To avoid surface runners on power bank shell molds, venting structures must be designed comprehensively, considering seven aspects: vent location selection, vent groove design, venting method matching, venting and mold strength balance, venting and molding process compatibility, venting structure optimization, and venting effect verification. This meticulous design achieves both efficient venting and product quality assurance.
The selection of vent location is fundamental to venting structure design. Power bank shells typically consist of multiple surfaces, including the front, back, sides, and end faces. Runner formation is closely related to the area of gas retention during melt filling. Venting locations should ideally be placed in the last areas filled by the melt, such as the far end of the shell, areas of abrupt wall thickness changes, or around inserts. For example, when reinforcing ribs exist on the side of the shell, gas retention can easily occur due to flow obstruction when the melt fills the base of the ribs. In this case, vent grooves should be placed on the mold core or cavity side corresponding to the base of the reinforcing ribs to guide the gas out. Furthermore, for venting at the parting line, the mold opening and closing direction must be considered to ensure the vent groove is located at the end of the melt flow, preventing gas from being re-entered into the melt.
The design of the vent grooves directly affects venting efficiency. The dimensions of the venting groove need to be determined comprehensively based on the melt flowability, mold temperature, and molding pressure. Generally, the depth of the venting groove should be less than the melt overflow value to avoid melt overflow and resulting burrs; the width needs to ensure smooth gas passage while avoiding a decrease in mold strength due to excessive width. For example, for common power bank shell materials such as PC or ABS, the venting groove depth can be controlled between 0.02-0.05 mm, and the width can be designed to be 2-5 mm, which can effectively vent and prevent overflow. In addition, the cross-sectional shape of the venting groove can be trapezoidal or semi-circular. The angle of the inclined side of the trapezoidal groove should be 30°-45° to reduce melt residue in the venting groove.
Combining venting methods can improve the venting effect. In addition to traditional venting grooves, auxiliary methods such as vent plugs, permeable steel, or vacuum venting can also be used. Venting plugs are suitable for venting deep cavities or narrow areas, venting gas through a porous structure. Permeable steel, a porous metal material, can be directly embedded in the mold core or cavity, utilizing its microporous structure to achieve uniform venting, especially suitable for high-gloss surfaces requiring high venting performance. Vacuum venting, on the other hand, uses a vacuum pump to remove air from the mold cavity before melt filling, reducing gas generation at its source, but requires a complex vacuum system and is more expensive. In practical design, single or combined venting methods can be selected based on the shell structure and cost requirements.
Balancing venting with mold strength is crucial in design. The venting structure must avoid weakening the mold's rigidity, especially in areas subjected to high-pressure injection. For example, when venting grooves are placed at the edges of the core or cavity, strength loss must be compensated for by increasing local wall thickness or using insert structures. For the use of permeable steel, its embedding area must be strictly controlled, typically not exceeding 30% of the core or cavity surface area, to prevent mold deformation due to the porous structure. Furthermore, the edges of the venting grooves must be chamfered or rounded to avoid stress concentration and cracking.
Adapting venting to the molding process can improve venting efficiency. Process parameters such as mold temperature, injection speed, and holding pressure affect gas generation and venting. For example, high-temperature molds can reduce melt viscosity and gas entrainment, but excessively high temperatures can lead to material degradation and the generation of more gas; high-speed injection can shorten filling time, but it easily entraps air, requiring a more efficient venting structure; excessively high holding pressure may compress gas into the product, forming bubbles. Therefore, venting structure design needs to be optimized in conjunction with molding process parameters, such as by adjusting the size of venting channels or venting methods to adapt to venting requirements under different process conditions.
Optimization of venting structures requires a combination of simulation and experimentation. Injection molding simulation software such as Moldflow can analyze the gas distribution during melt filling and predict the location of gas streaks, thus allowing for targeted optimization of the venting structure. For example, if simulation reveals excessively high gas concentration in a certain area, venting channels can be added at that location or a breathable steel can be used; for complex shell structures, rapid prototyping can be used to create trial molds to verify the venting effect, and the venting design can be adjusted based on the trial mold results.
Verification of venting effectiveness is the final step in the design process. Visual inspection of the product surface for defects such as air bubbles, silver streaks, or scorching can provide a preliminary assessment of venting adequacy. For high-requirement products, cross-sectional analysis or X-ray inspection is also necessary to confirm the presence of internal air bubbles. If insufficient venting is found, improvements can be made by enlarging the venting grooves, adding venting methods, or adjusting process parameters until the product surface quality meets requirements. Through continuous iteration, a highly efficient venting structure design for the power bank shell mold was ultimately achieved.
