Gating: Getting the Metal In Without Wrecking the Mould

The SPRUE is the vertical channel the metal is poured into. It should be TAPERED, wider at the top, because falling metal accelerates and narrows — a parallel sprue leaves a gap around the stream that sucks in air. The RUNNER is the horizontal channel at the bottom. It distributes metal and slows it down, and it is usually in the drag with the gates in the cope so that any floating dross stays in the runner. The GATES are the small openings from runner into cavity. Several small gates fill more gently than one large one and let the mould fill from more than one place, which matters on anything long or thin.

A POURING BASIN at the top of the sprue lets you fill it and keep it full, so the sprue never runs empty mid-pour. Pouring straight into a bare sprue hole guarantees an interrupted, air-entraining stream at the start. A runner extension beyond the last gate — a DIRT TRAP — gives the first, dirtiest metal somewhere to go that is not the casting. It costs a few centimetres of runner and catches the slag that came down the sprue first. Ceramic foam FILTERS do the same job far better and are cheap. They also slow and smooth the flow, which is why a filter often improves a casting for two reasons at once.

Start the pour and bring the basin up to full quickly, then hold it there until the mould is full. A pour that starts timidly and then speeds up entrains air at the very start, which is the metal that ends up deepest in the casting. Never stop and restart. A pause lets the stream break, lets air in, and can leave a COLD SHUT where the metal that stopped has skinned over before the rest arrives. Have enough metal melted before you start. Running out halfway is the single most avoidable failure in a foundry, and it is prevented by weighing the pattern, multiplying by the alloy's density ratio, and adding a generous allowance for the gating and risers — which is usually a third of the total again.