How to solve the exhaust problem of injection molds

The gas inside the injection mold in the injection mold factory includes not only the air in the mold cavity, but also the air in the flow channel and the decomposition gas generated by the plastic melt, as well as the water vapor formed by the moisture in the plastic at high temperatures. When processing injection molds, these gases should be stored in a timely manner to minimize the following hazards. ① Defects such as flow marks, water splashes, and weld marks form on the surface of plastic parts. ② Defects such as bubbles and loose structure inside the plastic parts can lead to incomplete filling in severe cases. ③ Difficulty in filling the melt or local occurrence of burrs. ④ Gas generates high temperature under high temperature and pressure, causing localized carbonization and burning marks on the product. ⑤ Reduce the filling speed of the melt. When the injection speed decreases, not only does it prolong the molding cycle, but the melt temperature also quickly decreases. At this time, if the injection pressure is increased, the residual stress will also increase, and the possibility of warping and deformation of the plastic part after demolding will increase. If the injection pressure is lowered and the material temperature is increased, it will cause plastic cracking. Properly opening exhaust slots can greatly reduce injection pressure, decrease injection time and holding time, and reduce mold locking pressure, thereby improving production efficiency, reducing production costs, and lowering machine consumption. For transparent plastic parts or plastic parts with strict surface requirements, special attention should be paid to the design of the exhaust system for injection mold processing. 1. Location of trapped gas in the cavity The location of trapped air in the cavity is usually in the following areas. ① Thin walled structural cavity, the end of the melt flow. ② The point where two or more molten materials converge. ③ The place where the melt reaches in the mold cavity. ④ The bottom of the blind hole in the injection mold cavity is mostly the end of the solid column in the product. ⑤ The bottom of reinforced ribs and screw columns in molded products. ⑥ The dead corners of complex injection mold cavities. It should be noted that the accurate position of the exhaust groove can only be determined after the trial mold, and it is important not to assume that the exhaust groove has been opened before the trial mold. 2. Injection mold exhaust method The exhaust in this chapter refers to the exhaust of the mold cavity and pouring system. The exhaust methods in injection molds include the following. ① Classification surface (including open exhaust groove). ② Fitting surface of inlay. ③ The mating surface between the push rod or push tube and the inner mold insert. ④ Side core pulling mechanism for exhaust. ⑤ Add exhaust needles or inserts to exhaust trapped air. ⑥ Breathable steel exhaust. ⑦ Air valve exhaust. 3. Design principles for exhaust system (1) The exhaust groove can only allow gas to be discharged, not allow plastic melt to flow out. (2) Different plastics have varying depths of exhaust ducts due to their different viscosities. The cavity should be designed with exhaust grooves, and the flow channel and cold material cavity should also be designed with exhaust grooves to minimize the gas in the pouring system from entering the injection mold cavity. (3) The exhaust groove must be connected to the outside of the mold frame, especially when exhausting through inserts, exhaust needles, or exhaust inserts, be sure to pay attention. (4) The exhaust groove should be machined with a milling machine as much as possible, and after machining, polished with 320 grit sandpaper to remove knife marks; Avoid using a grinder to process the exhaust groove, as the surface machined by the grinder is too flat and smooth, which often results in poor exhaust performance. (5) The exhaust groove on the parting surface should be set on one side of the mold cavity, usually on the fixed mold insert. (6) Both sides of the exhaust groove should be chamfered at a 45 ° angle.