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Waxwork
The simplest rubber mold is open-faced like the one below, for a decorative coverplate for a hinge. This is the exception rather than the rule because the surface detail is extremely intricate. Usually a simple piece like this would be sand-molded.
Very large pieces are divided into sections, and in these cases wax is "slush cast". The mold is rocked and turned to coat the interior with wax. The excess is poured out as it cools and hardens. Several layers of wax are built up in this way, creating a hollow copy of the original.
The wax pattern is allowed to cool and harden enough to remove it by gently pulling away the plaster mother and rubber molds that envelop it.
The wax must now be finished to remove bubbles or any other imperfections, including the parting lines where the sections of the mold came together. This is a meticulous process that greatly reduces the need for surface finishing of the final metal piece. A combination of heat and tools (often clay-working or dental tools) is used to achieve the proper detail in the wax.
When the wax pattern faithfully replicates the original, it is attached to a support structure, a gating system that includes a cup (at the top in the picture at right) where the metal will be poured, runners and sprues from the cup to the gate, which while in wax hold the patterns together but will be the gateway for the metal in the mold cavity. The small wax rods are vents, which will allow air to escape as metal rushes through the gate into the mold cavity.
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