ceramic extrusion
Squeeze toothpaste from a tube and it comes out as a continuous ribbon whose cross-section is the shape of the nozzle. Ceramic extrusion is that on an industrial scale: a stiff, plastic ceramic paste is forced through a shaped die, and out comes a continuous length of material with a constant cross-section — the die's outline printed on it. Cut that endless column to length and you have bricks, pipes, tubes, rods, and, most impressively, the fine honeycomb blocks that clean car exhaust and filter diesel soot.
The material being pushed is a plastic paste: powder (or clay) mixed with just enough water or organic binder and plasticizer to make a dough that flows under pressure but holds its shape the instant it leaves the die. The push comes from a ram (piston) or, in high-volume plants, an auger (screw) that also de-airs and homogenizes the paste in a pug mill. The paste's rheology is the whole art — it must have a yield stress: stiff and self-supporting at rest, so a wet extruded tube does not sag, yet fluid enough to flow through the die under a few MPa without tearing. Honeycomb dies, with their lattice of thin slots and pins, are a feat of tooling, and the paste must flow through them evenly or the thin walls tear.
Extrusion is cheap, continuous, and fast, which is why it dominates high-tonnage traditional ceramics — the whole brick and clay-pipe industry — and also makes the catalyst supports and diesel particulate filters that are extruded by the millions. Its signature defects come from the flow itself: laminations where paste streams fail to knit, die swell and distortion where flow is uneven, and surface tearing when the paste is too stiff or too fast. As always, whatever unevenness the paste carries out of the die is inherited by the fired part.
A cordierite honeycomb for a catalytic converter is extruded by forcing a stiff, plasticized paste through a die whose fine slotted lattice prints hundreds of parallel square channels per square inch; the endless honeycomb log is cut to length, dried, and fired — its thin walls only workable because the paste flowed evenly through every slot.
Push a plastic paste through a shaped die for continuous constant-section parts: bricks, tubes, and honeycombs.
Extrusion only makes shapes with a constant cross-section along one axis — no undercuts, no closed ends. And the paste's yield stress is a tightrope: too soft and the wet part sags, too stiff and it tears at the die.