1.It is difficult to remove the gate. During the injection molding process, the gate sticks to the sprue bushing and is not easily peeled off. When the mold is opened, the product is cracked. In addition, the operator must use the tip of the copper rod to knock out from the nozzle, so that it can be released after being loosened, which seriously affects the production efficiency. The main cause of this failure is the poor smoothness of the gate cone and the presence of tool marks in the circumferential direction of the bore. Secondly, the material is too soft. After a period of use, the small end of the tapered hole is deformed or damaged, and the spherical curvature of the nozzle is too small, so that the gate material produces a rivet head here. The taper hole of the sprue bushing is difficult to process. Standard parts should be used as much as possible. If you need to process it yourself, you should also make a special reamer. The tapered hole needs to be ground to Ra0.4 or higher. In addition, a gate pull rod or gate ejection mechanism must be provided.
2. The guide column is damaged. The guide post mainly plays a guiding role in the mold to ensure that the molding surfaces of the core and the cavity do not touch each other under any circumstances, and the guide post cannot be used as the force receiving member or the positioning member. In the following cases, when the injection is moving, the fixed mold will produce a huge lateral offset force:
(1) When the wall thickness of the plastic part is not uniform, the velocity of the material flowing through the thick wall is large, and a large pressure is generated here;
(2) The side of the plastic part is asymmetrical. For example, the mold of the stepped parting surface is not equal to the back pressure of the opposite sides.
3. Large molds are subject to dynamic and fixed mode offset due to different filling rates and the influence of mold weight during mold loading. In the above cases, the lateral offset force will be applied to the guide post during injection. When the mold is opened, the surface of the guide post will be pulled, damaged, and the guide post will be bent or cut in severe cases, and even the mold cannot be opened. In order to solve the above problem, a high-strength positioning key is added on each side of the mold parting surface, and the most convenient and effective one is to use a cylindrical key. The verticality of the guide post hole and the parting surface is very important. When the machining is carried out, the fixed mold is clamped at the position and clamped once on the trampoline, so as to ensure the concentricity of the moving and fixed hole, and Minimize the verticality error. In addition, the heat treatment hardness of the guide post and the guide sleeve must meet the design requirements.
4. The moving template is bent. When the mold is injected, the molten plastic in the cavity generates a huge back pressure, generally 600 to 1000 kg/cm2. Moldmakers sometimes do not pay attention to this problem, often change the original design size, or replace the moving template with a low-strength steel plate. In the mold with the top bar topping, the template is bent down during injection due to the large span of the two sides. The moving template must be made of high-quality steel. If it has sufficient thickness, it is not allowed to use low-strength steel plates such as A3. If necessary, support columns or support blocks should be placed under the moving plate to reduce the thickness of the template and improve the carrying capacity.