Silver Streaks: Main Causes and How to Solve Them?

Silver streaks are a frequent surface defect in injection molding, appearing as silvery, white, or metallic-looking lines on finished parts. While these streaks may not always compromise mechanical strength, they significantly affect visual appeal.

Causes of Silver Streaks

Silver streaks primarily result from material, process, and mold-related issues. Common causes include:

Moisture in Raw Materials

Hygroscopic plastics such as polycarbonate (PC), nylon (PA), or ABS can absorb moisture from the environment. During injection, the moisture rapidly vaporizes, forming small steam pockets that create streaks on the surface. This is particularly evident in thin-walled sections or areas with rapid flow.

Excessive Injection Speed

High injection speeds can generate excessive shear stress within the molten plastic. This rapid flow can stretch entrapped air or moisture, leading to streak formation along the flow lines. Overly fast injection also increases friction between the melt and the mold wall, sometimes causing localized overheating and surface defects.

Contaminated or Degraded Materials

Foreign particles, degraded resin, or material contamination can interrupt the smooth flow of plastic, leaving streaks or lines on the surface. Degraded material may result from repeated exposure to high temperatures or improper storage.

Excessive Melt Temperature

Overheating the polymer can reduce its viscosity excessively, increasing the likelihood of trapped air or volatile gases. High melt temperature can also accelerate material degradation, further contributing to silver streak formation.

Poor Mold Venting

Trapped air in poorly vented cavities can cause localized vapor pockets, which appear as streaks on the part surface. Inadequate venting is particularly problematic in thin or long flow paths.

Solutions to Silver Streaks

Addressing silver streaks requires a combination of material preparation, process control, and mold optimization:

1. Dry Raw Materials Thoroughly: Ensure hygroscopic materials are dried according to manufacturer recommendations before molding. Use desiccant dryers or ovens to reduce moisture content to an acceptable level, preventing vapor formation during injection.
2. Optimize Injection Speed and Pressure: Adjust injection speed to avoid overly fast filling, which can induce shear stress and stretch air pockets. Properly balancing injection pressure helps fill cavities without causing turbulent flow or overheating.
3. Control Melt Temperature: Maintain melt temperature within the recommended range for each material. Avoid excessive heating that could degrade the resin or increase the release of volatile components.
4. Use Clean and Quality Materials: Ensure resins are free from contamination, dust, or degraded material. Proper storage, handling, and filtering can prevent foreign particles from affecting surface finish.
5. Improve Mold Venting: Add or optimize venting channels to allow trapped air or gases to escape. This reduces the likelihood of surface streaks and improves overall part quality, especially in long or complex flow paths.
6. Regular Equipment Maintenance: Keep the injection molding machine, barrel, screw, and hopper clean to avoid material degradation. Regular maintenance ensures consistent melt quality and minimizes defects.

Prevention Strategies

Proactive measures can help prevent silver streaks before production begins:

• Conduct pre-production drying and moisture testing for hygroscopic resins.
• Use CAE simulation to analyze flow patterns and identify areas prone to high shear or trapped air.
• Maintain consistent material handling and storage to avoid contamination or degradation.
• Ensure proper mold design, including sufficient vents and uniform wall thickness, to reduce vapor entrapment.
• Train operators to monitor injection speed, melt temperature, and holding pressure during production.

Impact on Part Quality

Silver streaks can significantly affect the aesthetic, functional, and commercial value of molded parts:

• Visual Appearance: Metallic or white streaks diminish surface finish, which is critical for consumer electronics, automotive interiors, and high-end appliances.
• Surface Functionality: In some applications, surface defects can interfere with coating adhesion, painting, or labeling.
• Customer Perception: Visible defects may lead to product rejection or returns, affecting brand reputation and increasing costs.
• Cost Implications: Parts exhibiting silver streaks may require secondary finishing or scrapping, adding time and expense to production.

Conclusion

Drying hygroscopic plastics, adjusting injection speed and melt temperature, maintaining clean materials, and optimizing mold venting can significantly reduce streak formation. Implementing these measures ensures high-quality, aesthetically appealing parts, reduces waste, and improves production efficiency.