Determination of Plastic Injection Mold Parting Surface

- Apr 17, 2018-

          In the plastic injection mold manufacturing process, the problem of determining the parting surface is always encountered. It is a very complex issue and is constrained by many factors. Therefore, the main contradiction should be grasped when choosing the parting surface, and secondary factors should be abandoned. Different designers sometimes have different understandings of major factors and are related to their own work experience. The selection of parting surfaces for some plastic parts is simple, clear and unique; some plastic parts have many options. According to my work experience, I can determine according to the following principles:a) Ensure that the plastic product can be demolded         This is a primary principle because the purpose of setting up the parting surface is to be able to smoothly remove the product from the cavity. According to this principle, the parting surface should preferably be on the largest contour line of the plastic product, preferably on a plane, and this plane is perpendicular to the mold opening direction. The entire profile of the parting should show a trend of shrinkage, and there should be no uneven shape that affects the mold release so as not to affect the mold release.

b) Make the cavity shallowest         The depth of the mold cavity has three impacts on the mold structure and manufacturing:          1) At present, the machining of the mold cavity uses EDM machining. The deeper the cavity is, the longer the machining time is, affecting the mold production cycle and increasing the production cost.           2) The depth of the mold cavity affects the thickness of the mold. The deeper the cavity, the thicker the moving and fixed mold. On the one hand processing is more difficult; on the other hand, various injection machines have certain limitations on the maximum thickness of the mold, so the depth of the cavity should not be too large.

           3) The deeper the depth of the cavity, the greater the difference in actual dimensions between the upper and lower ends of the same dimension at the same draft angle.            Although the plastic mold is very delicate, after the plastic part is released from the mold, there will be a circle of burrs on the parting surface. We call it flash. Even if these burrs are cut off immediately after demoulding, they will still leave marks on the plastic parts and affect the appearance of the plastic parts.

d) Avoid side core pulling           Injection molds should avoid lateral core pulling as much as possible, because the structure of the transverse core-pulling mold is complex, which directly affects the size of the plastic parts and the precision of the fittings. It takes time and money, and the manufacturing cost is greatly increased. Can be usede) Make the parting surface easy to handle           The precision of the parting surface is an important part of the precision of the whole mold. The flatness and parallelism of the mating surface of the movable and fixed molds are within the tolerance range. Therefore, the parting surface should be flat and perpendicular to the peeling direction to ensure machining accuracy. If the selected parting surface is an inclined surface or a curved surface, the difficulty of the processing increases, accuracy cannot be guaranteed, and a flicker phenomenon is easily caused.f) Shorten the transverse pull core            The shorter the core pulls, the shorter the distance the inclined suction moves. On the one hand, it is possible to reduce the thickness of moving and fixing the mold and reduce the dimensional error of the plastic parts; on the other hand, demoulding is beneficial.

g) ensure the accuracy of plastic parts             As the plastic parts of mechanical parts, the parallelism, concentricity, and coaxiality are all required to be high. To ensure the precision of plastic parts, in addition to improving the precision of mold manufacturing, it has a lot to do with the choice of the parting surface. h) conducive to exhaust              For medium and small plastic parts due to the small cavity, the amount of air is not much, can use the gap of the parting surface exhaust. Therefore, choosing the parting surface should be beneficial to the exhaust. In accordance with this principle, the parting surface should be located at the point where the molten plastic finally arrives at the time of injection, and the cavity should not be closed.

i) Leave the plastic part in the movable mold             When the mold is opened, the plastic parts in the cavity do not normally come out on their own, and they need to be ejected by the ejector mechanism. The ejector has an ejector device and is located on the side of the movable mold. Therefore, when designing the mold parting surface, the mold should be opened. The post-mold plastic part can be left in the movable mold so that the ejector mechanism can directly eject the plastic part. If the plastic part is left in the fixed mold, another ejector device must be designed to release the mold. The mold structure is much more complex, and the cost increases and the processing cycle is prolonged.

j) Make the total pressure in the cavity larger than the parting plane              Plastic injection pressure P in the cavity in the same direction, so a certain direction of the total pressure F = PxS, S is a certain direction of the projected area, when S is greater, then F is greater, select the direction and the total pressure is greater Vertical profile, using the injection molding machine's clamping force to withstand greater injection pressure. Therefore, the mold structure is simple. Otherwise, the locking mechanism needs to be designed. The mold structure is complicated, the cost is increased, and the processing cycle is prolonged.               In summary, there are many factors affecting the selection of the injection mold parting surface. The general requirement is to smoothly release the mold, ensure the technical requirements of the plastic parts, and the mold structure is simple and easy to manufacture. When you select a parting plane plan, there may be some shortcomings, and then take other measures to make up for the existing problems, in order to choose close to the ideal parting surface.