Injection molding parting surface venting
Venting at the injection mold parting surface is a crucial aspect of injection mold design. It involves installing a specialized venting structure at the mold’s parting surface to expel air trapped in the cavity during filling and gases generated by the decomposition of the plastic melt. During injection molding, if the gas in the cavity isn’t promptly expelled, it can lead to a range of quality issues, such as material shortages, bubbles, burns, and noticeable weld marks. In severe cases, the compressed gas can even generate high temperatures, potentially damaging the mold. Therefore, properly designing the parting surface venting structure to ensure smooth venting is a key factor in ensuring part quality and improving production efficiency.
The importance of parting surface venting is primarily reflected in its impact on the part filling process. When molten plastic is injected into the mold cavity at a certain speed, the existing air in the cavity and the gases generated by the thermal decomposition of the melt need to be discharged promptly. Otherwise, these gases will be compressed in the corners or thin-walled areas of the cavity, forming high-pressure areas that hinder further filling of the melt and lead to part shorting (i.e., partial failure of the melt to fill the cavity). Even if the melt can barely fill the cavity, the compressed gas will cause localized scorching of the part due to the high temperature, resulting in black spots or carbonization marks, seriously affecting the part’s appearance. For example, when molding parts with deep cavities, narrow gaps, or complex patterns, gas is more likely to be trapped inside the cavity. If venting is not smooth, part shorting and scorching will be more prominent, resulting in a large amount of scrap. Furthermore, the presence of gas can affect the melt flow rate and filling sequence, causing uneven melt flow in the cavity, increasing the probability and visibility of weld marks, and reducing the mechanical properties of the part.
The design of the parting surface vent structure must be determined based on the structural characteristics of the plastic part and the cavity’s venting requirements. The most common venting method is to create a vent groove at the cavity edge of the parting surface. The vent groove should be located in the area where the melt is last to fill, at the end or corner of the cavity, for the most effective venting. The vent groove’s size is crucial to the design. The groove width is generally determined by the material properties of the part. For plastics with good flow properties, such as polyethylene and polypropylene, the vent groove width can be larger, typically 3-5mm. For plastics with poor flow properties, such as polycarbonate and ABS, the vent groove width can be controlled to 1-3mm. The groove depth must be strictly controlled. Too deep can cause melt to overflow the vent groove, resulting in flash, while too shallow can prevent effective venting. Typically, the vent groove depth for amorphous plastics is 0.02-0.05mm, and for crystalline plastics it is 0.01-0.03mm. Furthermore, the vent groove length is generally 5-10mm. Excessive length increases gas flow resistance, while too short can result in incomplete venting.
In addition to providing venting grooves, the gaps between the mold parting surfaces can also be used for venting. This method is suitable for simple plastic parts where venting is not critical. During mold processing and assembly, gaps between the parting surfaces inevitably exist, which can provide a certain degree of venting. However, it is important to note that the size of this gap is difficult to precisely control. If the gap is too large, melt overflow and flash will form; if the gap is too small, it will not meet the venting requirements. Therefore, for complex plastic parts or applications with high venting requirements, venting cannot rely solely on the gaps between the parting surfaces; dedicated venting grooves must be used in conjunction. Furthermore, in multi-cavity molds, each cavity requires a separate venting structure to ensure consistent venting across the cavity and avoid significant variations in part quality due to uneven venting. For example, in a mold for small plastic parts with multiple cavities, venting grooves should be provided at the end of each cavity to ensure smooth venting across all cavities.
The processing and maintenance of the vent structure on the parting surface can also affect its venting effectiveness. When machining the vent groove, ensure the groove is smooth and free of burrs, sharp edges, and other defects, which can hinder gas flow and even scratch the surface of the plastic part. Processing methods such as electrospark machining and milling can be used, followed by post-processing grinding and polishing. During mold use, the vent groove can easily become clogged with plastic debris, oil stains, and other debris, resulting in poor venting. Therefore, the vent groove needs to be cleaned regularly. Specialized cleaning tools, such as a fine copper wire brush and compressed air, can be used to remove debris from the groove and ensure a clear venting path. Also, before each production run, the vent structure on the parting surface should be inspected for blockage, wear, and other issues. Any problems should be addressed promptly to avoid compromising production quality. For example, when producing glass fiber reinforced plastics, glass fiber can easily accumulate in the vent groove, causing blockage and requiring more frequent cleaning and inspection.
In some special cases, simple parting surface venting may not meet the exhaust requirements. At this time, it is necessary to combine other exhaust methods, such as setting exhaust gaps at movable parts such as ejectors and cores, or using auxiliary means such as vacuum exhaust. The fitting gap between the ejector and the template, the gap between the core and the cavity, etc., can all be used as auxiliary exhaust channels to help exhaust gas deep in the cavity. For large and complex plastic part molds, vacuum exhaust technology is an effective solution. It connects a vacuum system to the cavity to extract the air in the cavity before injection, fundamentally solving the exhaust problem. However, this method is expensive and is only suitable for occasions with extremely high requirements for plastic part quality. In short, the exhaust design of the injection molding parting surface is a systematic project. It is necessary to comprehensively consider multiple factors such as the plastic part structure, material properties, and mold structure, reasonably design the exhaust structure, and do a good job of processing and maintenance to ensure smooth exhaust and improve plastic part quality and production efficiency.
