Single-point hot runner injection mold
A single-point hot runner injection mold is a mold structure that injects molten plastic directly into the mold cavity through a single hot runner system. Its core feature is the elimination of the traditional cold runner, allowing the melt to remain molten throughout the entire process from the nozzle to the cavity, reducing material waste and shortening the molding cycle. Compared to multi-point hot runner molds, the single-point structure is simpler and less expensive, making it suitable for the injection molding production of small and medium-sized plastic parts, single cavities, or symmetrical multi-cavities, such as bottle caps, cups, and small electrical appliance housings. The key components of this mold include the hot runner nozzle, heater, thermostat, and sprue bushing, which precisely control the melt temperature and pressure within the hot runner to ensure stable part quality. The design of the single-point hot runner must match the shape and size of the plastic part, especially ensuring the rationality of the gate position to avoid pressure loss and uneven cooling caused by an overly long melt flow path.
The structure of a single-point hot runner injection mold is relatively simple, primarily consisting of a hot runner system, cavity, core, cooling system, and ejection mechanism. The hot runner system is the core, comprising a hot nozzle, manifold (optional for single cavities), heater, and temperature sensor. The hot nozzle is directly connected to the mold cavity, and its head shape must match the part’s gate location. Common types include needle-valve, open, and latent hot nozzles. Open hot nozzles are simple and low-cost, but they are prone to gate marks. Needle-valve hot nozzles, which use a pneumatic or hydraulic cylinder to control the needle valve, eliminate gate marks and are suitable for parts requiring high aesthetics. The heater typically uses a coiled heating wire or heating rod wrapped around the hot nozzle and manifold. The power density is controlled at 10-20W/cm² to ensure uniform melt temperature. Temperature sensors (such as thermocouples) are installed on the hot nozzle head and manifold to monitor temperature in real time and provide feedback to the thermostat. Control accuracy must reach ±1°C to prevent overheating, degradation, or cooling and solidification of the melt.
The key design considerations for single-point hot runner molds lie in controlling thermal balance and pressure loss. The hot nozzle should be kept as short as possible (typically less than 100mm) to minimize heat and pressure losses. Its diameter, determined based on the weight of the part, is typically 8-16mm to ensure smooth melt flow. The clearance between the hot nozzle and the mold cavity should be controlled within 0.02-0.05mm. Thermal insulation (such as asbestos or mica sheets) should be used to minimize heat transfer to the mold, keeping the mold temperature independent of the hot runner temperature and preventing mold overheating that can make demolding difficult. For example, a PP bottle cap mold uses a 0.1mm-thick mica sheet for insulation between the hot nozzle and cavity. The hot runner temperature is controlled at 200°C, while the mold temperature is maintained at 40°C. This ensures melt fluidity while preventing the cap from sticking to the mold. The gate position should be selected to ensure the shortest and most uniform flow path of the melt in the mold cavity. For symmetrical plastic parts, the gate should be located in the center. For asymmetrical plastic parts, the optimal gate position can be determined through CAE simulation analysis to ensure balanced filling.
The process parameters for single-point hot runner molds must balance melt fluidity and stability. The hot runner temperature is typically 5-10°C higher than the barrel temperature. For example, for PE, the barrel temperature is 180-200°C, while the hot runner temperature is 190-210°C to ensure that the melt does not cool within the hot runner. The injection pressure should be 5%-10% higher than that of traditional cold runner molds to compensate for pressure losses in the hot runner system. For example, the injection pressure of an ABS part in a cold runner mold is 80 MPa, but this needs to be increased to 85-90 MPa with a single-point hot runner. The injection speed should be adjusted based on the gate size and part thickness to avoid excessive shearing of the melt at the gate. For example, for a 3mm diameter gate, the injection speed should be controlled between 30-50 mm/s. During the holding phase, since the melt in the hot runner can continue to feed into the cavity, the holding time can be shortened by 20%-30% compared to cold runner molds. However, the holding pressure must be maintained stable to prevent backflow at the gate. In addition, the hot runner system needs to be preheated before starting the machine (usually 30-60 minutes) to ensure that the temperature reaches the set value before starting production to avoid material shortage defects caused by insufficient temperature.
The application and maintenance of single-point hot runner molds require careful attention to several key details. They are suitable for large-volume production (over 100,000 pieces per year) of plastic parts with high raw material costs, such as engineering plastics (PC, PA). Eliminating the cold runner can save 10%-30% of raw material. For example, using a single-point hot runner for a PA66 gear can yield approximately 500 more parts per ton of raw material. For small, multi-cavity molds (such as an 8-cavity bottle cap mold), single-point hot runners utilize a manifold to evenly distribute the melt, ensuring consistent part quality across cavities. During routine maintenance, the hot nozzle should be regularly cleaned to prevent carbide buildup and clogging of the gate. The cleaning cycle is determined by the production batch (typically every 10,000 pieces). Heaters and temperature sensors should be regularly inspected to ensure uniform heating and accurate temperature measurement, and replaced promptly if damaged. During shutdown, the hot runner temperature should be lowered to below 150°C before turning off the power to prevent component damage caused by thermal expansion and contraction. Through reasonable application and maintenance, the service life of single-point hot runner molds can reach more than 500,000 molds, significantly reducing production costs.
