In precision molds, the design of the venting structure directly affects product quality in molding processes such as injection molding and die casting. Poor venting can easily lead to defects such as incomplete filling, burning, air bubbles, and silver streaks. A well-designed venting structure requires comprehensive consideration from multiple dimensions, including venting location selection, venting groove type, size control, material compatibility, and process optimization, to ensure timely gas discharge without melt leakage.
The selection of the venting location is the first step in the design. In precision molds, gas mainly accumulates at the end of the melt flow, in the gap between the core and the cavity, and at the mold parting surface. For example, for thin-walled products with complex shapes, the melt may experience localized stagnation due to differences in flow resistance during filling, where gas easily accumulates. Furthermore, if the gap between the core and the cavity is not properly sealed, air can be drawn into the cavity. Therefore, venting grooves should be preferentially placed in the last areas of melt filling, such as the ends of the product, corners, or areas of abrupt thickness changes, while also considering the mating surfaces of the core and cavity to ensure no dead zones for gas accumulation.
The type of venting groove needs to be flexibly selected based on the mold structure and molding process. Common venting types include parting surface vents, core vents, ejector pin vents, and permeable steel vents. Parting surface vents are suitable for most molds, using shallow grooves on the parting surface to allow gas to escape through the small gaps during mold opening and closing. Core vents target gas around the core, using small grooves on the core surface or the gap between the core and the mold platen. Ejector pin vents utilize the clearance between the ejector pin and the mold platen to guide gas to the outside of the mold. Permeable steel is a porous material that can be directly machined into the mold core or cavity; gas escapes through its internal pores, making it suitable for deep cavities or areas with difficult venting.
Controlling the dimensions of the vents is crucial to preventing melt leakage. Vents that are too deep will cause melt overflow, forming flash; vents that are too shallow will result in poor venting and ineffective gas removal. Generally, the depth of the venting groove needs to be determined based on the melt flowability and molding pressure. For plastics with good flowability or low-pressure molding processes, the venting groove depth can be controlled at 0.005-0.02 mm; for metals with poor flowability or high-pressure molding processes, the depth can be appropriately increased to 0.03-0.05 mm. Simultaneously, the width of the venting groove should be greater than its depth, generally 1-3 mm, to increase the venting area and improve venting efficiency.
Material suitability also significantly impacts the performance of the venting structure. Mold materials need to possess high hardness, high wear resistance, and good thermal stability to ensure that the venting groove is not easily worn or deformed during long-term use. For example, for high-speed injection molds, the core and cavity often use high-hardness steel (such as H13, S136, etc.) and undergo quenching treatment to improve wear resistance; the venting steel needs to be selected with moderate porosity and sufficient strength to prevent collapse under high pressure. Furthermore, the mold surface needs to be polished to reduce surface roughness, decrease gas adsorption and melt adhesion, and further optimize the venting effect.
Process optimization is a crucial means of improving the performance of venting structures. During mold manufacturing, strict control of machining precision is essential to ensure that the dimensions of the venting grooves match the design values, preventing venting problems or melt leakage due to machining errors. Simultaneously, during the mold debugging phase, trial molding is necessary to observe the molding process. If defects such as gas streaks or scorching are found, the position or size of the venting grooves must be adjusted promptly until the defects are eliminated. Furthermore, optimizing molding process parameters (such as injection speed, pressure, and temperature) can indirectly improve venting effectiveness. For example, reducing injection speed can decrease gas entrapment, and increasing mold temperature can reduce melt viscosity, promoting gas expulsion.
The venting structure design of a precision mold must balance venting efficiency and melt sealing. By rationally selecting venting locations, optimizing the form and size of venting grooves, adapting to mold materials, and optimizing process parameters, molding defects such as incomplete filling and scorching can be effectively avoided, improving product quality and production efficiency.
