This rubber cap seals the 4" opening at the bottom of the vacuum chamber in the V.I.C. 12 casting machine. It must be removed in order to perform regular maintenance on the chamber. Periodically (and always after a flask blowout), clean the accumulated investment and metal from the bottom of the vacuum chamber. To access the chamber, remove the adjustable clamp band from the black rubber cap. In the cap or immediately inside the vacuum chamber, is a soft fiber ceramic-like insulation material. If the fiber pad is damaged, replace it. If not, remove debris from it and re-install it in the cap. After cleaning out the chamber, with the fiber insulation pad in place at the bottom of the cap, reinstall the cap at the bottom of the vacuum chamber, and tighten the clamp band to secure the assembly.