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Sprue gate runner

Sprue gate runner

Sprue, gate and runner are the 3 most important design factors that determine the quality of a conventional plastic injection mould. The material flow path between the injection moulding machine’s nozzle and the final component is composed of these three components, which deliver molten plastic from the injection moulding machine barrel to the mould’s cavity to create the desired product.

Sprue gate in injection moulding

A good mould design will have the sprue, gate and runner designed to have as little waste as possible. This means that the runner, gate, and sprue should be as small as possible, while still large enough to fill and pack the cavity to get an acceptable moulded part.

SPRUE – What is sprue in injection moulding?

Sprues typically have a diameter of about 6mm and taper with a 5-degree angle. This angle on the sprue helps it release from the sprue bush of the mould when it opens, so it can be ejected and recycled. To achieve faster production and reduce the quantity of regrind material, the sprue is kept in hot condition with an external heater. This system is called a Hot Sprue system or a Hot Runner system.

Sprue in Plastic injection mould


In a mould, if the runner is too small, this will result in the cavity filling very slowly, which is not desirable. We recommend a large runner with a diameter between 3mm and 6mm. The circular profile of a runner reduces the friction and loss of heat during the injection of the material into the mould. For multi-cavity moulds, balancing the runner to fill all the cavities is very critical and important. The flow length between the machine nozzle to the cavities must be balanced and equal for all the cavities to ensure every cavity is filled with the same pressure, speed, and pressure.

Runner in Plastic injection mould


The end of the runner in any mould is the gate, which is the point of entry of the molten material into the cavity of the mould. This gate leaves a small blemish on the surface of the final part. The gate is the most important and critical part of the mould design as it must freeze before the part, runner, and sprue. This helps in keeping the cavity packed with sufficient pressure and avoids sink marks on the surface of the part during cooling. The component sticks to the cavity if excessive pressure is used in the gate while injection of the material.

Gate in Plastic injection mould

What is sprue runner & gate for?

The position of the gate is best when it is located on the thickest portion of the component. This helps in reducing the heat loss, and premature freezing of the gate and lowers the pressure that is required to fill the mould completely. The placement of the gate also determines the flowlines or weld lines on the component. These flowlines or weld lines can be avoided if the material temperature is maintained suitably. The wrong location of the gate or any reduction in the material temperature will form a weld line/ flow line defect on the surface of the component.

Various computer software programs can simulate the design and suggest different moulding parameters and gate positions before manufacturing the mould.

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