A hot runner system is a set of components used in injection molds that continuously heat the molten plastic, ensuring it remains fluid throughout the runner system. It replaces the "cold slug" found in traditional three-plate molds with cold runners, fundamentally eliminating the sprue waste generated after each injection—thereby shortening cycle times and saving raw materials. Based on structural characteristics, the industry typically divides hot runner systems into three main categories: open-type, needle-valve type, and micro semi-hot runner systems. The open-type design is the simplest, consisting of three core components: a hot nozzle, a distributor plate, and a temperature-control box. In this system, the melt is injected directly into the mold cavity at a constant temperature, making it ideal for applications such as transparent parts, thin-walled components, and micro-precision parts—scenarios that demand exceptionally high surface quality but exhibit low shear sensitivity. The needle-valve type adds a pneumatically or hydraulically actuated needle valve at the nozzle’s front end, enabling the gate to close instantly after injection, resulting in minimal gate marks and effectively preventing flow marks and stringing. This type is particularly well-suited for high-gloss casings, automotive interior and exterior trim parts, and multi-color/multi-material co-injection molding applications. The micro semi-hot runner system strikes a balance between “fully hot” and “cold runner” designs: part of the runner system is heated while another part is cooled. With lower system costs and simpler maintenance, it’s an ideal choice for mass-produced parts such as household appliances, electronic connectors, and small gears.
At the application level, hot runner systems have become deeply entrenched in nearly all industries that require precision injection molding, including automotive, medical, packaging, consumer electronics, and optical lenses. Take car headlight lenses as an example: PMMA or PC materials are highly sensitive to molecular orientation and internal stresses. By adopting an 8-point sequential valve hot runner system, it’s possible to achieve low-pressure, high-speed molding of 0.3-mm-thick walls and 800-mm-long light-guiding strips in molds with a clamping force of 8,000 kN, with optical distortion below 5 milliradians. In a 96-cavity mold for medical pipette tips, the hot runner system, combined with an electric injection molding machine, reduces the cycle time to just 4.2 seconds—a 35% improvement over cold-runner solutions. Moreover, there’s no need for manual sprue trimming; the parts can be directly fed into the sterile packaging line. Looking ahead, as molds evolve toward larger sizes, higher precision, multi-cavity designs, and greater intelligence, hot runner systems will further integrate with technologies such as in-mold sensors, real-time closed-loop temperature control, and AI-based predictive maintenance, becoming an indispensable “melt highway” in Industry 4.0 injection molding workshops.