Molding is typically used for plastic materials that are injected, transferred under pressure, or poured into a mold. Just as some shapes are easier to create using additive or subtractive manufacturing methods, molding and casting are often the best options when a part is needed with complex surface features or contours that would not be easily created using other prototyping methods. Filling the mold with a soft or liquid material and allowing it to harden results in a positive part with the desired shape. Both involve using a cavity (usually called a mold or die) that represents a negative of the final part to be created. Molding and casting are similar methods of producing a part. The tolerances and surface finishes achievable by investment casting are such that machining is not generally required.
When the part has cooled, the mold is destroyed and the part removed. To facilitate filling of the mold the melt is poured while the mold is still hot. To make parts, the mold cavity is filled with molten metal that is allowed to solidify. Following this the mold is usually placed in a second oven to cure for 12 to 24 hours. The completed mold is placed in an oven and the wax removed by melting and evaporation. A metal hollow tube is now placed over the wax patterns and a slurry-such as plaster of Paris-is poured to entirely cover the patterns. If the production volume is large enough several wax patterns are attached to a tree that contains the runners, gates, and other features that will feed and distribute the molten metal. The wax pattern is removed from the mold and attached to a wax base that contains a gate. The external shape of the wax pattern resembles the internal shape of the mold. After the mold is formed, wax is injected to form a pattern.
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