The powder metallurgy manufacturer is often confronted by a drawing for a component designed with an alternative manufacturing process in mind. It is not sensible or desirable for the powder metallurgy component manufacturer to attempt to quote or produce to these drawings or designs as it is likely that certain features cannot be produced. It is desirable to redesign the component so that it can fulfil its design function, as well as take full advantage of the powder metallurgy process, in particular cost effective manufacture to near nett shape with close dimensional tolerances.
There are a few simple factors to consider:
• the need to avoid feather-edged tooling. Stresses on the edge would cause it to deform under pressing loads and bind against the mating parts of the tooling. (This problem can be overcome by the use of a small flat section.)
• the inability of the powder metallurgy process to introduce re-entrant angle and cross holes. Such features would have to be machined using a post processing step.
• sharp corners should be avoided, being replaced by small radiuses.
• the need to be able to eject the part from the tools after pressing
Other design rules relate to the practicalities of producing certain tooling configurations and have to be considered on a component-by-component basis.
Practicalities of Production
There are constraints on the height and wall thickness of parts. The initial height of powder in the die is about 2.5 times that of the pressed compact. Consequently components with final dimensions of height to wall thickness of greater than about 16:1 are not possible. Filling also becomes a problem with this height to wall ratio when the wall thickness itself is small. Usually wall thicknesses of less than 2 mm at this ratio are not possible.
Normally a powder metallurgy component is a single, pressed and sintered part. It is possible to produce a complex component, impossible to press in one operation, by pressing two simpler sub-components and co-sintering, or sinter-brazing them together during the sintering process.
Co-sintering relies upon the interdiffusion of the two parts during sintering with no additional joining aid, whereas sinterbrazing uses an intermediate layer of a braze material between the two parts, which joins them together whilst they are at the sintering temperature. A third option is to resistance weld the two parts together after sintering.
It is highly desirable to seek the advice of a powder metallurgy component manufacturer to ascertain the most cost effective design for the component by discussing the function of the component and all of the critical features.