How to solve common failures of injection molds?-part 1

2022-04-02

1. Difficulty in gate stripping

During the injection molding process, the gate sticks 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 the 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, resulting in the riveting head of the gate material.

The taper hole of the sprue bushing is more difficult to process. 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 a Ra0.4 or more. In addition, a gate pull rod or gate ejection mechanism must be provided.

2. Guide post damage

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, and 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 passes through the thick wall at a large rate, and the result is relatively high here. Large pressure; (2). The side of the plastic part is asymmetrical, such as the back pressure on the opposite sides of the mold of the stepped parting surface is not equal.

3. Large mold

Due to the different filling rate in each direction and the influence of the die weight during mold loading, the moving and fixed mold deviations occur. 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. In severe cases, the guide post may be bent or cut off, and 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. In the process, the moving is used. After the fixed mold is aligned with the position and clamped, the boring is completed on the boring machine at one time, so as to ensure the concentricity of the moving and fixed mold holes, and Minimize the verticality error. In addition, the heat treatment hardness of the guide post and guide sleeve must meet the design requirements.

4. Movable template bending

When the mold is injected, the molten plastic in the cavity produces a huge back pressure, generally 600~1000 kg/cm. Mold makers sometimes do not pay attention to this problem, often change the original design size, or replace the movable template with a low-strength steel plate. In the mold with ejector pins, 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 cannot be used. When necessary, support columns or support blocks should be set under the movable formwork to reduce the thickness of the formwork and improve the bearing capacity.

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