June 15, 2026
For an injection molding engineer, few issues are more disruptive than setting up a high-volume production run of PC/ABS components only to discover silver streaks (splay), gas marks, or surface blistering radiating outward from the gate. These cosmetic defects do more than ruin the appearance of premium electronic enclosures or automotive trims; they often indicate underlying structural weaknesses that can lead to part failure under mechanical stress. Fortunately, diagnosing and fixing silver streaks in PC/ABS is a highly systematic process. The root causes almost always trace back to two primary operational errors: Moisture Hydrolysis or Thermal/Shear Degradation.
The first step is always to eliminate moisture hydrolysis, which is the core threat. Polycarbonate (PC) is inherently hygroscopic, meaning it actively absorbs water molecules directly from the surrounding atmosphere. If PC/ABS granules are processed with even a fraction of a percent of absorbed water, the intense heat inside the injection barrel turns that trapped moisture into superheated steam. This steam initiates a chemical reaction called hydrolysis, which permanently severs the molecular chains of the Polycarbonate. The trapped steam is then compressed and stretched along the mold wall during injection, forming classic silver streaks.
Standard hot-air hopper dryers are completely inadequate for PC/ABS. The material must be dried using a dedicated desiccant dehumidifying dryer. The moisture content of the granules must be brought down to less than 0.02% prior to injection. The optimal parameters require baking the granules at 90°C to 100°C for 3 to 4 hours. Ensure that the dryer's air loop is maintaining a steady dew point of -40°C or lower. Be careful not to leave the material in the dryer for more than 10 hours, as this can cause the ABS phase to oxidize and turn yellow.
The second step involves diagnosing and correcting thermal and shear degradation. If your moisture analyzer confirms that the granules are thoroughly dry, yet silver streaks persist, the material is likely undergoing thermal breakdown or trapping gas due to excessive shear energy inside the barrel. Check your barrel temperature configuration and ensure your actual melt temperature falls strictly within the recommended 240°C to 270°C window. If the barrel profile exceeds 280°C, or if the material residence time inside the barrel is too long, the butadiene component within the ABS phase will decompose, releasing gas that registers as splay. Reduce the nozzle and front-zone temperatures slightly to see if the streaks disappear.
Additionally, evaluate your injection velocity and screw dynamics. High injection speeds through restricted or sharp gates generate massive frictional heat (shear stress). This localized overheating scorches the polymer matrix. Transition your injection profile to a slow-to-fast sequence. Injecting slowly through the gate allows the material to establish a stable flow front before increasing velocity to fill the cavity. Keep the screw rotation speed moderate (30 to 60 RPM) to prevent excessive mechanical shearing during plasticization. Finally, raise the back pressure slightly to a range of 0.5 to 1.5 MPa. This mild resistance forces any trapped air or process gases out through the hopper, ensuring a dense, homogeneous melt.
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