Metal injection molding
Introduction of PIM
Powder injection molding (PIM) technology can be broken down into two parts:
- metal injection molding (MIM),
- ceramic injection molding (CIM).
The different stages of the MIM process can be broken down into several steps, the preparation of the feedstock, the injection stage, the debinding stage and the last stage which is sintering.
Step 1: Preparation of the feedstock:
The first step of the MIM process is the elaboration of the feedstock. This initial step is the elaboration of a two-phase mixture between the metal/ceramic powder and the thermoplastic binders. It is obtained by extrusion or by using a twin screw mixer (Figure 3). For most raw materials, the best compromise is to use a volume loading rate of 60% powder and 40% binder.
Step 2: Injection molding // Injection molding
The forming phase of the parts, i.e. molding, is carried out on injection molding machines (Figure 3). The “green” parts thus obtained are thus composed of a mixture of metal or ceramic and polymer (Figure 4).
Step 3: Debinding
The debinding step consists in the progressive elimination of organic compounds present inside the green components. There are two ways to get rid of them, the first being the use of a solvent that will dissolve and extract the polymer from the sample, and the second is the thermal degradation of the polymer (Figure 6).
Step 4: Sintering
The last operation on the component is the complete densification of the powder. It is obtained by heating the component at a sufficiently high temperature (below the melting temperature of the metal powder) and maintained for a certain time. As there is about 40% of binder in the green part, it is normal to have a shrinkage during the sintering phase for which the residual rate of closed porosities will be about 1-2%. This shrinkage is of the order of 20% and is dependent on the binder content of the injected mixture.