Precision injection molding process
Precision injection molding is an advanced manufacturing technology capable of producing plastic parts with high dimensional accuracy, excellent surface quality, and stable mechanical properties. It is widely used in high-end industries such as automotive, electronics, medical, and aerospace. Compared to conventional injection molding, precision injection molding has more stringent requirements for mold accuracy, process parameter control, raw material properties, and production environment. Its core goal is to precisely control the entire molding process to maintain a very small dimensional tolerance for plastic parts, typically requiring dimensional accuracy of 0.01-0.001mm to meet the assembly and use requirements of precision components.
High-precision mold design and manufacturing are fundamental to precision injection molding. The dimensional accuracy of the mold cavity and core directly determines the dimensional accuracy of the plastic part. Therefore, during the mold design phase, the shrinkage characteristics of the plastic must be fully considered. Cavity dimensions must be precisely calculated and determined, typically with dimensional compensation based on the shrinkage of different plastics (e.g., PE has a shrinkage of 1.5%-3.0% and PC has a shrinkage of 0.5%-0.7%). Mold machining accuracy is also crucial. High-precision machining equipment (such as high-speed machining centers, EDM machines, and wire-cut machines) must be used to ensure a cavity surface roughness Ra ≤ 0.8μm and a clearance between parting surfaces within 0.005-0.01mm to avoid defects such as flash and burrs. Furthermore, the mold’s guide mechanisms (such as guide pins and guide bushings) must exhibit extremely high precision, ensuring that the relative position error between the movable and fixed molds does not exceed 0.005mm to prevent dimensional deviations in the plastic part due to mold misalignment.
Strict control of process parameters is crucial for achieving precision injection molding. During the injection phase, the stability of injection speed and pressure directly impacts the melt filling state. Precision injection molding typically utilizes a multi-stage injection process. By controlling the injection speed in stages (e.g., low speed in the initial stage to fill blind spots in the cavity, high speed in the middle stage to improve filling efficiency, and low speed in the final stage to maintain pressure and prevent overflow), this ensures smooth melt flow and avoids excessive internal stress in the plastic part caused by turbulence. The holding pressure and holding time must also be precisely set. The holding pressure is generally 60%-80% of the injection pressure, and the holding time is determined based on the thickness of the plastic part to compensate for volume changes caused by plastic cooling shrinkage and reduce dimensional errors. Furthermore, the fluctuation range of the melt temperature and mold temperature must be strictly controlled. The melt temperature fluctuation should not exceed ±2°C, and the mold temperature fluctuation should not exceed ±1°C. High-precision temperature control systems (such as PID thermostats) can achieve real-time adjustment to ensure the stability of the melt viscosity and cooling rate.
The selection and processing of high-performance plastic raw materials significantly impact the quality of precision injection molding. Precision plastic parts typically require raw materials with stable melt flow rates, low shrinkage, and good dimensional stability. Therefore, specialized materials that have been rigorously screened and modified should be preferred. For example, engineering plastics reinforced with materials like glass fiber and mineral fillers can effectively reduce shrinkage (for example, the addition of 30% glass fiber to PA66 can reduce shrinkage from 1.5%-2.5% to 0.3%-0.8%). Raw materials must be thoroughly dried before use, especially hygroscopic plastics (such as PA and PC). These materials require drying in a dedicated dryer. The drying temperature and time are determined by the plastic type (for example, PC is typically dried at 120°C for 4-6 hours). The moisture content of the raw materials should be kept below 0.02% to prevent defects such as bubbles and silver streaks in the molded parts due to moisture evaporation. Furthermore, the purity of the raw materials must be strictly controlled to avoid contamination with impurities or different plastic grades, which can prevent uneven melt flow and fluctuating part properties.
Precisely controlling the production environment is essential for precision injection molding. Precision injection molding workshops require stable temperature and humidity, generally maintained between 20-25°C and 50%-60% relative humidity. Excessive temperature fluctuations (exceeding ±1°C) can cause thermal expansion and contraction of the mold and equipment, affecting the dimensional accuracy of the molded parts. Excessive humidity can absorb moisture from the raw materials, affecting molding quality. The workshop must also be kept clean to prevent dust and impurities from entering the mold cavity or raw materials. A dust-free workshop design with air cleanliness levels reaching Class 10000 or higher is recommended. Furthermore, the stability of the injection molding equipment is crucial. High-precision injection molding machines should be selected, with a clamping force repeatability of ≤±1% and an injection volume repeatability of ≤±0.5%. Regular maintenance and calibration of the equipment is essential to ensure the precise fit and coordinated movement of all moving parts, minimizing part quality deviations caused by equipment fluctuations.
Precision testing and quality feedback mechanisms are crucial for ensuring the continued stability of precision injection molding processes. During production, high-precision testing equipment (such as coordinate measuring machines, laser diameter gauges, and surface roughness meters) is required to conduct 100% or spot checks on the dimensions, shape tolerances, and surface quality of plastic parts. The coordinate measuring machine’s measurement accuracy should reach 0.001mm to ensure timely detection of out-of-tolerance products. Furthermore, a comprehensive quality feedback mechanism should be established to compare and analyze test data with mold design and process parameters. If dimensional deviations are detected, compensation can be achieved by fine-tuning process parameters such as mold temperature and hold time. Mold wear caused by long-term production requires prompt repair or replacement to ensure process stability. Furthermore, statistical process control (SPC) technology can be implemented to monitor and analyze key process parameters and part quality indicators in real time, predicting quality fluctuations and implementing preventative quality control.
