Although research in the area has been carried out for more than a decade the focus on making hollow ground and polished spheres has increased in the last three years. Now staff at the Fraunhofer Institute for Manufacturing and Advanced Materials (IFAM) in Dresden are working co-operating with others at a subsidiary of the Glatt pharmaceuticals and chemicals group, Hollomet GmbH Dresden.
Glatt were pioneers of fluid-bed coating techniques, originally developed for coating pharmaceutical products and since applied to metal spheres. Together with IFAM, they have created the technology for the manufacture of rapidly reacting metal ball valves and bearings. Commercial operations are expected to begin next year.
“In an injection valve the movement of a ball causes the valve to open and close. The lighter the ball, the quicker it moves,” explains Dr Ing Hartmut Göhler, project manager at the IFAM. 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. “For the first time we’ve been able to produce metal hollow spheres in the required diameter of between two to 10 millimetres. The hollow spheres are between 40% and 70% lighter than solid ones.”
The process begins with polystyrene balls being lifted and held in an air current over a fluidized bed while a suspension consisting of metal powder and binder is sprayed onto them. When the metal layer on the balls is thick enough, heat treatment begins, in which all the organic components, the polystyrene and the binder evaporate. The residual materials are gaseous and escape through the pores in the metal layer. Fragile balls of metal remain.
“These are now sintered at just below melting temperature, and the metal powder granules bind together, forming a hard and cohesive shell,” says Göhler. “The sphere is now stable enough to be ground in a machine, but the pressure must not be too high as otherwise the hollow body will deform. The wall thickness can be set to between a few tenths of a millimeter and one millimetre.
Göhler sees applications for the technique wherever a low mass inertia is required. “Hollow spheres will create applications which have not been possible up to now,” he says. The scientists have already produced ground spheres made of steel, other metals such as titanium and various alloys are envisaged for the future.
“Mostly we use 316L, other Cr-Ni-steels, Cr-steels, but low alloyed steels (Fe-C) as well. It’s also possible to manufacture these spheres from the most other metals and alloys. Titanium needs a little more research, but aluminium, zinc, tin and other low-melting metals are not possibilities.
“We use a small commercial grinding machine for bearing balls – and our special know-how to handle the fragile steel hollow spheres. We have no repeated strength data so far and there is work to do to increase their strength.”
