Development of Metal Powder Injection Molding Equipment

Development of Metal Powder Injection Molding Equipment

Metal Injection Molding (MIM) is the most rapidly developing field in powder metallurgy and industry in recent years. It is a new powder metallurgy near-net molding technology formed by combining modern advanced plastic injection molding technology with traditional powder metallurgy technology.

1、 MIM molding technology

The basic process of MIM is to uniformly mix fine metal or ceramic powder with organic adhesive to form a rheological material, inject an advanced injection machine into a mold cavity with the shape of a part to form a blank, and use new technology to remove the adhesive and sinter it to make it highly dense into a product. If necessary, post processing can also be performed. "The IHI technology not only has the advantages of conventional powder metallurgy technology such as high production efficiency, good product consistency, less or no cutting, and economic efficiency, but also overcomes the shortcomings of traditional powder metallurgy products such as low density, uneven material, low mechanical properties, and difficulty in forming thin-walled complex parts. It is particularly suitable for the production and processing of large quantities, small sizes, complex, and metal components with special requirements." Since the industrialization of this process technology in the mid-1980s, it has achieved rapid development. Injection molding products have been widely used in the computer information industry, automobile and motorcycle industry, medical and health equipment, household appliances, instrumentation, machinery manufacturing, chemical engineering, textile, national defense and military industries, and other fields. Up to now, hundreds of companies in more than 20 countries and regions have been engaged in the product development, research and sales of this process technology. As a result, powder injection molding technology has become the most actively developed cutting-edge technology field in the new manufacturing industry, known as the pioneering technology in the world's powder metallurgy field, representing the main direction of the development of powder metallurgy technology.

The main characteristics of this process are as follows:

(1) This process technology utilizes an injection molding machine to inject product blanks to ensure that materials fully fill the mold cavity, which ensures the realization of complex structure of the part. This is incomparable with traditional mechanical processing and conventional powder metallurgy technology, and is a strong foundation for the development of injection molding technology.

(2) Injection molding products have high dimensional accuracy, and injection molding processes can directly mold thin-walled and complex structural components. The shape of the product can already meet or approach the final product requirements, and the product does not require secondary processing or only a few finishing processes. The dimensional tolerance of parts is generally maintained at about ± 0.1% to ± 0.3%. Especially for reducing the processing cost of hard alloys that are difficult to machine and reducing the processing loss of precious metals, it is particularly important.

(3) Compared with traditional powder pressing processes, injection molded products have uniform microstructure, high density, and good performance.

2、 Necessity of continuous sintering equipment

With the large-scale industrialization of MIM technology, general production equipment and various specialized metal injection molding equipment in the traditional powder metallurgy and injection molding industries have been widely used in the industrial production of metal injection molding. The improvement of enterprise requirements for industrial production efficiency, equipment automation, processing continuity, and equipment performance has promoted the industrialization process of metal injection molding. The comprehensive development of the MIM industry requires more production equipment to improve the production efficiency of enterprises. Correctly selecting and mastering various equipment in the MIM production process can improve product quality, output, and labor productivity, and accelerate the development of industrialization.

Currently, the mixing process mainly uses traditional twin planetary mixers, single screw extruders, piston extruders, twin screw extruders, eccentric wheel mixers, and z-shaped impeller mixers, which can ensure the uniformity and efficiency of mixing.

The injection process can also draw on traditional injection equipment, such as dual loop injection molding machines, dual template injection machines, rod less injection machines, fully automatic injection machines, electromagnetic dynamic injection molding machines, etc., which can better meet the technical requirements of filling.

For the degreasing process, as degreasing is a field that has never been involved before in the relevant industry, its principle is: under the premise of ensuring that the parts obtained by injection molding do not deform, the various components in the adhesive gradually change into gaseous or liquid substances with the increase of temperature, using the principle of continuous physical and chemical changes, to remove the injection molding blank, in order to achieve the purpose of removing the adhesive. Therefore, the position of this process in the entire MIM technology is particularly special and important. The degreased parts have almost no strength, and a slight vibration can cause damage to the parts. At the same time, consider the degreasing and sintering stages to minimize the energy waste caused by repeated heating of parts, and consider integrating traditional single processes such as degreasing, sintering, and heat treatment into a comprehensive process. This can reduce uncertain factors in production, improve the quality of molded parts, and greatly improve production efficiency.

