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Variations

As we mentioned earlier, injection molding is an alternative for the compacting step in the powder metallurgy process.

Compacting

Loose powder is compressed and densified into a shape known as a green compact. Most compacting is done with mechanical and rigid tools. Hydraulic and hybrid (combination of mechanical, hydraulic, and pneumatic) presses can also be used. Compacting pressures generally range between 5 and 50 tons per square inch ( 70 to 700 MPa).

In most cases, the prepared powder flows under gravity into the die until there is some excess. The excess is scraped off and the press closes to compact the powder.

During compacting, the powder does not flow like a liquid but simply compacts until an equal and opposing force is developed by the friction between the particles and the die surfaces. The resulting density is a strong function of both the thickness and width of the part being pressed, as side-wall friction is a key factor in compaction.

It is seldom possible to transmit uniform pressures and, since maximum density occurs below the punch and decreases down the column, it is very hard to produce uniform density throughout the compact. When nonuniform thickness is desired, more complicated presses or methods must be used.

Compaction rates are extremely slow (range from 6 pieces per minute to 100 pieces per minute on average). However, parts up to several hundred pounds can be effectively compacted.

By means of mechanisms such as bulk movement of particles, deformation of individual particles, and particle fracture or fragmentation, the density of the powder has been raised to about 80% of an equivalent cast or forged metal.

Ceracon Process

It is a newly developed process designed to raise conventional pressed and sintered powder metallurgy products to full density.

Hot granular material capable of transmitting pressure in a pseudo-uniform manner surrounds a heated preform totally. A conventional hydraulic press compacts the entire assembly.

Cycle times are on the order of several seconds. The part and the pressuring medium separate freely, and the pressure-transmitting granulates are reheated and reused.

Osprey Process

The molten metal is atomized and propelled toward shaped collector molds by streams of inert or harmless gas ( nitrogen or carbon dioxide). The droplets are cooled in a controlled manner to strike the mold in a semisolid state and rapidly freeze.

The product differs from conventional powder metallurgy in that the structure exhibits uniform fine grain size, uniform chemistry, and little, if any porosity.

Deposition rates can be as high as 40 to 50 pounds per minute.

Source: DeGarmo, Black, Kohser, Materials and Processes in Manufacturing.