Miniature hollow spheres

Production engineers at the Fraunhofer Institute in Germany have manufactured 2mm to 10mm diameter metallic hollow spheres that could be used for an array of high-tech applications.

The team claims that it had been nearly impossible to produce metallic hollow spheres of this size before this latest development.

The hollow spheres, which were developed at the Fraunhofer Institute for Manufacturing and Advanced Materials (IFAM) in Dresden, can be used to make magnetic ball valves.

These valves would be integral parts of new drive technologies for more fuel-efficient aircraft and automobiles in the future.

For a magnetic ball valve to react quickly, the balls must be as light as possible. The same applies to moving bearings rapidly.

Until now it has only been possible to produce balls of this size as solid spheres, but a solid body is relatively heavy and therefore reacts slowly in a ball valve.

‘In an injection valve, the movement of a ball causes the valve to open and close,’ explained Hartmut Göhler, project manager at IFAM. ‘The lighter the ball, the quicker it moves.’

Göhler said the new 2mm to 10mm diameter metal hollow spheres are 40 to 70 per cent lighter than solid ones.

The fabrication process starts with polystyrene balls, which are lifted up and held by an air current over a fluidised bed. Meanwhile a suspension consisting of metal powder and binder is sprayed onto them.

A heat treatment is applied when the metal layer on the balls is thick enough. After this, all the organic components, the polystyrene and the binder evaporate.

Any residual gaseous materials escape through the pores in the metal layer. The result is a fragile ball of metal.

The ball is then sintered at just below melting point, and the metal powder granules bind together – forming a hard and cohesive shell. The sphere is then stable enough to be ground in a machine.

The researchers note that the pressure must not be too high or the hollow body would deform. The wall thickness is then set between a few tenths of a millimetre and one millimetre.

Göhler said applications for the technique could be wherever a low mass inertia is required. ‘Hollow spheres will create applications that have not been possible up to now,’ he added.

The team has already produced ground spheres made of steel, while metals such as titanium and various other alloys are envisaged for the future.