The concept of continuous sintering equipment was born with the introduction of comprehensive processes. In order not to defeat China in the fierce international competition and occupy a leading position in the international industry, it is necessary to actively develop MIM technology, especially to integrate and integrate traditional single processes to form effective integrated processes, and to carry out research and development of integrated processing equipment as soon as possible.

3、 Continuous sintering equipment and its control technology

A large number of thermal degreasing studies have shown that the key to thermal degreasing is to control the temperature of degreasing at a low temperature stage (150~350%) and slowly increase the temperature (1~C/min), without producing deformation or defects. Therefore, it is necessary to ensure that a true degreasing furnace has good temperature stability and uniformity. Compared to atmospheric thermal degreasing, vacuum thermal degreasing has a lower vacuum pressure, which is conducive to the volatilization of the binder and the elimination of decomposition products. Therefore, the degreasing rate is higher than atmospheric thermal degreasing under normal pressure. Due to this characteristic, there are significant differences between MIM degreasing and other related processes. Several brands of continuous sintering equipment on the market are introduced.

There are two types of sintering furnaces in terms of operation: vertical and horizontal. The disadvantage of vertical sintering furnaces is that they tend to be very uneven in temperature in the presence of atmosphere; There is also a temperature deviation between the curved end of the storage body of the horizontal sintering furnace and the internal temperature, which greatly reduces the quality of the sintered product.

The degreasing and sintering integrated furnace consists of the following six parts: the capture system, the vacuum system, the inflation system, the external circulation system, the electrical control part, and the vacuum control part. The furnace body adopts a sandwich water cooled structure, and the furnace liner is composed of a small rust steel corrugated outer insulation blanket, a zirconium blanket, a heating element, and a high-temperature resistant stainless steel corrugated inner insulation shield from the inside out. The internal heat shield can prevent the escape of lipid substances to other parts of the furnace body, and is convenient for cleaning. The furnace adopts an inner sealed door, which can effectively prevent the loss of heat and the escape of lipids. The trap system consists of a multistage water cooled disc trap, a degreasing tank, a multistage filter, and a starting valve. Lipid substances can flow smoothly into the degreasing tank. The vacuum system consists of a two-stage vacuum system. The rotary vane vacuum pump and the Roots pump can be selected and used according to the product material and the vacuum degree required for degreasing. The inflation system can be smashed through three glass rotations on the flowmeter to achieve wide flow regulation. The external circulation system is composed of sealed fans and heat exchangers, enabling rapid cooling. The electrical control system consists of a furnace temperature control system, a vacuum control system, an inflation control system, and a cooling circulation system. The actual temperature is measured by thermocouples and compared with the set temperature, and the current and equipment heating power are changed to achieve temperature control, allowing the three heating zones to rise simultaneously. During operation, vacuum thermal degreasing constantly introduces protective gas, forming a small pressure difference between the inner and outer furnaces, achieving one-way gas flow, effectively avoiding lipid contamination of the heating body and deformation of the inner furnace due to excessive temperature difference, With the continuous development of metal injection molding technology, the technical level of degreasing has become increasingly broad, with Germany developing a rapid catalytic degreasing technology. This technology requires high requirements for degreasing furnaces, requiring specialized acid resistant degreasing equipment, and environmental issues should be considered when designing furnaces. The strength of the parts after degreasing with this technology is very low and easily damaged (in fact, the strength of any degreased parts is not high); And before sintering, there will always be adhesive thorns remaining in the blank. In this case, reducing the intermediate links of the product plays a very important role in improving the product yield.

