Problems caused by using long nozzles and their remedies
Long nozzles (typically exceeding 100mm in length) are commonly used in deep-cavity molds or hot runner systems in injection molding. However, their unique structure can easily lead to a series of problems, such as melt retention, excessive pressure loss, and temperature fluctuations, which can lead to product defects or equipment failure. These problems are closely related to the long nozzle’s large heat dissipation area, high flow resistance, and significant heat loss. Targeted remedial measures, such as optimizing structural design, adjusting process parameters, and strengthening maintenance, are needed to mitigate or eliminate these adverse effects and ensure stable production.
Melt retention and degradation are the most common problems with long nozzles. The increased nozzle length prolongs the melt’s residence time in the flow channel (30%-50% longer than with standard nozzles). This is particularly true with improper nozzle temperature control or when using heat-sensitive raw materials (such as PVC and POM). Thermal degradation can easily occur, resulting in defects such as black specks and burnt particles. For example, in the production of PVC pipe fittings using a 150mm long nozzle, the melt residence time reached 8 minutes, far exceeding the thermal stability time of PVC (5 minutes). This resulted in dense black specks on the finished product and a defect rate of 15%. Remedial measures include: increasing the nozzle temperature by 10-15°C (for example, from 170°C to 185°C for PVC nozzles), reducing melt viscosity to shorten residence time, shortening the production cycle (for example, from 60 seconds to 45 seconds) to reduce the cumulative residence time of the melt in the nozzle, and installing spiral grooves in the nozzle flow channel to enhance melt fluidity and reduce retention. For severe degradation, the nozzle should be disassembled and the internal burnt material cleaned with a copper wire brush, followed by a 30-minute acetone soak to remove residual degradation products.
Excessive pressure loss is another prominent issue with long nozzles. Increasing the runner length significantly increases melt flow resistance (pressure loss is proportional to length, increasing by 10%-15% for every 50mm increase in length). This can easily lead to defects such as underfill and noticeable weld lines. For example, a PC lampshade using a 200mm long nozzle requires an increase in injection pressure from 120 bar to 160 bar compared to a standard 80mm nozzle to fill the cavity. This not only increases energy consumption but also leads to increased internal stress in the part. Remedial measures include: increasing the nozzle runner diameter (e.g., from 4mm to 6mm) to reduce flow velocity gradients, which can reduce pressure loss by 20%-30%; adopting a streamlined runner design (taper of 5°-10°, with a transition radius R ≥ 2mm) to avoid right angles or abrupt tapers; and increasing the melt temperature (e.g., from 280°C to 295°C for PC) to reduce melt viscosity and minimize flow resistance. If the mold allows, a heating jacket (power 500~1000W) can be added between the nozzle and the mold to maintain a stable flow channel temperature and further reduce pressure loss.
Large temperature fluctuations are an inherent drawback of long nozzles. Due to their large heat dissipation area (50%-100% greater than standard nozzles), the front end of the nozzle is prone to experiencing low temperatures (10-20°C below the set point), causing the melt to cool and thicken. Meanwhile, the rear end, closer to the barrel, experiences higher temperatures, creating a significant temperature gradient (up to 15-25°C). This fluctuation can lead to unstable melt flow, product weight deviations exceeding 1%, and increased dimensional fluctuations. For example, using a 180mm long nozzle, the nozzle front end temperature was 200°C, while the rear end temperature was 220°C, a 20°C temperature difference. The standard deviation of product weight increased from 0.3g to 0.8g. Remedial measures include: using a segmented heating nozzle (independent temperature control at the front and rear ends), with the front end temperature 5~10℃ higher than the rear end to offset heat dissipation losses; wrapping the outside of the nozzle with an insulation layer (such as glass fiber cotton, 5~10mm thick) to reduce heat loss and control temperature fluctuations within ±5℃; increasing the nozzle heating power (power per centimeter length ≥10W), such as the total power of a 150mm long nozzle ≥1500W, to improve the heating response speed.
Poor nozzle-mold fit can easily lead to material leakage and misalignment. Long nozzles, due to their lack of rigidity, are prone to bending and deformation under injection pressure (deflection can reach 0.1-0.3mm). This increases the clearance between the nozzle and the mold’s sprue bushing (exceeding 0.1mm), allowing melt to accumulate in the gap and cause secondary blockage. For example, a 500-ton injection molding machine used a 250mm-long nozzle. After mold closing, the nozzle deviated 0.2mm from the mold center. This caused material leakage during production, requiring hourly machine shutdown for cleaning. Remedial measures include increasing nozzle wall thickness (from 8mm to 12mm) or using high-strength alloys (such as H13 steel) to improve rigidity and control deflection to within 0.05mm; adding a support sleeve between the nozzle and barrel to reduce bending; adjusting the concentricity of the nozzle and mold (deviation ≤ 0.05mm) to control the clearance to 0.03-0.05mm; and applying high-temperature grease (temperature resistant ≥ 300°C) to the mating surfaces to reduce wear.
Difficult maintenance and shortened lifespan are hidden issues with long nozzles. Due to their complex structure (e.g., segmented heating and spiral runners), cleaning and repair are challenging. Dead corners within the runners are prone to accumulation of debris, exacerbating wear and corrosion over time. For example, a long hot runner nozzle developed corrosion pits on the runner surface after three months due to residual PC degradation products not being promptly cleaned. This hindered melt flow. Remedial measures include: establishing a dedicated maintenance plan, disassembling the nozzle for cleaning every 3,000 molds, and using a dedicated clearing needle (0.5mm smaller in diameter than the runner) to clear the runner; regularly inspecting the heating element (every 2,000 molds) to ensure uniform heating across all sections and a resistance deviation of ≤5%; installing a filter (80-120 mesh) at the nozzle inlet to prevent impurities from entering the runner; and choosing a detachable nozzle structure that divides the nozzle into two or three sections to facilitate the replacement of worn parts (such as the front nozzle tip), thereby extending the overall service life. By implementing these measures, the average lifespan of long nozzles can be extended from six months to over 12 months.
