Metal Injection Moulding (MIM) involves manipulating metal powders to behave like a plastic by mixing them with polymer binders to form a feedstock. This feedstock is used to injection mould net shaped, precision components. Moreover, parts are then thermally processed to remove the binder system and then sintered to a high-density metal component.
Preparing the Feedstock
Primary MIM raw materials include metal powders and a thermoplastic binder. The binder is only an intermediate processing aid and therefore must be removed from the products after injection moulding.
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 feedstock, which is granulated with granule sizes of several millimetres.
A custom-made tool is fitted into the moulding machine and the feedstock is injection moulded into it to produce a net shaped ‘Green’ component. 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 like the great variety of plastic components.
The moulded ‘Green’ component is transferred to the debind ovens where they are either thermally or solvent debound. Once debound the parts are very brittle and absorbent as only one binder remains. At this stage parts are referred to as a ‘Brown’ component.
The ‘Brown’ component then reaches the final stage of the process where it is sintered in a controlled atmosphere. Temperatures are taken to just below melting point of the material, allowing menisci to form between the particles. Controlled, uniform shrinkage of on average 17% occurs at this stage and the component densifies to its finished size, shape and also tolerance parameters. After sintering, the high density ‘White’ component has equivalent properties to wrought materials and can be heat treated, polished, plated or welded.
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.
Post Sintering Operations
Goudsmit UK’s experience and attention to detail enables us to produce quality precision components that don’t require costly secondary operations. The MIM process has a typical tolerance range of ±0.5%, depending on factors such as material, location of the injection point, thermal treatment and in general, the shape of the part. However, if requirements do exceed even our process capabilities, we work with our clients to develop the most cost-effective solution.
Dimensional tolerances of ±0.3% are common and machining is required for closer tolerances. MIM can produce parts where it’s difficult to professionally manufacture an item through other means of fabrication. Increased costs for traditional manufacturing methods essential to part complexity, such as internal and external threads, miniaturization, or identity marking, typically do not increase the cost in a MIM operation due to the flexibility of injection moulding.
Other design capabilities that can be implemented into the MIM operation include:
- Product codes
- Part numbers
- Date stamps
- Parts manufactured to their net weight reducing material waste and cost
The ability to combine several operations into one process ensures MIM is successful in saving lead times as well as costs, providing significant benefits to Goudsmit UK’s customers.