Key Considerations for PEI Plastic Injection Molding
PEI (Polyetherimide) is a high-performance amorphous engineering plastic with high heat resistance, high strength and low moisture absorption. However, it has a narrow processing window and is highly sensitive to drying, temperature, equipment, molds and process parameters. Strict compliance with specifications is required for injection molding; otherwise, defects such as silver streaks, bubbles, internal stress cracking and scorching are prone to occur. The following sorts out the key points of each core link to ensure stable production.
1. Raw Material Drying: Core Premise to Avoid Hydrolysis
Though PEI has low water absorption (0.25%-0.35% in 24h), trace moisture will cause severe hydrolysis at high temperatures, leading to reduced product strength and appearance defects. A dehumidifying dryer is a must: dry at 140-150℃ for 4-6h (or 185℃ for 2-3h to avoid pre-oxidation) to reduce moisture content below 0.02%. After drying, keep the material warm at 90-100℃ in the hopper to prevent secondary moisture absorption, and use materials immediately in humid environments. Dry different batches and grades separately; re-dry opened and unused raw materials before processing.
2. Equipment and Molds: Basic Guarantee for High-Temperature Molding
PEI melt features high viscosity and high processing temperature, so equipment and mold matching is critical. For injection molding machines, select reciprocating screw types with precise temperature control (±2℃) and stable injection capacity. The screw should have a length-diameter ratio of 18:1-24:1 and a compression ratio of 1.5:1-2.5:1; the injection volume should be 30%-80% of the maximum (≥60% for reinforced PEI). Set back pressure at 5-15MPa and screw speed at 30-60r/min to balance plasticization and avoid degradation.
For molds, design short and thick runners with sufficiently large side or fan gates (add gates for complex products) to reduce shear stress. Set exhaust grooves (0.01-0.03mm deep, 5-10mm wide) at gas trapping positions and clean them regularly. Control mold temperature at 120-150℃ (150-180℃ for thin-walled/complex parts) to reduce internal stress and weld lines. Ensure a demolding taper ≥1°, polish the surface to below Ra0.05μm, and design uniform cooling water channels with a cooling time of 20-60s (prolong for thick-walled parts).
3. Process Parameters: Precise Control to Avoid Degradation
PEI’s processing temperature range is 340-400℃, requiring precise parameter setting by material grade and product structure. Adopt sectional barrel temperature control: 320-340℃ at the rear, 340-360℃ in the middle, 360-380℃ at the front, with the nozzle temperature 5-10℃ higher than the front. Control melt temperature at 340-370℃ for unreinforced PEI and 360-380℃ for reinforced PEI (strictly no more than 380℃ to prevent thermal degradation). Limit melt residence time in the barrel to ≤5min; clean the barrel with PEEK for color changes or shutdowns.
Set injection pressure at 100-180MPa (150-180MPa for thin-walled/complex parts, 100-130MPa for simple thick-walled parts) and injection speed at 20-80mm/s to balance fluidity and shear heat. The packing pressure is 60%-80% of the injection pressure, with a packing time of 10-30s (prolong for thick-walled parts) to compensate for shrinkage and reduce internal stress.
4. Common Defects and Targeted Countermeasures
Quickly locate and resolve defects during production: silver streaks are caused by insufficient drying or high barrel temperature (extend drying, lower temperature); scorching results from poor exhaust or high screw speed (optimize exhaust, reduce speed to 30-40r/min); internal stress cracking is due to low mold temperature or insufficient demolding taper (raise temperature to 140-150℃, increase taper to ≥1.5°); obvious weld lines come from low melt temperature or few gates (raise temperature, add gates); mold sticking is caused by high mold temperature or long packing time (lower temperature, shorten packing); warpage is due to uneven cooling (optimize water channel layout, raise mold temperature to ≥120℃).
5. Post-Processing, Safety and Mass Production Control
Anneal PEI products to release internal stress: heat at 180-200℃ (10-20℃ higher than service temperature, 20-30℃ lower than heat distortion temperature) for 1-4h, then cool with the oven, especially for high-precision or high-temperature service parts. Ensure good workshop ventilation and wear protective gear, as overheating (＞380℃) produces pungent gases; regularly maintain equipment and cool molds before cleaning. Collect degraded waste separately and avoid mixing with new materials.
For mass production, test 3-5 parameter combinations to record product performance, form a standardized process card, and conduct random inspections every 2h to monitor moisture content, appearance and dimensions. Stop production immediately for troubleshooting if abnormalities occur, and track barrel temperature and screw speed to prevent parameter drift.
In summary, PEI injection molding relies on strict drying control, precise temperature regulation, suitable equipment and molds, optimized process parameters and standardized post-processing. Flexible adjustment based on material grade and product structure, combined with repeated mold testing, achieves stable mass production and gives full play to PEI’s performance advantages.
