Extending the cross runner can reduce the spray marks and chatter marks of PC injection molded parts
PC (polycarbonate) is widely used in high-end plastic parts such as optical lenses and medical devices due to its excellent light transmittance, impact strength, and heat resistance. However, PC is prone to producing shot and chatter marks during injection molding, which can affect product quality. Shot marks appear as silver streaks extending from the gate, while chatter marks are wavy patterns on the surface of the part. Both are caused by unstable melt flow and excessive shear stress. Extending the runner has proven to be an effective method to mitigate these defects. By increasing the melt flow path, it allows for full plasticization, smooths pressure transmission, and reduces stress concentration. For example, in the production of PC eyeglass lens molds, extending the runner length from 50mm to 100mm reduced the incidence of shot and chatter marks by over 60% and increased light transmittance by 3%-5%. Therefore, a thorough understanding of the impact of runner length on PC material flow and the rational design of runner parameters are crucial for improving the quality of PC molded parts.
The mechanism of the formation of shot marks and chatter marks is closely related to the flow characteristics of PC materials. PC material melt has high viscosity and is sensitive to shear rate. When the melt enters the cross runner from the main channel and then passes through the gate, the sudden change in flow rate causes a sharp increase in shear stress, uneven orientation of the molecular chains, and the formation of shot marks. At the same time, fluctuations in injection pressure can cause the melt to vibrate in the mold cavity, forming periodic chatter marks. When the cross runner is too short, the melt enters the mold cavity before it stabilizes, the shear stress cannot be fully released, and defects are more likely to occur. Extending the cross runner can gradually stabilize the melt during the flow process, the shear stress dissipates through friction and heat transfer, the molecular chains are rearranged, and the orientation difference is reduced. For example, when the cross runner is too short, the shear rate of the PC melt can reach more than 10,000 s⁻¹ , resulting in obvious shot marks; after extension, the shear rate drops to below 5,000 s⁻¹ , and the shot marks are significantly reduced.
The optimal design of the cross-runner length requires a comprehensive consideration of part size and gate type. Generally speaking, for PC materials, the cross-runner length should be no less than 1.5-2 times the maximum part dimension, or 10-15 times the sprue diameter. For small parts (such as a 50mm diameter lens), the cross-runner length should be 80-120mm; for large parts (such as a 300mm long baffle), the length should be increased to 150-250mm. The cross-runner length-to-diameter ratio should also be controlled, typically 8-12 times the diameter, to ensure stable laminar flow. For example, for an 8mm diameter cross-runner, the length should be between 64-96mm to keep the melt flow Reynolds number below 2000 and avoid turbulence. Furthermore, the cross-runner length must be compatible with the injection pressure. Excessive length increases pressure loss, necessitating an appropriate increase in injection pressure by 5%-10% to ensure the melt fills the cavity.
The cross-sectional shape and surface quality of the runner are equally important in reducing defects. A circular cross-section is superior to a trapezoidal or rectangular cross-section, ensuring uniform melt flow and symmetrical shear stress distribution, thereby reducing stress concentration caused by uneven flow. The diameter of the circular cross-section is determined by the weight of the part, generally ranging from 6-12mm, with the smaller diameter used for small parts and the larger for large parts. For example, for a 100g PC part, a cross-section diameter of 10mm and a length of 120mm ensures stable melt flow. The surface roughness of the cross-section must achieve Ra ≤ 0.4μm. Fine grinding is used to reduce surface friction, ensuring smooth melt flow and avoiding pressure fluctuations and chatter marks caused by excessive frictional resistance. Furthermore, the connection between the cross-section, main runner, and gate must utilize an arc transition with a radius of at least 3mm to prevent eddy currents and pressure loss in the melt at corners.
Extending the cross runner requires optimizing supporting process parameters to minimize defects. The injection speed should be controlled incrementally: a slow initial speed (20-40 mm/s) to ensure smooth melt entry into the cross runner and minimize impact. The speed should be gradually increased (50-80 mm/s) during the intermediate stages to establish stable flow within the cross runner. The speed should be reduced again (30-50 mm/s) upon passing through the gate to avoid a sudden increase in shear stress. The barrel temperature should be appropriately elevated (280-300°C) to reduce PC melt viscosity and compensate for the pressure loss caused by the extended cross runner. The mold temperature should be controlled between 80-100°C to ensure slow cooling of the melt after entering the cavity, minimizing molecular chain orientation. The holding pressure should be set to 60%-70% of the injection pressure, and the holding time should be extended by 10%-20% to help release internal stress in the melt. For example, increasing the holding time from 8 seconds to 10 seconds after extending the cross runner can reduce internal stress in PC parts by 25%, further minimizing chatter marks.
Considerations and practical application techniques for extending the cross-runner are crucial for ensuring optimal results. A longer cross-runner is not necessarily better. Excessive length increases the melt’s residence time in the barrel, leading to PC material degradation and the formation of silver streaks and black specks. Therefore, the length should be kept within a reasonable range, while simultaneously increasing the screw speed (30-60 rpm) to reduce residence time. For multi-cavity molds, the length of each cross-runner should be consistent, with a variation of no more than 5% to ensure uniform part quality across cavities. In actual production, the cross-runner length can be gradually adjusted through trial molds, increasing by 20-30 mm at a time, and observing the changes in shot and chatter marks until optimal results are achieved. For example, if shot marks are evident at an initial cross-runner length of 50 mm, they will decrease after increasing to 80 mm and are virtually eliminated at 100 mm, confirming that 100 mm is the optimal length. By combining cross-runner optimization with process parameter adjustment, the shot and chatter mark defect rates for PC injection molded parts can be reduced to below 5%, meeting the quality requirements of high-end products.
