PS (polystyrene) is a brittle material with high sensitivity to shear and temperature. Silver streaks, silk lines and gas marks are common appearance defects in injection molding production. The following is a complete set of practical solutions that can be directly implemented in the workshop, to completely eliminate silver streaks from material, process, mold and equipment aspects.
Material Drying and TreatmentAlthough PS has low moisture absorption, moisture, high recycled material ratio or humid storage environment will directly cause silver streaks. Sufficient drying must be carried out before production. Use a hot air dryer at 70 to 80 degrees Celsius for 2 to 3 hours. Control the ratio of recycled material, mix evenly with virgin material before drying together, and do not use damp recycled material alone. Check whether there is condensed water or dew inside the hopper, and keep hot air circulation at the bottom of the hopper to prevent re-absorption of moisture.
Injection Temperature and Screw Speed ControlExcessively high temperature will cause thermal decomposition of PS material and produce gas to form silver streaks. Excessively low temperature will lead to poor flow and high shear, and silk lines will also appear. It is recommended to control the barrel temperature in sections: first stage 160 to 170 degrees Celsius, second stage 170 to 185 degrees Celsius, third stage 180 to 190 degrees Celsius, and nozzle temperature 5 to 10 degrees Celsius lower than the third stage. Properly reduce screw speed and back pressure to reduce melt shear overheating and air entrapment. Control back pressure at 0.3 to 0.6 MPa to avoid excessive heating decomposition of raw material in the barrel.
Mold Venting and Runner OptimizationPS fills fast, and insufficient gas exhaust is an important cause of silver streaks. Focus on improving venting conditions near the gate, flow end and wall thickness change position. Properly set vent grooves with depth of 0.015 to 0.025 mm and width of 5 to 10 mm to ensure smooth exhaust of cavity gas. Avoid sharp corners and sudden cross-section structures in mold runners and gates to prevent shear tearing of melt flow. Try to avoid direct appearance surface for gate position to reduce jetting and turbulence.
Injection Speed and Pressure AdjustmentExcessively high injection speed will cause serious melt turbulence and air entrapment, and increase shear force to cause silver streaks. Excessively low speed will cause cold material marks. Adopt slow-fast-slow sectional injection in production: slow speed for mold sealing at feeding section, medium speed for filling section, and slow down for pressure holding at the end. Properly reduce the first-stage injection pressure and shorten holding time to avoid excessive internal stress of PS products and formation of stress-type silver streaks.
Mold Temperature and Cooling ControlExcessively low mold temperature will cause rapid cooling of PS surface and stretching of molecular chains to form stress silk lines. Properly increasing mold temperature can significantly improve silver streaks. For ordinary PS products, mold temperature is controlled at 30 to 50 degrees Celsius. For products with high appearance requirements, it can be increased to 50 to 60 degrees Celsius to make melt flow more stable and reduce internal stress and surface silver streaks. Keep uniform and slow cooling to avoid stress concentration and reoccurrence of silver streaks caused by rapid cooling.
Mold and Equipment Cleaning and MaintenanceLong-term residual PS decomposed and carbonized materials in barrel, nozzle and hot runner will periodically cause silver streak defects. Regularly disassemble and clean screw, barrel and nozzle to remove carbon deposits and carbides. Keep mold cavity and parting surface clean to avoid residual volatilization of oil and rust inhibitor, forming gas marks similar to silver streaks and affecting product appearance.
Quick Judgment of Silver Streak Types and Corresponding SolutionsWatermark-like silver streaks are mostly caused by damp raw material, so focus on strengthening drying process. Foggy dispersed silver streaks are mostly caused by poor venting, optimize venting structure immediately. Streak silver streaks along flow direction are mostly caused by excessively high injection speed and excessive shear, properly reduce injection speed. Silver streaks gradually increasing with production time are mostly caused by thermal decomposition, reduce barrel temperature and clean internal carbon deposits of equipment.
