Investigating Metal Injection Moulding

Materials World magazine
,
24 Apr 2020

Idha Valeur takes a closer look at what metal injection moulding has to offer and the shape of things to come. 

The potential of metal injection moulding (MIM) ‘lies in its ability to capture the shaping advantage offered by injection moulding…[It also] provides advantages of cost reduction at volumes ranging from a few thousand to over millions of parts per year due to its ability to hold close tolerances’, asserts the Encyclopaedia of Materials: Science and Technology.

This certainly sounds like a good combination. The manufacturing technique is after all a merged process of two commonly used methods – plastic injection moulding and powder metallurgy. Widely accepted as the inventor of MIM, state-side-based Raymond Wiech honed the method to process metal powders in the 1970s.

It involves manipulating these powders to behave like a plastic by mixing them with polymer binders using a kneader or shear roll extruder, to form a feedstock. ‘This feedstock makes the mixture capable of being handled by plastic processing equipment before [the binder] is then removed and the powder particles are sintered boned together in a high-temperature furnace, leaving behind a solid metal component,’ says Rachel Garrett, Managing Director of CMG Technologies, in Suffolk, UK. 

Iain Todd, Professor of Metallurgy at the University of Sheffield, adds, ‘Fine powders are mixed with a polymer carrier that softens when heated and allows the mixture to flow into and fill the mould cavity to give a perfect reproduction of the required shape. This then cools and “binds” the powders together.  
‘The powders used tend, in general, to be a very fine, average particle size of 10 microns or so, but much larger powders can be used without great detriment. This small size is largely a hangover from the original powders used in injection moulding for ceramics and these are generally finer.’

Jason Dawes, Technology Manager at The Manufacturing Technology Centre (MTC) in Coventry, UK, and member of the IOM3 Particulate Engineering Committee, explains that the most common metals used in MIM are 316L and 17-4PH stainless steels. ‘But nickel, titanium and many other alloys are widely used. Theoretically speaking, any metal that could be turned into fine powder is suitable for MIM, provided that the right feedstock recipe is developed.'

He notes that, in the UK, there are several companies that do research into MIM as a process or use it as a regular production process. These include ‘the MTC and CMG, High Performance Moulding in Nottingham, Dean Group in Manchester, Metal Injection Moulding Ltd in Cheshire, Rencol in Bristol and Sinotech in Market Harborough. There may be a handful more companies with full MIM capabilities,’ Dawes says. 

Design freedom

Producing elaborate and complex 3D metal shapes has historically been challenging using traditional processes that involve cutting and manipulating the material. Garrett notes that this often results in more waste. 

‘MIM offers greater design freedom than many other production processes because it alleviates the common constraints associated with trying to shape metal. This flexibility means you can integrate several elements into a single moulded piece, incorporating texture, knurling, threads, lettering and company logos. It offers a manufacturing capability of producing precise, complex parts in large quantities in metal.’ 

The technique is suitable for creating parts, products or components for sectors including healthcare, automotive, industrial, firearms and consumer goods.

In a bind

Removing the polymer binder prior to sintering is an important step for producing the final product. ‘The polymer/binder poses one of the biggest challenges,’ explains Todd. ‘Once we have an object, the binder becomes something we no longer need or indeed want. Polymers contain all sorts of elemental constituents – such as carbon, nitrogen, hydrogen – that if incorporated into metals and alloys would mean that the properties of the material in the sintered condition would be impaired.’ 

The de-binding process is commonly conducted through either heat or a chemical process using solvents or other chemicals that will react with the binder material. Dawes adds that, in some cases, even hot water will do the trick. 

Porous parts

As with other techniques, such as computer numerical control (CNC) machining or casting, parts made with MIM will still contain some porosity. ‘Generally, after the sintering stage, the surface finish is around 0.6-0.8 microns and the theoretical density of the components we produce will be controlled within the range of 95-98% dense,’ notes Garrett. ‘CMG technologies manufacture parts from micron powders, enabling us to produce parts with wall sections as thin as 0.5mm, which are hermetic to greater than 10-10 torr for applications in gas analysis and hermetic packaging.’

Sintering in a hydrogen atmosphere helps to prevent certain metals forming an oxide layer, while hot isostatic pressing post-sintering can be used to fully remove any porosity, which is beneficial for applications in aerospace, medicine and automotive.

Thickness issues 

Although MIM offers the manufacturing industry several benefits, including minimal scrap and waste, there are limitations too. 
‘There are a number of significant technological challenges and limitations that are inherent within MIM processes, such as high tooling and equipment costs, the physical dimensions of MIM components which are limited due to the nature of the production process, and production limitations which are restricted 
to thin-walled parts,' Garrett says.  

‘We aim for a maximum weight of 50g and overall dimensions of less than 100mm. Thick sections are difficult to debind, the thicker they are the longer it takes and therefore more costly. Plus, if the binders aren’t fully removed it affects the composition of the final part. Also, the nature of our process allows us to core material out to reduce wall thickness and therefore cost. Machining cored sections out adds cost whereas with MIM it doesn’t,’ she adds. 

Todd notes that MIM works best if the production volume is quite high as the costs of the moulds are relatively high too, especially for complex components. 

Refining the recipe

One of many sectors benefiting from the customisation, design freedom and the ability to manufacture complex and intricate parts that MIM offers is healthcare. 
The material recipe depends on the application. ‘Our engineers look at the design requirements of the component and compile the right blend of metal powders and polymer for that individual piece,’ says Garrett. 'The intricacy of the design and the durability of the component are all taken into consideration to ensure the right material for the part.’ 

Todd continues, ‘The knack is to match the binder and design and sintering method with the alloy you are using – some are relatively robust, such as 316L stainless steel, others like Grade 2 and Grade 5 titanium, less so. 

‘In many ways the reproducibility is key. But there are other advantages – parts can be made with controlled porosity to encourage bone integration for example. Reduction of part count, component reproducibility and standardisation of parts are really important. Some components that are complicated to manufacture by conventional routes can be made relatively easily this way. And that has to be a good thing.’ 

The smallest part CMG has created, for example, was the size of a full stop, to stop chaffing at the end of fixed metal braces. ‘We [also] worked with sterile single-use manufacturer DTR Medical Ltd, Swansea, to develop a sharp cutting jaw for their rotating cervical biopsy punch,’ Garrett says. 

She adds that probably the most exciting project was helping upper-body prosthetics manufacturing company, Covvi, based in Leeds, UK, to design and create knuckle joints, finger links and drive gears for robotic hands. 

Young at heart

Although MIM was established in the 1970s, it is still a fairly young process, says Garrett, and not as established in the UK. But she believes this is changing as the manufacturing process is being included in the higher education curriculum. ‘We are seeing new graduates come through who have heard of the process which is a positive step for the industry.’

Dawes also echoes Garrett’s view that the outlook for MIM is promising and that developments are ongoing. ‘MIM has helped developing additive manufacturing (AM) – Binder Jet, for instance, is a form of MIM without using tooling, but a powder bed. Some of the AM applications can be focused to reduce costs of MIM processes, like printing the tooling. With AM, MIM is only growing in use also because the metallic powders developed could be used in MIM, as well as much of the post processing development,' he adds. 'MIM is a well-established manufacturing method with more collaborative than competing technologies and its use is growing worldwide.’