Maintenance of injection molding screw and barrel
Maintenance of injection molding screws and barrels is fundamental to extending equipment life and ensuring production stability. Its core focus is to reduce wear, prevent corrosion, and maintain proper clearances. Proper maintenance can extend equipment life by 30-50%. On one production line, through standardized maintenance, the screw replacement interval was extended from 15,000 hours to 25,000 hours. Maintenance requires a cyclical schedule: daily checks for temperature and pressure anomalies, weekly cleaning of the hopper and feed port, monthly clearance measurements, and an annual comprehensive overhaul. Neglecting maintenance can have serious consequences. For example, a PE production line developed 0.5mm-deep screw scratches due to inadequate cleaning of the feed port, resulting in uneven melt plasticization and a 15% black spot defect rate in plastic parts. On a PVC production line, failure to regularly replace the check ring resulted in leakage and a 20% drop in production efficiency.
Daily cleaning is a fundamental part of maintenance and requires thorough cleaning during each shutdown or material change to prevent residual material degradation or cross-contamination. The melt in the barrel should be drained before shutdown to prevent solidification after cooling. This operation reduced the scrap rate of an ABS production line from 10% to 2% the next time the machine was started. The appropriate cleaning method should be selected based on the material: heat-stable materials can be cleaned with new material, while heat-sensitive materials require special cleaning materials and a lower temperature. After using cleaning materials on a POM production line, the amount of residual degradation products in the barrel was reduced by 80%. Cleaning of the feed port is crucial; dust and foreign matter must be removed daily. A PA6 production line experienced screw scratches due to a damaged hopper filter and the mixing of metal impurities. Replacing the filter and adding a magnetic separation device eliminated similar problems.
Monitoring and adjusting the clearance is crucial for screw and barrel maintenance. Excessive clearance can lead to poor plasticization and inaccurate metering, while too little can exacerbate wear. The normal clearance is 0.1-0.3mm (depending on diameter). A 50mm Φ screw had an initial clearance of 0.15mm, but after 10,000 hours of use, it increased to 0.3mm. This increased melt pressure fluctuations and part weight deviation from ±0.5% to ±2%. After replacing the screw to restore the clearance, stability improved. Clearance can be measured using a feeler gauge or lead pressure method. Weekly measurements should be taken and trends recorded. Trend analysis at a precision injection molding workshop enabled early detection of abnormal screw wear, avoiding unplanned downtime. Clearance adjustment requires professional assistance and can be achieved by grinding the screw or replacing the bushing. In one case, grinding the screw diameter by 0.1mm restored the clearance from 0.4mm to 0.2mm, at a cost of only one-fifth that of replacing a new screw.
Preventing wear and corrosion is a crucial maintenance task. Targeted measures should be implemented based on the material’s characteristics to minimize screw and barrel wear. When processing glass fiber or mineral-reinforced materials, high-wear-resistant alloy screws (such as nitrided 38CrMoAlA) are essential. Their surface hardness reaches HV900 or higher, making them 3-5 times more wear-resistant than ordinary screws. A PA66+GF30 production line using this screw reduced the wear rate from 0.01mm/1,000 hours to 0.003mm/1,000 hours. When processing corrosive materials like PVC and POM, anti-corrosion coatings (such as chrome plating or nickel-phosphorus alloy) are necessary. Chrome plating on a PVC production line reduced barrel corrosion by 90%. Furthermore, avoiding idling (limiting each idling period to no more than 30 seconds) can reduce dry friction. An operator mistakenly left the screw idling for 5 minutes, resulting in 0.2mm of surface wear and direct losses of 20,000 yuan.
Regular maintenance and component replacement are effective means of preventing failures. Replacement cycles should be determined based on usage and operating conditions. Consumable parts such as check rings and seals should be replaced every 3,000-5,000 hours. On one production line, failure to replace the check rings in a timely manner resulted in a 5% fluctuation in melt volume, which returned to ±1% after replacement. The thickness of the screw’s nitriding layer should be regularly inspected. When the thickness drops from the initial 0.5mm to 0.3mm, re-nitriding is indicated. A 65mm Φ screw was found to have an extended service life of 10,000 hours after re-nitriding. Scratches or corrosion exceeding 0.2mm on the barrel’s inner wall require repair. Laser cladding technology can be used. In one case, a barrel repaired with this technology achieved performance comparable to new at only 60% of the cost of replacement. After maintenance, a test run is required to verify the plasticization and part quality during 50 consecutive production runs to ensure satisfactory maintenance. One workshop, through rigorous verification, achieved an overall equipment efficiency (OEE) of over 90% after maintenance.
