Troubleshooting methods for common faults of injection molds

The structural form of Online Injection Molding Service molds and the quality of mold processing directly affect the quality of plastic products and production efficiency. The most common and most frequently occurring mold faults in the production of injection molds and plastic products and their main causes are analyzed and eliminated as follows.
1. Difficulty in removing the gate. During the injection molding process, the gate sticks to the gate sleeve and is not easy to remove. When the mold is opened, cracks and damage appear on the product. In addition, the operator must knock it out from the nozzle with the tip of a copper rod to loosen it before demolding, which seriously affects production efficiency. The main cause of this failure is that the finish of the gate taper hole is poor and there are knife marks in the circumferential direction of the inner hole. Secondly, the material is too soft, and the small end of the taper hole is deformed or damaged after a period of use, and the spherical curvature of the nozzle is too small, causing the gate material to produce a rivet head here. The taper hole of the gate 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 purchase a special reamer. The taper hole needs to be ground to Ra0.4 or above. In addition, a gate pull rod or a gate ejection mechanism must be set.
2. Damage to the guide pin. The guide pin 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 pin cannot be used as a force-bearing part or a positioning part. In the following cases, the dynamic and fixed molds will generate huge lateral offset forces during injection: (1). When the wall thickness requirements of the plastic part are uneven, the material flow rate through the thick wall is large, and a large pressure is generated here; (2). The side of the plastic part is asymmetrical, such as the counterpressure on the two opposite sides of the mold with a stepped parting surface is not equal.
3. Large molds will produce dynamic and fixed mold offsets due to different filling rates in all directions and the influence of the mold’s own weight during mold installation. In the above cases, the lateral offset force will be added to the guide pin during injection, and the surface of the guide pin will be roughened and damaged when the mold is opened. In severe cases, the guide pin will bend or cut off, and even the mold cannot be opened. In order to solve the above problems, high-strength positioning keys are added on each side of the mold parting surface. The simplest and most effective way is to use cylindrical keys. The verticality of the guide pin hole and the parting surface is crucial. During processing, the dynamic and fixed molds are aligned and clamped, and then bored at one time on the boring machine. This ensures the concentricity of the dynamic and fixed mold holes and minimizes the verticality error. In addition, the heat treatment hardness of the guide pins and guide sleeves must meet the design requirements.
4. The dynamic 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, often changing the original design size, or replacing the dynamic template with a low-strength steel plate. In the mold with a push rod, the large span of the two side seats causes the template to bend during injection. Therefore, the dynamic template must be made of high-quality steel with sufficient thickness. Low-strength steel plates such as A3 must not be used. If necessary, a support column or support block should be set under the dynamic template to reduce the thickness of the template and improve the bearing capacity.
5. The push rod is bent, broken or leaked. The quality of the self-made ejector is good, but the processing cost is too high. Now, standard parts are generally used, and the quality is poor. If the gap between the ejector and the hole is too large, leakage will occur, but if the gap is too small, the ejector will expand and get stuck due to the increase in mold temperature during injection. What is more dangerous is that sometimes the ejector cannot be pushed out of the general distance and breaks. As a result, the exposed ejector cannot be reset during the next mold closing and crashes into the die. In order to solve this problem, the ejector is re-grinded, and a 10-15 mm matching section is retained at the front end of the ejector, and the middle part is ground down by 0.2 mm. After assembly, all ejectors must be strictly checked for matching clearance, which is generally within 0.05-0.08 mm 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. For example, poor cooling will cause large shrinkage of the product, or uneven shrinkage and warping and deformation. On the other hand, if the mold is overheated as a whole or in part, the mold cannot be formed normally and production is stopped. In severe cases, the ejector and other moving parts are damaged due to thermal expansion and jamming. The design and processing of the cooling system are determined by the shape of the product. Do not omit this system because the mold structure is complex or the processing is difficult. In particular, large and medium-sized molds must fully consider the cooling problem.

7. The fixed-distance tensioning mechanism fails. Fixed-distance tensioning mechanisms such as swing hooks and buckles are generally used in fixed mold core pulling or some secondary demolding molds. Because these mechanisms are set in pairs on both sides of the mold, their movements must be synchronized, that is, the mold is closed and the buckle is released at the same time, and the mold is opened to a certain position and unhooked at the same time. Once the synchronization is lost, the template of the pulled mold will inevitably be skewed and damaged. The parts of these mechanisms must have higher rigidity and wear resistance, and the adjustment is also difficult. The life of the mechanism is short. Try to avoid using them and use other mechanisms instead. When the core pulling force is relatively small, the method of spring pushing out the fixed mold can be used. When the core pulling force is relatively large, the core can slide when the movable mold retreats. The structure of completing the core pulling action first and then separating the mold can be used. For large molds, hydraulic cylinder core pulling can be used. The inclined pin slider type core pulling mechanism is damaged. The most common problems of this mechanism are mostly inadequate processing and too small materials. There are mainly the following two problems. The advantage of a large inclined pin angle A is that it can produce a larger core pulling distance within a shorter mold opening stroke. However, if the inclined angle A is too large, when the extraction force F is a certain value, the bending force P=F/COSA on the inclined pin during the core pulling process is also larger, and the inclined pin is prone to deformation and inclined hole wear. At the same time, the upward thrust N=FTGA generated by the inclined pin on the slider is also greater. 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. 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, 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 and reset. Especially when the mold is closed, the slider is not reset smoothly, causing the slider to be damaged or even bent. According to experience, the length of the slider left in the slide groove after the core pulling action is completed should not be less than 2/3 of the total length of the guide groove.
9. Finally, design. When manufacturing the mold, it should be based on the specific conditions such as the requirements of the plastic part quality, the size of the batch, and the requirements of the manufacturing period. It can not only meet the product requirements, but also be the simplest and most reliable in the mold structure, easy to process, and low in cost. This is the most perfect mold.