Injection molded insert exhaust
Venting of injection molding inserts is a crucial step in ensuring part quality during the injection molding process. Inserts, as metal or non-metal parts embedded in the mold, are used to create a specific part structure or enhance local mold strength. However, their presence can make it difficult for gases within the mold cavity to escape. Poor venting can result in defects such as bubbles, burns, missing material, and weld marks, impacting the part’s performance and appearance. Therefore, effective venting measures must be implemented to ensure the smooth venting of gas around the inserts, ensuring the melt can fully fill the mold cavity and improve part quality.
The reasons for poor exhaust of injection molded inserts are mainly related to the structural design of the inserts, the installation method, and the exhaust system design of the mold. When the inserts are complex in shape and large in size, they will occupy part of the space in the cavity, narrowing the gas flow channel and increasing the difficulty of exhaust. If the clearance between the inserts and the mold cavity is too small, the gas will not be able to be discharged through the gap and will accumulate around the inserts. The position of the mold exhaust groove is not set properly, and it does not cover the trapped gas area near the inserts, or the size of the exhaust groove is unreasonable, such as too shallow depth and too narrow width, which will affect the exhaust effect. In addition, the injection molding process parameters are not set properly, such as too fast injection speed and too high melt temperature, which will cause the melt to fill the cavity faster than the gas exhaust speed, causing the gas to be trapped around the inserts and form defects.
To solve the exhaust problem of injection molded inserts, we can start with the mold structure design and take various measures to improve the exhaust effect. The most common method is to set exhaust grooves on the mating surface of the insert and the mold cavity. The exhaust grooves should be opened in the trapped gas area around the insert, such as the edge and corner of the insert. The depth is determined according to the type of plastic, usually 0.02-0.05mm, the width is 5-10mm, and the length should extend to the outside of the mold to ensure that the gas can be discharged smoothly. For inserts with complex shapes, exhaust holes can be set inside the insert to guide the gas inside the insert to the outside of the mold. The diameter of the exhaust hole is generally 0.5-1mm, and the melt must be prevented from entering the exhaust hole. In addition, breathable steel can be used to make inserts or local parts of inserts. Breathable steel has a large number of interconnected micropores that can effectively exhaust gas. It is especially suitable for complex inserts where it is difficult to set exhaust grooves.
Properly selecting the insert material and installation method can also help improve venting. The choice of insert material should take into account its thermal conductivity and processing properties. Materials with good thermal conductivity (such as copper alloys) can accelerate the cooling of the melt and reduce gas generation. Materials that are easy to process facilitate the creation of venting grooves or holes in the insert. When installing the insert, ensure an appropriate clearance between the insert and the mold cavity, typically 0.01-0.03mm. This prevents melt overflow while allowing gas to escape through the gap. For large inserts, a segmented installation method can be used, reserving a small gap between each segment as a venting channel. At the same time, ensure the coaxiality and verticality of each segment to avoid affecting the dimensional accuracy of the plastic part.
Adjusting injection molding process parameters also has a significant impact on insert venting. Optimizing these parameters can reduce gas generation and accumulation. Appropriately reducing the injection speed allows the melt to fill the mold cavity at a steady rate, giving gas ample time to escape. Especially when the melt is filling around the insert, the injection speed should be reduced to prevent turbulent flow and excessive air entrainment. Properly increasing the melt and mold temperatures can reduce melt viscosity and improve fluidity, allowing the melt to better fill the mold cavity and reducing gas entrapment caused by poor flow. Furthermore, appropriately increasing back pressure can increase the density of the melt, venting gases contained in the melt and reducing the amount of gas entering the mold cavity. During the production process, process parameters should be continuously adjusted through mold trials to find the optimal combination to ensure good insert venting and improve part quality. Additionally, regularly clean the venting slots and holes around the insert to prevent plastic debris from clogging the venting channels and affecting venting efficiency.
