“Hot runner” is a term used in injection molding that refers to the system of parts that are physically heated such that they can be more effectively used to transfer molten plastic from a machine’s nozzle into the various mold tool cavities that combine to form the shell of your part. Sometimes they are called “hot sprues.” You can contrast the term “hot runner” with its opposite, and the historically more common “cold runner.” Cold runners are simply an unheated, physical channel that is used to direct molten plastic into a mold tool cavity after it leaves the nozzle. The primary difference is that hot runners are heated while cold runners are not.

While hot runners are not required for injection molding processes, they can be useful to ensure a higher quality part. They are particularly beneficial with challenging part geometries that require lower margin of error in the flow properties of the molten plastic (i.e. where inopportune cooling or temperature deltas might result in uneven flow). Further, hot runners can be beneficial in reducing wasted plastic during high volume shoots. Because cold runners are unheated, the channel needs to be larger and thus more plastic needs to be shot during each cycle. If you are shooting a large number of parts while iterating to get the design correct you could easily run up the cost of plastic above the cost of a hot runner assembly. The downside to hot runner technology, absent the aforementioned example, is that it is more expensive by default than a cold runner setup.

The advantage of hot runners is that, if designed properly, the plastic will flow from the machine’s nozzle more uniformly into the gate locations. A gate location is the point at which molten plastic enters the injection mold tool cavity. Gate location, plastic temperature, the design of internal mold cavities, and the material properties of the plastic itself as well as that of the mold tool all have an important impact on the success or failure of the injection molding process. Hot runner technology has been around since the middle of the 20th century but it was used only sporadically until technological improvements and market forces surrounding the price of material inputs made it more viable in the 1990s.

Hot runners are designed to maximize manufacturing productivity by reducing cycle time. One of the reasons they didn’t take hold when they first came out is that they needed to maintain the molten plastic at a uniform temperature while the injection mold cavity is simultaneously being cooled. This requires a fair level of complexity. The initial (now obsolete) designs implemented internal heating with isolated heaters inside channel cavities. Internally heated hot runner designs resulted in solidified plastic on the internal boundaries of the channel with molten plastic much more localized to the specific heater location. By contrast, externally heated runners utilize heated nozzles and a heated manifold and based on the high thermal conductivity of metal they are able to maintain much more even flow properties for the internal plastic.