uniaxial die pressing
Think of how an aspirin tablet or a coin is made: pour a measured pinch of powder into a hole, then squash it flat between a top and a bottom stamp. Uniaxial die pressing is exactly that, done to ceramic powder. You fill a hard steel cavity with free-flowing granules and squeeze them along one axis between two punches until the loose powder becomes a solid shape you can pick up. It is the fastest, cheapest, most automated way to give ceramic powder a geometry, which is why the tiles on your wall, the insert on a cutting tool, and the white body inside a spark plug are almost all made this way.
In practice the powder is not the raw fine particles — those do not flow or pack well — but spray-dried granules: little soft spheres of many particles glued by a few percent of organic binder, so they pour into the die like sand. As the punch descends and pressure climbs, roughly 20 to 150 MPa, densification happens in stages: first the granules slide and rearrange, then they deform and crush at their contact points, and finally the primary particles slot into tighter packing. Green density typically climbs to about 50 to 60 percent of the material's theoretical density. In single-action pressing only the top punch moves; in double-action both punches close inward, which spreads the compaction more evenly through the depth.
The strength of the method is throughput and precision on simple shapes — flat discs, rings, short cylinders, plates — pressed at parts-per-second rates with tight thickness control. Its limits are honest and important. Tall, thin parts (high length-to-diameter) are hard because the pressure cannot reach the far end past the wall; sharp internal features and undercuts are impossible; and the same die-wall friction that makes filling easy leaves behind density gradients that the later firing must live with. Whatever unevenness you press in, you inherit as warping and cracks after sintering.
To make an alumina cutting-tool insert, an operator drops a fixed volume of spray-dried Al2O3 granules into a square die, a top and bottom punch close to about 100 MPa in a fraction of a second, and the pressed square is ejected at roughly 55 percent of theoretical density — a green body strong enough to handle but still full of the pores that firing must remove.
Fill, squeeze along one axis, eject: fast and precise for simple flat shapes, but shape and uniformity are limited by the rigid die.
Pressing does not mix or heal — it only rearranges what you poured in. A hard agglomerate or a lump of foreign matter in the powder becomes a flaw in the pressed part, and the die's wall friction guarantees the density is never perfectly uniform.