The design of the venting system in a precision injection mold is crucial for ensuring product molding quality. Its core objective is to prevent defects such as bubbles, scorching, and incomplete filling caused by gas stagnation, while simultaneously considering mold structural strength and machining feasibility. The design of the venting system requires comprehensive consideration of seven aspects: venting location selection, venting groove size control, venting method diversification, mold structure adaptability, material property matching, process parameter coordination, and trial molding verification and optimization.
The selection of the venting location requires precise positioning of gas accumulation areas. In precision injection molds, gas typically accumulates at the ends of the molten material flow, such as in cavity corners, deep ribs, narrow grooves, and at the ends of runners and cold slug cavities. During the design phase, venting grooves or vents must be created at the final gas accumulation points based on molten material flow path analysis to ensure smooth gas discharge. For example, in gear molds, the areas at the ends of the melt flow, such as the tooth tip and tooth root, are key areas for venting design.
The size control of the venting grooves must balance venting efficiency with the risk of overflow. The depth of the venting groove needs to be determined based on the fluidity of the plastic material: for plastics with good fluidity (such as PP and PE), the venting groove depth can be controlled to a shallower range to prevent overflow; for plastics with poor fluidity (such as PC and PBT), the venting groove depth needs to be appropriately increased to ensure smooth venting. The width of the venting groove is usually designed to be a larger value to increase the venting area, but excessive width should be avoided as it can reduce mold strength. The length of the sealing area needs to be strictly controlled; too long can easily cause overflow, while too short will reduce sealing performance and affect the venting effect.
Diversified venting methods can improve the adaptability of the venting system. In addition to traditional parting surface venting grooves, venting pins, insert gap venting, and permeable steel venting can be used in combination with the mold structure. For example, venting pins can be prioritized in areas where gas easily accumulates, such as deep cavities or narrow grooves; venting gaps or micro-venting grooves can be set at the mating surfaces of side core pullers and cavities; for molds with internal reinforcing ribs or complex core structures, venting channels can be designed between the reinforcing ribs or at appropriate locations on the core.
Mold structure adaptability design is key to the success of the venting system. Precision injection molds may have complex structures, such as side core-pulling mechanisms and angled ejector structures. Gases near these structures require well-designed venting systems. For example, venting gaps or micro-venting channels should be installed at the mating surfaces of the side core-pulling mechanism and the mold cavity to ensure that gas trapped in this area can be smoothly discharged during the side core-pulling process, preventing localized defects on the product's tooth surface due to gas residue.
Material property matching design must consider the plastic's flowability and gas generation. Different plastics produce varying amounts and compositions of gas during injection molding. For instance, some high-performance engineering plastics may release more volatiles at high temperatures, requiring a more powerful venting system. For heat-sensitive plastics (such as PA and PC), the venting channel gap needs to be appropriately increased to prevent gas retention and material decomposition.
Collaborative design of process parameters can optimize venting. Changes in parameters such as injection speed, pressure, and temperature alter the melt flowability and gas generation and discharge. Higher injection speeds and pressures make it more difficult for gas to escape from the mold cavity. In such cases, the venting system needs optimization, such as appropriately increasing the depth of the venting channels or adjusting the distribution of venting holes. Excessive injection molding temperature may cause plastic decomposition, generating more gas; therefore, temperature factors must be fully considered when designing the venting system.
Trial molding verification and optimization are the ultimate guarantee for the venting system design. The accurate location and size of the venting channels can usually only be determined after trial molding. Through the trial molding process, the effectiveness of the venting points can be verified, areas of gas stagnation can be identified, and the location, depth, or number of venting channels can be adjusted in a timely manner. Combining mold flow analysis software to predict gas flow within the mold can further optimize the venting configuration, ensuring consistency between theoretical design and practical application.
