Andrew Almond of European Powder Metallurgy Association* explains the metal injection moulding process and its benefits.
What is metal injection moulding?
Metal injection moulding (MIM) is a development of the traditional powder metallurgy (PM) process and is rightly regarded as a branch of that technology. MIM technology consists of a chain process that involves moulding, debinding and sintering. Subsequent treatments can include machining, hardening, deburring or surface treatment. Each of these processes has an influence on the final properties, and some of these properties, which are influenced by all of the processes, can only be identified in the finished part.
MIM technology, with its many sub-processes, has hundreds of parameters that need to be controlled, from feedstock qualification, including the individual ingredients and their mixing, through to moulding, which contains many more parameters that need to be set correctly, and tool quality and longevity to maintain a stable moulding process. Furthermore, the debinding and sintering steps create a number of variables that need to be considered, such as time, temperature and atmosphere, to ensure optimal MIM parts are produced during the production run.
What materials can the MIM process use?
The MIM technique is a very versatile mass production method as almost any metal that can be produced in a suitable powder fo
rm can be used to manufacture MIM components. The main exceptions are aluminium and magnesium, due to the inability to sinter these materials. Stainless steel, low-alloy steels, tool and high-speed steels are widely used, with alternatives such as titanium, intermetallics and magnetic/cobalt/copper alloys can also be used with this process. Depending on the component and its intended application, this can result in specific mixes and blends being formulated, helping to tailor the components’ composition for additional benefits, such as weight reduction, porosity or increased wear resistance, for example.
What size of components can be made?
MIM is mainly used to produce smaller components, with the ideal part weights ranging from 0.2-30g. Although heavier or larger parts can be manufactured, there are some cost considerations, as the total cost of the powder is a linear function of the weight of the part, compared with machined solid bar stock, where the machining cost increases mainly with machining time and, to a lesser extent, with material utilisation. Parts that require a lot of time are better candidates for MIM than turned parts.
What are the benefits of using MIM components?
MIM components have been applied and used in a range of sectors over the years, from medical applications through to automotive and consumer goods. The main benefit of using them is the complexity of the part that can be manufactured, from narrow walls and internal geometries though to external or internal threads. These can be designed into the part in the initial production stages, which reduces the need for potentially expensive secondary operations. Additional to this level of complexity is the wide range of materials that can be used in production to offer consistent repeatability and high quality parts.
The future of MIM components
Over recent years the MIM process has been blended with emerging manufacturing techniques, to help push the boundaries of the components, materials and scope of applications. Bi-material and binder jet 3D printing are two examples of how MIM has adapted to meet the challenge of tomorrow’s components.
Bi-material MIM parts
Bi-material MIM parts are the direct combination of two materials with different properties in a single production step, which eliminates the need for a subsequent joining process. The variety of parts that can be manufactured ranges from hollow items with complex internal structures to flexible, non-detachable joints, and combinations of different materials in the same MIM part.
Binder jet 3D printing for prototyping and small-scale MIM production
Binder jet 3D printing is an additive manufacturing process in which powder material is deposited in thin layers and selectively joined according to CAD data, hence parts are generated without the need for special tools or moulds. Instead of melting the material, as it is done in selective laser melting or electron beam melting, in the binder jetting process the metal powder is joined by an adhesive ink and after thermal debinding either directly sintered to full density or partially sintered and infiltrated. These two process routes offer a wide choice of materials and flexibility in attainable properties.
Since the process chain for binder jetting and sintering to full density is generally similar to the MIM process, this technology holds the potential to be used in prototyping or small-scale production for MIM parts. A printed prototype of an MIM part could be used to test the sintering conditions, adjust the shrinkage geometry and characterise the mechanical properties, even before the mould is produced. The end result is to significantly shorten the time required to design new MIM components, develop tools or alter the parts to specific customer needs.
*The European Powder Metallurgy Association (EPMA) was founded in 1989 for the then-niche powder metal industry to give it a louder voice with EU legislators and create synergy for the powder metal sector across Europe. Over the years, the EPMA has expanded the processes it represents, ensuring that as the metal powder industry develops and reacts to manufacturing changes and advancements, so has the association. This is evident as powder-related processes have been absorbed into the association, starting with hard materials in 1999, metal injection moulding 2001, hot isostatic pressing in 2009 and, more recently, additive manufacturing in 2013.