Rapping involves tapping a wooden or metal pattern with a mallet or rapping bar before drawing it out of the sand. The vibration slightly enlarges the mold cavity and breaks the surface tension, allowing the pattern to lift out cleanly without tearing the sand walls. Excessive rapping can distort the final casting dimensions.
Heard on the job
Give the corebox a good rap before you try to pull it, or you'll tear the sand.
Insider tip
Rookie moulders often rap too hard in one direction, creating an oval or oversized cavity instead of just freeing the pattern.