What are Runners and Sub-Runners in Injection Molding?
In the injection molding process, the runner system is the "bridge" connecting the injection machine nozzle and the mold cavity. It is responsible for transporting molten plastic to each cavity stably and uniformly, directly affecting the molding quality, production efficiency and cost control of plastic parts. As the core components of the runner system, runners and sub-runners have clear division of labor and work together, which are the key links to ensure stable and efficient injection molding production. Below is a detailed analysis of their definitions, structures, functions and core differences.
I. Runners: The "Main Channel" of the Runner System
In injection molding, the runner is the first channel that molten plastic flows through after entering the mold from the injection machine nozzle. It is also the "starting end" and "main delivery pipe" of the entire runner system. It is directly connected to the injection machine nozzle and undertakes the core task of transporting high-temperature and high-pressure molten plastic from the machine to the sub-runners or directly into the cavity (for single-cavity molds). It is the part with the largest diameter and the largest flow capacity in the runner system.
1. Structural Characteristics of Runners
The structural design of the runner needs to adapt to the size of the injection machine nozzle and the flow characteristics of the plastic. It usually adopts a conical structure with a taper of 2°-5°. This design can not only reduce the flow resistance of molten plastic, but also facilitate the smooth removal of runner scrap during mold release. One end of the runner is the feed port, which is closely attached to the injection machine nozzle. The diameter of the feed port is slightly larger than that of the nozzle to avoid material leakage, material accumulation and other problems; the other end is connected to the sub-runners or the cavity to realize the smooth transition of plastic.
In addition, the runner is usually set at the center of the mold (corresponding to the center of the injection machine nozzle) to ensure uniform force when the plastic enters the mold, reduce pressure loss and temperature loss during the flow process, and is especially suitable for the runner layout of multi-cavity molds.
2. Core Functions of Runners
Transportation function: Quickly and stably transport the molten plastic injected by the injection machine to the sub-runners or cavities, ensuring sufficient plastic flow to meet the molding needs.
Transition function: Alleviate the pressure difference and temperature difference between the injection machine nozzle and the mold cavity, so that the molten plastic can smoothly transition from a high-pressure and high-speed state to a state suitable for cavity filling, reducing defects such as shrinkage marks, bubbles and material shortage in plastic parts.
Guiding function: Provide a clear flow direction for molten plastic, avoid disorderly flow of plastic in the mold, ensure the filling process is carried out in an orderly manner, and improve the dimensional accuracy of plastic parts.
II. Sub-Runners: The "Branch Pipes" of the Runner System
Sub-runners are branch channels connecting the runner and the mold cavity, mainly used in multi-cavity injection molds. After the runner transports the molten plastic to the designated position, the sub-runners will evenly distribute the plastic to each cavity, ensuring that the filling speed, pressure and temperature of each cavity remain consistent, thereby ensuring the dimensional consistency and quality stability of multi-cavity plastic parts.
1. Structural Characteristics of Sub-Runners
The structural design of sub-runners needs to be flexibly adjusted according to the number of cavities, layout and plastic part size. Common cross-sectional shapes include circular, trapezoidal, rectangular, etc., among which the circular cross-section is the most widely used—it has the smallest flow resistance and uniform heat dissipation, which can minimize the energy loss of plastic during the flow process; trapezoidal and rectangular cross-sections are convenient for mold processing and the removal of runner scrap.
The diameter of sub-runners is usually smaller than that of the main runner, and the length needs to be reasonably designed according to the cavity layout, trying to shorten the flow path and reduce bends, so as to avoid excessive pressure loss and too fast temperature drop during the plastic flow process. At the same time, the layout of sub-runners must follow the principle of "equal distance and equal diameter" to ensure consistent flow resistance of each branch and realize uniform distribution of plastic.
2. Core Functions of Sub-Runners
Distribution function: Evenly distribute the molten plastic transported by the runner to each cavity, ensuring that the filling volume and filling speed of each cavity are consistent, and avoiding material shortage in some cavities and material overflow in others.
Pressure reduction and stabilization function: Further reduce the pressure and flow rate of molten plastic, so that the plastic enters the cavity in a more stable state, reducing defects such as flow marks and gate marks on the surface of plastic parts, and improving the appearance quality of plastic parts.
Auxiliary heat dissipation function: The surface area of sub-runners is relatively large, which can assist in dissipating part of the heat of molten plastic, allowing the plastic to cool gradually during the filling process, and avoiding plastic part deformation, uneven shrinkage and other problems caused by excessive temperature.
III. Core Differences Between Runners and Sub-Runners
Although both runners and sub-runners belong to the runner system, they have obvious differences in function, structure and design requirements. The specific comparison is as follows:
| Comparison Dimension | Runners | Sub-Runners |
| Core Function | Connect the injection machine to sub-runners/cavities and mainly transport plastic | Connect the runner to cavities and distribute plastic to each cavity |
| Structural Shape | Mostly conical with a taper of 2°-5° | Circular, trapezoidal, rectangular, etc., with uniform cross-section |
| Size Characteristics | Large diameter, the thickest part in the runner system | Diameter smaller than the runner, adjusted according to cavity layout |
| Application Scenario | Required for both single-cavity and multi-cavity molds | Mainly used for multi-cavity molds; can be omitted for single-cavity molds |
| Design Focus | Adapt to nozzle size, reduce material leakage and pressure loss | Equal distance and equal diameter to ensure uniform plastic distribution |
IV. Key Design Points of Runners and Sub-Runners
The rationality of the design of runners and sub-runners directly affects the efficiency of injection molding production and the quality of plastic parts. The core design points are as follows:
- Match plastic characteristics: Adjust the diameter, length and taper of the runner according to the fluidity, melting point and other parameters of the plastic—for plastics with poor fluidity (such as PC, ABS), the runner diameter needs to be increased to reduce flow resistance; for plastics with good fluidity (such as PE, PP), the runner diameter can be appropriately reduced to reduce raw material loss.
- Optimize layout design: The runner must be aligned with the center of the injection machine nozzle, and the sub-runners must follow the principle of "equal distance, equal diameter and shortest path" to reduce bends and excessively long flow paths, avoiding pressure loss and temperature drop.
- Facilitate mold release and cleaning: The runner design must consider the convenience of taking out the runner scrap. The conical runner and smooth inner wall of the runner can reduce the adhesion of runner scrap and lower the cleaning cost.
- Control cost loss: On the premise of ensuring molding quality, try to reduce the runner size to reduce the loss of molten plastic (runner scrap needs to be recycled and reused), thereby reducing production costs.
V. Summary
Runners and sub-runners are two indispensable core parts of the injection molding runner system. They work together to form a complete transportation path for molten plastic from the injection machine to the cavity. As the "main channel", the runner undertakes the core transportation task; as the "branch pipes", the sub-runners are responsible for uniformly distributing plastic. The rationality of their design directly determines the quality, production efficiency and cost of plastic parts.
In actual injection molding production, it is necessary to optimize the structure and layout of runners and sub-runners in combination with the plastic part structure, the number of cavities, plastic characteristics and injection machine parameters, so as to achieve efficient, stable and high-quality injection molding production and escort product quality.
