Back at the compaction chamber, two alternative pressing systems are available. In the hot pressing method, elevated temperatures are used. It’s a dual-process operation, with the powdered metal compacting, then it sinters. Almost solid, just slightly porous, the combined processing approach yields tool quality results. Alternatively, cold pressing is done without heat. That latter phase of the process is left until the compacted product reaches the sintering stage.
Merging Powder Metallurgy Stages
Two of the discrete powder metallurgical stages can be combined. This is hot pressing, with the compaction work and sintering operation done in one chamber. First, the die creates the Green part, which is still a mass of individual metal particles. Next, the grains “neck” and sinter. The elevated temperature is held just below the critical melting point of the selected alloy, so the particles combine. There’s no liquidizing, no melting metals flowing. Again, two separate sources of energy are utilized inside a single chamber. Immense pressures give the powder its geometry, then the applied thermal energy encourages the compacted powder to coalesce.
The Cold Pressing Difference
Unlike the above example, there are no heating elements to transform the Green product into a solid mass. High pressures are in evidence in the compaction chamber, where upper die sections and their lower components come together. A core element and a feeder shoe complete the die architecture. If there’s any heat at all in here, it’s just applied as a low-temperature bake, which burns off the pressing lubricant. From here, the Green compact moves forward and into the sintering furnace. But why are there two approaches to the same process? Even the die architecture is different in hot pressing, with graphite cores replacing the hardened steel variants.
Detailing the Hot and Cold Benefits
Hot press compaction/sintering chambers are used to produce densified tools. Pressed into heat-distributing graphite dies, the thermal energy and compaction process improves the sintering characteristics of metal powders that would otherwise resist the process. By combining the two stages, low-porosity, densified carbide cutting tools can be produced in quantity within a machine form that wasn’t originally intended for such purposes. Alternatively, when it comes to cold pressing, the two discrete forming stages produce geometrically detailed products, which exhibit a calculated porosity quotient, plus some kind of an internally stored fluid/wax. Lubricating oils are popularly stored inside self-lubricating bearings in this manner.
Results-wise, hot pressing techniques remove potential material weaknesses. As the porosity quotient drops, a part becomes stronger, denser. This option, where a single chamber applies compaction pressure and thermal energy, is used to create sintered carbide tools, among other specialized items. Back with the cold pressing equipment, finely detailed components are created. They look solid, but they possess predetermined voids, capillary pathways, and lubricating fluids.