Traffic light shells serve as crucial visual markers for urban traffic management, and their appearance directly impacts road safety and the city's image. During traffic light shell injection molding, gate marks, as remnants of molten plastic entering the mold cavity, often manifest as protrusions, depressions, or color variations, severely degrading the product's aesthetics. To effectively reduce this defect, a multi-dimensional approach is needed, encompassing gate design, process parameter optimization, mold structure improvement, and post-processing, to achieve a balance between appearance quality and production efficiency.
The scientific selection of gate type is paramount in minimizing marks. Submerged gates, due to their concealment within the product's non-surface surfaces or edges, are the preferred solution for traffic light shells. These gates automatically cut off during mold opening, and residual marks can be eliminated through subsequent grinding or hot-melt treatment. For complex-shaped shells, a combination of point gates and hot runner systems can be used to balance injection pressure through multiple gates, avoiding weld lines and stress concentration caused by a single gate. Furthermore, horn gates are suitable for curved shells, as their curved channels can conceal marks within the product; however, strict control of the runner length is necessary to prevent melt degradation. Precise design of the gate location and size is crucial for controlling defects. Gates should ideally be placed in non-visually centered areas of the product, such as edges, bottoms, or reinforcing ribs, avoiding locations in the light transmission path of transparent lampshades or on the reflective surfaces of reflectors. For multi-cavity molds, ensure symmetrical distribution of gates to prevent product deformation and secondary defects caused by uneven injection. Gate size must be determined based on a combination of plastic flowability and product wall thickness; excessively large gates will result in noticeable defects, while excessively small gates may lead to incomplete filling or jetting marks. Optimization is typically achieved using empirical formulas combined with CAE simulations to ensure the melt fills the cavity smoothly in a laminar flow state.
Dynamic control of traffic light shell injection molding process parameters directly impacts defect formation. Injection speed should be set according to gate type and product structure. High-speed injection should be used when the melt passes through the gate to reduce material degradation caused by shear heat, switching to medium-low speed filling after entering the cavity to avoid eddies and bubbles. Precise matching of holding pressure and time is essential; excessively high holding pressure will increase residual stress at the gate, leading to raised defects; insufficient holding pressure may cause shrinkage marks and weld lines. Mold temperature control is equally crucial. Appropriately increasing the mold temperature can reduce melt viscosity and decrease shear stress at the gate, but cooling efficiency must be balanced to prevent product deformation.
Optimized mold structure design is fundamental to eliminating flow marks. The concentricity of the sprue bushing and locating ring must be strictly controlled within 0.01mm to prevent asymmetrical melt flow due to eccentricity. The runner system should employ rounded transitions and polishing to reduce melt flow resistance and frictional heat. For transparent lampshades, a cold slug well can be installed near the gate to capture the low-temperature melt at the front end, preventing it from entering the cavity and forming flow marks. The layout of the mold venting channels must be reasonable to prevent scorching or insufficient filling due to trapped air, indirectly concealing gate marks. Furthermore, using nitrogen-assisted injection or sequential valve control technology can achieve controlled melt filling and reduce pressure fluctuations in the gate area.
Appropriate post-processing can further improve the appearance of flow marks. Residual gate protrusions can be repaired by manual grinding or automatic polishing machines, but the processing force must be controlled to prevent scratches on the product surface. Hot melt treatment melts gate residue by heating it, allowing it to flow naturally. This method is suitable for thermoplastics, but precise temperature and time control is crucial to avoid product deformation. For high-gloss products, flame treatment or plasma cleaning techniques can be used to eliminate color and gloss differences at the gate marks through surface modification. Chemical solvent wiping requires caution; it is only suitable for specific plastics and solvent concentration and contact time must be controlled to prevent corrosion of the product surface.
Material selection and formulation optimization provide fundamental support for gate mark control. Using high-flow, low-shrinkage engineering plastics, such as PC/ABS alloys or modified PMMA, can reduce stress concentration and shrinkage differences at the gate. Adding lubricants or flow promoters can improve melt flow and reduce material degradation caused by shear heat. For lampshades requiring high light transmittance, in-mold transfer technology without gate marks can be used to directly mold the decorative layer onto the product surface, completely avoiding the impact of the gate on the appearance.
Controlling gate marks in traffic light shell injection molding is a systematic engineering process involving design, process, mold, and materials. By optimizing the selection of gate type, precisely designing its location and dimensions, dynamically controlling process parameters, improving and innovating mold structure, and rationally applying post-processing techniques, the appearance quality of products can be significantly improved, meeting the dual requirements of safety and aesthetics for urban traffic facilities. This technical approach is not only applicable to the field of traffic lights but can also provide valuable insights for the injection molding of other outdoor lighting equipment.
