How is the finished metal part recovered from the refractory mould in the investment casting process?

• A. The refractory mould is destroyed by vibration to leave the metal part
• B. The mould is dissolved in acid
• C. The wax remains
• D. The part is cut from the runner

Answer: (A)

In investment casting, the metal part is formed inside a brittle ceramic shell. After the metal solidifies, the shell must be removed without damaging the part. The shell is designed to break away from the metal when the assembly is vibrated or shaken, so the ceramic can be shattered and peeled off, leaving the finished part free. This takes advantage of the shell’s fragility and its relatively weak adhesion to the solidified metal. After the shell comes off, the part is then separated from the gating system, typically by cutting off the runner and finishing as needed. The other options don’t fit because the ceramic shell isn’t dissolved in acid, the wax is already removed during burnout, and cutting from the runner occurs after the shell has been removed rather than as the mold is broken away.