Sprue (molding)

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In casting, a sprue is the passage through which a molten material is introduced into a mold, and the term also refers to the excess material which solidifies in the sprue passage. In sand casting, the sprue is formed by a dowel which is removed from the sand to make the hole into which the metal is poured.

[edit] Injection Molding

In injection molding, the term sprue refers to the passages through which a liquid material (such as polystyrene) flows into a die, where it solidifies to form parts. It also refers to the material which solidifies in these passages, forming a framework attaching the parts together in a roughly planar arrangement.

Many scale model kits are made from injection-molded plastic. Hobbyists, such as builders of scale models, typically remove the parts of a model kit from the sprue using a sharp craft knife or razor saw. They may also use the sprue as a raw material to fabricate additional parts, such as railings on model ships, or antenna wires on airplanes.

Sprues in model kits often include engravings to identify the parts by number.

Some moldmakers make a distinction between three separate entities: the gate, the runner, and the sprue. The gate is the location at which the molten plastic enters the mold, and is often seen as a small nub or projection (the "gate mark") on the outside edge of the molded piece. The runners are large-diameter channels through which plastic flows, usually around the edges of the part or along straight lines. Finally, in this naming scheme, the sprue represents only the smaller channels that divert from the runners to the individual part. An analogy to the runner/sprue system might represent the runners with city water mains, and sprues with the smaller pipes leading to individual houses.