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What is a Hot Runner?

Paul Scott
Paul Scott

A hot runner is a heated nozzle and manifold assembly installed on injection molding equipment. This assembly allows the plastic charge material left in the feed mechanism to remain fluid after injection while the part itself cools and solidifies. Injection molds equipped with hot runners are more economical, featuring faster cycle times and less material wastage. These savings are possible due to the fact that the manifold and nozzles keep the plastic in them fluid between injection cycles, eliminating the wasted time and material associated with the solidified “runners” in conventional cold molds. Installing a heated runner assembly adds significantly to the cost of any mold, limiting the viable use of the devices to high-production volume processes.

Injection molding is a production process where granulated materials, typically various grades of plastic, are melted and injected into a mold under pressure. Once the injection process is completed, the mold and parts are allowed to cool and solidify, allowing the product to be ejected from the mold cavity. The path within the mold followed by the melted plastic prior to reaching the actual cavity is known as a manifold or runner. Typically, these consist of one or more narrow channels. each ending in a nozzle which forms the cavity entrance. In conventional cold mold processes, the plastic left in these channels and nozzles cools and solidifies along with the molded part.


These solidified sprues or “runners” are then discarded prior to commencement of the next injection cycle. This not only represents material wastage, particularly in cases where the runners can't be recycled, but also adds a production step to the process. This increases cycle times, cuts down on productivity and increases the unit cost of the parts produced. The use of a hot runner system almost entirely eliminates these problems by keeping the plastic in the manifold channels and nozzle fluid between injection cycles. This is achieved by including electrical heating elements in the manifold and nozzles, which keeps those parts at a constant temperature of approximately 550 to 590 °F (290 to 310°C).

Hot runner heating elements fall into two categories: internal and external types. External hot runner heaters are located within the manifold body immediately adjacent to the channels and around the outside of the nozzles. Internal heater elements are located within the channels and nozzles. Although the internal heating method is used in some specialist applications and older machines, the more efficient external heating method has largely replaced it. While the use of hot runner systems represents significant savings, the high associated installation costs restrict their use.

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