During the injection molding process, the venting performance of the gating system directly impacts product quality. As one of the most important venting areas within the mold, the rationality of its venting design is crucial. The parting surface is the interface between the movable and fixed molds. When the melt fills the cavity, if the existing air, gases generated by plastic decomposition, and gases released by the release agent are not promptly exhausted, defects such as bubbles, burns, and material shortages can form inside or on the surface of the product, and may even damage the mold. Therefore, properly designing venting structures on the parting surface to ensure smooth gas discharge is crucial for a smooth injection molding process and product quality.
The basic principle of parting surface venting is to utilize the fluidity of the melt and the compressibility of the gas. As the melt fills the cavity, the gas is forced out of the mold through venting grooves or gaps on the parting surface. As the melt enters the cavity, it pushes the gas within toward the parting surface. If a suitable venting structure is provided on the parting surface, the gas can be smoothly discharged under the pressure of the melt. The depth and width of the venting groove should be designed according to the characteristics of the plastic. For low-viscosity plastics (such as PE and PP), the venting groove should be relatively shallow, generally between 0.02-0.05mm, to prevent melt overflow. For high-viscosity plastics (such as PC and POM), the venting groove depth can be increased, generally between 0.05-0.1mm, to improve venting efficiency. Furthermore, the venting groove should be sufficiently long, generally extending to the outside of the mold, to prevent gas accumulation within the venting groove.
The design location of the vent grooves on the parting surface is a key factor influencing the venting effect. Typically, vent grooves should be located at the last point filled by the melt, i.e., at the end or corner of the cavity, as these areas are where gas is most likely to accumulate. For example, in a rectangular cavity, vent grooves should be located at two diagonal points away from the gate; in cavities with complex shapes, vent grooves should be located at the confluence of the various melt sections based on an analysis of the melt’s flow path. At the same time, the vent grooves should be located at an appropriate distance from the runners and gates to prevent the melt from entering the vent grooves too early during the filling process, causing overflow. Furthermore, for large plastic parts, multiple vent grooves can be provided on the parting surface to form a venting network, ensuring that gas from each area can be discharged promptly.
There are various types of venting structures on the parting surface, and the selection must be based on the mold structure and the requirements of the plastic part. The most common is the straight groove vent, which is directly located on the parting surface. It has a simple structure, is easy to process, and is suitable for most plastic parts. The straight groove vent is usually composed of several sections. The section closest to the cavity is shallower to prevent melt overflow, and gradually deepens outward to facilitate gas discharge. For plastic parts with higher precision requirements, gap venting can be used. That is, the gap between the movable mold and the fixed mold is used for exhaust. This method does not leave exhaust marks on the surface of the plastic part, but requires higher processing precision for the mold. In addition, for deep-cavity plastic parts, a vent plug can be installed on the parting surface. The vent plug is made of porous material and can effectively exhaust while preventing melt overflow. It is suitable for occasions with high gas content.
Maintenance and optimization of the exhaust system of the parting surface are crucial to ensuring the exhaust effect. During the use of the mold, the exhaust groove is easily blocked by plastic debris, oil stains, etc., resulting in poor exhaust. Therefore, the exhaust groove needs to be cleaned regularly to ensure its smooth flow. At the same time, exhaust can be assisted by adjusting the injection molding process parameters. For example, the injection molding speed can be appropriately increased to allow the melt to quickly fill the cavity and reduce the time for gas accumulation; the mold temperature can be appropriately increased to reduce the viscosity of the melt and enhance its fluidity, which helps gas discharge. In addition, if poor exhaust defects are found in the product during the production process, the exhaust structure of the parting surface should be checked in time, and the size and position of the exhaust groove should be adjusted if necessary to optimize the exhaust effect.
Combining parting surface venting with other venting methods can further improve venting efficiency. In actual production, in addition to installing venting structures on the parting surface, other venting methods can be combined, such as installing venting in the clearances between moving parts like ejectors and cores, or installing auxiliary venting grooves near the gate. For example, for parts with slender cores, a 0.01-0.03mm gap can be reserved between the core and the mold plate for venting. For multi-cavity molds, venting grooves can be installed at the end of the runner to exhaust gas within the runner. The synergistic effect of multiple venting methods ensures that gas is promptly exhausted throughout the entire gating system and cavity, minimizing product defects.
