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Hot runners revolutionized the molding process by making part cooling the chief element dictating cycle time and not the sprue. Because they dramatically reduce scrap, hot runners have benefited molders during times of rising resin prices. The technology has also opened the door to coinjection and multi-component molding within the same machine.

Until the 1950s, molders accepted the necessity of generating a large percentage of scrap on every shot, in the form of sprues and runners that had to be discarded (in-plant recycling was yet to come into fashion). Worse yet, the sprue was usually thicker than the part, so it lengthened cycle time unproductively. In addition, secondary operations were required to separate the sprue from the part, often leaving an unattractive gate mark. The cold runner affected mold filling and packing due to the loss of heat from the melt while passing through the runner system. This caused pressure drops that could result in sink marks or under-filled parts.

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Efforts to remedy this limitation date back at least to 1940, when E.R. Knowles patented a runnerless molding device. Relatively little activity in this area is recorded until 1952, when Protective Closures Co. of Buffalo, N.Y., offered a hot-runner system for molding small caps. It used an electric heater plate bolted to the runner plate on the fixed platen. Throughout the 1950s, hot-runners saw limited use. Because they were built as an integral part of the mold, they were inaccessible when starting up or when a gate plugged, and they were hard to insulate from the cold mold. Thermocouple placement was also difficult. (This history is summarized in The Development of Plastics Processing Machinery and Methods by Joseph Fred Chabot, Jr.; John Wiley & Sons, 1992.)

A solution, in the form of an “external hot runner,” was introduced by Improved Machinery (Impco), Nashua, N.H., as early as 1954. It eliminated sprues and runners by gating directly into the cavity or cavities via multiple injection nozzles.

Watlow patented a cartridge-style heater in 1954, designed to keep material molten longer in hot runners. In 1958, Gerald D. Gilmore, one of the founders of Incoe, a builder of injection molds at the time, created an internally heated sprue bushing designed to keep the melt hot until it entered the mold cavity. Moldmaker Caco-Pacific says it also developed a hot-runner mold in 1959 that boosted productivity by 107% over cold-runner designs.

Mold-Masters Ltd. in Georgetown, Ont., patented what it claims to be the first truly commercial hot-runner system in 1963. Company founder Jobst Gellert created aluminum and beryllium nozzle components with cast-in heating elements that were fused to the body of the nozzle for improved heat transfer. He designed fused copper-alloy nozzles as well.

Hot runners didn’t really catch on until the late 1970s. Only 15% of new molds reportedly were built with hot runners in 1978. That number is believed to be 50% to 60% today, according to Incoe sources. One of the limitations was the large size of the probes or “torpedoes,” which ruled out hot-runners for very small parts. Since the 1990s, the trend has been to more and more compact nozzles, and nozzles that can edge-gate multiple parts. Gate pitch distances are now down to 7 mm.