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Development history and technology of metal powder injection molding

time:2020-04-28Views:219 Author:Best Seiko

Metal powder injection molding technology (Metal Injection Molding, abbreviated as MIM) is a field of rapid development in the field of powder metallurgy and industry in recent years. It is a new type of powder metallurgy formed by the combination of modern advanced plastic injection molding technology and traditional powder metallurgy technology. Near-net-shape molding technology: 1. MIM molding technology: MIM's basic process is to mix fine metal or ceramic powder with organic binder into a rheological substance, and use an advanced injection machine to inject a mold with a part shape The cavity forms a blank, and the new technology removes the binder and sinters it to make it highly dense and become a product. It can also be post-processed when needed. This technology not only has the conventional powder metallurgy technology, high production efficiency, good product consistency, and less cutting Or without cutting, economical and efficient advantages, and overcome the shortcomings of traditional powder metallurgy products, such as low density, uneven material, low mechanical properties, and difficulty in forming thin-walled complex parts, especially suitable for large quantities, small, complex, and special requirements of metal zero The production and processing of components. This process technology has achieved rapid development since it was industrialized in the mid-1980s, and injection molded products have spread throughout the computer information industry, automobile and motorcycle industry, medical and health equipment, household appliances, instrumentation, In the fields of machinery manufacturing, chemical industry, textiles, national defense and military industry. So far, hundreds of companies in more than 20 countries and regions have been engaged in product development, development and sales of this process technology, and powder injection molding process technology has become The development of the new manufacturing industry is a very active frontier technology field, known as the pioneering technology in the field of powder metallurgy in the world, and represents the main direction of the development of powder metallurgy technology. The main characteristics of the process are as follows: (1) can form complex structures The process technology uses an injection machine to inject the product blank to ensure that the material is fully filled in 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. It is injection Strong foundation for the development of the molding process. (2) The injection molded 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 product requirements. The product does not require secondary processing or only a few. Precision machining. The dimensional tolerance of the 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 losses of precious metals. (3) )Compared with the traditional powder pressing process, the microstructure of the injected products is uniform, high density, and good performance. Second, the need for continuous sintering equipment: With the large-scale industrialization of MIM technology, the general production equipment of traditional powder metallurgy and injection molding industry and Various special metal injection molding: Industrial production equipment has been widely used in the industrial production of metal injection molding. Production efficiency and equipment automation, increased processing continuity and improved equipment performance requirements have promoted the industrialization of metal injection molding. The overall development of the MIM industry requires the use of production equipment to increase the production efficiency of enterprises. Correct selection and mastering of various equipment in the MIM production process can improve product quality, output and labor productivity, and accelerate industrial development. At present, the mixing process mainly uses traditional twin planetary mixers, single screw extruders, piston extruders, twin screw extruders, eccentric mixers, z-shaped impeller mixers, etc. Uniformity and high efficiency. The injection process can also draw on traditional injection equipment, such as double-loop injection molding machine, double-template injection machine, tie-bar-less injection machine, fully automatic injection machine, electromagnetic dynamic injection molding machine, etc., all of which can better meet the technical requirements of filling. For the degreasing process, since degreasing is an area that has never been involved in the related industry before, the principle is: on the premise of ensuring that the parts obtained by injection molding are not deformed, the various components of the adhesive are used continuously as the temperature increases The principle of physical and chemical changes gradually changes into a gaseous or liquid substance, and is released from the injection molding blank to achieve the purpose of removing the binder. Therefore, the position of this process in the entire MIM technology is particularly special and important. The parts after degreasing have almost no strength, and 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 the parts as much as possible, and consider integrating the single processes of traditional degreasing, sintering, and heat treatment into a comprehensive process, which can reduce uncertain factors in production and improve the quality of molded parts It also greatly improves production efficiency. The proposal of the integrated process gave birth to the concept of continuous sintering equipment. In order not to defeat China's fierce peer competition and occupy the leading position in the international industry, it is very necessary to actively develop MIM technology.It is necessary to integrate and integrate the traditional single process to form an effective integrated process, and R&D of comprehensive Shangyi equipment as soon as possible. Third, 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 (150~350% slow temperature rise (1~C/min, no cattle deformation or defects, Therefore, the true degreasing furnace is required to have good temperature stability and uniformity. Compared with atmospheric thermal degreasing, vacuum thermal degreasing has a low vacuum pressure, which is conducive to the volatilization of the binder and the discharge of decomposed products, so the degreasing rate is greater than that under normal pressure. Atmospheric degreasing. Due to this feature, MIM degreasing is very different from other related processes. Several continuous sintering equipment on the market are introduced. Various sintering furnaces are divided into vertical and horizontal operation methods. The disadvantage of the vertical sintering furnace is that it is easy to be very uneven in temperature in the presence of the atmosphere; the curved end of the furnace body of the horizontal sintering furnace also has a deviation between the temperature and the internal temperature, which makes the quality of the sintered product greatly reduced. Degreasing The sintering integrated furnace is composed of the following six parts: the capture system, the vacuum system, the aeration system, the external circulation system, the electrical control part and the vacuum control part. The furnace body adopts a sandwich water cooling structure, and the furnace liner is made of small rust steel corrugations from inside to outside The outer heat insulation felt, zirconium felt, heating element and high temperature resistant stainless steel corrugated inner heat insulation screen. The inner heat insulation screen can prevent the fat material from escaping to other parts of the furnace body, and it is easy to clean. The furnace adopts the inner sealing door, which can effectively Prevent heat loss