Main Features and Components of High Performance MIM Vacuum Sintering Furnace

Metal Injection Molding technology (Metal Injection Molding, referred to as MIM) is the fastest-growing field in Powder Metallurgy and industry in recent years. It is a new type of powder formed by combining modern advanced plastic injection molding technology and traditional powder metallurgy technology. Metallurgical near net shape forming technology. 1. MIM molding technology The basic process of MIM is: uniformly mix fine metal or ceramic powder and organic binder to form a rheological substance, use advanced injection machine to inject mold cavity with part shape to form blank, new technology removes binder and After sintering, it becomes a highly dense product, and it can be post-processed if necessary. ihai technology not only has the advantages of conventional powder metallurgy technology, such as high production efficiency, good product consistency, less cutting or no cutting, and high cost efficiency, but also overcomes traditional powder metallurgy products with low density, uneven material, low mechanical properties, and is not easy to form thin Due to the shortcomings of complex wall parts, it is especially suitable for the production and processing of large-volume, small, complex and metal parts with special requirements. Since the industrialization of this process technology in the mid-1980s, it has developed by leaps and bounds. It has covered computer information industry, automobile and motorcycle industry, medical and health equipment, household appliances, instrumentation, machinery manufacturing, chemical industry, textile, national defense and military industry and other fields. So far, hundreds of companies in more than 20 countries and regions have been engaged in the product development, development and sales of this process technology, and the powder injection molding process technology has therefore become the most active frontier technology field in the new manufacturing industry. Known as the pioneering technology in the field of powder metallurgy in the world, it represents the main direction of the development of powder metallurgy technology. The main features of the process are as follows: (1) Parts with complex structures can be formed. This process technology uses the injection machine to inject the product blank to ensure that the material is fully filled with the mold cavity, which also ensures the realization of the complex structure of the part. This is unmatched by 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. The injection molding process can directly form thin-walled and complex structural parts. The shape of the product can meet or approach the requirements of the final product, and the product does not need secondary processing or only less finishing. The dimensional tolerance of parts is generally maintained at about ±0.1% to ±0.3%. It is especially important to reduce the processing cost of hard alloys that are difficult to machine, and to reduce the processing loss of precious metals. (3) Compared with the traditional powder pressing process, the injected product has uniform microstructure, high density and good performance. 2. The necessity of continuous sintering equipment With the large-scale industrialization of MIM technology, general-purpose production equipment in the traditional powder metallurgy and injection molding industries and various specialized metal injection molding: industrial production equipment has been widely used in the industrial production of metal injection molding. Enterprises' improvement of industrial production efficiency and equipment automation, continuous processing and equipment performance requirements has promoted the industrialization process of metal injection molding. The overall development of the MIM industry needs to improve the production efficiency of enterprises through production equipment. Correct selection and mastering of various equipment in the MIM production process can improve product quality, output and labor productivity, and accelerate industrialization development. At present, the mixing process mainly uses traditional double planetary mixers, single-screw extruders, piston extruders, twin-screw extruders, eccentric wheel mixers, z-shaped impeller mixers, etc., which can ensure the uniformity of mixing. Uniformity and high efficiency. The injection process can also learn from traditional injection equipment, such as double-circuit injection molding machines, double-platen injection machines, tie-bar-less injection machines, automatic injection machines, electromagnetic dynamic injection molding machines, etc., which can better meet the technical requirements of filling. For the degreasing process, since degreasing is a field that has never been involved in related industries before, the principle is: under the premise of ensuring that the parts obtained by injection molding are not deformed, use the various components in the binder to continuously increase the temperature. The principle of physical and chemical changes, gradually changing into gaseous or liquid substances, detached from the injection molding blank, in order to achieve the purpose of detaching the binder. Therefore, the position of this process in the whole MIM technology is particularly special and important. The degreased parts have almost no strength, and a slight vibration may damage the parts. At the same time, consider the degreasing and sintering stages to reduce the energy waste