Hot top, sprue and riser: the vocabulary of a casting
Most of the metal poured into a mould is never part of the finished part. The words for the sacrificial bits explain how casting actually works.
A casting starts as a shape cut into sand or held in a die, but the shape of the part is only half the design. The other half is the plumbing that gets liquid metal into the cavity and manages what happens as it freezes.
Metal shrinks as it solidifies. If nothing compensates, the last region to freeze pulls material from its surroundings and leaves a void inside the part. Almost every strange word in casting exists to deal with that one fact.
The path in
The sprue is the vertical channel the metal is poured down. From there it travels along runners and through gates into the cavity itself. A spray is a side branch of the runner, used to distribute metal to more than one point.
The reservoir
A riser is a reservoir of molten metal attached to the casting, positioned so that it stays liquid longer than the part does. As the part shrinks, it draws from the riser instead of from itself, and the shrinkage void ends up in the sacrificial metal where nobody cares about it.
Keeping that reservoir liquid is the job of the hot top, a live open riser fitted with a ceramic liner that slows heat loss. It is a small piece of insulation doing a structural job, because a riser that freezes early stops feeding and the defect moves back into the part.
What gets cut off
When the casting is broken out, all of that plumbing comes with it and has to be removed. Flash is the thin fin of metal squeezed into the joint between mould halves, and it marks the parting line for anyone reading the part later.
Read a casting drawing with those words in mind and the geometry starts making sense. Heavy sections get risers. Thin sections get gated early so they do not freeze before filling. The part shape is a requirement, and everything else on the drawing is the solidification plan.