problem becomes more and more prominent. Uniform feeding is a key factor for dimensional accuracy and deformation control of final sintered products. High powder compaction density can reduce sintering shrinkage, and is also conducive to the sintering process and dimensional accuracy control. For iron-based and stainless steel products, there is also a problem of carbon potential control 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 to study the strengthening sintering technology of coarse powder billets. This technology is an important research aspect of metal powder injection molding at present.