Open hot injection nozzle
An open hot-runner nozzle is a crucial gating system component in injection molding, primarily used in hot-runner molds. Its function is to transport molten plastic from the injection molding machine nozzle to the mold cavity while maintaining a stable melt temperature within the runner to prevent blockage caused by melt cooling and solidification. Compared to closed hot-runner nozzles, open hot-runner nozzles offer a simpler structure, lower cost, and easier maintenance. They are widely used in the production of small and medium-sized plastic parts, thin-walled parts, and products requiring high surface quality. However, these nozzles are prone to melt overflow and require high sealing requirements, requiring rational design and use to maximize their advantages.
The structure of an open hot-dip nozzle is relatively simple, consisting primarily of a nozzle body, a heating element, a thermocouple, a retaining ring, and a sprue bushing. The nozzle body is typically constructed of high-strength alloy steel (such as H13 steel) for excellent thermal conductivity and wear resistance. It features an internal flow channel. The flow channel diameter is determined by the part size and plastic flowability, typically ranging from 3 to 10 mm. The flow channel surface is polished (Ra ≤ 0.4 μm) to reduce melt flow resistance. A heating element (such as a heating coil or heating rod) is tightly wrapped around the nozzle body and electrically maintains the nozzle temperature, ensuring the melt remains molten within the flow channel. Heating power is determined by nozzle size and the plastic melting point, typically ranging from 50 to 300 W. A thermocouple is installed in the nozzle head to monitor the nozzle temperature in real time and transmit the temperature signal to the thermostat for precise temperature control, typically achieving an accuracy of ±1°C.
The operating principle of an open hot-shooting nozzle is to continuously heat the nozzle using a heating element, keeping the plastic melt in the runner within the set melt temperature range. During injection, the melt, under the action of injection pressure, enters the mold cavity directly from the gate at the nozzle head. After filling, the melt cools and solidifies at the gate, creating a seal and preventing backflow, as there is no valve structure. This operating method makes the open hot-shooting nozzle faster, reduces the residence time of the melt in the runner, and reduces the risk of plastic decomposition due to prolonged heat exposure. It is particularly suitable for the molding of heat-sensitive plastics (such as PVC and POM). However, due to its open structure, during the pressure holding phase and between injections, if the nozzle temperature is too high or the gate size is not properly designed, the melt can easily overflow from the gate, affecting the appearance of the plastic part and the cleanliness of the mold.
The application scope and selection criteria for open hot-shooting nozzles are determined based on the characteristics of the plastic part and molding requirements. Regarding the plastic part material, open hot-shooting nozzles are suitable for plastics with good fluidity and rapid cooling rates, such as PE, PP, and PS. These plastics cool and solidify quickly at the gate, forming an effective seal and reducing the risk of flash. High-viscosity plastics (such as PC and PMMA) cool more slowly and are prone to flash, requiring caution or the use of specially designed gates. Regarding the plastic part structure, open hot-shooting nozzles are suitable for small and medium-sized parts (typically weighing less than 500g), thin-walled parts (wall thickness ≤2mm), and products requiring a gate-free surface. Because the gate of an open hot-shooting nozzle is directly located on the part surface, it will leave a gate mark (typically 1-3mm in diameter). When selecting the nozzle, it is necessary to determine the nozzle temperature range according to the plastic type (e.g., the nozzle temperature for PE is 180-220°C, and the nozzle temperature for PC is 280-320°C), and select a single-nozzle or multi-nozzle system based on the number and layout of cavities to ensure that the melt can evenly fill each cavity.
The installation and commissioning of open hot-type nozzles significantly impact their performance. During installation, ensure concentricity between the nozzle and the mold’s retaining ring and cavity gate. Concentricity deviation should be within 0.05mm. Otherwise, melt flow will deviate, leading to uneven part filling and increased wear at the gate. The clearance between the nozzle and the mold must be strictly controlled, typically between 0.02-0.05mm. Excessive clearance can easily cause melt flash, while too small a clearance can interfere with thermal expansion and damage the nozzle or mold. During commissioning, temperature calibration is first required to ensure that the thermocouple temperature is consistent with the actual nozzle temperature, with a deviation of no more than ±3°C. Then, set the heating temperature based on the plastic’s characteristics. The heating process should be slow (no more than 10°C per minute) to prevent nozzle cracking due to thermal stress. Furthermore, injection and holding pressures should be adjusted during mold trials. Generally, injection pressure should be 5%-10% lower than that of conventional runner molds to reduce the risk of flash. Observe the melt flow at the gate to ensure smooth filling.
Common problems and maintenance measures for open hot-dip nozzles are crucial for ensuring long-term stable operation. Common problems include gate flash, nozzle clogs, and temperature runaway. Gate flash is often caused by overheating the nozzle, excessive holding pressure, or an oversized gate. Solutions include lowering the nozzle temperature (by 5-10°C at a time), reducing the holding pressure, or replacing a smaller sprue bushing. Nozzle clogs are often caused by impurities in the raw material, insufficient heating temperature leading to melt cooling and solidification, or char accumulation from plastic decomposition. These problems can be addressed by cleaning the flow channel, increasing the heating temperature (within the plastic’s thermal stability range), or improving raw material filtration (using a 100-200 mesh filter). Regarding maintenance, the hot-dip nozzle should be cleaned regularly (every 5,000-10,000 molds) using a dedicated cleaner or a copper brush to remove residual melt and char from the flow channel. The heating element should be inspected for damage; the insulation resistance of the heating coil should be ≥1MΩ; otherwise, it should be replaced promptly. Thermocouples should be calibrated regularly to ensure accurate temperature measurement. In addition, before long-term shutdown, the nozzle temperature must be lowered to room temperature to avoid damage to components due to sudden temperature changes.
