MIM (Metal Injection Moulding)

1. INTRODUCTION

Metal injection moulding (MIM) has over the past decade established itself as a competitive manufacturing process for small precision components which would be costly to produce by alternative methods.

It is capable of producing

• in both large and small volumes
• complex shapes
• from almost all types of materials including metals, ceramics, intermetallic compounds, and composites

Metal injection moulding (MIM) is a development of the traditional powder metallurgy (PM) process and is rightly regarded as a branch of that technology.

• The standard PM process is to compact a lubricated powder mix in a rigid die by uniaxial pressure, eject the compact from the die, and sinter it.

Quite complicated shapes can be and are regularly being produced by the million, but there is one significant limitation as regards shape.

• After compaction in the die the part must be ejected, i.e. pushed out of the die cavity.

  It will be obvious, therefore, that parts with undercuts or projections at right angles to the pressing direction cannot be made directly.

  That limitation is substantially removed by the metal injection moulding process developed during the last decade and now expanding rapidly.

The use of injection moulding for the production of quite intricate parts in a number of plastic materials has been known for many years, and most of us come into contact with them in some form or other every day.

One important feature of such parts is that they are relatively cheap.

However, for engineering applications these thermo-plastic materials have quite inadequate mechanical properties.

Some improvement is made possible by the use of solid fillers - ceramic or metal powders - but the real breakthrough occurred when it was found possible to incorporate a very high volume fraction of metal powder in a mix so that, instead of a filled plastic part, a plastic-bonded metal or ceramic part is produced.

Careful removal of the plastic binder leaves a skeleton of metal or ceramic which, although fragile, can be handled safely and sintered in much the same way as traditional die compacted parts.

After sintering densities of 95% or more are reached and the mechanical properties are, for that reason, generally superior to those of traditional PM parts.

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2. COMPARISON WITH COMPETITIVE TECHNOLOGIES

MIM (Metal Injection Moulding) is essentially a technology for producing complex shape parts in high quantities. If the shape allows the production of the part by, for example, conventional pressing and sintering, MIM would in most cases be too expensive.

However, if the required number of complex parts is higher than a certain amount MIM is cheaper than machining.

The effect of the volume production on cost shows that, for example, for the smallest part weighing 4.5g the cost per part falls from $1.4 for an annual production of 250,000 pieces to $0.2 for 3 million or more.

This figure also shows the influence of part size on the cost factor - the bigger the part the smaller is the gap between the cost of 250,000 and 3 million pieces.

• A typical competing process to MIM is investment casting and the table below compares the characteristics of parts produced by the two processes.

Table:Comparison of parts manufacturing processes in terms of shaping capabilities.

In regard to many features MIM comes out on top.

However this does not tell the whole story,and many shapes that are possible by MIM cannot be produced by other routes.

MIM certainly has advantages compared with investment casting in the case of high part numbers of castings, and of course in non-castable alloys.

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3. 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.
  • This eliminates the possibility of warpage that can result from non-uniform density in a die-compacted part.
  • The rheological properties of the feedstock, that is the powder/binder mix, are of major importance.
  • 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 (Courtesy of BASF, Germany)

Almost 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).

• 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.

• 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.

• 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.

There are two basic processes:

• Heating of the green compact to cause the binder to melt, decompose, and/or evaporate.
• 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.
• 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.

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

The attached table lists typical mechanical property data for a range of materials processed by MIM.

Comparison with wrought materials is not straightforward because data for identical compositions are not available, but the data in the table below are indicative.

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4. MIM PRODUCTS

Metal injection moulding (MIM) has over the past decade established itself as a competitive manufacturing process

• for small precision components which would be costly to produce by alternative methods.
• It is capable of producing in both large and small volumes
• complex shapes
• from almost all types of materials including metals, ceramics, intermetallic compounds, and composites.

Components made by MIM technology are finding new applications in industry sectors such as automotive, chemical, aerospace, business equipment, computer hardware, bio-medical and armaments.

MIM and Powder Metallurgy :Metal injection moulding (MIM) is a development of the traditional powder metallurgy (PM) process and is rightly regarded as a branch of that technology.

• The standard PM process is to compact a lubricated powder mix in a rigid die by uniaxial pressure, eject the compact from the die, and sinter it.
• Quite complicated shapes can be and are regularly being produced by the million, but there is one significant limitation as regards shape.
• After compaction in the die the part must be ejected, i.e. pushed out of the die cavity. It will be obvious, therefore, that parts with undercuts or projections at right angles to the pressing direction cannot be made directly.
• That limitation is substantially removed by the metal injection moulding process developed during the last decade and now expanding rapidly.

PLASTIC MATERIAL The use of injection moulding for the production of quite intricate parts in a number of plastic materials has been known for many years, and most of us come into contact with them in some form or other every day.

• One important feature of such parts is that they are relatively cheap.
• However, for engineering applications these thermo-plastic materials have quite inadequate mechanical properties.

METAL AND CERAMIC MATERIAL Some improvement is made possible by the use of solid fillers - ceramic or metal powders - but the real breakthrough occurred when it was found possible to incorporate a very high volume fraction of metal powder in a mix so that, instead of a filled plastic part, a plastic-bonded metal or ceramic part is produced.

• Careful removal of the plastic binder leaves a skeleton of metal or ceramic which, although fragile, can be handled safely and sintered in much the same way as traditional die compacted parts.
• After sintering densities of 95% or more are reached and the mechanical properties are, for that reason, generally superior to those of traditional PM parts.

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5. MIM WORK SEQUENCES ANIMATION

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Information courtesy of ARBURG GmbH

The production of an injection-moulded part from the feedstock is comparable with the injection-moulding of plastics. The binder component of the compound is melted in the injection unit and is again kneaded through the screw during dosage. It is then injected under high pressure into the cavity of the mould inserted into the clamping unit. After the feedstock has hardened there, the mould is opened by opening the clamping unit, the moulded part is ejected by the ejector and is picked up by a robotic handling unit.

Due to the fact that material and mould changing can be carried out manually in as short a time as under 20 minutes allows just-in time production in line with requirements. The wide range of automation possibilities means that uncomplicated series production of components made of metal powder is easily possible.

In general, a normal screw-type injection moulding machine consists of a clamping unit, an injection unit and a controller.

The mould, consisting of two halves, is securely fitted in the clamping unit. The clamping unit itself has a stationary platen, referred to as the fixed mounting platen, and also a moving mounting platen. When the clamping unit and therefore the mould is closed, the material can be injected. If the mould is opened due to the clamping unit being open, the moulded part can be removed

The machine's injection unit principally consists of the screw, which transports the compound and compresses it so that is free of bubbles, the heating system which controls the temperature of the compound, and the nozzle out of which the compressed and heated material is injected under pressure into the mould.

Finally, the controller coordinates all movement and production sequences of the powder injection moulding machine.

The injection moulding machines are equipped with computer controllers and monitors to allow ease of use.

All defined adjustment parameters can be saved on data mediums, thereby guaranteeing that executed production cycles can be reproduced. As early as during the production process, reject and good parts can be identified and automatically separated.

To inject powder materials, it is possible to use moulds with the features normally used for working plastics, such as sliding bars, core pulls, unscrewing units, cavity pressure transducers etc.

However, due to the abrasive properties of the powder / binder melts, attention should be paid to providing protection against wear (e.g. by way of special hardening or alloys).

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