Metal Injection Moulding

Topics Covered

Background

The Process

Applications

Effect of Raw Materials

Controlling Carbon Content

Post-Sintering Operations

Design Considerations

Shape Restrictions

Dimensional Tolerances

MIM and Large Scale Production

Economics of MIM

Summary

Background

Metal injection moulding (MIM) is a relatively new manufacturing process for the large-scale net shape forming of high integrity, multifunctional metal parts. It combines the geometrical complexity possible from injection moulded plastic with the mechanical properties of high performance metallic alloys. MIM embraces the best practices of injection moulding and powder metallurgy, and because machining operations are reduced or eliminated, there are real associated economic advantages. Components can be produced in a wide range of metals - low alloy steels, stainless steels, wear or heat resistant steels, and soft magnetic irons - and can be subjected to secondary treatments.

The Process

In the MIM process metal powders such as carbonyl iron or fully-alloyed, inert gas atomised powder are mixed with thermoplastic binders and plasticisers, to obtain a homogeneous feedstock. The mean diameter of the primarily spherical powder grains is in the range of about 5-15µm. At this point, the powdered metal represents 50-70% of the total volume of the mixture. This feedstock can be fed as thermoplastic material into inline injection moulding machines, working at temperatures between 100°C and 250°C. Moulded parts exhibit all the geometric features of the finished article, apart from the fact that they have an enlarged volume. These enlarged ‘green’ bodies have sufficient rigidity for them to be handled by automated ‘pick-and-place’ equipment. In the subsequent debinding stage, the binder and plasticisers are mostly removed - the sintering operation that follows removes the last traces. Binder removal is usually performed by thermal decomposition and evaporation, chemical decomposition, or extraction with liquid chemicals.

Sintering takes place in vacuum furnaces or in continuous working furnaces under protective atmosphere. During sintering, a linear shrinkage takes place affecting all dimensions of the MIM part. The parts are then ready to be assembled or to undergo secondary operations such as surface hardening or electroplating, according to their final specification.

Applications

A variety of complex shaped, multifunctional parts of high standard steels are made by metal injection moulding. MIM parts have been successfully introduced into the car industry, as well as in electrical hand tools, household machines, locking systems, measuring and control technology, precision mechanics, and other applications.

Effect of Raw Materials

The properties of powders, such as hardness, do not affect the MIM process - the rheology of the feedstock is the only limiting factor. Elementary powders may be used as well as fully alloyed powders. Due to the high sintering activity of the fine-grained powders, MIM offers a great potential for microstructure engineering. The sintering leads to very stable microstructures, which allow nearly all the secondary operations that are carried out on conventional steels.

Controlling Carbon Content

Carbon control during sintering is a challenge, as oversupply, of carbon can occur due to the binder system. For steels with very low or high carbon content, the problem may be overcome by using reactive or neutral atmospheres respectively. Close attention is required when processing medium carbon steels, e.g. with a controlled carbon content of 0.1-0.5%.

Post-Sintering Operations

A wide range of secondary operations can be used to optimise the material properties to suit the service conditions for the part. Typical operations are:

•        Heat treatment - through hardening or case hardening

•        Surface coatings

•        Hot isostatic pressing to full density.

A range of MIM steels have been developed by Schunk Sintermetalltechnik GmbH, table 1. Depending on the heat treatment conditions, a tensile strength of up to 1600MNm^-2 can be reached. A breaking elongation of up to 14% at a lower tensile strength level can be achieved with low alloyed steels. Due to the closed pores, case hardening can be done when the base carbon content is controlled at low levels within narrow tolerance bands. On material MP5-0012 with a carbon content of 0.2± 0.05%, a surface hardness of 750 HV0.1 and a bulk hardness of 400 HV0.1 have been observed. On the high-alloyed austenitic stainless steel AISI 316L, a tensile strength of about 500MNm^-2 and a breaking elongation of about 50% has been found in the as-sintered state. The mechanical properties of MIM steels cannot be altered by means of heat treatment, so they are therefore better compared with wrought steels rather than with die pressed (conventional powder metallurgy) steels.

