What is a Sprue in Die Casting - Difference Between Sprue and Runner | Diecasting-mould

The design of die casting mold allows manufacturers to get a complete product that does not need assembly, however, it may leave some excess material that needs to be removed from the casting. Sprue is one of the causes. What is a sprue in die casting? What is the difference between sprue, runner and gate?

What is a Sprue in Die Casting?

A sprue is a vertical passage through which the molten material flows into the casting mold, it is the first channel for liquid metal to enter the mold cavity, and it is also the primary part of pressure transmission, so its size will affect the flow speed and filling time of liquid metal. The sprue connects the pouring basic to the runner, it is usually tapered downwards to minimize turbulence and the formation of air bubbles. The sprue is used to guide the metal down from the sprue gate into the runner, inner gate, or directly into the cavity. It provides enough pressure to make the liquid metal overcome all kinds of flow resistance under the action of gravity, and fill the cavity within the specified time. Common shapes of sprue including the cone shape with a big top and small bottom, the column shape with equal section, and the inverted cone shape with a small top and big bottom. For magnesium and aluminum alloy castings, serpentine, flaky and slit sprues are also used. In the die casting process, the material in the sprue will solidify and need to be removed from the finished part.


Here is the simple structure of the gating system, you can find the sprue, runner, and gate in it. (image is from google)

Difference Between Sprue and Runner

The flow path into the mold cavity consists of a sprue, runner and gate, how do they work, and how to distinguish them?

1. The sprue is the first channel for liquid metal to enter the casting mold, then the metal flows into the runner. 

2. A sprue is a large-diameter channel through which the material enters the mould. A runner is a smaller diameter channel that directs the molten metal towards the individual part. A similar example can be found in a water system that uses a water main (gate) and a smaller pipe (runner) to each house.

3. A trapezoid runner or round runner design is desirable, there is a wide variety of shapes of the sprue. 

Difference Between Sprue and Gate

Sprue and gate are different parts of the gating system and have different functions. So the die casting sprue and gate differ in shape, size, position and more aspects. 

1. The sprue is the channel of the material that leads into the runner, and then flows into the mold cavity through the gate. The sprue may be considered the continuation of the mold to the nozzle of the casting machine. The sprue needs to be large enough to fully pack the cavity. The cone-shaped sprue with a big top and a small bottom is in the full flow state, while the sprue with equal cross-section and inverted cone-shaped sprue with small top and the big bottom is in the nonfull flow state.

2. The gate is at the end of the runner, leading into the mold cavity. It is small and designed to freeze before the part, runner, or sprue freeze. Usually, the gate is smaller than the wall thickness of the part. But too small gate can cause defects, tool wear, and incomplete filling. Gate size is determined by filling speed and component thickness.