Some Experiences in Improving the Quality of Injection Molded Products
Strict control of raw materials is fundamental to improving the quality of injection molded products. The purity, particle size distribution, and moisture content of plastic particles directly impact the molding process and the performance of plastic parts, so a comprehensive raw material inspection process must be established. For example, each batch of raw materials must undergo a melt index (MI) test to ensure that its fluidity fluctuation is within ±5%. For highly hygroscopic plastics such as polyamide and polycarbonate, they must be dried before molding at a temperature of 80-120°C for 4-8 hours to reduce the moisture content to below 0.02%, thereby avoiding defects such as bubbles and silver streaks during the molding process. In addition, raw materials must be stored in a moisture-proof and sun-proof manner, and the storage time should not exceed 6 months to prevent performance degradation due to raw material aging.
Meticulous mold maintenance is key to ensuring consistent quality of injection molded products. The mold cavity surface roughness must be controlled below Ra0.8μm, and regular polishing must be performed to prevent cosmetic defects in plastic parts caused by surface roughness. Guide pins, guide bushings, and other mating parts must be checked for lubrication weekly and precision calibrated monthly to ensure smooth mold opening and closing, with positioning accuracy deviations within 0.01mm. For hot runner molds, the temperature distribution of the hot runner system must be checked daily to ensure that the temperature deviation of each gate is within ±2°C to prevent uneven filling caused by temperature variations. For example, a home appliance company established a mold maintenance log and conducted a comprehensive mold overhaul every 1,000 production runs, increasing its product qualification rate from 90% to 97%.
Optimizing and consolidating process parameters is key to improving the quality consistency of injection molded products. Orthogonal testing is required to determine the optimal combination of process parameters, including barrel temperature, mold temperature, injection pressure, injection speed, holding pressure, and holding time. These parameters must then be consolidated into the injection molding machine’s control system to avoid parameter fluctuations caused by human adjustments. For example, for polypropylene parts, the optimal parameters determined through orthogonal testing are: barrel temperature 210°C, mold temperature 50°C, injection pressure 70MPa, injection speed 60mm/s, holding pressure 50MPa, and holding time 10 seconds. Using these parameters, the dimensional tolerance of parts produced can be controlled within ±0.03mm, with no appearance defects. At the same time, a parameter monitoring mechanism must be established, recording key parameters hourly and implementing timely adjustments if deviations are detected.
Real-time monitoring of the molding process and the ability to handle exceptions directly impact the stability of product quality. Pressure sensors, temperature sensors, and visual inspection systems can be installed on injection molding machines to monitor melt pressure, cavity pressure, barrel temperature, and part appearance in real time. When an anomaly is detected, the system automatically issues an alarm and adjusts relevant parameters. For example, if cavity pressure suddenly drops by 10%, the system automatically increases injection pressure by 5% and extends the hold time by 2 seconds to prevent underfill. If visual inspection reveals sink marks on the part surface, the system automatically raises the mold temperature by 3°C and increases the hold pressure by 10%. After implementing this intelligent monitoring system, an automotive parts manufacturer reduced its process defect rate from 3% to 0.5%, significantly reducing scrap and production costs.
Cultivating employee operational skills and quality awareness is a long-term guarantee for improving injection molding product quality. Regular operator training is essential to ensure they master the molding characteristics of different plastics and how to adjust process parameters. For example, for plastics with poor flowability, operators should be trained to improve filling efficiency by increasing barrel temperature and injection pressure. For plastics prone to internal stress, they should be trained to eliminate stress through annealing. Furthermore, a three-tiered quality inspection system, consisting of self-inspection, mutual inspection, and specialized inspection, has been established. Operators must conduct self-inspections every 10 products produced, checking their dimensions and appearance. Teams conduct mutual inspections to ensure consistent quality across upstream and downstream processes. Quality inspectors conduct specialized inspections, using equipment such as coordinate measuring machines and tensile testing machines for comprehensive testing. These measures can reduce quality issues caused by human factors by over 60%, fundamentally improving product quality consistency.
