A simple concept, a complex process
The idea to plastify powdered raw materials with the help of thermoplastic additives and subsequently use injection moulding to form complex components was first developed for ceramic components.
Fig. 2 The Metal Injection Molding process
In the 1970s this process was developed to allow the processing of metal powders by Raymond Wiech in the US, widely considered the inventor of the Metal Injection Molding process. The flow diagram in Fig. 2 shows the sequence of processing steps.
The principal technological problems that had to be solved before the MIM process could be industrialised included:
- Production of a homogeneous powder-binder mix with a high metal powder loading and sufficient viscosity for injection moulding
- Development of economical binder removal processes providing shape retention
- Sintering to high density and dimensional accuracy.
1: Preparing the feedstock
The primary raw materials Metal Injection Molding are metal powders and a thermoplastic binder. The binder is only an intermediate processing aid and must be removed from the products after injection moulding. The properties of the powder determine the final properties of the Metal Injection Molded product.
The blended powder mix is worked into the plastified binder at an elevated temperature using 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 plastic injection moulding industry.
Feedstock can either be purchased “ready to mould” from a number of international suppliers, or it can be manufactured in-house by a MIM producer if the necessary skills and knowledge are available.
2: Injection moulding
The ‘green’ MIM parts are formed in an injection moulding process equivalent to the forming of plastic parts. The variety of part geometries that can be produced by this process is similar to the great variety of plastic components.
3: Binder removal
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 later in this introduction. 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 the evaporation of 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, MIM parts exhibit a substantial shrinkage during sintering. The linear shrinkage is usually as high as 15 to 20% (Fig. 3). If required, sintered MIM parts may be further processed by conventional metalworking processes such as heat treatments or surface treatments in the same way as cast or wrought parts.
For certain applications, such as the automotive, medical and aerospace sectors, Hot Isostatic Pressing (HIP) can be used to completely remove any residual porosity. As MIM parts are typically small, this can be relatively cost effective for critical components.
Fig. 3 MIM part a) as moulded, b) after binder removal, c) after sintering (Courtesy BASF AG, Germany)