。
The casting came cleanly out of the mold.
Literal
Casting [subject-が] mold [from-から] completely came-out.
鋳物 (いもの, 'casting, cast metal object') is metalworking vocabulary. 金型 (かながた, 'metal mold/die') is the manufacturing term. すっぽり here means 'cleanly, completely' — the casting separated from the mold without sticking. とれた (from 取れる, 'to come off, to be removed') describes the successful release. A satisfying manufacturing moment.