How to solve common failures of injection molds?

2021-11-04

Injection mold is a tool for producing plastic products; it is also a tool for giving plastic products a complete structure and precise dimensions. 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 common causes of injection mold failures and troubleshooting methods in the production of injection molds and plastic products are described in detail as follows:

1. Difficulty in gate stripping

During the injection molding process, the gate sticks to 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 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, fixed mold is aligned and clamped, and the boring machine is finished at one time on the boring machine, which can 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 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 improve the load-bearing capacity.

5. The ejector rod is bent, broken or leaking

The self-made ejector rods are of better quality, but the processing cost is too high. Nowadays, standard parts are generally used and the quality is poor. If the gap between the ejector pin and the hole is too large, leakage will occur, but if the gap is too small, the ejector pin will become stuck due to the increase in mold temperature during injection.

What's more dangerous is that sometimes the ejector rod will not move and break when it is ejected for a normal distance. As a result, the exposed ejector rod cannot be reset during the next mold clamping and will damage the die. In order to solve this problem, the ejector rod was re-ground, and the mating section of 10~15 mm was retained at the front end of the ejector rod, and the middle part was ground 0.2 mm smaller. All the ejector rods must be strictly checked after assembly, and the clearance is generally within 0.05~0.08 mm. It is necessary to ensure that the entire ejector mechanism can move forward and backward freely.

6. Poor cooling or water leakage

The cooling effect of the mold directly affects the quality and production efficiency of the product, such as poor cooling, large shrinkage of the product, or uneven shrinkage, resulting in defects such as warping deformation. On the other hand, the whole or part of the mold is overheated, so that the mold cannot be formed normally and the production is stopped. In severe cases, the movable parts such as the ejector rod will be damaged due to thermal expansion.

The design and processing of the cooling system depends on the shape of the product. Don't omit this system because of the complex structure of the mold or the difficulty in processing. Especially for large and medium-sized molds, the cooling problem must be fully considered.

7. The fixed-distance tensioning mechanism fails

Fixed-pitch tensioning mechanisms such as swing hooks and buckles are generally used in fixed mold core pulling or some secondary demolding molds. Because such mechanisms are set in pairs on both sides of the mold, their actions must be synchronized, that is, The mold is buckled at the same time when the mold is closed, and the mold is unhooked at the same time when the mold is opened to a certain position.

Once the synchronization is lost, it will inevitably cause the template of the drawn die to be skewed and damaged. The parts of these mechanisms must have higher rigidity and wear resistance, and it is difficult to adjust. The life of the mechanism is short. Avoid using it as much as possible. You can switch to other mechanisms.

When the core pulling force is relatively small, the spring can be used to push out the fixed mold. When the core pulling force is relatively large, the core can be used to slide when the movable mold is retracted. A hydraulic cylinder can be used to pull the core on the mold. The oblique pin slider core pulling mechanism is damaged. The more common problems of this kind of organization are mostly inadequate processing and too small materials. There are mainly the following two problems.

The oblique pin inclination angle A is large, and the advantage is that it can produce a larger core pulling distance in a shorter mold opening stroke. However, if the inclination angle A is too large, when the extraction force F is a certain value, the bending force P=F/COSA of the inclined pin during the core pulling process is larger, and the inclined pin deformation and the inclined hole wear are prone to occur.

At the same time, the upward thrust N=FTGA generated by the inclined pin on the slider is also greater, which increases the positive pressure of the slider on the guide surface in the guide groove, thereby increasing the frictional resistance of the slider when sliding. It is easy to cause uneven sliding and wear of the guide groove. According to experience, the inclination angle A should not be greater than 25

8. Some molds are limited by the area of the template

The length of the guide groove is too small, and the slider is exposed outside the guide groove after the core pulling action is completed. This will easily cause the slider to tilt in the post core pulling stage and the initial stage of mold closing reset, especially when the mold is closed, the slider reset is not smooth. Damage the slider, or even break it under bending. According to experience, after the slider completes the core pulling action, the length left in the chute should not be less than 2/3 of the full length of the guide groove.

9. Finally in the design

When manufacturing molds, it should be based on the requirements of the quality of the plastic parts, the size of the batch, the requirements of the manufacturing period, etc., which can not only meet the requirements of the product, but also the most simple and reliable mold structure, easy to process, and low cost. The mold is excellent.

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