In traffic light shell injection molding, flash (also known as overflow or burrs) is a common defect affecting the product's appearance and structural integrity. Its formation is closely related to mold design, injection molding process parameters, and equipment condition. Reducing flash requires a comprehensive approach encompassing mold precision, clamping force control, injection parameter optimization, and equipment maintenance. The following is a detailed technical analysis.
Mold design is the primary step in preventing flash. Traffic light shells typically have complex geometries, and the parting surface design must balance sealing and ease of demolding. If there are gaps or insufficient fit at the parting surface, molten plastic will seep into the gaps under high pressure, forming flash. Therefore, high-precision machining processes (such as CNC machining) should be used in mold design to ensure the flatness of the parting surface and the fit clearance is less than 0.02mm. Simultaneously, for deep cavity structures or thin-walled areas, the location and number of gates must be rationally designed to avoid localized mold deformation due to pressure concentration during melt flow. Furthermore, the venting system design of the mold also needs optimization. If the venting grooves are too deep or improperly distributed, the melt may overflow from the parting surface due to poor venting; conversely, if the venting grooves are too shallow, gas cannot be effectively discharged, resulting in insufficient melt filling. It is generally recommended that the depth of the venting grooves be controlled between 0.005-0.01 mm and evenly distributed along the edge of the mold cavity.
Clamping force is a key process parameter for controlling flash. During injection molding, molten plastic forms high pressure (typically 50-150 MPa) within the cavity. If the clamping force is insufficient, the mold parting surface will be forced open, causing plastic to overflow. The clamping force setting needs to be calculated comprehensively based on the product's projected area and the material's fluidity, generally covering 1.2-1.5 times the melt pressure. For example, for large traffic light housings, an injection molding machine with matching clamping force (such as a 500-1000 ton machine) should be selected, and the rigidity of the clamping mechanism should be ensured to prevent clamping force attenuation due to wear after long-term use. In addition, the clamping force must be monitored in real time during injection molding. If pressure fluctuations exceed 5%, the machine must be stopped immediately to check the mold or equipment status.
Optimizing injection molding process parameters is crucial for reducing flash. Excessive injection speed can cause turbulence at the melt front, increasing the instantaneous pressure on the mold parting surface; conversely, insufficient injection speed may lead to incomplete filling due to melt cooling, forcing operators to increase injection pressure and indirectly causing flash. Therefore, a segmented injection process should be adopted based on material properties (such as melt flow index) and product structure: initially filling the cavity at high speed, then switching to low-speed holding pressure near full mold filling to balance filling efficiency and pressure control. The setting of holding pressure and time also requires careful consideration. Excessively high holding pressure will intensify stress on the mold parting surface, while excessively long holding time may cause the melt to seep into gaps under pressure. It is generally recommended to control the holding pressure at 70%-80% of the injection pressure, and the holding time should be adjusted according to the product wall thickness (e.g., 1-2 seconds for thin-walled parts, 3-5 seconds for thick-walled parts).
The coordinated control of mold temperature and melt temperature is also crucial. Too low a mold temperature leads to rapid melt cooling, increased surface hardness, and increased flow resistance, forcing operators to increase injection pressure. Conversely, too high a mold temperature can cause increased parting line clearance due to material thermal expansion. For PC (polycarbonate) or ABS (acrylonitrile-butadiene-styrene copolymer) materials commonly used in traffic light housings, the mold temperature is typically controlled between 60-80℃, while the melt temperature is adjusted according to the material grade (e.g., 260-300℃ for PC, 220-260℃ for ABS). Furthermore, residual plastic or oil on the mold surface must be cleaned regularly to prevent parting line seal failure due to foreign matter.
Equipment maintenance is fundamental to ensuring long-term production stability. The hydraulic system of the injection molding machine needs regular inspection to ensure no leaks in the clamping cylinder and that the hydraulic oil is clean enough. Moving parts such as mold guide pillars and guide sleeves need regular lubrication to prevent parting line misalignment due to wear. The ejection mechanism must maintain synchronized operation to prevent localized mold deformation due to uneven ejection. For example, a traffic light shell manufacturer reduced the flash occurrence rate from 3% to below 0.5% by implementing a maintenance system of "daily inspection, weekly maintenance, and monthly overhaul."
Reducing flash in traffic light shell injection molding requires a systematic approach encompassing mold design, clamping force control, process parameter optimization, and equipment maintenance. Improving mold precision, matching clamping force, optimizing injection and holding pressure processes, coordinating temperature parameter control, and strengthening equipment maintenance can significantly reduce the risk of flash formation and improve product yield and production efficiency.
