Prevention of Stress Cracking in PSU Injection Molding

PSU is a high-performance engineering plastic widely used in automotive, electronics, medical, and structural components. However, its rigid molecular structure makes it highly sensitive to internal stress, leading to cracking, crazing, and whitening after molding. More than 40% of PSU defects are caused by stress cracking. Effective prevention requires integrated control of materials, mold design, processing parameters, and post-treatment.

Raw Material Handling and Control

PSU absorbs moisture easily and must be fully dried before molding. The recommended drying condition is 120–140°C for 3–4 hours, with moisture content controlled below 0.02%. Excessive moisture causes degradation and increases internal stress. The proportion of recycled material should not exceed 15%, and recycled material must be redried. Mixing with other plastics must be avoided to prevent poor compatibility and stress concentration.

Mold Structure Optimization

Gate position and type significantly affect stress distribution. Gates should be set at thick sections, and direct gates should be avoided to reduce impact force. Runner and gate size should be sufficient to lower shear stress. A draft angle of at least 1° is required for smooth ejection. Sharp corners must be replaced with fillets of R0.5 or above to reduce stress concentration. Balanced ejection and sufficient venting are also necessary to avoid uneven stress.

Injection Molding Parameter Control

Melt temperature is generally set between 320°C and 350°C. Lower temperatures are preferred to reduce degradation. Mold temperature should be increased to 100–120°C to relax molecular orientation and reduce internal stress. Injection speed should be moderate to avoid high shear. Injection and holding pressures should be minimized while maintaining full filling. Sufficient cooling time helps avoid warping and stress caused by uneven shrinkage.

Post-Treatment for Stress Relief

Annealing is the most effective method to eliminate internal stress in PSU parts. The recommended process is 120–150°C for 2–4 hours, followed by slow cooling to room temperature. Rapid cooling must be avoided to prevent thermal stress. Parts should also be protected from contact with organic solvents, oils, and chemicals, which accelerate stress cracking.

Production and Application Precautions

During production, inspect gates, corners, and ejection areas for whitening or micro-cracks. Assembly stress should be minimized to avoid forcing parts into position. Products should be protected from extrusion and bending during storage and transportation. Proper handling greatly reduces the risk of delayed cracking.

Conclusion

Stress cracking in PSU is caused by material, mold, processing, and application factors. Comprehensive control from drying, mold design, parameter adjustment, to annealing and assembly is required. Standardized processing and proper post-treatment can effectively reduce cracking and ensure the reliability and service life of PSU products.