gel casting
Think of how liquid gelatine sets a fruit jelly: you pour a runny liquid into a mould, a chemical network forms all through it, and minutes later the whole thing is a firm solid that holds the mould's shape when you turn it out. Gel casting does that with ceramic. You pour a fluid, high-solids ceramic slurry that carries dissolved organic monomers into a mould, trigger the monomers to polymerize in place, and the liquid sets into a stiff gel that locks the powder in position — giving a strong green body you can demould, machine, and fire.
The elegance is in how little binder it needs. Unlike injection molding, which carries 40 volume percent of wax just to flow, gel casting builds strength from a thin three-dimensional polymer web threaded through the packed powder, so only a few weight percent of organics are present. That means far less to burn out and much higher green density and green strength — strong enough that a gel-cast blank can be turned on a lathe or milled while soft, then fired. The original chemistry used acrylamide monomers, which are neurotoxic and pushed developers toward safer monomer systems, but the principle is unchanged: a slurry that must stay low-viscosity and pourable right up until the moment the initiator and catalyst kick off the gelation.
Gel casting shines for large, complex, high-quality parts where the binder burden of injection molding is unwelcome: turbine rotors, crucibles, sputtering targets, and intricate structural ceramics. It marries the free-form geometry of casting with the high, uniform green density usually associated with pressing. Its demands are a well-behaved, deflocculated slurry (the colloid chemistry it borrows) and careful control of the polymerization so the whole part gels uniformly without premature setting in the pipes or slow setting that lets particles settle.
To make a large silicon-nitride turbine part, a low-viscosity Si3N4 slurry premixed with monomer is poured into a mould, an initiator sets off polymerization, and within minutes the slurry gels into a rigid green body strong enough to demould and green-machine. With only a few percent organics, burnout is quick and green density is high and uniform.
In-situ polymerization sets a poured slurry into a strong green body with little binder — casting's freedom, pressing's density.
The classic acrylamide monomer system is toxic and demands care; safer monomer chemistries now exist. Either way the slurry must gel uniformly, so an initiator that fires too early clogs the line and one that fires too late lets particles settle into a gradient.