Problems that may arise from using long nozzles and their remedies
Long nozzles (usually referring to injection nozzles with a length exceeding 100mm) are widely used in injection molding production because they can adapt to the molding requirements of deep-cavity molds or special-structured plastic parts. However, their slender structure can easily cause a series of problems, affecting molding stability and plastic part quality. The core challenges of long nozzles are large heat loss, high melt flow resistance, and difficulty in installation and debugging. If used improperly, defects such as cold slug spots, nozzle blockage, and plastic part shortages can occur. For example, when producing deep-cavity plastic containers (depth of 200mm), a 150mm long nozzle is used. Due to excessive heat loss, the melt temperature at the front end of the nozzle is 30°C lower than that of the barrel, resulting in a significant cold slug spot at the bottom of the container. Due to the high flow resistance, the injection pressure needs to be increased by 20%, increasing energy consumption and mold loss.
The most common problems with long nozzles are melt temperature fluctuations and cold slug generation. As the nozzle length increases, the contact area with the cold mold expands, causing heat to dissipate rapidly through metal conduction. This can cause the temperature to drop by 20-50°C, particularly at the nozzle tip where it contacts the mold. This can cause the melt to cool and thicken prematurely. Once this cold slug enters the mold cavity, it can form cold slug spots (typically 1-3mm in diameter) on the surface of the part or clog the gate, leading to insufficient filling. For example, when using a 120mm long nozzle to produce ABS electrical housings, the nozzle tip temperature drops from 220°C to 180°C, below ABS’s optimal flow temperature of 200-210°C. This results in continuous cold slug streaks near the housing gate, leading to a defect rate as high as 15%. Furthermore, temperature fluctuations can lead to unstable melt viscosity, causing part weight deviations exceeding ±3%, impacting dimensional accuracy.
Remedial measures for temperature issues focus on strengthening nozzle insulation and precise temperature control. Wrapping the nozzle with a high-density fiberglass insulation sleeve (5-10mm thick) reduces heat radiation and convection losses, which can increase the nozzle front temperature by 10-15°C. A “stepped heating” design is employed, increasing the heating power at the front section of the nozzle (near the mold end) (e.g., 20% higher than at the rear section) to compensate for conductive heat dissipation. For example, for a 150mm long nozzle, the rear section has a heating power of 50W, while the front section has a heating power of 60W, improving the overall nozzle temperature uniformity to within ±5°C. An independent thermocouple and thermostat are installed at the nozzle front to monitor the temperature in real time and quickly adjust the heating output to avoid localized overcooling or overheating. For heat-sensitive materials, an insulating gasket (such as a 0.5mm thick mica sheet) is added between the nozzle and the barrel to reduce heat transfer from the barrel to the nozzle and prevent melt degradation at the rear end of the nozzle. For example, after a company installed an insulation sleeve and enhanced front-end heating on a 180mm long PC nozzle, the nozzle temperature fluctuation dropped from ±10°C to ±3°C, and the cold material spot defect rate dropped from 12% to 0.5%.
Another prominent issue with long nozzles is excessive melt flow resistance. The slender flow channel increases melt pressure loss within the nozzle by 30-50%, leading to increased injection pressure and slower filling speeds, which can easily cause defects such as part shorts and noticeable weld marks. Furthermore, high resistance exacerbates melt shear heating, causing premature degradation of heat-sensitive materials (such as PVC) within the nozzle, resulting in charred material that blocks the flow channel. For example, when using a 200mm long, 4mm diameter nozzle to produce POM gears, the melt pressure loss within the nozzle reaches 40MPa, requiring an increase in injection pressure from 90MPa to 130MPa to fill the cavity. Furthermore, shear overheating can cause char marks to appear within the gear bore. Flow resistance also leads to unstable melt flow rate at the nozzle outlet, increasing part weight fluctuations and affecting batch consistency.
Solving the problem of flow resistance requires optimizing the nozzle structure and adjusting the process. Increasing the nozzle flow channel diameter (such as from 4mm to 5mm). According to Poiseuille’s law, if the flow channel diameter increases by 25%, the flow resistance can be reduced by more than 50%. However, it should be noted that a diameter that is too large will increase the risk of cold material and requires enhanced heating. Design the nozzle flow channel to be “gradually expanding”, with a small inlet diameter (such as 4mm) and a large outlet diameter (such as 6mm) to reduce the pressure loss at the outlet and avoid the sudden expansion of the melt at the outlet to generate vortices. Increase the melt temperature and mold temperature and reduce the melt viscosity. For example, increasing the barrel temperature of PP from 200°C to 220°C and the mold temperature from 50°C to 70°C can reduce the flow resistance by 20%. Use ” high pressure and low speed” In the injection process, while ensuring filling, the shear rate of the melt in the nozzle can be reduced (for example, from 1000s⁻¹ to 500s⁻¹ ) to minimize degradation risks. For example, one company increased the flow channel diameter of a long nozzle from 4.5mm to 5.5mm and raised the barrel temperature by 15 °C, reducing the injection pressure by 30MPa and eliminating the problem of charred material blockage.
Improper installation and maintenance of long nozzles can also lead to a range of problems, such as misalignment between the nozzle and the mold, leaks caused by poor sealing, and contamination caused by residual char inside the nozzle. If the nozzle center deviates by more than 0.1mm from the center of the mold’s main flow channel during installation, melt flow can be obstructed, local pressure surges can occur, and even damage can occur at the nozzle-mold interface. Poor sealing can cause melt to leak through the gap, forming hard lumps upon cooling. These can scratch the mold surface or clog the flow channel during subsequent injection. Over time, char accumulates on the nozzle’s inner wall, affecting melt flow uniformity and causing optical defects, especially in transparent plastic parts such as PC lenses. For example, one company failed to calibrate the concentricity of a long nozzle during installation, resulting in a 0.3mm deviation. After 30 mold runs, a crack developed at the nozzle tip, causing leakage and a four-hour production stoppage.
Remedial measures for installation and maintenance issues require standardized operating procedures and regular maintenance. Before installation, use a dial indicator to calibrate the concentricity of the nozzle and mold to ensure a deviation of ≤0.05mm. When tightening the nozzle mounting bolts, apply uniform force diagonally to prevent misalignment due to uneven force. When replacing nozzle seals (such as O-rings), use high-temperature-resistant materials (such as fluororubber, with a temperature resistance of over 200°C) and ensure that the seal groove is clean and burr-free to prevent seal failure. Disassemble the nozzle weekly and clean the inner wall of the nozzle with a copper brush. If necessary, soak it in a specialized cleaning agent (such as an alkaline cleaning agent for PA) for 30 minutes to thoroughly remove any residual impurities. Regularly inspect the nozzle tip for wear and replace it promptly if wear exceeds 0.2mm to avoid compromising the seal and flow. For example, one company implemented a maintenance plan for long nozzles: daily concentricity calibration, weekly cleaning of the inner wall, and monthly seal replacement. This plan reduced downtime due to installation and maintenance by 80%.
