Injection nozzle leakage and solution
Injection nozzle leakage is a common problem in injection molding production. It occurs when melt leaks from the gap between the nozzle and the mold’s sprue bushing during the injection or pressure-holding phases. This not only wastes raw materials but also leads to defects such as missing material and flash in the molded parts. In severe cases, the leaked material can wear out the nozzle and sprue bushing after solidification, shortening the service life of the mold and equipment. Causes of leakage include poor nozzle-sprue bushing fit, improper temperature control, excessive injection pressure, or damage to the nozzle seal. Specific solutions must be implemented to ensure a stable injection molding process.
Inadequate fit between the nozzle and sprue bushing is a primary cause of leaks. A reliable seal must be achieved by optimizing the structural parameters and clearance between the two. The nozzle head is typically designed as a spherical or conical shape, mating with the concave spherical or conical surface of the sprue bushing. The radius tolerance for the spherical fit must be controlled within ±0.05mm, and the angular tolerance for the conical fit must be controlled within ±10° to ensure tight contact. Clearance is a critical parameter. For small-diameter nozzles (diameter ≤10mm), the clearance should be ≤0.02mm; for large-diameter nozzles (diameter >10mm), the clearance can be relaxed to 0.02-0.03mm. Too large a clearance will prevent sealing, while too small will cause metal friction and wear. During installation, the nozzle and sprue bushing must be aligned coaxially (≤0.05mm). This can be avoided by adjusting the position of the injection molding machine nozzle or installing a locating ring guide on the mold to prevent localized clearance caused by eccentricity. In addition, the contact surface of the nozzle head and the sprue bushing needs to be polished (roughness Ra ≤ 0.8 μm) and hardened (hardness ≥ HRC50) to improve wear resistance and sealing performance.
Improper temperature control can cause the melt to cool prematurely or become over-plasticized at the nozzle, increasing the risk of leakage. Precise control of the nozzle and barrel temperatures is essential. When the nozzle temperature is too low, the melt cools and solidifies within the nozzle, forming a solid plug that obstructs melt flow. This can cause a sudden increase in pressure during injection and lead to leakage. In this case, the nozzle temperature should be increased (for example, from 280°C to 300°C for PC) to ensure good melt fluidity within the nozzle. When the nozzle temperature is too high, the melt viscosity decreases, resulting in excessive fluidity and a tendency to leak through the gap during the holding phase. The nozzle temperature should be appropriately lowered (for example, from 180°C to 170°C for PE) while also shortening the melt’s residence time within the nozzle. For heat-sensitive plastics (such as PVC), nozzle temperature fluctuations must be strictly controlled (within ±2°C) to prevent excessive temperature-induced material decomposition and gas generation, which can increase cavity pressure and cause leakage.
Setting the injection and holding pressures too high can increase the impact of the melt on the nozzle sealing surface, leading to leaks. Process optimization is necessary to reduce the pressure. The injection pressure should be appropriately set based on the complexity of the part and the melt flowability. For thin-walled, complex parts, increasing the injection speed (e.g., from 50 mm/s to 100 mm/s) rather than increasing the pressure can improve filling, keeping the injection pressure within 70% of the equipment’s rated pressure. The holding pressure is typically set between 50% and 70% of the injection pressure, and the holding time is determined by the part thickness (approximately 1-2 seconds per millimeter of thickness) to avoid excessive holding pressure that could cause melt backflow into the nozzle. For example, for a 3mm-thick ABS part, the holding pressure can be set to 60 MPa for 4 seconds, ensuring dimensional stability while reducing the risk of leaks. Alternatively, a graded holding pressure strategy can be employed, with an initial high holding pressure (e.g., 70 MPa) followed by a one-second decrease to 50 MPa to prevent initial pressure peaks from impacting the sealing surface.
Damage or wear to the nozzle seal is a direct cause of leaks, requiring regular inspection and replacement of relevant components. The check ring and gasket inside the nozzle are consumable parts. Wear on the check ring’s sealing surface can cause melt backflow during the pressure-holding phase, leading to leakage through the gap between the nozzle and the sprue bushing. Check the check ring regularly for wear (≤0.1mm) and replace it promptly if wear exceeds the limit. Replace it with a wear-resistant material that matches the barrel material (such as carbide). For nozzles with spring seals, check the spring’s elasticity for attenuation. If the spring force drops by more than 10%, replace it with a spring of the same specification to ensure adequate sealing pressure. Furthermore, loose nozzle bolts can lead to insufficient sealing pressure. Regularly check the bolt tightening torque (e.g., 80-100 N · m for M16 bolts ) and use a locknut or apply threadlocker to prevent loosening.
Structural defects or improper installation of the mold’s sprue sleeve can also cause glue leakage, requiring corresponding improvements to the mold. The inlet of the sprue sleeve needs to be chamfered (such as C1-C2) to prevent sharp edges from scratching the nozzle sealing surface; the sprue sleeve needs to be long enough (usually extending 5-10mm beyond the fixed mold plate) to ensure contact area with the nozzle. Too small a contact area will result in excessive local pressure and damage the seal. When installing the sprue sleeve, ensure that its axis is perpendicular to the mold parting surface (verticality ≤ 0.01mm/m) and secure it with a locating pin to prevent displacement under the action of injection pressure. For molds that often leak glue, a glue collection groove can be set around the sprue sleeve. The groove should be 2-3mm deep and 5-10mm larger in diameter than the sprue sleeve to collect leaked melt and prevent it from flowing into other parts of the mold and causing contamination or damage. The glue collection groove needs to be cleaned regularly to prevent glue accumulation and solidification.
