The thixotropic process is gaining ground for its ability to create strong yet light magnesium parts for a range of applications. Dave Wilson reports
Despite the fact it is a method widely used to manufacture housings for mobile phones and portable computers in the Far East, engineers in the UK would be forgiven for being unfamiliar with ’thixotropic moulding’.
However, as product designers become more aware of the versatility of the process to produce lightweight, strong, complex moulded magnesium parts, this could soon change. Many may soon be considering the use of magnesium alloy parts as a replacement for those made from engineering plastics and die-cast magnesium and aluminium alloys.
According to Charles Maltby, technical and commercial director at Cambridgeshire-based Shearline, although the thixomoulding process combines elements of conventional die casting and plastic injection moulding, it is more closely akin to the latter.
In a typical thixotropic manufacturing process, chippings of a magnesium alloy are placed into a hopper at one end of a machine. The magnesium alloy is then heated to a point just below its melting point, at which point it goes into a semi-solid state where certain fractions within the alloy start to become liquid and act as a lubricant, while other parts of it remain in solid solution.
The semi-solid material is then fed into a barrel with a reciprocating screw, which moves forward, applying a shearing action to the thixotropic magnesium alloy, effectively injection-moulding it into a mould cavity where the component is to be manufactured. Once it has been moulded, it is removed from the tool, where any secondary operations can be performed on it, if needed.
“If you could make train seats from magnesium alloy, they would weigh substantially less than existing ones”
However, the improved tolerance and thinner wall sections achievable via the process have the effect of reducing – indeed eliminating – many second operation machining processes, saving time and energy.
Maltby said the process has two major advantages: ’It does not demand as much energy and it does not produce the byproducts so typical of other processes. The result is that a thixotropic moulding machine can be used in any factory environment.’
What is more, because the magnesium alloy is forced into the mould under pressure, the resultant parts have a better density and lower porosity than those obtained from conventional die-casting processes.
Presently, the thixotropic process has been limited to high-volume production runs as tooling costs prohibit its use in low-volume manufacturing.
’It’s hardly surprising then that most of the thixotropic manufacturing machines are based in Japan, China and Korea, where consumer products are made in large numbers,’ said Maltby. ’In total, there are less than 500 machines in the world – and of those fewer than 10 are in Europe; around 20 in North America. None are in the UK.’
The machines that are currently producing parts are usually only capable of making relatively small components, and that’s something that Maltby would like to see change.
’There are a number of applications that would benefit from the weight-saving characteristics of the process,’ he said. ’One application is for train seats – if you could make such parts from magnesium alloy, they would weigh substantially less than existing seats, and hence less fuel would be needed to propel the train.’
Before parts of such a size can be made using the thixotropic process, added Maltby, work is needed to define, and resolve, any remaining technical issues involved in using the machines to make parts from newer magnesium alloys that offer reduced flammability and increased fatigue life or corrosion resistance.
As this is unchartered territory, Shearline has teamed up with academics at Sheffield University and the Advanced Manufacturing Research Centre with Boeing in a two-year KTP project to assess those issues further. Maltby sees this as a first step towards the day when he may be able to demonstrate such a machine in use at his headquarters in Cambridge.