What is powder metallurgy metalworking
MIM technology (Metal Injection Molding) is a modern technological process for the production of small metal parts. It is suitable for applications that require precise dimensional parameters, with an annual production volume of at least 5000 pieces. Nowadays it is possible to use a wide variety of metal materials - from structural steel and stainless steel to titanium or tungsten. MIM technology describes five key features: high quality of the finished surface, material flexibility, cost-effective production, complexity of the shape and reproducibility.
The process of MIM technology
MIM combines two commonly used methods - thermoplastic injection molding and conventional powder metallurgy. In classic powder metallurgy, fine metal powders are pressed into molds under high pressure and sintered at high temperatures (around 1300 ° C). The material is only connected during sintering, where lattice diffusion occurs. When using MIM technology, the mixture of fine metal powder and the binder is injected using transfer presses, which are almost identical to those used for thermoplastic injection. The starting material for MIM is a mixture of a binding agent and a fine metal powder, the so-called feedstock. The metal powder is produced by atomizing metal, the metal particle size is in the range from 5 to 20 μm. The binder forms about 10-20% by weight of the mixture. The binder component is formed by a polymer, the type of which the next step in the removal of the binder depends on. The binder is combined with the powder by kneading and mixing, and the resulting product is granulated. The feedstock is usually a commercially available product that is suitable for direct processing as it is supplied in the form of granules. Today there are a variety of metals on the market that are processed for MIM technology: iron alloys, nickel, alloy steel, titanium, tungsten, etc. The most common binders include polyethylene glycol (PEG), which is first drawn in water and then has to be thermally decomposed. Other binders used are paraffins, which have to be decomposed thermally, and also polyoxymethylene (POM), which has to be decomposed catalytically.
The processing of feedstock by injection is the same as for the injection molding of thermoplastics. In the heated injection unit of the press, the feedstock is plasticized by the action of friction and heat between 150 and 200 ° C. The molten material is injected into the mold through a nozzle. The cooled product is removed from the mold and taken out. The fragile product is called the "green body" at this stage in the process.
Handling of MIM parts after molding
Injection molding machines for processing metal powders require special screw geometry. In the case of processing hard metal, it is necessary to ensure protection against wear on the screw, screw tip, on the cylinder and the blocking of the return flow by coating. Abrasion of the press materials listed above would lead to contamination of the starting material. The mold for the part must be tempered evenly. When designing molds for MIM, it is important to consider the shrinkage of the product, which can be up to 30% of the original volume of the part, depending on the binder system.
As mentioned in the introduction, the method of removing the binder depends on the type of binder system. There are four basic types of binder removal: thermal binder degradation, catalytic binder decomposition, binder dissolution and gelation, followed by binder evaporation. In this step most of the binder is removed and the product is called a "brown body".
The final step is a sintering process that results in secondary binder removal, where bridges are created between the particles of the metal and the internal pores that were created during the binder removal shrink. Sintering is carried out in a controlled atmosphere (nitrogen, hydrogen, argon ...) and a sintering temperature range at the limit of the melting point of the metal. At this stage of the process it is necessary to pay attention to the product that is being placed in the oven. If the goods are not properly supported, deformation and warping will occur. Sintering is very important in terms of shrinkage, density and dimensional accuracy of the product.
When checking the quality of the product, the following parameters are monitored: defects on and below the surface, exact size, hardness and density. The resulting product of MIM technology is a part, the density of which is 95-99% of the theoretical density. It has mechanical and physical properties comparable to those of the products of metalworking processes. Products can be further refined through heat and surface treatments.
When is it appropriate to use MIM?
MIM is suitable for the production of components with a maximum weight of 200g, the achievable accuracy of the production process is ± 0.3%. Basic tolerances are defined in the ISO standard (ISO 7151). It is possible to produce parts with complex shapes, such as pressed parts, which in conventional methods are produced from several parts which are then pressed together under pressure. It is now possible to manufacture the product in one step. Products can have reinforcing ribs, blind holes, protrusions, points, and internal and external threads. It is also possible to make parts with different wall thicknesses in different places on the part, the minimum wall thickness is 0.2 mm. From an economic point of view, MIM is ideal for series production of orders from 5000 pieces / year. The price of complex components is much lower with MIM than with traditional processes such as metalworking or casting.
Today, products made with MIM can be found in almost every industry, from electronics, automotive to medical applications. The first parts produced were for the military industry, where this technology was used to produce parts for weapons. Metal housings for clockworks and their parts can be cited as an example of everyday use.
- GERMAN, Randall M a Animesh BOSE. Injection molding of metals and ceramics. Princeton, N.J., U.S.A .: Metal Powder Industries Federation, c1997, 413 p. ISBN 18-789-5461-X.
- PETZOLDT, Frank. Managing quality in the complete metal injection molding process chain. Powder injection molding. 2014, year. 8, No. 2, pp. 37-45.
- Catamold® base material for powder injection molding: Processing - Properties - Applications, technical information, BASF Aktiengesellschaft, 2007.
- Kinetics: Metal Injection Molding [on-line]. 2014 [cit. 2014-07-04]. Available at: http://www.kinetics.com/
- MIM Advantages. Indo-MIM [online]. [cit. 2014-07-04]. Available at: http://www.indo-mim.com/mimAdvantages.html
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