The ceramic injection moulding (CIM) sector is also experiencing similar growth, although at lower volumes. In addition, cemented carbides (hardmetals) are increasingly being processed via the powder injection moulding route. Powder Injection Moulding is the term used to cover the metal injection moulding, ceramic injection moulding and carbide injection moulding industries.

Here we focus on metal injection moulding processing technology, materials and properties, and give examples of applications. The processing of injection moulded ceramics and carbides follow very similar routes.

Metal Injection Moulding: an established and proven manufacturing technology

The idea to plastify powdered raw materials with the help of thermoplastic additives and subsequently using injection moulding to form complex components was first developed in ceramics technology. In the 1970’s this process was adapted to metal powders by Raymond Wiech in the USA. He is widely considered the inventor of the new metal forming process which was named metal injection moulding.

Since its beginnings it has developed into a well-established manufacturing technology. The flow diagram in Fig. 1 shows the sequence of processing steps.

The metal injection moulding process in brief

The raw materials for metal injection moulding are metal powders and a thermoplastic binder. The blended powder mix is worked into the plastified binder at elevated temperature in a kneader or shear roll extruder. The intermediate product is the so-called feedstock. It is usually granulated with granule sizes of several millimetres, as is common in the plastics injection moulding industry.

The ‘green’ metal injection moulded parts are formed in an injection moulding process equivalent to the forming of plastic parts. The variety of parts geometries that can be produced by this process are similar to the great variety of plastics components.

The subsequent binder removal process serves to obtain parts with an interconnected pore network without destroying the shape of the components. The types of binder removal processes applied are further explained below. At the end of the binder removal process there is often still some binder present in the parts holding the metal powder particles together, but the pore network allows to evaporate the residual binder quickly in the initial phase of sintering at the same time as sintering necks start to grow between the metallic particles.

The sintering process leads to the elimination of most of the pore volume formerly occupied by the binder.

As a consequence, metal injection moulded parts exhibit a substantial shrinkage during sintering. The linear shrinkage is usually as high as 15 to 20% (Fig. 2).

If required, sintered metal injection moulded parts may be further processed by conventional metalworking processes like heat treatments or surface treatments the same way as cast or wrought parts.

The pore structure of most metal injection moulded materials has a characteristic appearance as shown in Fig. 3. The residual porosity is usually less than 5% by volume and consists of very fine, isolated, almost spherical pores. At the part surface the porosity has been completely eliminated. Metal injection moulded parts are therefore impermeable for liquids and gases.

Injection moulding machines from the plastics industry

A wide selection of various sizes of fully automatic injection moulding machines, experienced mould makers, mould design software, parts handling devices and more are easily available at moderate cost.

Even the higher abrasiveness of metal injection moulding feedstock as compared to common thermoplastic materials is not a real problem as injection moulding can handle materials with even much more abrasiveness. Waste material after injection moulding such as runners and reject parts can be recycled by adding the crushed material to fresh feedstock.

3-D of all simulation assists with all stages of the metal injection moulding process

Computer simulation of the injection moulding process is far advanced and allows to study every detail of mould filling in 3-D. The design engineer can analyse the filling dynamics in thick and thin areas on his computer, study the effects of gate position, thermal gradients in the mould and phenomena such as jetting and binder segregation. One can optimise the kinetics of mould filling and determine shrinkage and distortion of the green compact relative to the mould dimensions.

A careful analysis and optimisation of the mould cavities can reduce design time and cost while significantly improving yield and quality. Simulation systems for the debinding and sintering stages are also well advanced.

Continue to next page: Binders and binder removal techniques in metal injection moulding