Compacting or Pressing of Metal Powders: How is it Done?

March 29, 2018

At one end of the workbench, a bronze bearing is awaiting inspection. At the other end, a clutch plate is on display. A pair of surgical scissors sits between the two different items. What do they all have in common? The answer, as you’d expect, is that they were all sintered. A metal powder was compacted, the metal-forming process took hold, and the press-and-sinter technique produced a beautifully crafted item.

Powder Metal Compaction: An Intermediate Guide 

First of all, this technology requires an absolutely pure environment. No atmospheric intrusion is allowed. Perform the compacting or pressing operation in a vacuum or with an oxidization-inhibiting gas. Next, the atomization stage has done its work; the powder is currently piled in a finely ground mass. It may even have been mixed with a lubricant or some other process additive, and now the powder metal is on its way to the compaction stage. A rigid toolset, a die, or some other form-pressing mechanism finally presses the powder until it assumes the desired shape.

Selecting Axial or Isostatic Compaction 

In plain English, a linear force is applied when a loose cylinder of powder metal is forced into a closed die. That’s the axial method, a rod-like piston that consolidates the fine particles. As for isostatic compaction, the term applies to a force that uniformly presses every surface of the compacted object simultaneously. Like a hand crushing a lump of soft clay, the entirety of the mass is squeezed from all sides. This is a more complex operation, one that requires a form shaping envelope and submersion in a vat of non-compressible fluid.

Compaction Influencing Variables 

If the atomization stage doesn’t properly reduce a metal block into the required mass of fine powder, the pressing stage could go awry. Evenly atomized metal grains move over each other uniformly, so part density is predictable. Furthermore, particle shape impacts the pressing action, as does the composition of the metal. Lastly, the inner surface of the die must be as close to frictionless as possible. Die friction causes dimensional inconsistencies.

Whether the compaction force pushes from one direction, or it uniformly presses the fine powder into its final shape, that squeezing energy must be mechanically applied in a controlled and consistent manner. Otherwise, more dimensional inconsistencies are likely. The compaction operation is also a little like a three-dimensional jigsaw, with die sections and cores and upper/lower punch segments. As those parts bear down, a bronze bushing or some other essential component takes shape, then it moves forward to the sintering stage.

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