The structural form of the injection mold and the processing quality of the mold directly affect the quality and production efficiency of the plastic parts. The most common and most common mold failures in injection mold production and plastic product production practice and their main reasons are analyzed and excluded as follows.
1. It is difficult to strip the gate. During the injection molding process, the gate sticks to the gate sleeve and is not easy to come out. When the mold is opened, the product has crack damage. 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 the 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 spherical arc of the nozzle is too small, causing the gate material to produce a rivet head here. The taper hole of the sprue sleeve is difficult to process, and standard parts should be used as much as possible. If you need to process it yourself, you should also make or buy a special reamer. The taper hole needs to be ground to above Ra0.4. In addition, gate pulling rods or gate ejection mechanisms 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-bearing part or a positioning part. In the following cases, when the injection is moved, the fixed mold will generate a huge lateral deflection force:
(1) When the wall thickness of the plastic parts is not uniform, the flow rate of the material passing through the thick wall is large, and a large pressure is generated here;
(2) The sides of the plastic parts are asymmetrical, such as a mold with a stepped parting surface, the back pressure on the opposite sides is not equal.
3. For large molds, due to the different filling rates in each direction and the influence of the self-weight of the mold when the mold is installed, the moving and fixed molds are offset. In the above cases, the lateral offset force will be applied to the guide post during injection, and the surface of the guide post will be rough 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 convenient and effective way is to use a cylindrical key. The verticality of the guide hole and the parting surface is very important. In the process of processing, the movable and fixed die are clamped at the position, and then the boring machine is finished at one time, so as to ensure the concentricity of the movable and fixed die holes and ensure the Minimize squareness error. In addition, the heat treatment hardness of the guide post and guide bush must meet the design requirements.
4. The moving template is bent. When the mold is injected, the molten plastic in the mold 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 moving template with low-strength steel plates. In the mold with ejector ejector, due to the large span between the seats on both sides, the template bends down during injection. Therefore, the moving formwork must be made of high-quality steel, with sufficient thickness. A3 and other low-strength steel plates must not be used. When necessary, support columns or support blocks should be set under the moving formwork to reduce the thickness of the formwork and improve the bearing capacity.
5. The ejector rod is bent, broken or leaked. The quality of the self-made ejector is better, but the processing cost is too high. Now, standard parts are generally used, and the quality is poor. If the gap between the ejector pin and the hole is too large, there will be leakage, but if the gap is too small, the ejector pin will be stuck due to the increase of mold temperature during injection. What's more dangerous is that sometimes the ejector pin is pushed out of the normal distance and will not move and break. As a result, the exposed ejector pin cannot be reset and the die will be damaged when the mold is closed next time. In order to solve this problem, the ejector rod is re-ground, retaining a 10-15mm mating section at the front end of the ejector rod, and grinding the middle part down by 0.2mm. After all ejector rods are assembled, the matching clearance must be strictly checked, generally within 0.05~0.08 mm, to ensure that the entire ejector mechanism can advance and retreat 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 and deformation of the warped surface. On the other hand, the whole or part of the mold is overheated, so that the mold cannot be formed normally and production is stopped. The design and processing of the cooling system depends on the shape of the product. Do not omit this system because the mold structure is complex or difficult to process. Especially for large and medium-sized molds, the cooling problem must be fully considered.
7. The fixed-distance tensioning mechanism fails. Fixed-distance tensioning mechanisms such as swing hooks and hasps are generally used in fixed mold core pulling or some secondary demolding molds. Because such mechanisms are arranged in pairs on both sides of the mold, their actions must be synchronized, that is, When the mold is closed, it is buckled at the same time, and when the mold is opened to a certain position, the hook is released at the same time. Once the synchronization is lost, the template of the drawn mold will be skewed and damaged. The parts of these mechanisms must have high rigidity and wear resistance, and the adjustment is also difficult. The service life of the mechanism is short. When the core pulling force is relatively small, the method of pushing out the fixed mold by a spring can be used. When the core pulling force is relatively large, the core sliding when the moving mold retreats, and the core pulling action is completed first and then the mold is split. Hydraulic cylinders can be used to pull cores on the mold. The inclined pin slider core pulling mechanism is damaged. The most common faults of this kind of mechanism are that the processing is not in place and the materials used are too small. There are mainly the following two problems.
The inclination angle A of the inclined pin is large, and the advantage is that a large core pulling distance can be generated in a short mold opening stroke. However, if 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 oblique pin during the core pulling process is also larger, and the oblique pin deformation and oblique hole wear are prone to occur. At the same time, the upward thrust N=FTGA generated by the oblique pin on the slider is also greater, and this force increases the positive pressure of the slider on the guide surface in the guide groove, thereby increasing the friction resistance when the slider slides. It is easy to cause uneven sliding and wear of the guide groove. As a rule of thumb, the inclination angle A should not be greater than 25.
8. Some molds are limited by the area of ​​the template, and the length of the guide groove is too small. When the mold is closed, the reset of the slider is not smooth, so that the slider is damaged or even damaged by 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, 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 and other specific conditions, which can not only meet the requirements of the product, but also be the most simple and reliable in terms of mold structure, easy to process, and low cost. , this is the most perfect mold.