In order to achieve a truly continuous operation between the removal of adhesive, the removal of residual adhesive, and the sintering process, Germany has developed a MIM-MASTER catalytic debonding and sintering system. This system includes a catalytic debonding section, a continuous sintering section, and auxiliary devices, including exhaust gas burning, gas convection drying device, bypass conveyor belt, acid injection system, electrical control cabinet, and a whole process control system (PIC). The continuous catalytic degreasing section is designed as a muffle mesh belt structure using Ni-Cr heating elements. The metal injection molded parts are placed on a conveyor belt and heated to a certain temperature in the preheating zone, so that the acid does not condense on the workpiece when passing through the debonding belt. When passing through the debonding belt, the upper part removes the adhesive under the action of carrier gas (typically nitrogen) and catalyst (commonly used nitric acid). The flow direction of the atmosphere in the furnace is very important. In the preheating zone, the flow direction of the atmosphere is the same as the movement direction of the workpiece until it enters the exhaust gas combustion chamber. During the removal of the adhesive belt, the flow direction of the atmosphere in the furnace is opposite to the movement direction of the workpiece, ensuring that the parts that have basically removed the adhesive can encounter the highest concentration of acid. The size of the burning device of this furnace can be smaller than that of a batch furnace with the same production rate, because the exhaust gas is generated continuously in the middle of the entire removal process, and a large amount of exhaust gas will not be generated within a certain period of time, as in a batch furnace. The burning device is designed as a two-stage structure: in the first stage, fuel gas, such as natural gas, is used to interact with formaldehyde (one of the components of the exhaust gas) to burn under the condition of insufficient oxygen, Reducing oxides of nitrogen and residual nitric acid; In the second stage, the remaining formaldehyde and fuel gas are mixed with excess air and fully combusted to generate carbon dioxide and water. After passing through the degreasing furnace, metal injection molded parts are fed into a continuous sintering furnace through a sealed transverse conveyor belt. During the process of removing residual adhesive and sintering, parts should avoid vibration, so a specially designed walking beam transmission structure is adopted. The sintering section is mainly divided into three stages: heating up, sintering, and cooling. The heating section is responsible for removing the remaining adhesive and pre firing. Ni-cr coils are used as heating elements, with a general maximum temperature of 800 ℃. The sintering belt bears the main sintering function, and the heating element is wire, with a maximum temperature of up to l600oC. Metal powder injection molded parts are sintered in an inert or reducing atmosphere, and the exhaust gas generated during production is discharged after combustion through an exhaust stack located in the population section. The cooling belt is designed as a double wall water cooling structure, and the cooling water flow rate and temperature can be manually adjusted.

Although sintering quality is related to each process, the most important factor is determined by the uniformity of temperature and the stability of the sintering process. Therefore, sintering equipment used for metal powder injection molding is required to have excellent temperature uniformity to achieve isotropic shrinkage of MIM products, thereby reducing sintering deformation and improving product accuracy; The sintering furnace is required to have good sealing performance, low air leakage rate, and ensure the required temperature, pressure, and atmosphere to achieve densification of the sintering material; Accurate temperature and sensitive control are required to achieve stable batch production of MIM products. Moreover, the main problem with sintering furnaces currently produced in China is the low accuracy of temperature control, which makes it difficult to determine a stable production process during the production process. Continuous sintering furnaces produced in Germany are at the forefront of the industry in terms of control accuracy, but there are also drawbacks. Highly automated equipment requires very standardized operation. A slight error can delay the operation of the entire equipment, resulting in huge losses. In addition, the lipid waste materials generated during the degreasing sintering process are easily attached to various components in the furnace, which can also have a significant impact on the performance of the equipment. Overall, although the sintering furnace has also achieved the integration of degreasing and sintering, there are still problems such as insufficient flexibility in temperature control, unstable pressure in the preheating section between degreasing and sintering, and no consideration is given to the feasibility of integrating with subsequent heat treatment.

In summary, the ideal goals for continuous sintering equipment are:

(1) Integrating traditional single processes to achieve the integration of degreasing, sintering, heat treatment, and other processes. Adding a heat treatment function section to directly heat treat the parts after sintering can greatly save production costs, reduce production cycles, and ensure production quality.

(2) Realize flexible control of the temperature and product residence time in the degreasing area and high-temperature sintering area, which can meet the production needs of various products with different process requirements, and also improve the situation of delayed production due to inflexible control.

(3) Improve equipment automation control and self adjustment capabilities, improve equipment operation reliability, reduce operator labor intensity, and improve production efficiency.

4、 Conclusion

Based on the analysis of the MIM molding process and the characteristics of powder injection molded parts, it is necessary to integrate traditional single processes such as degreasing, sintering, and even post processing into a comprehensive process. The structure and control mode of the continuous sintering equipment are given.