Hot runner

A hot runner is an injection mold component containing a series of channels that distributes molten plastic within a mold. Unlike an ordinary cold runners, the hot runners are heated, so the plastic melt in the hot runners never freeze.

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Hot runners are reasonably complicated devices, so they are usually assembled from the premanufactured units. The two main types of hot runners are the externally heated and internally heated. The later have some slender heaters inside the runners; the outside boundaries of the runners usually have the temperature below the melt-freeze, so there is a layer of a frozen plastic that insulates the internal heater and the molten plastic from the cold outside boundaries.

Hot runners usually make the mold more expensive to manufacture and run, but allow savings by reducing the plastic waste and sometimes by reducing the cycle time (do not have to wait until the runners would freeze).