News in Brief
Advanced Materials //
Hollow Spheres Made of Metal
esearchers at the Fraunhofer Institute for Manu- facturing and Advanced Materials IFAM in Dres-
den, Germany, working in cooperation with German
company hollomet GmbH Dresden, Germany, have
created a technology for the manufacture of rapidly
reacting ball valves and bearings.
“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 the- refore reacts slowly in a ball valve. “For the first time we’ve been able to produce metal hollow spheres in the required diameter of just two to ten millimeters.
The hollow spheres are 40% to 70% lighter than solid ones.”
The process starts with polystyrene balls which are lifted up and held by 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 po- lystyrene and the binder evaporate. The residual ma- terials are gaseous and escape through the pores in the metal layer. A fragile ball of metal remains. This is now sintered at just below melting temperature, and the metal powder granules bind together, forming a hard and cohesive shell. 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 millimeter.
10-03 :: March 2010
Hollow spheres made of ground steel, measuring just two to ten millimeters. © Fraunhofer IFAM