Risers: Metal Shrinks When It Freezes, and the Hole Goes Somewhere

Gravity does the feeding, so a riser feeds downward and sideways over a limited distance — roughly four times the section thickness along a plate, and less in alloys that freeze over a wide temperature range. The connection between riser and casting — the riser NECK — must stay liquid until the casting has finished shrinking. A long thin neck freezes first and cuts the riser off from the part it was supposed to feed. So the neck is short and generous, and the riser sits directly on or beside the heavy section. A riser placed at the end of a thin plate in the hope of feeding a boss at the other end feeds nothing at all.

An OPEN riser reaches the top of the mould, so air can push down on the metal as it feeds. It also lets you see when the mould is full and top it up at the end of the pour, which is genuinely useful. A BLIND riser is enclosed in the sand. It stays hotter because it has no exposed surface, but as the metal drains out it needs air to replace it — otherwise a vacuum forms and stops the feeding. That is what a WHISTLER or a permeable core in the riser top is for: a small passage that lets atmospheric pressure into the riser cavity. It is a small detail that makes the difference between a blind riser working and doing nothing at all.

The gating and risers can easily weigh as much as the casting. That metal is not wasted — it is cut off and remelted — but it must be MELTED, and running out mid-pour ruins the casting and the mould together. Weigh the pattern, multiply by the ratio of alloy density to pattern-material density, and add the gating and riser volumes. Then add a margin, because a crucible is never poured completely empty. Write the actual numbers down after the pour: what went in, what came out as casting, what came off as gating. After a few pours those figures let you predict a melt accurately, which is the difference between a foundry that works and one that improvises.