Talk:Metal injection molding
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Metal Injection Molding (MIM)can produce complex-shaped three-dimensional precision metal parts without any compromise in strength. This avoids most of the secondary operations that contribute to acting as time and cost barriers. Current applications encompass disc drive components, computer peripherals, automotive components, components for handguns, fire arm parts, tungsten cubes / spheres for various applications, jewellery, surgical tools, sporting goods, watch industry, musical instruments & instrumentation electronics.
MIM is a fast growing manufacturing method that bridges the manufacturing gap in the other metal working technologies / shaping process because of technology or cost. MIM can be a very cost effective method for producing large quantities of precision parts.
In fact, the main value of MIM comes from its inherent ability to manufacture an a limitless array of geometries in a variety of different alloys. In many cases, MIM is able to deliver equal or superior performance to investment casting or machining processes, & at a significantly lower cost. The design freedom of MIM makes it possible to form features that may not be feasible with other metal forming technologies.
The objective of MIM is to look at miniaturisation of mechanical assemblies, combining multiple parts, & reducing assembly operations. More efficient material usage, part consolidation opportunities, & reduced manufacturing steps to a net-shape component are all examples of savings MIM can deliver.
Merge with Powder Metallurgy
I wonder if this article should be merged with Powder Metallurgy as they seem to describe the same thing, but I am not sure. --Jrash 21:59, 26 March 2006 (UTC)
Leave it here
Actually - the process is rather different - although they are related processes. MIM mixes particles of metals (steel, stainless steel) with wax and thermoplastics and are injected into a mold using equipment similar to traditional plastic injection molding equipment. This article does need significantly more content however.--Blanco71 20:36, 14 September 2006 (UTC)
"''''I am leaving below a description of what MIM actually is.Also it is a newer process to make metal parts and is different from Powder metallurgy but in both cases metal powder is used as raw material''''"
[[ == THE MIM PROCESS ==]]
Fundamental requirements of MIM in each step of the process:· Metals Powders · Binders · Mixing · Moulding · De-binding · Sintering · Post-sintering operations · Mechanical prpperties of MIM components:
Introduction In the traditional PM process it is normal to produce after sintering a part having dimensions very close to those of the original compact.In this way it is not difficult to ensure close dimensional tolerances. With injection moulding, however, the situation is quite different. · The 'green' compact, as the as-moulded part is called, contains a high volume percentage of binder - as much as 50% - and during sintering a large shrinkage occurs. It is, therefore, a major requirement of the sintering process to ensure that this shrinkage is controlled. · In this regard, MIM has an advantage over conventional PM in so far as the density of the metal in the compact is, if the mix has been made correctly, uniform throughout and the shrinkage, though large, is also uniform. o This eliminates the possibility of warpage that can result from non-uniform density in a die-compacted part. o The rheological properties of the feedstock, that is the powder/binder mix, are of major importance. o The viscosity at the moulding temperature must be such that the mix flows smoothly into the die without any segregation, and the viscosity should be as constant as possible over a range of temperature. However, the mix must become rigid on cooling. These requirements dictate the properties of the binders used, and to some extent, the granulometry of the powder. Let us look first at the powders..
Carbonyl Iron Powder OM
Metal PowdersAlmost any metal that can be produced in a suitable powder form can be processed by MIM. Aluminium is an exception because the adherent oxide film that is always present on the surface inhibits sintering. The list of metals that have been used includes many common and several less common metals and their alloys - plain and low alloy steels, high speed steels, stainless steels, superalloys, intermetallics, magnetic alloys and hardmetals (cemented carbides). o However, the most promising candidates from the economic point of view are the more expensive materials. This is accounted for by the fact that, unlike alternative processes that involve machining, there is practically no scrap which helps to offset the high cost of producing the powder in the required form. Scrap is of lesser significance in the case of inexpensive metals. The term 'suitable powder form' deserves clarification, and it can be seen that the issue is not clear cut - there are conflicting requirements. Particle shape is important for a number of reasons:· It is desireable to incorporate as high a proportion of metal as possible, which means that powders having a high packing density are indicated. · Spherical or near spherical shape should, therefore, be preferred, but the risk of the skeleton going out of shape during the debinding process is increased: (there is no metallurgical bonding between the particles as happens in a die pressed compact). Average particle size and particle size distribution are also important:· Fine powders which, as is well known, sinter more readily than coarser powders would, therefore, seem to be desireable, but there are a number of limiting factors. The table below compares the different powder production techniques and their relative cost for MIM powders.
