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What are the reasons for the failure of the injection mold

update:03-12-2020
abstract:

The structure of the injection mold and the quality of […]

The structure of the injection mold and the quality of mold processing directly affect the quality and production efficiency of plastic parts. The most common and most common mold failures in the production of injection molds and plastic products and their main reasons are analyzed and eliminated as follows.

1. Difficulty in gate stripping. During the injection molding process, the gate is stuck in the sprue sleeve and is not easy to come out. When the mold is opened, the product is cracked and damaged. In addition, the operator must knock out the tip of the copper rod from the nozzle to loosen it before demoulding, which seriously affects production efficiency. The main reason for this failure is the poor finish of the gate taper hole, and there are knife marks in the circumferential direction of the inner hole. The second is that the material is too soft, the small end of the taper hole is deformed or damaged after a period of use, and the curvature of the nozzle spherical surface is too small, causing the gate material to produce a riveting head here. The taper hole of the sprue bushing is difficult to process, so standard parts should be used as much as possible. If you need to process it yourself, you should also make your own or buy a special reamer. The taper hole needs to be ground to Ra0.4 or more. In addition, a gate pull rod or gate ejection mechanism must be provided.

2. The guide post 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 collide with each other under any circumstances. The guide post cannot be used as a force-bearing part or a positioning part. In the following situations, 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 material flow through the thick wall has a large speed, and a larger pressure is generated here;

(2) The side surface of the plastic part is asymmetrical, such as the stepped parting surface mold, the opposite pressure on the two opposite sides is not equal.

3. For large molds, due to the different filling rates in all directions, and the influence of the mold's own weight during mold assembly, the movable and fixed molds will deviate. In the above cases, the lateral offset force will be added to the guide post during injection, and the surface of the guide post will be roughed and damaged when the mold is opened. If the guide post is severely bent or cut off, the mold cannot even be opened. In order to solve the above problems, a high-strength positioning key is added on each of the four sides of the mold parting surface. The most simple and effective way is to use a cylindrical key. The verticality of the guide pin hole and the parting surface is very important. During processing, the moving is used. After the fixed mold is aligned and clamped, the boring machine is finished once on the boring machine. This can ensure the concentricity of the moving and fixed mold holes. Minimize the verticality error. In addition, the heat treatment hardness of the guide post and guide sleeve must meet the design requirements.
4. The movable template is bent. When the mold is injected, the molten plastic in the cavity produces a huge back pressure, generally 600 ~ 1000 kg/cm2. Mold makers sometimes do not pay attention to this problem, and often change the original design size, or replace the movable template with low-strength steel plate. In the mold with ejector pin ejection, the large span between the two sides of the seat causes the template to bend during injection. Therefore, the movable formwork must be made of high-quality steel, with sufficient thickness, and low-strength steel plates such as A3 should not be used. When necessary, support columns or support blocks should be set under the movable formwork to reduce the thickness of the formwork and increase the bearing capacity.