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Metal Injection Molding (MIM)

… a revolutionary molding process for
the production of intricately shaped metal
micro and small parts.

 

Tools

A very important prerequisite for drawing
conformal MIM parts is an absolutely
precise and abrasion resistant injection tool.

  Metal Injection Molding is a novel method for the production of
metallic micro and small parts. The method combines two well-known
production technologies, which until today had nothing to do with
each other. These technologies are
  1. Plastic injection molding
  This technique has been known for many years and is used in the field of the production of complex shaped plastic parts. In particular, the use of sliding split moulds enables the forming of undercuts. Plastic pellets are melted in the plastic injection moulding machine and then, while in a liquid state, pressure injected into a mould. The part sets (solidifies) in the mould and can subsequently be removed as the finished article. Plastic injection moulding has revolutionised moulding in the field of plastics, and cannot be ignored in industrial manufacture today.  
  2. Metal powder sintering (P/M process)

 

 

The technology of sintering has also been used for many years, to produce metal parts of many various alloys of metal powders. Metal powder is power pressed into a mould, whereby a shaped part build up from the powder is formed. This part is then removed from the mould and subjected to heat treatment, which causes the metal particles to fuse into a whole via diffusion processes. Thereby a metallic part is formed which corresponds to that of the compresion mould. P/M is an economic process for the production of relatively simply shaped metal parts, and is well recognised in industrial production.  
  Metal Injection moulding combines both technologies
  First, an injectionable starting material is prepared, consisting of plastic and very fine metal powder, containing more than 90% by weight of metal powder. Like plastic, this material is processed into moulded parts with a conventional plastic injection machine (plastic injection moulding). After injection, the total plastic content which made the shaping possible is leached out of the moulded parts without the parts themselves losing their shape. In a further step the parts are sintered, thereby obtaining metallic properties. Parmaco works with the Parmatech-Process, which has been in industrial use to great success for the last 15 years in the USA.  
  Advantages of the MIM process
  Excellent shaping possibilities
  Very complex-shaped parts can be manufactured without or with only very little secondary finishing. Undercuts, which are not possible with conventional sintering processes, can be realised with MIM without problems. One particular advantage of the Parmatech process is the extremely accurate reproduction of even the finest mould cavities. The extremely flowable material also allows the forming of sharp edges.  
  Very good material properties
  In contrast to conventional compression and sintering, which usually allows densities of 90% of the theoretical material density. The MIM process reaches densities of between 96% and 100% of the theoretical material density. Thereby, material properties are reached which largely correspond to those of parts processed entirely of metal.  
  Excellent surface quality
  The surface of MIM parts is far superior to that of precision cast parts. A roughness height of Ra 3.2 is reached without polishing. Thereby, finishing and polishing costs can be eliminated or substantially reduced.  
  Low tolerance limits
  The MIM process allows an accuracy of better than +/- 0.3% of the required dimensions. This means that with respect to extremely small parts, the accuracy can be in the range of hundredths of a millimetre. Calibrating, or secondary finishing may become necessary with parts of larger dimensions.  
  Material selection
  MIM parts usually do not have to be mechanically refinished. The harder it is for a material to be machined, the more advantageous the MIM process. In particular stainless steel, soft magnetic alloys, iron/nickel materials but also tool steel and special purpose alloys can be employed. Case hardening of MIM parts is also possible without problems and is often usual. All surface treatment processes such as nickel plating, hard chroming etc, can also be applied.  
  Economic efficiency
  The economic efficiency strongly depends on how much of the characteristics and possibilities of the process are required for the end use of the produced part. The process as such is by no means inexpensive, however it can enable enormous savings when used selectively. Whether the process is economic for a particular use, has to be examined/verifies, case by case. Thereby it is most important that the design engineer fully understands the possibilities of the MIM process. A part already designed for manufacture by a purching or stamping process is almost certainly more expensive when manufactured with the MIM process, if however, the MIM process is included initially while designing, the possibility of this process can enable the construction of functions of the part which make possible substantial simplifications of the whose constructional unit (assembly group). Often it is possible to reduce the number of parts required for a constructional unit, to eliminate the machining costs for conventionally manufactured parts or to substantially increase the quality of the constructional unit by selecting the appropriate material. All of this can lead to substantial cost savings.  
  Positive effects on the economic efficiency
of the MIM process:
 

– ideal shaping possibilities
– good mechanical properties
– low tolerance limits
– high surface quality

The MIM process is most economic where complex formed, small parts with high tolerance limits, good mechanical properties and high surfaces quality are required.

When, for example, precision casted parts which subsequently have to be machined, can be manufactured without machining, owing to the MIM process, a substantial economic advantage in favour of MIM can result. In particular when the parts are extremely small (less than 20g) which makes machining difficult and therefore raising costs.

When the P/M process is out of question because of the complexity of that part or because of material properties, the MIM process offers an alternative.

When mechanical machining is limited by the material, the shaping possibilities of the MIM process are especially in the foreground.

MIM is a process which fundamentally opens new possibilities for the design engineer. It's very unique characteristics enable it to be applied where other processes reach their limits due to costs or quality reasons.
Or where parts just cannot be manufactured by other process.

 
  Negative effects on the economic efficiency
of the MIM process:
 

– high material costs
– high process costs
– high mould costs

Compare to other cost elements, material costs are the more of consequence the heavier the partis. MIM works with very fine metal powders which accordingly are very expensive. This means that the MIM process is best used with small and extremely small parts, there, the material costs are of comparatively small importance. This disadvantage of high material costs can however be alleviated by the fact that with the MIM process the material can be utilised 100% with no loss whatever, due to the fact that the feedstock can be re-cycled. Usually, the upper weight limit of a parts is in the region of 20g the lower limit being ca. 0.03g.

The process costs also have a strong effect on the end price of MIM produced parts. Understandable, the larger the part is, the fewer will be able to fit the process furnace at one time. This means, as with the weight of the parts, that the costs per part are lower the smaller the parts are. The upper limit of part size is in the region of 400mm. Extremely small parts, with dimensions in the region of 10 to 15 mm are, however, on principle more suitable.

The relatively high initial costs of mould production (as with plastic injection moulding) lead to the necessity to realise a minimal production run. It is however, difficult to define a minimal number as the total costs, and not the isolated mould costs determine economic efficiency. But as a rule, numbers of 10'000 parts per year constitute a lower limit, which can, however, under particular circumstances, such as a very simply mould construction, be crossed.

 
  Construction criterion
 

As the MIM process fundamentally opens new possibilities for the construction of metallic small parts, it is imperative that MIM is included in the considerations already at the initial phase of the development. The more consequently a part is developed of the MIM process, the greater the functional and economic advantages of the end product will be.

Principally., the same guidelines apply to the design of MIM parts as that of plastic moulded parts. This means that thick cross-sections may have a negative effect on the dimensional accuracy, even when using the MIM process. The parts have to be designed in such a way that they can be de-moulded with slides or cores etc. Naturally, the less complicated the mould tool in, the more economical it is to build.

Parmaco will gladly advise you in the initial phase of new development with regard to the MIM process.