
A sprue is the vertical passage through which liquid material is introduced into a mold and it is a large diameter channel through which the material enters the mold. It connects the pouring basin to the runner. In many cases it controls the flow of material into the mold. During
casting
Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a ''casting'', which is ejected ...
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
Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a ''casting'', which is ejected ...
, a sprue is the passage through which a molten material is introduced into a
mold
A mold () or mould () is one of the structures certain fungi can form. The dust-like, colored appearance of molds is due to the formation of spores containing fungal secondary metabolites. The spores are the dispersal units of the fungi. Not ...
, and the term also refers to the excess material which solidifies in the sprue passage.
Function
Sprues can serve as filters, as
heat sink
A heat sink (also commonly spelled heatsink) is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is dissipated away from the device, t ...
s, 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
injection molding
Injection moulding (U.S. spelling: injection molding) is a manufacturing process for producing parts by injecting molten material into a mould, or mold. Injection moulding can be performed with a host of materials mainly including metals (for ...
, sprue refers to the passage through which a liquid material (such as
polystyrene
Polystyrene (PS) is a synthetic polymer made from monomers of the aromatic hydrocarbon styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin per unit weight. It is ...
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.
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
scale-model kits are made from injection-molded plastic. Hobbyists typically remove the parts of a model kit from the runner using a sharp
craft knife or
razor saw. The sprues usually form a rectangle with the runners and parts inside which makes them easier to box.
Model maker
A model maker is a professional craftsperson who creates a three-dimensional representation of a design or concept. Most products in use and in development today first take form as a model. This "model" may be an exacting duplicate (prototype) of ...
s sometimes use sprues or runners as raw material to fabricate additional parts, such as railings on
model ships,
antenna wires on
airplanes
An airplane or aeroplane (informally plane) is a fixed-wing aircraft that is propelled forward by thrust from a jet engine, propeller, or rocket engine. Airplanes come in a variety of sizes, shapes, and wing configurations. The broad spectr ...
, or
greeble
A greeble ( ), or "nurnies", is a part harvested from plastic modeling kits to be applied to an original model as a detail element. The practice of using parts in this manner is called " kitbashing".
Etymology
The term "greeblies" was first us ...
s on fictional spacecraft.
Sprues in model kits often include engravings to identify the parts by number.
See also
*
Chvorinov's rule
References
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Casting (manufacturing)
Plastics industry