Slip Casting and Tape Casting

Two Ways to Cast a Powder

Guide 2 shaped a part by force: a punch drove dry powder into a rigid die, fast and simple, but the die wall dragged on the powder and left a density gradient baked into the green body. Slip casting and tape casting take the opposite tack entirely. Instead of squeezing a dry powder, they suspend it in a liquid and let a fluid find the shape — you literally pour or spread a slurry, and the powder settles into a wall or a sheet as the liquid is drawn away. It is casting in the plain sense of the word: the shape is cast from something that flows.

The reward for going liquid is shape freedom that pressing simply cannot reach: a plaster mould can wrap around a complex hollow curve, and a blade can lay a sheet only tens of microns thick. But the common thread of this whole rung still runs through both. Every method ends in a fragile green body with some green density, and the rule that governs the rung is unforgiving — any nonuniformity you cast in will survive firing and come back as a defect. A trapped bubble, a settled streak, a lump of clumped powder: fire it, and there is your flaw.

Slip Casting: The Plaster Mould Drinks the Water

Slip casting is the oldest fluid route and still the workhorse behind sanitaryware — the toilets, sinks, and basins in every building. You pour a fluid slip into a mould made of plaster of Paris, which is riddled with fine, thirsty pores. Capillary suction in those pores pulls the water out of the slip, and against the mould face the stranded powder packs into a solid layer — the cast wall. The picture is exactly a river depositing silt on its bank: the mould drinks the water and leaves the powder behind, and the wall thickens inward from every surface the slip touches.

How fast does the wall grow? Not steadily — it slows as it thickens, because the water now has to seep out through the wall it has already built before it can reach the plaster. The thickness grows as the square root of time, the same sqrt(time) law you met for diffusion: a wall that reaches 5 mm in 30 minutes needs four times as long, about 2 hours, to reach 10 mm. So thick pieces are slow. Two variants split here: in solid casting you let the slip fill the piece right through, while in drain casting you wait until the wall is thick enough and then pour the surplus slip back out, leaving a hollow shell — which is exactly how a teapot, a crucible, or a toilet bowl gets its inside.

1. Deflocculate the slip so it pours freely, then fill the plaster mould.
2. Wait while capillary suction draws water into the plaster and a cast wall builds against the mould face, its thickness climbing as the square root of time.
3. When the wall reaches the thickness you want, pour the surplus slip back out (drain casting) to leave a hollow shell — or, for a solid piece, let it fill through.
4. Let the piece firm up and shrink slightly as more water leaves, so it pulls away from the mould.
5. Demould the fragile green body and dry it slowly and evenly before firing.

What Makes a Slip Pour: Zeta Potential and Deflocculants

Now the paradox from the first section comes due. A slip may be 45 percent solid by volume, yet it has to flow. Left to themselves, fine particles in water attract one another through van der Waals forces and clump together — they flocculate. A flocculated slip is the caster's nightmare: it thickens into a paste, it traps pockets of water and air inside its clumps, and it packs into a loose, uneven wall. Pour that, and you have cast a weak, porous body full of the very nonuniformity the rung warns against.

The fix is to charge the particles so they push apart instead of clumping. A deflocculant — a small amount of sodium silicate, a polyacrylate, or simply a shift in pH — adsorbs onto the particle surfaces and raises the magnitude of the zeta potential, the net charge across the electric double layer wrapped around each particle. Push the pH well away from the isoelectric point (where the charge, and so the zeta potential, falls to zero) and every particle now carries a like charge; like charges repel, the clumps break up, and the slip flows freely even at high solids while packing into a denser, more even wall. A well-deflocculated slip can drop in viscosity by a factor of ten or more for the same solids loading.

Tape Casting: A Doctor Blade Lays a Ribbon

Tape casting is the fluid route reinvented for thin, flat sheets. A slurry sits in a reservoir; a long carrier film, usually Mylar, slides continuously underneath; and a precisely set steel blade — the doctor blade — skims across the top, leaving behind a wet film whose thickness is set by the blade gap and the carrier speed. As the film moves on, the solvent evaporates and the powder-plus-binder left behind stiffens into a flexible green tape. The whole picture is spreading icing on a moving cake with a knife, or a painter drawing a scraper across a wall to leave an even skin.

TAPE CASTING  (side view of the doctor blade)

         slip reservoir
              |||
            [ ||| ]  <-- doctor blade; the gap h sets wet thickness
     _________|   |________________________________
    |         |###|====== green tape =============  |
    |_________|   |________________________________|  --> carrier moves this way
              (Mylar carrier film)

   wet gap h  ->  solvent evaporates  ->  flexible green tape
   typical fired-layer thickness:  ~5 to 50 microns
   handle it, punch vias, print an electrode, stack, laminate.

The magic that makes a ceramic sheet bend without snapping is in the recipe. Besides powder and solvent, the slurry carries a binder — a polymer such as polyvinyl butyral that glues the particles into a leathery, foldable sheet — and a plasticizer that keeps that binder soft rather than brittle. That flexibility is the whole point: the green tape can be handled, punched with holes, screen-printed with a metal electrode, then stacked many layers deep and pressed into a laminate. This is precisely how a multilayer ceramic capacitor is built — hundreds of barium-titanate dielectric layers, each only a few microns thick, interleaved with electrodes — and how alumina chip substrates and the cells of a solid-oxide fuel cell are made.

The Organics Must Leave — and Uniformity Survives

Both routes pay for their shape freedom in the same coin: a low green density. All those organics take up room, and the particles arrive gently in a fluid rather than being rammed home, so a slip-cast or tape-cast body packs looser than a part that was pressed and then cold isostatically pressed. But there is an honest upside worth stating plainly. Because a well-dispersed slip has no die wall dragging on it, it can pack more uniformly than a pressed body, which always fights the density gradient friction leaves behind. Casting often trades a little absolute density for better evenness — and evenness, this rung keeps insisting, is what firing actually cares about.

Before the part can sinter, though, every trace of that binder, plasticizer, and deflocculant has to come out. That is debinding, or binder burnout — a slow, patient heat treatment, typically over a few hundred degrees C, that decomposes the organics and lets the gases seep out through the still-porous body. Rush it and you are in trouble: gas generated faster than it can escape will bloat the piece, crack it, or peel a laminated tape stack apart at its layers. Debinding a thick slip-cast wall or a many-layer capacitor can take hours to days, and it is unforgiving of haste.

So the same rule closes this guide as opened it: any nonuniformity you cast in survives firing and returns as a forming flaw — a trapped bubble becomes a pore, a settled or streaky region becomes differential shrinkage that warps the part. The craft, in both routes, is a clean, well-dispersed, bubble-free slip or tape. Between them they cover a huge range: slip casting owns complex hollow shapes like sanitaryware, tape casting owns thin flat sheets like substrates and capacitor layers. Guide 4 turns to the stiff-paste routes — extrusion, injection molding, and gelcasting — and guide 5 gathers everything under the one idea that has quietly run through the whole rung: the green body and its green density.