and the escape of lipid substances. The trapping system consists of a multi-stage water-cooled disc trap, a degreasing tank, a multi-stage filter, and a start valve. The lipid substance can be smoothly whitened: it flows into Inside the degreasing tank. The vacuum system is composed of a two-stage vacuum system. Rotary vane vacuum pumps and roots pumps can be selected and used according to the product material and the vacuum degree required for degreasing. The inflation system can be dropped by three glass transfer meters Wide flow adjustment. The external circulation system consists of a sealed fan and heat exchanger, which can achieve rapid cooling. The electrical control system consists of furnace temperature control system, vacuum control system, inflation control, and cooling circulation system. The actual temperature is measured by thermocouple and Compared with the set temperature, the current and the heating power of the device are changed to achieve temperature control, so that the three heating zones heat up at the same time, and the vacuum thermal degreasing continuously introduces protective gas during operation, so that the inner and outer furnace chambers form a small pressure Poor, to achieve one-way flow of gas, effectively avoiding the deformation of the heating element and the inner furnace due to the excessive temperature difference caused by the contamination of the lipid substance, and the purpose of degreasing is becoming more and more extensive with the continuous development of metal injection molding technology. Among them, Germany developed a rapid catalytic degreasing technology. This technology has higher requirements for degreasing furnaces and requires special acid-resistant degreasing equipment. Environmental protection issues must be considered when designing the furnace. The strength of the parts after degreasing is very strong. Low, very easy to damage (in fact, the strength of any blank after degreasing is not high; and there will always be stick thorns remaining in the blank before sintering. This situation Next, the reduction of product intermediary links plays a very important role in improving product yield. In order to realize the true continuous operation of removing the binder, removing the remaining binder and the sintering process, Germany developed the MIM-MASTER catalytic debonding and sintering system. This system includes catalytic debonding section and continuous sintering section and its ancillary devices, including exhaust gas burning, gas convection drying device, bypass conveyor belt, acid injection system, electrical control cabinet and full process control system (PIC. Continuous catalytic degreasing section Designed as a muffle-type mesh belt structure, using Ni-Cr heating elements. Metal injection molded parts are placed on the conveyor mesh belt and heated to a certain temperature in the pre-heat belt, so that when passing the debonding agent belt, the acid will be It will not condense on the workpiece. When passing through the debonding agent belt, the upper part is carried out in the carrier gas (generally nitrogen and catalyst (commonly used nitric acid to remove the binder. The direction of the atmosphere flow in the furnace is very important. The movement direction of the workpiece is the same until it enters the exhaust gas burning equipment. When the binder is removed, 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 been removed of the binder can encounter a high concentration of acid. The size of the burning device can be smaller than the batch furnace of the same production rate, because the exhaust gas is continuously generated in the middle of the entire removal process, and unlike the batch furnace, a larger amount of exhaust gas will be generated in a certain period of time, which burns the exhaust gas The device is designed as a two-stage structure: in the first stage, fuel gas such as natural gas, etc., is combined with formaldehyde (one of the components of the exhaust gas to burn in the absence of oxygen, reducing nitrogen oxides and residual nitric acid; second stage The remaining formaldehyde and fuel gas are mixed with excess air and burned sufficiently to produce carbon dioxide and water. After the metal injection molded parts pass through the degreasing furnace, they are sent to the continuous sintering furnace through a sealed horizontal conveyor belt. The parts are removed from the remaining binder and sintering Vibration should be avoided during the process, so a specially designed walking beam transmission 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-burning, and uses Ni-cr coils as heating Components, the general high temperature is 800 ℃. The sintering belt assumes the main sintering role, the heating element is silk, and the high temperature can reach l600oC. The metal powder injection molded parts are sintered in an inert or reducing atmosphere. Exhaust gas is discharged from an exhaust chimney located in the population section after combustion. The cooling belt is designed as a double-wall water cooling structure, and the cooling water flow rate and cooling water temperature can be manually adjusted. Although the sintering quality is related to each process, it is mainly It is also determined by 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 achieves isotropic shrinkage, thereby reducing sintering deformation and Improve product accuracy; require good sealing performance of the sintering furnace, low air leakage rate, ensure the required temperature, pressure and atmosphere to achieve sintering density; require accurate temperature, sensitive control, and achieve stable mass production of MIM products. At present, the main problem of sintering furnaces produced in China is that the temperature control accuracy is not high, so that in the production process It is difficult to determine a stable production process. The continuous sintering furnaces produced in Germany are at the forefront of the same industry in terms of control accuracy, and they also have drawbacks. Highly automated equipment requires extremely standard operations. A slight error will delay the operation of the entire equipment, resulting in great losses. In addition, the lipid waste generated during the degreasing and sintering process is easily attached to the various components in the furnace, which will also have a great impact on the performance of the equipment. As a whole, although the sintering furnace has also achieved 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 no consideration is given to integration with subsequent heat treatment. Oral sex. In summary, the ideal goals of continuous sintering equipment are: (1 Fusion of traditional single process to achieve the integration of degreasing, sintering, heat treatment and other processes. Increase the heat treatment function section, directly heat treat parts after sintering, which can greatly save production Cost, reduce the production cycle, and at the same time can ensure the production quality. (2 Realize the flexible control of the temperature in the degreasing area and high temperature sintering area and the residence time of the product in the area, so that it can meet the production needs of various products with different process requirements. It can also improve the situation of delaying production due to inflexible control. (3Improve equipment automatic control and self-adjustment capabilities, improve equipment operation reliability, reduce operator labor intensity, and improve production efficiency. IV. Conclusion: Through MIM molding process The analysis of the characteristics of powder and injection molded parts should integrate the traditional degreasing, sintering and even post-processing into a single process, and give the structure and control mode of continuous sintering equipment.

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