Common failure reasons and troubleshooting methods for injection molds

The 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 processing quality of the mold directly affect the quality and production efficiency of the plastic parts. The injection mold production and plastic product production practices are the most common, and the most common causes of injection mold failures and their troubleshooting methods are described in detail as follows:

1. Difficult to remove the gate. In the injection molding process, the gate sticks to the gate sleeve and is not easy to come off. When the mold is opened, the product has crack damage. In addition, the operator must use the tip of the copper rod to knock out from the nozzle, so that it can be demolded after loosening, which seriously affects the production efficiency. The main reason for this kind of failure is the poor finish of the gate cone 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 cone hole is deformed or damaged after a period of use, and the spherical radius of the nozzle is too small, which causes the gate material to produce rivets. The taper hole of the gate sleeve is more difficult to process, and standard parts should be used as much as possible. If you need to process it yourself, you should also make your own or purchase a special reamer. The cone hole needs to be ground to above Ra0.4. 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 touch each other under any circumstances, and the guide post cannot be used as a force member or a positioning member. In the following cases, when the injection is moving, the fixed mold will produce a huge lateral deflection force: (1). When the wall thickness of the plastic part is not uniform, the velocity of the material flow through the thick wall is large, resulting in a relatively high Large pressure; (2). The side of the plastic part is asymmetric, such as the opposite pressure on the opposite sides of the mold of the stepped parting surface.

3. For large molds, due to the different filling rates in each direction and the influence of the mold's own weight when the mold is installed, dynamic and fixed mold offsets occur. In the above cases, the lateral deflection force will be applied to the guide post during injection. The surface of the guide post is roughened and damaged during mold opening. In severe cases, the guide post is bent or cut off, and even the mold cannot be opened. In order to solve the above problems, a high-strength positioning key is added on each side of the mold parting surface. The most simple and effective one is the cylindrical key. The verticality of the guide column hole and the parting surface is very important. During the processing, the moving is used. After the fixed mold is clamped to the position and clamped, the boring is completed at a 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. The moving template is bent. When the mold is injected, the molten plastic in the mold cavity generates a huge back pressure, generally 600~1000 kg/cm?. Mold manufacturers sometimes do not pay attention to this problem, and often change the original design size, or replace the moving template with a low-strength steel plate. In the mold that uses the ejector rod to push the material, due to the large span of the two seats, the template is bent down during injection. Therefore, high-quality steel must be used for the moving template, and it must have sufficient thickness. Low-strength steel plates such as A3 must not be used. When necessary, support columns or support blocks should be placed under the moving template to reduce the thickness of the template and increase the load-bearing capacity.

5. The top rod is bent, broken or leaking. The quality of the self-made jack is better, that is, the processing cost is too high, and now standard parts are generally used, and the quality is poor. If the gap between the plunger and the hole is too large, leakage occurs, but if the gap is too small, the plunger expands and becomes stuck due to the increase in mold temperature during injection. What is more dangerous is that sometimes the ejector pin is pushed off at a normal distance and is broken. As a result, the ejector pin exposed during the next mold clamping cannot be reset and damage the die. In order to solve this problem, the plunger is re-grinded, and the matching section of 10-15 mm is reserved at the front end of the plunger, and the middle part is ground by 0.2 mm. After assembly, all ejector rods must be strictly checked for fit clearance, generally within 0.05 to 0.08 mm, to ensure that the entire ejector mechanism can move forward and backward freely.

6. Poor cooling or water leakage in water channel. 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 and warpage deformation and other defects. On the other hand, the whole or part of the mold is overheated, which prevents the mold from forming properly and stops production. In severe cases, the movable parts such as the ejector pin are stuck due to thermal expansion and damaged. The design and processing of the cooling system depends on the shape of the product. Do not omit this system because of the complicated mold structure or difficult processing. Especially for large and medium-sized molds, the cooling problem must be fully considered.

7. The fixed tension mechanism fails. Fixed-length 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 action requirements must be synchronized, that is, Simultaneously close the mold and buckle, open the mold to a certain position and release the hook at the same time. Once the synchronization is lost, it will inevitably cause the template of the drawn mold to be skewed and damaged. The parts of these mechanisms must have high rigidity and wear resistance, and adjustment is difficult. The life of the mechanism is short. Avoid using it as much as possible. You can use other mechanisms. In the case of relatively small core-pulling force, the spring can be used to push out the fixed mold.In the case of relatively large core-pulling force, the core can be used to slide when the moving mold retreats. The structure of the core is first divided before the mold is divided. The hydraulic cylinder can be used for core pulling on the mold. The inclined pin slider core pulling mechanism is damaged. The most common problems of this kind of mechanism are that the processing is not in place and the materials are too small. There are two main problems.

The oblique pin has a large inclination angle A. The advantage is that it can produce a relatively large core-pulling distance within a short opening stroke. However, when the inclination angle A is too large, when the pulling force F is a certain value, the bending force P=F/COSA received by the slant pin during core pulling also increases, and the slant pin deformation and slant hole wear are likely to occur. At the same time, the upward thrust N=FTGA generated by the inclined pin to the slider is also greater. This force increases the positive pressure of the slider against the guide surface in the guide groove, thereby increasing the frictional resistance when the slider slides. It is easy to cause slippage and guide groove wear. According to experience, the inclination angle A should not be greater than 25

8.Some molds are limited by the template area, the length of the guide groove is too small, the slider is exposed outside the guide groove after the core pulling operation is completed, so that the slider is likely to tilt at the post-core pulling stage and the initial stage of mold clamping reset, especially at When the mold is closed, the slider is not reset smoothly, causing damage to the slider and even damage due to 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 total length of the guide groove.

9. Finally, when designing and manufacturing the mold, according to the specific requirements of the quality of the plastic parts, the size of the batch, the requirements of the manufacturing period, etc., it can not only meet the product requirements, but also be the most convenient and reliable in the mold structure, easy to process, and make the cost Low, such a mold is excellent.

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