Metal Injection Moulding (MIM) is an emerging technology in the manufacturing world across the globe. Though has its origins from early 1920’s and WWII times, when the first parts were made, it came to the industrial forefront only during 1970’s. Even after 45 years, it is still considered a relatively unknown, exotic and specialised manufacturing process. One of the main reasons being the emergence of only a handful of companies in the world, who have mastered this technology and invested enough time and resources to transform the science behind the process into an almost art-like form.
MIM is the process of injecting fine metal powders mixed with appropriate thermoplastic and wax binders at high pressures and elevated temperatures into an engineered mould, to form the desired component shape. The temperatures are high enough to only melt the binders, which form a viscous slurry with the metal powders and upon cooling, are ejected from the mould as a solid part. Subsequently, a controlled process of ‘debinding’ is carried out to remove the binders using a chemical solvent. The still fragile parts are then subjected to ‘sintering’ – a metal densification process, where they are heated to high temperatures close to the melting point of the metal alloy in a controlled atmosphere, resulting in a fusion of the metal powders together and forming a near net shape with densities ≥ 96% of theoretical density.
Although the described process is simplistic in nature, the real-world complexities are manifold. These are interlinked and can broadly be classified into the following categories:
Material: The proportion of metal powders to binders in the mixture drives the flow characteristics and part shrinkage during sintering. This not only needs to be determined exactly beforehand, but also readily altered to suit the requirement.
Tool design: Tool design is invariably complex and intricate due to the nature of parts made by MIM. In addition to accommodating the part design, the tool has to provide necessary provisions for uninterrupted material flow and uniform cooling. Add to it, the various slides for part features that need to perfectly match and act as a single unit in the closed condition, it is not difficult to imagine the complexity involved.
Machine parameters: Sophisticated moulding machines with a horde of parameters and options, require a thorough understanding of the physics of mould filling. It is essential to be equipped with this knowledge to avoid common moulding related defects of sink, incomplete filling, weld lines or flow lines.
Shrinkage: The shrinkage characteristics of a part are a function of both the initial material mixture and ‘staging’ during the sintering process. Staging refers to orientation and placement of the parts in specifically designed ceramic holders called stagers. The material mixture determines the percentage of shrinkage whereas the staging method controls the distortion, since the part would naturally tend to shrink towards its Centre of Gravity (CG). Even with other things being done correctly, an incorrect staging method would seriously hinder achieving the required net shape and dimension. Then, there is the sintering cycle itself with its carefully determined temperature range and ramp rates. A bit too much and the parts might melt; a bit less and insufficient shrinkage might take place. It is a balancing act and learnt from a history of trials and analysis.
Manufacturing complex parts
MIM process has no parallels when it comes to manufacturing small, highly complex and intricate parts with profiles that are either impossible or cost-prohibitive with conventional techniques. The process also provides a natural flexibility to adapt to varying production demands in terms of quantities. Volume requirements driven by changing market conditions can be easily accommodated at minimal risk. With the raw material in a powder form, it is also easier to meet the strength requirements of the part by simply altering the chemistry and addition of required alloying elements. The range of materials currently used covers various grades of ferrous alloys, stainless steels and exotic metal alloys of tungsten, titanium and inconel. MIM material can be ‘engineered’ in the true sense of the word without any change in the overall process and still produce a reliable and high performance part. The parts produced by MIM have mechanical properties approaching those of wrought metals and can be practically treated as such. They can be subjected to any kind of machining operation, heat treatment, surface finishing operations of glass bead blasting, sand blasting, barrelling and all kinds of plating and passivation processes without extensive surface preparation.
MIM, for all its capabilities has its share of limitations too. Working economics generally limit the part weight to around 50 gms and size to under 60 mm. Thinner or thicker wall thicknesses other than those specified might also pose a problem. In terms of the tolerances directly achievable from the MIM process, a thumb rule of ±0.5% of nominal dimension is usually employed. MIM is preferable for high production volumes, small quantities may not yield the perceived cost advantage.
Applications of MIM
MIM fits everywhere. It is not limited by any market segment or application. Everywhere and anywhere, if there is a requirement for small and complex parts with intricate geometries and, conventional methods are not cost effective, MIM would come out triumphant. Today, MIM has its presence in all the broad market segments and well known applications. However, automotive, industrial, and medical applications are the main segments of MIM application. Majority of applications using MIM have sophisticated design involving multiple interlocking parts and are driven by the need to combine several parts into one. The automotive industry uses various parts with irregular profiles and holes that are at an angle to each other. MIM process has been able to introduce Value Engineering/Value Addition (VE/VA) in such cases and eliminate multiple intermediate machining/joining processes.
Understanding the future
The future of this branch of manufacturing is bright and has great potential for development. MIM has experienced a rapid growth in the last decade with annual sales exceeding $1 billion per year by some estimates and a compound growth rate of as high as 20%. However, the engineering industry is still largely unfamiliar about MIM process and its capabilities and applications. With an increased awareness and acceptance of the technology from high volume manufacturers of metal components, an explosive growth rate can be expected in the future. There are still numerous possibilities in material choices and process parameter optimisation that are waiting to be explored and integrated into industrial production. Another important aspect is the integration of the technology and its potential at the preliminary concept or design stage itself, which would open a whole new world of possibilities and even widerspread of MIM in the relevant branches of engineering.
The article is authored by Sujith V Sukumaran, Territory Manager, Indo-MIM