Root Cause Analysis and Solution of Silver Streak Defects in Injection Molded Products
2026-05-20 13:11:27
Injection Mold
Silver streak is one of the most frequent appearance defects in injection molding production, presenting continuous silvery linear traces along the melt flow direction on product surfaces. This defect not only destroys surface finish and assembly appearance, but also easily causes internal structural looseness, reduced mechanical strength and unstable dimensional consistency, which seriously affects product delivery qualification rate and actual service performance.
The essential cause of silver streak formation is that various types of mixed gas inside molten plastics cannot be discharged smoothly during filling process. Air bubbles are stretched into strip gaps under material flow shearing force, and form visible silvery marks after light reflection after cooling and shaping. Comprehensive sorting out all gas generation sources and implementing targeted improvement measures are effective ways to completely eliminate silver streak defects.
1. Raw Material Moisture Absorption and Vaporization (Primary Cause)Most commonly used injection molding engineering plastics have strong moisture absorption characteristics. Raw materials such as ABS, PC, PA, PET and PBT easily absorb ambient moisture during storage and transportation. Undried damp raw materials enter high-temperature barrels, and internal moisture is instantly converted into water vapor to form dense bubbles evenly mixed in melts.
Water vapor bubbles are stretched continuously with high-speed material flow during injection filling, and fixed into obvious silver streaks after mold cooling shaping. Recycled crushed materials have larger specific surface area and faster moisture absorption speed. Mass production of silver streak defects is easily caused by large proportion mixing of undried recycled materials. In addition, auxiliary materials such as color masterbatch, filling powder and glass fiber also carry trace moisture, which will induce tiny silver streaks and water grain defects after vaporization.
Improvement Measures: Formulate classified constant-temperature drying standards for different raw materials, extend drying time sufficiently for hygroscopic materials, and adopt sealed heat preservation storage after drying to prevent secondary moisture absorption. Control mixing proportion of recycled materials strictly, and carry out secondary drying treatment for recycled materials before use. Eliminate damp auxiliary materials fundamentally to cut off water vapor generation sources.
2. Excessive Barrel Temperature Causing Thermal Decomposition and Gas PrecipitationEach plastic raw material has matched safe processing temperature range. Once barrel segmented temperature or nozzle temperature is set excessively high exceeding material tolerance limit, plastic molecular chains will break and degrade, releasing a large number of low-molecular volatile mixed gases. These chemical decomposition gases are mixed in melts to form irregular silver streaks with dark color.
Long-term shutdown standby causes raw materials to stay in high-temperature barrels for a long time, leading to carbonization deterioration and continuous precipitation of smoke gas, which will further aggravate silver streak distribution range and defect severity. Random mixing of different types of raw materials with inconsistent heat resistance will also produce composite reaction gas under high temperature conditions.
Improvement Measures: Reset segmented barrel temperature strictly in accordance with raw material processing manuals to avoid local overheating. Reduce idle standby time of injection machines, and empty residual deteriorated materials inside barrels timely after shutdown. Standardize raw material use specifications and prohibit random mixed use of different brand models to avoid chemical reaction gas generation.
3. Unreasonable Process Parameters Leading to Air EntrainmentUnreasonable process parameter matching is an important artificial inducement of silver streaks. Too low back pressure cannot realize sufficient compaction and stirring of melts, and a large amount of external air is sealed inside raw material particles during plasticizing process. Excessively fast injection speed makes melts fill cavities rapidly in a short time, directly blocking mold exhaust passages, and internal cavity air cannot be discharged in time to form compressed air silver streaks.
Unstable screw retraction speed and blocked hopper feeding easily cause intermittent air suction during plasticizing. Unreasonable holding pressure switching timing will also cause secondary gas backflow mixing, increasing internal gas content of finished products.
Improvement Measures: Appropriately increase production back pressure to realize sufficient melt defoaming and homogenization. Adopt multi-segment graded injection speed setting, use low speed for feeding buffer and slow down filling speed at terminal positions to reserve sufficient exhaust time. Optimize feeding smoothness and stabilize plasticizing rhythm to reduce external air entrainment probability.
4. Poor Mold Exhaust Performance Causing Cavity Air EntrapmentMold is a key channel for gas discharge during molding. Insufficient quantity and insufficient depth of exhaust grooves set at mold parting surfaces, runner ends, product dead corners and rib positions will directly lead to poor gas discharge effect. Long-term production accumulation of glue dirt, carbon deposits and sundries will easily block reserved exhaust grooves and completely lose exhaust function.
Unreasonable gate position design leads to excessively long material flow paths, and gas is easy to gather at product terminal areas to form concentrated silver streak defects. Excessively low mold temperature accelerates rapid solidification of melt surfaces, and internal bubbles are frozen before being discharged smoothly. In addition, excessive spraying of release agent and mold lubricant will produce volatile oil-gas under high temperature, forming irregular scattered silver streaks.
Improvement Measures: Increase exhaust groove layout positions appropriately for products prone to silver streaks, regularly clean internal blockages of exhaust passages, and optimize gate layout to shorten gathering paths of internal gas. Raise mold temperature properly to delay surface solidification speed of melts. Standardize release agent spraying dosage to reduce oil-gas mixing hidden troubles.
5. Equipment Failure and On-site Environmental Influencing FactorsSerious wear of injection machine screws and check valves increases internal matching clearance, easily causing melt backflow and disordered flow field, and repeatedly entraining external air to form intermittent silver streak defects. Incomplete internal barrel cleaning and residual carbonized old materials will continuously precipitate mixed gas during remelting process.
High ambient humidity in production workshops especially in rainy and humid seasons will accelerate re-moisture absorption of dried raw materials in feeding links, indirectly increasing internal water vapor content of melts. Excessively low cooling water temperature will also aggravate rapid shaping of bubbles and fix silver streak defects.
Improvement Measures: Inspect vulnerable wearing parts of injection machines regularly and replace severely worn accessories timely. Complete thorough barrel cleaning when switching production products. Control workshop ambient humidity reasonably, and equip feeding insulation and moisture-proof equipment to maintain stable raw material drying state.
Summary AnalysisAll silver streak defects in injection molding production are derived from two core factors: excessive internal gas content of melts and unsmooth overall exhaust channels. Gas sources are divided into water vapor generated by raw material moisture, thermal decomposition volatile gas, external entrained air and oil volatile gas generated by auxiliary supplies.
In actual production improvement, priority shall be given to realizing thorough drying of raw materials to cut off moisture sources, then optimize temperature parameters to avoid material decomposition gas, match graded injection speed and reasonable back pressure to reduce artificial air entrainment, cooperate with mold exhaust optimization and standardized on-site equipment maintenance, and eliminate silver streak defects from multiple links including raw materials, processes, molds and equipment, so as to steadily improve appearance quality and comprehensive qualified rate of injection molded products.
