rap
To free (a pattern) in a mould by light blows on the pattern, so as to facilitate its removal.
rap: tap a pattern loose from its sand mould
In sand casting, rapping is the controlled tapping of a pattern against the walls of its mould cavity to break the adhesive bond between pattern and sand. The pattern, typically made from wood, plastic, or metal, sits in compacted molding sand; without this loosening step, extraction risks tearing the mould and ruining the casting. A rapping bar, a flat or tapered steel tool, is struck lightly against the pattern's edges and surfaces, vibrating it enough to separate sand particles without jarring the entire mould apart.
The technique demands judgment. Too gentle, and the pattern sticks; too forceful, and the mould collapses, the pattern splinters, or the drag (lower mould half) shifts relative to the cope (upper half), causing a step in the final casting. Experienced molders develop feel for the correct blow force and rhythm, often rapping in a methodical sequence around the pattern's perimeter. Machine-operated rapping arms now standardize this in high-volume foundries, using pneumatic or hydraulic strikers calibrated to specific pattern weights and sand types.
Pattern material and mould condition
Wooden patterns, common in small jobbing shops, rap more readily than plastic or metal because wood flexes slightly; metal patterns used in production casting require more aggressive rapping. Molding sand composition affects adhesion too: greensand (unbaked, moisture-bound) releases differently than chemically bonded core sand. Parting compound, a dry talc or graphite powder dusted onto the mould surface before pattern insertion, reduces sticking and cuts rapping force needed by half.
Rapping also serves a secondary function: it settles the sand slightly, improving dimensional accuracy and reducing subsurface porosity near the pattern walls. The vibration closes microvoids that would otherwise trap gas during metal pour. For this reason, even in foundries with mechanical pattern extraction (stripper plates), light manual rapping is often retained as a finishing step before the pattern is withdrawn.
The term itself reflects the metalworking world's onomatopoetic naming habit: the rap is the actual sharp blow struck, and the verb "to rap" describes both the blow and its effect. It appears in foundry manuals from the 19th century and remains standard trade language, though the technique has not fundamentally changed since sand casting emerged in the medieval period.