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Considerations between the hot runner and cold runner systems
2025-07-31
In the injection molding process, the runner system is the "bridge" connecting the injection molding machine nozzle and the mold cavity. Its design directly affects the quality, production efficiency and cost of plastic products. According to the state of the plastic melt in the runner, it can be divided into two types: hot runner system and cold runner system. Selecting a suitable runner system is a key link in optimizing injection molding production.
Cold Runner Injection Molding
The cold runner system is the most traditional and widely used runner form in injection molding. Its core feature is that the runner and the mold cavity use the same cooling method - cooling through the cooling water channel in the mold, so that the plastic melt in the runner is cooled and solidified together with the product.
Structure and working principle
The cold runner system usually consists of a main runner, a branch runner, a gate and a cold material hole:
Main runner: directly connected to the nozzle of the injection molding machine, it is the first channel for the melt to enter the mold;
Branch runner: distributes the melt from the main runner to each cavity, and the cross-section is mostly circular or trapezoidal;
Gate: a narrow channel connecting the branch runner and the cavity, controlling the speed and pressure of the melt entering the cavity;
Cold material hole: located at the end of the main runner, used to collect the cold material at the front of the melt to prevent the cold material from entering the cavity and affecting the quality of the product.
During the injection molding process, after the melt fills the cavity and runner, it is completely solidified under the action of the cooling system. When the mold is opened, the solidified runner condensate (called "gate material") is ejected together with the product, and needs to be separated manually or mechanically later.
Hot runner injection molding
The hot runner system (also known as the "runnerless system") uses an external heating device to keep the plastic melt in the runner in a molten state at all times, and only the melt in the cavity cools and solidifies. When the mold is opened, there is no condensate left in the runner, and the product can be ejected directly.
Structure and working principle
The hot runner system consists of three parts: heating components, runner components and temperature control system:
Heating components: including heating coils, heating rods, etc., wrapped or embedded in the runner plate and the outside of the nozzle to maintain the runner temperature (usually 10-30℃ higher than the melting point of the plastic);
Runner components: composed of hot runner plate (distributing melt) and hot nozzle (connecting the runner plate and the cavity), the inside of the runner is smooth and has no dead corners, reducing melt retention;
Temperature control system: real-time monitoring of runner temperature through thermocouples, precise control of heating power, avoid local overheating or insufficient temperature.
During injection molding, the melt enters the cavity directly after being distributed by the hot runner plate, and the melt in the runner remains in a flowing state due to continuous heating. After molding, only the product in the cavity solidifies, and there is no condensate in the runner, which can achieve continuous production.
Considerations of runner system
When choosing a hot runner or cold runner system, the following factors should be considered comprehensively:
Production batch: For products with an annual output of more than 100,000 pieces, hot runner is preferred (lower long-term cost); cold runner is selected for small batches or trial production;
Material cost: When using plastics with a unit price of more than 50 yuan/kg (such as PEEK, LCP), the material saving advantage of hot runner is significant;
Product requirements: Hot runner is selected for appearance parts (such as automotive interiors, electronic housings); cold runner can be considered for structural parts (such as brackets, gears);
Plastic properties: Cold runner is recommended for heat-sensitive plastics such as PVC and POM; both are acceptable for general plastics such as PP, ABS, and PA;
Mold complexity: Hot runner can ensure uniform feeding and reduce the difficulty of debugging for multi-cavity molds (such as more than 32 cavities in one mold).