Expanded Polystyrene Lost Foam Casting—Modeling Bead Steaming Operations

The retention of pattern voids observed in the production of expandable polystyrene patterns for lost foam casting can be traced to conditions developed during mold filling and subsequent steaming. Void formation and closure, or healing, was observed using high-speed video imaging through a clear acrylic sheet cut to match one-half of a test pattern mold. Two processing conditions, i.e., the initial bead packing density and the velocity of steam as it passes between beads, were shown to significantly impact the ability of a void to heal during steaming. A model is proposed to predict conditions where voids will heal based on three criteria that relate to a limitation of the processing window, the void size, and the ability of the bead to swell.