What is Runners?

Runners, also known as channels, are fundamental elements in the injection molding process. They are the flow paths that carry molten plastic from the injection machine’s nozzle to the mold cavities. Although they are not part of the final product, runners in injection molds play a critical role in process efficiency, part quality, and production cycle optimization.

Proper design of molten plastic distribution channels is essential to ensure uniform delivery of material to all cavities, with the right temperature, pressure, and flow rate. Poor runner design can lead to issues such as flash, short shots, visible weld lines, or internal defects caused by air entrapment or uneven flow.

There are various types of runners depending on the mold’s complexity and the part requirements. The most common are cold runners and hot runners. In cold runner systems, the plastic solidifies in the channel and is ejected along with the molded part, creating waste that must be recycled or discarded. Hot runner systems, on the other hand, maintain the plastic in a molten state via heated elements, eliminating runner waste and shortening cycle times.

The design of gating systems must consider multiple factors: type of polymer, material viscosity, number and arrangement of cavities, injection pressure, fill time, and shrinkage during cooling. A well-balanced runner system improves productivity by ensuring uniform cavity filling and avoiding uneven injection that could result in out-of-spec parts.

Another important aspect is the geometry of the runners in injection molds. Cross-sections may be circular, trapezoidal, or semicircular depending on machining convenience, material type, and flow efficiency. Gate design and secondary channels also influence how the cavity fills and how the part behaves during ejection.

From an efficiency standpoint, hot runner systems involve higher initial investment but offer major advantages for long runs or high-value parts: they eliminate the need to reprocess runner waste, improve surface finish, and enable shorter cycle times. However, they also require tighter thermal control and more complex maintenance.

At companies like Plásticos Lezo, with extensive experience in technical molds and customized series, optimizing molten plastic distribution channels is a key phase in every project. Flow simulation tools (such as Moldflow) are used to predict material behavior inside the mold, refine runner design before tooling, and prevent costly modifications or delays during startup.

In summary, runners in injection molds are not just flow channels—they are core components of gating system design that directly affect part quality, process efficiency, and the sustainability of plastic manufacturing. Careful planning and execution of runner systems allow full optimization of injection molding, ensuring functional, precise, and repeatable plastic components.