Moulding Sand: Holding a Shape and Letting the Gas Out

Silica sand is the bulk and the refractory — it is what survives the heat. Bentonite clay, 5 to 10% by weight, is the binder. Water, 2 to 4%, is what makes the clay sticky. Too dry and the mould crumbles when the pattern is drawn. Too wet and the metal flashes the water to steam faster than it can escape, which blows gas back through the casting and in bad cases throws metal out of the mould. The squeeze test is the traditional measure and it works: squeeze a handful, and it should hold its shape and show your finger marks sharply, then break cleanly in two rather than crumbling or smearing. That is a real test of exactly the two properties that matter.

Fill the flask in layers and ram each one. Under-rammed sand collapses and washes away when the metal runs; over-rammed sand has no permeability left and traps gas. Ram harder at the edges of the flask and more gently over the pattern. The pattern face needs enough strength to hold detail and enough openness to breathe, and it is the one place where over-ramming does the most harm. Strike the surface off level with a bar and cut VENTS — fine holes pushed down almost to the pattern with a wire. They cost seconds and give the gas somewhere to go other than through the metal.

Dust the parting face with a fine powder before ramming the second half, or the two halves of sand bond to each other and the mould cannot be opened without destroying it. Before drawing the pattern, RAP it — tap it gently in several directions to loosen it from the sand. Then lift it straight up, slowly, without tilting. A tilted draw drags the mould wall and the damage is usually not repairable. Repair small damage with a wet sand slurry and a fine tool, and accept that a badly torn mould is quicker to re-ram than to patch. Patching a mould is where beginners lose an afternoon to a casting that was never going to come out.

Green sand can be used again and again. Break up the used mould, remove the burnt-black sand that touched the metal if it is badly scorched, riddle the rest to break the lumps, and re-temper with water. It changes over time. Repeated heating burns out some clay, so a working sand needs occasional new bentonite; it also picks up fines that reduce permeability, so it needs occasional fresh sand. Keep it covered and it keeps its moisture; leave it open and it dries out overnight. A foundry's sand is a living working material rather than a consumable, and its condition is worth a line in the log alongside the pour.