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Exciting, light and strong: that’s injection moulded magnesium

02 September 2010

Ultra lightweight, very strong and becoming more affordable, magnesium is coming of age and provides an exciting metal for the aerospace, automotive and consumer products markets. Charles Maltby describes how his company embraced a particular magnesium moulding technology following a successful client project

Although magnesium is widely used for mass-produced goods such as computer and mobile phone casings, cost and availability have limited smaller volume applications. Precision manufacturing company Shearline is looking to address this with a programme that it is creating with the support of the University of Sheffield and the Advanced Manufacturing Research Centre (AMRC) with Boeing. The company has secured a Knowledge Transfer Programme (KTP), appointed a KTP associate and is looking to provide a number of services, including design-for-manufacture, tool design, moulding, finishing and assembly.

Shearline’s interest in magnesium came when it started working with a client that incorporates injection moulded magnesium components within its products. The client, a designer of high specification electric bikes had several of the main components created in single pieces using a thixomoulding process. This allowed a complex structure to be moulded in one piece with the benefit of lightness and strength.

Shearline has now set up a cell to support its client and provides tooling, handling, finishing and assembly. Triggered by this activity knowledge of the versatility of the process and the design-for-manufacture process that is required to maximise its potential, the company started to investigate the wider applications of the metal.

Automotive pioneers
Magnesium moulding removes some constraints from designers. It has long been used within the automotive industry – the early Volkswagen beetle had magnesium gearbox and engine housings – and it is used in high performance cars where the traditional higher cost has not been an issue when weight savings are taken into account.

However, the lightness of magnesium is creating new opportunities where fuel economy is important. A report by Husky has found that magnesium component adoption in automobiles is growing at an annual rate of 12-15%. The main drivers are its lowered cost and the potential for weight reduction by incorporating thinner walls into powertrain, drivetrain, safety systems and seating.

The attraction of magnesium is that it has an excellent strength-to-weight ratio compared with that of the more widely used structural metals. It is one-quarter the mass of steel and two-thirds the mass of aluminium and can be machined faster with the potential for less wear on the casting tools, given appropriate tool design.

Low energy moulding
Magnesium can be put into a thixotropic state, which is the property of semi-solid materials to become more fluid with heat and shear. With conventional casting it is necessary to melt the metal material to provide the right properties of flow. However with the thixomoulding process lower temperatures can be used as both shear and temperature are applied simultaneously to control the viscosity.

The magnesium thixomoulding machine functions in a similar way to a plastics injection moulding machine using chips of metal alloy instead of plastic pellets. The chips are heated and pressurised on demand within a screw injection unit under a blanket of inert gas. After the material has achieved a pre-determined shot size it is injected into the mould cavity at very high pressure. The screw meters a precisely controlled ratio of slurry ensuring the consistent flow of material into the mould.

Tooling for this process can also be created using new additive layer methods that reduce the overall cost of tooling and its construction - especially important when only low volumes of product are required Thixomoulding allows complex structures to be created that would otherwise require multiple parts, and it therefore minimises the need for secondary machining and sub assembly stages. The results of this process include reduced porosity, better surface finish and tighter dimensional tolerances than has previously been obtained with typical magnesium alloy casting. This offers the potential for development of new structural components incorporating complex parts with closer tolerances for more demanding industries such as aerospace.

Making thixomoulding available to a smaller production scale within a total manufacturing environment will allow Shearline to evaluate the process on behalf of clients, particularly for prototypes and early production cycles. There are currently only a few sites capable of this in Europe and they are almost all in research labs.

Environmentally sound
Interest is also coming from the fact that magnesium is 100% recyclable. This will help the automotive industry meet European guidelines, which state that new cars must be 85% recyclable by weight rising to 95% by 2015. In addition, according to the International Magnesium Association, unlike conventional processes, Thixomoulding produces no emissions of sulphur hexafluoride, an ozone depleting gas. The entire process takes place in a closed machine which can be monitored and controlled by one person.

Supplies of magnesium are increasing as production in Australia joins that of China, Russia and Canada – a factor that will reduce its cost over time. Future sources of magnesium might include salt water, liberated as a by-product of seawater desalination processes.

A total manufacturing capability
Shearline’s projects cover anything from scientific equipment and ink-jet printers to aerospace assemblies and remote weapon controls. The company provides bespoke manufacturing for technology-based companies, from design for manufacturing, prototyping and first batch production, through to full-scale manufacture, packaging and shipping.
Its experience in exotic metals and ceramics originates from the work of its Hybrid Laser Technology division which manufactures and supplies precision laser cut ceramic substrates. It uses the most advanced CO2 laser systems to profile, cut, and scribe ceramic substrates including alumina, aluminium nitride, silica, piezo-ceramics and other specialised materials, each with their own unique mechanical, electrical, thermal and bio-chemical properties.

Magnesium alloy is a seriously ‘sexy’ metal and those manufacturing products that incorporate it should find a growing market as we enter an era where reducing energy consumption and recyclability are increasingly high on the list of manufacturers’ priorities.

Charles Maltby is technical and commercial director at Shearline Precision Engineering

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