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.
Function
Sprues can serve as filters, 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 it is large enough to retain its heat and stay liquid, as metal in the main casting cools and shrinks. The design of the sprue and runner system can be also utilized to trap unwanted dross and sand from continuing into the main cavity; this may include adding porous material to the runners, or designing the sprue to eject the dross to the side of the sprue using cyclonic separation.
Sprue design
The design of gating and runner is also essential for casting. The design can be done by using vertical and bottom gating.
For bottom gating
t_f = frac {2 A_m(sqrt {h_t}-sqrt {h_t-h_m})} {A_g sqrt {2g}}
where:
tf = Time for filling
Am = Area of mold
Ag = Area of gate
g = Acceleration due to gravity
ht = Total height
hm = Height of mold cavity
Injection molding
In injection molding, the term sprue refers to the passages through which a liquid material (such as polystyrene or polyvinyl chloride) 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.
Gates and runners
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 cavity and is often seen as a small nub or projection (the "gate mark") on 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.
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 runner as a raw material to fabricate additional parts, such as railings on model ships, or antenna wires on airplanes.
Runners in model kits often include engravings to identify the parts by number.
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