Injection molding silver streaks (silver wire, material flowers) and solutions
Injection molding silver streaks, also known as silver streaks or material smears, are white or silver streaks that appear on the surface of a product, severely impacting its appearance. Their formation is closely related to factors such as the raw material condition, process parameters, and mold design. Silver streaks can take various forms, ranging from radial distribution near the gate to irregular streaks across the surface of the product. These streaks are essentially bubbles formed on the surface of the product due to gases or volatiles mixed into the plastic melt that fail to be expelled promptly during the molding process. For example, when the raw material contains moisture, the moisture vaporizes at high temperatures, producing water vapor. This water vapor forms bubbles in the melt. Once the melt fills the mold cavity, the bubbles burst, leaving silver streaks on the surface. These streaks are particularly noticeable on transparent or light-colored products.
Raw material factors are a common cause of silver streaks and need to be controlled at the source. First, insufficient drying of the raw materials is the main cause. Hygroscopic plastics such as PA, PC, and ABS absorb moisture from the air during storage. If they are not dried enough, the moisture will vaporize due to heat in the barrel and mix with the melt to form bubbles. The solution is to develop a strict drying process based on the characteristics of the plastic. For example, PA66 needs to be dried at 100-120°C for 4-6 hours, and PC needs to be dried at 120-130°C for 6-8 hours. After drying, it needs to be transported to the hopper in a sealed state to avoid secondary moisture absorption. Secondly, impurities or incompatible recycled materials mixed in the raw materials can also cause silver streaks. Impurities decompose at high temperatures to produce gas, or are incompatible with the matrix resin to form interface bubbles. This needs to be avoided by screening the raw materials and controlling the proportion of recycled materials (no more than 20%).
Improper process parameter settings are a key factor in silver streaks, and these defects must be eliminated through parameter optimization. Excessively high barrel temperatures can cause plastic degradation, producing low-molecular-weight volatiles and forming silver streaks. For example, PC is susceptible to degradation when barrel temperatures exceed 320°C. The temperature should be lowered by 10-20°C and the silver streaks observed. Excessively fast injection speeds can create turbulent melt flow in the cavity, entraining air. This is particularly prone to silver streaks near the gate. Stepwise speed regulation can be employed, reducing the speed during the initial filling phase (e.g., from 80 mm/s to 50 mm/s) to minimize air entrapment. Insufficient holding pressure or a short holding time prevents the melt from compacting and discharging gases within the cavity. Appropriately increasing the holding pressure (by 10-15%) and the holding time should ensure that the melt tightly fills the cavity. Furthermore, excessively low mold temperatures can cause the melt to cool too quickly, preventing gases from escaping quickly. Increasing the mold temperature (e.g., from 50°C to 70°C for ABS) should improve melt flow and degassing.
Improper mold design can also exacerbate silver streaks, necessitating targeted structural improvements. Improper gate placement and form can lead to turbulent melt flow. For example, when the gate faces the cavity wall, the melt impacts the wall, generating vortices that draw air and form silver streaks. Adjusting the gate position to an area with smoother melt flow or using fan gates or latent gates instead of direct gates can reduce impact strength. Poor venting is another major cause of silver streaks. Adequate venting grooves should be provided at the mold parting surface and at the core-cavity interface. These grooves should be kept within a depth of 0.01-0.03mm (determined by the plastic’s overflow value) and a width of 5-10mm to ensure smooth gas discharge. For complex structural parts, venting pins or inserts can be added to the final filling area to enhance venting capacity. Furthermore, a runner with an undersized cross-section or rough surface can increase melt flow resistance and lead to gas entrapment. Enlarging the runner dimensions and polishing the surface (Ra ≤ 0.8μm) are necessary to reduce gas generation.
Addressing silver streaks requires a comprehensive assessment of multiple factors, and a step-by-step investigation approach can be employed on-site. First, check the raw material dryness. Re-dry the raw materials and test the mold to see if the streaks disappear. If no improvement is seen, adjust the barrel temperature and injection speed to reduce the risk of degradation and air entrapment. If streaks persist, check the mold venting and clean or deepen the venting grooves. Finally, verify the mold temperature and holding pressure parameters to ensure adequate melt fill. For stubborn streaks, test the raw material purity, replace new materials, or adjust the formula to avoid impurities. Through systematic investigation and targeted adjustments, silver streaks can be effectively eliminated and the product’s appearance quality restored.