caused by repeated heating of parts as much as possible, and consider integrating the traditional degreasing, sintering, heat treatment and other single processes into a comprehensive process, which can reduce uncertain factors in production and improve the quality of molded parts , also greatly improved production efficiency. The proposal of the comprehensive process gave birth to the concept of continuous sintering equipment. In order not to make our country lose in the fierce international competition and occupy the leading position in the international industry, it is very necessary to actively develop MIM technology, especially to integrate and synthesize the traditional single process to form an effective comprehensive process , and conduct research and development on the integrated technology 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 degreasing temperature in the low temperature stage (150-350%) and slowly increase the temperature (1-C/min), without deformation or defects, so it is required that the degreasing furnace has good temperature stability and uniformity. Compared with atmospheric thermal degreasing, vacuum thermal degreasing has low vacuum pressure, which is beneficial to the volatilization of binder and the discharge of decomposition products, so the degreasing rate is greater than that of atmospheric degreasing under normal pressure. Due to this feature, there is a big difference between MIM degreasing and other related processes. Here are several brands of continuous sintering equipment on the market. Various sintering furnaces are divided into two types: vertical and horizontal in terms of operation methods. The disadvantage of the vertical sintering furnace is that the temperature is very uneven in the presence of the atmosphere; the curved end of the horizontal sintering furnace body is also There is a deviation between the temperature and the internal temperature, which greatly reduces the quality of the sintered product. The integrated degreasing and sintering furnace is composed of the following six parts: capture system, vacuum system, gas charging system, external circulation system, electrical control part and vacuum control part. The furnace body adopts interlayer water-cooling structure, and the furnace is composed of small stainless steel corrugated outer heat insulation felt, zirconium felt, heating element and high temperature resistant stainless steel corrugated inner heat shield from the inside to the outside. The inner heat shield can prevent lipids from escaping to other parts of the furnace body, and at the same time, it is easy to clean. The furnace adopts an inner sealing door, which can effectively prevent the loss of heat and the escape of lipids. The capture system consists of a multi-stage water-cooled disc trap, a degreasing tank, a multi-stage filter and a start valve. Lipid substances can be smoothly whitened: received and flowed into the degreasing tank. The vacuum system consists of a two-stage vacuum system. The rotary vane vacuum pump and Roots pump can be selected according to the product material and the vacuum degree required for degreasing. The inflation system can be made by three glass rotors on the flow meter to achieve wide flow adjustment. The external circulation system is composed of a sealed fan and a heat exchanger, which can realize rapid cooling. The electrical control system consists of a furnace temperature control system, a vacuum control system, an air filling control system, and a cooling circulation system. The actual temperature is measured by a thermocouple and compared with the set temperature, the current and the heating power of the equipment are changed to realize temperature control, so that the three heating zones can be heated at the same time. The furnace forms a small pressure difference to realize the one-way flow of gas, which effectively avoids the lipid pollution of the heating element and the deformation of the inner furnace due to excessive temperature difference, and achieves the purpose of degreasing. With the continuous development of metal injection molding technology, its The technical level is also getting wider and wider, among which Germany has developed a rapid catalytic degreasing technology. This technology has high requirements on the degreasing furnace, and requires special acid-resistant degreasing equipment, and environmental protection issues should be considered when designing the furnace. The strength of the blank of the part after degreasing by this technology is very low, and it is very easy to damage (in fact, the strength of the blank after any degreasing is not high); and there will always be sticking thorns remaining in the blank before sintering. In this case, reducing the intermediary link of the product plays a very important role in improving the product yield. In order to realize the real continuous operation between removing the binder, removing the remaining binder and sintering process, Germany developed the MIM-MASTER catalytic debonding and sintering system. This system includes catalytic debonding part, continuous sintering part and its auxiliary devices, including waste gas burning, gas convection drying device, bypass conveyor belt, acid injection system, electrical control cabinet and whole process control system (PIC). The continuous catalytic degreasing part is designed as a muffle mesh belt structure and uses Ni-Cr heating elements. Metal injection molded parts are placed on a conveyor belt and heated to