Hot Runner
A hot runner is a mold system that keeps the runner channels carrying the melt from the machine nozzle to the cavity continuously at melt temperature with heaters. Unlike a cold runner, no runner is formed that solidifies and is discarded in each cycle: material waste, runner removal labor and runner cooling time are eliminated. Each zone (manifold, nozzle) is managed from the temperature controller through a separate control channel.
A hot runner mold requires as many control channels as it has zones. The controller reads the thermocouple signal of each zone and powers the heater by PID; it diagnoses conditions such as a broken cable, a reversed thermocouple connection and a triac failure. The quality of the controller shows in the part as the temperature balance between zones.
The price is the mold cost and the material left in dead zones during color changes; in jobs with frequent color changes the manifold design (flow-balanced, without dead volume) becomes important.
In high-volume, multi-cavity molds, with expensive raw material and in fully automatic production. In low-volume, multi-color jobs with frequent mold changes, a cold runner may remain economical.