Table 1. Chemical composition of steels produced by MIM,

Design Considerations

To manufacture high quality MIM parts with small size tolerance bands, it is important to apply some basic roles for their design. The injection moulding process allows a large amount of freedom with respect to geometry. In particular, the quasi-hydrostatic conditions during mould filling allow the use of complex tooling. Two or more directions of demoulding may be possible, which allows the construction of undercuts, as well as non-circular breakthroughs. Threads are also possible if high precision is not required.

Shape Restrictions

Shape restrictions are caused by the mould filling conditions as well as by the demoulding forces. To avoid distortion during debinding and sintering, the parts are ideally of a shape that enables them to be placed flat on the sintering plates, although parts of complex shape can be fully supported in ceramic powder. Production costs may restrict the weight of the parts because of the relatively high price of the raw material. The wall thickness of MIM parts should be in the range of 0.3-5mm. The weight of the parts is typically in the range of 0.1-100g. Some design rules for MIM parts are given in figure 1. In general, it is a good idea to contact the producer of MIM parts at an early stage of the concept phase, to optimise the design with respect to the part's function as well as to the MIM process.

Dimensional Tolerances

A frequently discussed aspect of MIM is the necessary width of the size tolerance bands of MIM parts. In an industrial large-scale production process, various effects influence tolerance bands, so the MIM process has to be carefully controlled in each step of production. The most critical process parameters are the composition and rheology of feedstock, the injection moulding parameters, and the heat treatment conditions during debinding and sintering. Possible distortions during debinding, as well as during sintering, have to be kept in mind. Quality assurance during production has provided information about the size tolerance bands needed for MIM. Measuring 1000 parts produced during one week led to a near-normal distribution of values for the overall length and the height of an actuator of the soft magnetic alloy MP-S-0018. Calculations indicated that a tolerance band of ±0.5% relative to the nominal dimension is needed for this part. A comparable value has been found for micro-gears of the austenitic stainless steel MP-S-0004. A tolerance band of ±0.5% of the nominal dimension can therefore be regarded as a good guideline for designing MIM parts.

MIM and Large Scale Production

The MIM process has particular features which render it suitable for large scale production:

•        Feedstock production is possible in kneading machines in large batches.

•        The moulding process uses in-line injection moulding machines that are similar to those used in plastics industries.

•        The process parameters of the injection moulding machine are fully controlled by microcomputers.

•        Depending on the design of the part, multicavity tools can be used - tools with up to 20 cavities are known.

•        The moulding process and the subsequent handling of green parts can be automated using handling robots.

•        The debinding and sintering can be done in batch-type furnaces - up to 50,000 parts per batch for smaller parts - or in continuous working furnaces.

•        Loss of raw material can be almost completely avoided by using hot runner systems or by recycling the sprue.

The combination of freedom in design with the features of a typical large-scale production gives an indication of the fields of successful application of MIM parts, especially with respect to economic conditions. Figure 2 shows typical fields of application correlated with the production quantity as well as with the geometric complexity.

Economics of MIM

From the economic point of view, machining is a competitive technology only when the part is not geometrically complex. If the parts become more complex, forgings, die-castings and especially die-pressed PM parts will be more economic. However, these technologies have a certain demand of production quantity, because of the tooling costs. Precision casting is a very competitive technology for very complex parts in small and medium quantities. However, for large quantities there are some restrictions, because of limitations in the automation of the process. The discussion above emphasises how MIM enables the production of smaller, very complex shaped, multifunctional parts, with excellent material properties, under very good economic conditions.

Summary

Metal injected moulded (MIM) parts are finding use in an ever-increasing number of industries, ranging from office equipment to industrial machines, from medical appliances to household goods. The annual growth rate in Europe has been predicted at 20-30% - a figure that seems likely to be exceeded as industry becomes more aware of MIM.

Primary author: Henri Cohrt

Source: Materials World, Vol. 7 no. 4 pp 201-03, April 1999

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