a pressing density gradient
Imagine spreading butter with one stroke of a knife: thick and firm where the knife first pressed, thin and soft where the force ran out. A die-pressed ceramic has the same problem hidden inside it — some regions are packed tighter than others, even though the outside looks uniform. A pressing density gradient is that map of non-uniform green density across a pressed part, and it is the direct fingerprint of die-wall friction plus the way the punches move.
Trace where it comes from. In single-action pressing the top punch pushes down and the die and bottom are fixed, so pressure is highest just under the moving punch and lowest at the far bottom corners, where wall friction has bled the pressure away. The result is a classic pattern: dense near the top and the outside, looser in the bottom-inner region. A part might run from 58 percent of theoretical density in the dense zones to 50 percent in the starved corners — a small number that causes big trouble. Double-action pressing pushes from both ends and moves the low-density zone to the middle; floating dies and lubrication flatten the gradient further, but never to perfectly flat.
Why obsess over a few percent? Because green density decides local firing shrinkage: a body that is 60 percent dense might shrink 15 percent on firing, while a neighbouring region at 50 percent must shrink more to reach the same final density. When one part of a single body wants to shrink more than the part next to it, the mismatch tears at the interior — the piece warps, bows, or cracks, and the defect is set in the green stage long before the kiln is lit. This is the everyday face of the honest principle that forming flaws are inherited by the fired part.
A single-action-pressed disc fired flat comes out slightly dished, thinner and denser on the punch side, thicker and more porous on the die-bottom side. The cure is not a hotter kiln but a better press cycle: switch to double-action, add a die-wall lubricant, and lower the height-to-diameter so friction has less depth to act over.
Uneven green density becomes uneven firing shrinkage, which becomes warp and cracks — set in the press, revealed in the kiln.
You usually cannot see a density gradient in the green body; it hides until firing shrinkage magnifies it into visible distortion. That is why controlling it at the press, not diagnosing it after the kiln, is the whole game.