a temperature in a preheated belt so that acid does not condense on the workpiece as it passes through the debinding belt. When passing through the binder removal belt, the upper part releases the binder under the action of carrier gas (generally 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 that of the workpiece until it enters the waste gas burning device. When the binder strip is removed, the gas flow direction in the furnace is opposite to the movement direction of the workpiece, so that the parts that have been basically removed from the binder can encounter the highest concentration of acid. The size of the incineration device of this furnace can be smaller than that of the batch furnace with the same production rate, because the waste gas is continuously generated in the middle of the whole removal process, unlike the batch furnace, a large amount of waste gas is generated within a certain period of time, and its combustion The exhaust gas removal device is designed as a two-stage structure: the first stage is fed with fuel gas such as natural gas, which works together with formaldehyde (one of the exhaust gas components), burns under the condition of oxygen deficiency, and reduces nitrogen oxides and residual nitric acid; In the second stage, the remaining formaldehyde and fuel gas are mixed with excess air and fully burned to generate carbon dioxide and water. After the metal injection molded parts pass through the debinding furnace, they are fed into the continuous sintering furnace via a sealed transverse conveyor belt. Parts should avoid vibration during the process of removing the remaining binder and sintering, so a specially designed walking beam transfer structure is used. The sintering part is mainly divided into three sections: heating, sintering and cooling. The heating section is responsible for removing the remaining binder and pre-firing, using Ni-cr coil as the heating element, and the maximum temperature is generally 800°C. The sintering belt undertakes the main sintering function, the heating element is wire, and the maximum temperature can reach l600oC. Metal powder injection molding parts are sintered in an inert or reducing atmosphere, and the waste gas generated during production is burned out by an exhaust chimney located in the entrance section. The cooling belt is designed as a double-wall water-cooling structure, and the cooling water flow rate and cooling water temperature can be adjusted manually. Although the sintering quality is related to each process, the most important thing is the uniformity of temperature and the stability of the sintering process. Therefore, the sintering equipment used for metal powder injection molding is required to have very good temperature uniformity, so that the MIM product can achieve isotropic shrinkage, thereby reducing sintering deformation and improving product precision; the sintering furnace is required to have good sealing performance and low air leakage rate. Ensure the required temperature, pressure and atmosphere to achieve densification of the sintered mass; require accurate temperature and sensitive control to achieve stable mass production of MIM products. Moreover, the main problem of the sintering furnaces currently produced in China is that the temperature control accuracy is not high, so it is difficult to determine a stable production process in the production process. The continuous sintering furnace produced in Germany is at the forefront of the same industry in terms of control accuracy, but there are also disadvantages. The highly automated equipment requires very standardized operation. A slight error will delay the operation of the entire equipment, causing huge losses. In addition, the lipid waste generated during the degreasing and sintering process is easy to attach to various components in the furnace, which will also have a great impact on the performance of the equipment. On the whole, although the sintering furnace has also realized the integration of degreasing and sintering, there are still problems such as inflexible temperature control, unstable pressure in the preheating section between degreasing and sintering, and the integration with subsequent heat treatment is not considered. oral sex. In summary, the ideal goals of continuous sintering equipment are: (1) Integrate the traditional single process to realize the synthesis of degreasing, sintering, heat treatment and other processes. Adding a heat treatment function section and directly heat treating the parts after sintering can greatly save production costs, reduce production cycles, and at the same time better ensure production quality. (2) Realize the flexible control of the temperature in the degreasing area and the high-temperature sintering area and the residence time of the product in the area, so as to meet the production needs of various products with different process requirements, and also improve the delay in production due to inflexible control . (3) Improve the automatic control and self-adjustment capabilities of equipment, improve the reliability of equipment operation, reduce the labor intensity of operators, and improve production efficiency. 4. Conclusion Through the analysis of the MIM molding process and the characteristics of the powder injection molding parts, it is necessary to integrate the traditional degreasing, sintering and post-processing into a comprehensive process, and the structure and control mode of the continuous sintering equipment are given .