Sprue (manufacturing)

A sprue is the passage through which liquid material is introduced into a mold. In many cases it controls the flow of material into the mold. During casting or molding, the material in the sprue will solidify and need to be removed from the finished part. This excess material is also called a sprue.

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.

Sprues can serve as filters, as heat sinks, and as feeders. Bronze, in particular, has a high shrinkage rate as it is cooling; a sprue can continue to provide molten metal to the casting, provided that it is large enough to retain enough heat to stay liquid, as metal in the main casting cools and shrinks. The design of the sprue and runner system can be also used to prevent dross and sand from continuing into the main cavity; this may include adding porous material to the runners or using cyclonic separation to move the dross to the side of the sprue.

The design of the sprue gating and runner is also essential for casting. The design can incorporate either bottom or vertical gating.

For bottom gating

where:

This equation may change if the height of gating is equal to height of casting material.

Then the equation will be:

or, simplified,

where:

In injection molding, sprue refers to the passage through which a liquid material (such as polystyrene or polyvinyl chloride) flows into a die, where the material solidifies to form parts. Sprue also refers to the material that solidifies in these passages, forming a framework that attaches the parts in a roughly planar arrangement.

Some moldmakers distinguish the sprue, the gate, and the runner. The sprue is a large-diameter channel through which plastic flows, usually around the edges of the part or along straight lines. The runner is a smaller channel from the sprue to the individual part. An analogy may be found in a water system that employs a water main (sprue) and smaller pipes (runners) to individual houses. The gate is the location at which the molten plastic enters the mold cavity and is often evidenced by a small nub or projection (the "gate mark") on the molded piece.

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Wikipedia