In nature, the simplest shape is a sphere. It has only one dimension and has the smallest surface for the volume. In conventional P/M it is not the simplest shape because we don't make it like a snow ball, pressing it more or less equally from all directions, but we press it vertically. Gravity causes us to fill the die cavity pretty much in the vertical direction and so that is the way we press it. The first illustration shows the basic simple tools trying to squeeze powder to a sphere. But there are a few obvious problems. The powder in the middle won't get compressed as much as the powder on the sides and if we bring the perfect half sphere punches right together to get the perfect spherical part, then the tips of the punches which would be theoretical razor edges, will come together with unfortunate results. We can put typical P/M flats on the tips of the punches but that will leave us with a cylindrical band around the part, and this is commonly done.
展开▼
