Venting of the cavity on the parting surface
Venting at the parting surface is a critical step in ensuring part quality during injection molding. Its purpose is to promptly expel air and gases generated by melt decomposition from the cavity during melt filling, preventing defects such as bubbles, burning, material shortages, and weld marks in the molded part caused by trapped gases. The parting surface, the junction between the movable and fixed molds, is also the primary channel for cavity venting. Its large area and direct connection to the cavity allow for efficient gas removal. Properly designed venting structures at the parting surface are crucial for improving part quality and shortening molding cycles.
The basic principle of parting surface venting is to use the tiny gap between the movable mold and the fixed mold to achieve venting. When the melt fills the cavity, the gas inside the cavity flows toward the parting surface under the push of the melt, and is discharged to the outside of the mold through the gap of the parting surface. The size of the venting gap is a key parameter affecting the venting effect. If the gap is too large, it will easily cause the melt to overflow from the gap and form flash; if the gap is too small, the gas cannot be effectively discharged. The reasonable value of the venting gap needs to be determined according to the type of plastic. For plastics with good fluidity (such as PE, PP), the gap should be controlled at 0.01-0.02mm; for plastics with poor fluidity (such as PC, PMMA), the gap can be appropriately increased to 0.02-0.03mm. In addition, the length of the venting gap also needs to be controlled, usually 5-10mm. Too long will increase gas flow resistance and reduce venting efficiency.
There are various structural forms of parting surface venting, the most common of which are natural venting, venting groove venting, and venting plug venting. Natural venting uses the clearance of the parting surface itself to vent air. It is suitable for plastic parts with simple cavity structures and low gas volume. It does not require additional venting structure processing and is low-cost, but the venting effect is limited. Vent venting is to create a special venting groove on the parting surface. It is suitable for most plastic parts, especially those with complex structures and those with high gas volume. Vents are usually opened in the area where the melt is last filled, such as the end of the cavity, corners, and where weld marks are formed. They are generally rectangular or trapezoidal in shape, with a depth of 0.01-0.03mm and a width of 5-20mm. The length extends to the outside of the mold to ensure smooth gas discharge. Vent plug venting is to install a vent plug in the gas-trapped area of the parting surface. The vent plug is made of porous material with good air permeability and can effectively discharge gas. It is suitable for complex cavity areas where it is difficult to create venting grooves.
When designing the exhaust structure of the parting surface, the flow path of the melt and the exhaust sequence must be considered to ensure that the gas can be discharged in an orderly manner. First, the filling process of the melt in the cavity should be simulated by mold flow analysis software to determine the gas gathering position, that is, the gas trapping point, and the exhaust structure should be set at these gas trapping points to improve the targeted exhaust. Secondly, the layout of the exhaust groove should be consistent with the flow direction of the melt, avoid being perpendicular to the flow direction of the melt, and prevent the melt from directly rushing into the exhaust groove to form flash. For large plastic parts, multiple exhaust grooves need to be set on the parting surface, evenly distributed around the cavity, to ensure that the gas in all parts can be discharged in time. In addition, the connection between the exhaust groove and the cavity should have a smooth transition to avoid sharp corners or steps to prevent the melt from generating turbulence at the entrance of the exhaust groove and entraining more air.
The machining and maintenance of the parting surface vent structure are crucial to effective venting. When machining the vent groove, dimensional accuracy and surface quality must be maintained. Precision machining methods such as electrical discharge (EDM) and wire cutting can be used to ensure that the groove depth and width meet design requirements, with a surface roughness Ra no greater than 1.6μm, to prevent machining errors from affecting venting. During mold assembly, the parallelism and flatness of the parting surface must be ensured to avoid uneven venting gaps caused by loose mold clamping. During production, the vent grooves and vent plugs must be regularly cleaned to remove residual plastic debris and oil to prevent blockage. If poor venting is detected in a plastic part, the vent structure must be inspected for blockage or improper dimensions, and appropriate cleaning and repairs must be performed. Through proper design, precision machining, and regular maintenance, the parting surface vent structure can effectively exhaust gas from the mold cavity, ensuring part quality.
