Hot runner injection molds are key equipment in modern injection molding. They maintain the molten state of plastic melt in the runner through precise temperature control to achieve runnerless injection molding. Compared with traditional cold runner molds, they can eliminate 10%–30% of runner waste while improving product molding accuracy and production efficiency, and have become the mainstream mold solution in fields such as precision electronics and auto parts. The core of this technology lies in the coordinated operation of the hot runner system and the mold body, as well as the precise control of temperature control accuracy, with all links requiring strict matching to ensure production stability.
The core working principle of hot runner injection molds is to keep the plastic melt in the mold runner in a molten state at all times through external heating devices, avoiding the generation of runner solidification in traditional cold runners. During injection by the injection molding machine, the melt directly enters the cavity through the hot runner system. After pressure holding and cooling, the plastic part is formed. During mold opening, only the plastic part needs to be ejected without cleaning the runner solidification, realizing continuous waste-free production.
The temperature control system is crucial for realizing hot runner functions. Heaters provide continuous heat for the runner, and thermocouples real-time monitor the runner temperature and feed it back to the temperature controller. The industry-recognized temperature control accuracy of hot runners must be within the range of ±1℃, ensuring stable melt fluidity and avoiding material degradation due to excessively high temperatures or melt solidification and insufficient injection pressure caused by excessively low temperatures.
After plastic in the injection molding machine barrel is plasticized and melted, it enters the manifold of the hot runner system through the injection nozzle. The manifold evenly distributes the melt to each hot nozzle, which then injects it into the mold cavity. After the cavity is filled with melt, the pressure holding system maintains a certain pressure to compensate for melt shrinkage. The cooling system then cools the cavity, and after the plastic part is solidified and formed, the mold is opened and the part is ejected, completing a production cycle.
The hot runner system is the core functional unit of hot runner injection molds, mainly including hot nozzles, manifolds and heating elements. Hot nozzles are directly connected to the cavity and are divided into open type and needle valve type. Needle valve hot nozzles can control the melt injection timing through needle valves, effectively solving the problems of stringing and drooling. Manifolds are responsible for evenly distributing the melt from the injection molding machine nozzle to each hot nozzle, and their internal runner design must follow the principle of minimum pressure loss. Heating elements mostly use electric heating rods or electric heating rings, which are closely attached to the manifold and hot nozzles to provide uniform and stable heat.
The temperature control system consists of temperature controllers, thermocouples and connecting wires. The temperature controller automatically adjusts the power output of heating elements according to the temperature signals fed back by thermocouples, realizing precise temperature control of the hot runner system. Thermocouples must be accurately arranged at key positions of the manifold and hot nozzles to ensure the accuracy of temperature monitoring, which is a prerequisite for the stable operation of hot runner injection molds.
The mold body needs special design for the hot runner system, including positioning structure, heat insulation structure and sealing structure. The positioning structure ensures the precise alignment of the hot runner system and the mold cavity, avoiding melt leakage or injection deviation. The heat insulation structure uses heat insulation gaskets or air gaps to reduce heat conduction from the hot runner system to the mold template, preventing excessive mold temperature from affecting plastic part cooling and molding accuracy. The sealing structure achieves sealing between the hot nozzle and the cavity through sealing rings, avoiding melt leakage during injection.
Under current technical trends, the temperature control system of hot runner injection molds is gradually integrating the Internet of Things and AI technologies to realize real-time temperature monitoring, remote debugging and fault early warning. Some advanced systems can automatically adjust temperature control parameters according to material characteristics and production conditions, further improving production stability and product qualification rate.
To meet the demand for small plastic parts such as precision electronic components, hot runner injection molds are developing towards miniaturization and multi-cavity. The diameter of micro hot nozzles can be reduced to less than 1 millimeter, and the number of cavities in multi-cavity molds can reach more than 100, greatly improving the production efficiency of small precision plastic parts.
With the wide application of special materials such as high-temperature engineering plastics and biodegradable plastics, the heat resistance and corrosion resistance of hot runner injection molds are continuously improved. Heating elements use high-temperature resistant alloy materials, and manifolds and hot nozzles use stainless steel or titanium alloy materials to meet the molding requirements of special materials and expand the application range of hot runner injection molds.
