Sprue (manufacturing)
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A sprue is a large diameter vertical channel through which liquid material is introduced into a mold. It connects the pouring basin to the runner. 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. It is usually tapered downwards to minimize turbulence and formation of air bubbles.
Casting
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.
Function
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 is tapered with its bigger end at the top to receive the liquid metal, the smaller end is connected to the runner.
Sprue design
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:
- = Time for filling
- = Area of mold
- = Area of gate
- = Acceleration due to gravity
- = Total height
- = Height of mold cavity
This equation may change if the height of gating is equal to height of casting material.
Then the equation will be:
or, simplified,
where:
- = Time for filling
- = Area of mold
- = Area of gate
- = Acceleration due to gravity
- = Total height (Height of gating + height of mold cavity)
- = Height of mold cavity
Injection molding
In
Sprues, runners, and gates
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.
Many
Sprues in model kits often include engravings to identify the parts by number.