Ideal powder is said to be as follows: · tailored particle size distribution, for high packing density and low cost ( (mixture of lower cost large particles and higher cost small particles) · no agglomeration predominantly spherical (or equiaxed) particle shape sufficient interparticle friction to avoid distortion after binder removal, · probably a an angle of repose over 55 degrees small mean particle size for rapid sintering, below 20 micron dense particles free of internal voids minimized explosion · and toxic hazards clean particle surface for predictable interaction with the binder. In the real world, of course, the choice is restricted to what is available, but growing demand has stimulated a major effort by powder manufacturers to produce powders to meet the special requirements of MIM..
Mixing · Tumbler mixes - double cone mixers for example - such as are widely used for the dry blending or mixing of powders are of little use for MIM mixtures. For these it is necessary that a shearing action takes place. · Several different types are available: · Z blade and planetary mixers are examples. · A major objective is to ensure that the whole of the surface of each particle is coated with binder. As has been indicated earlier the least possible amount of binder should be used, but the appropriate volume ratio of binder to powder depends on the powder characteristics. In industrial practice, the ratio varies from about 0.5 to 0.7.
Moulding The machines used for this part of the MIM process are substantially the same as those in use in the plastics industry. · Here it is usual to convert the mix into solid pellets by a process referred to as granulation. These pellets can be stored and fed into the moulding machine as required. The screw from which the mix is extruded into the die cavity is heated and the nozzle temperature carefully controlled to ensure constant conditions. · The die temperature also is controlled - it must be low enough to ensure that the compact is rigid when it is removed. · A method of reducing the unit cost of parts is to use a mould with multiple cavities so that several parts are produced at each injection. To be worthwhile, however, the saving must be such that it more than offsets the increased cost of the mould. It is, therefore, more relevant when very large quantities of a particular part are to be produced.
De-binding The removal of the binder from the green part is a key stage of the process and one that requires most careful control.There are two basic processes: · Heating of the green compact to cause the binder to melt, decompose, and/or evaporate. o This must be done with great care in order to avoid disruption of the as-moulded part, and in this connection the use of binders with several ingredients which decompose or evaporate at different temperatures is advantageous. o The process normally takes many hours, the time being dependent, inter alia, on the thickness of the thickest section. The recent introduction of catalytic debinding of polyacetal MIM feedstock using gaseous nitric acid or oxalic acid has greatly reduced the time for debinding, and equipment has been developed whereby catalytic debinding and sintering can be executed on a continuous production basis. · The second debinding process applicable to certain binder systems only, is to dissolve out the binder with suitable solvents such as trichlorethane. Normally heating is required as a final step to complete the removal by evaporation.Other less commonly used binding processes use gelation, e.g. with mixtures of cellulose and gums, and freezing of an aqueous slurry containing also organic ingredients. During debinding the strength of the compact decreases markedly and great care is necessary in handling the 'brown' parts as they are called.
Sintering This is the name given to the heating process in which the separate particles weld together and provide the necessary strength in the finished product. · The process is carried out in controlled atmosphere furnaces - sometimes in vacuum - at a temperature below the melting point of the metal. · Sintering in MIM is substantially the same as that used for traditional PM parts. · Because it is essential to avoid oxidation of the metal, the atmospheres used are generally reducing. Apart from protecting the metal, such atmospheres have the further advantage of reducing any oxide existing on the surfaces of the powder particles. This surface oxide is, of course, greater in total the finer the powder and so is of greater significance in MIM than it is with traditional PM.· The exact composition of the sintering atmosphere used depends on the metal being sintered. For many metals a straightforward atmosphere containing hydrogen is all that is required, but in the case of steels which have carbon as an essential alloying element, the atmosphere must contain a carbon compound or compounds so that it is in equilibrium with the steel, i.e. it must neither carburise nor de-carburise the steel. · The fact that the powders used are very much finer in MIM than those used in PM means that sintering takes place more readily by reason of the higher surface energy of the particles. · As the 'brown' part is extremely porous, a very large shrinkage occurs and the sintering temperature must be very closely controlled in order to retain the shape and prevent 'slumping'. · The final part has a density closely approaching theoretical, usually greater than 97%, and the mechanical properties are not significantly, if at all, below those of wrought metal of the same compositions
Post-Sintering Operations The properties of MIM components can be improved by many of the standard processes that are applicable to wrought metals and/or PM components
