Injection mold is a tool for producing plastic products; it is also a tool that gives the complete structure and precise dimensions of plastic products. The structure of the injection mold and the quality of the mold directly affect the quality and production efficiency of the plastic parts. Injection mold production and plastic product production practices are the most common. The most common causes of injection mold failure and its exclusion methods are described in detail as follows:
1. It is difficult to discharge the material from the gate. In the injection molding process, the gate is stuck in the sprue bushing and it is not easy to escape. When the mold is opened, the product is cracked. In addition, the operator must use the tip of the copper rod to knock out from the nozzle, make it loose and release it, which will seriously affect the production efficiency. The main cause of this failure is the poor finish of the tapered hole in the gate, and the tool hole in the circumferential direction of the inner hole. The second is that the material is too soft. After a period of use, the small end of the tapered hole is deformed or damaged, and the curvature of the nozzle's spherical surface is too small, causing the gate material to produce a rivet at this point. The taper hole of the sprue bushing is more difficult to process. Standard parts should be used as far as possible. If it is necessary to process it by itself, special reamers should be made or purchased. Taper holes need to be ground to Ra0.4 or more. In addition, a gate puller or gate ejector must be provided.
2. Damage to the guide post. The guide pillar mainly plays a guiding role in the mold to ensure that the molding surface of the core and the cavity do not touch each other in any case, and the guide post cannot be used as a force member or a positioning member. In the following situations, the fixed mold will generate a huge lateral offset force when the injection is started: (1) When the wall thickness of the plastic part is not uniform, the velocity of the material flow through the thick wall is large, and the ratio is greater here. Large pressure; (2). Asymmetrical side of the plastic part, such as the opposite side of the mold of the step-shaped parting surface is not equal to the counter pressure.
3. Large molds, due to the different charge rates, and the impact of the weight of the mold in the mold, resulting in dynamic, fixed mold offset. In the above cases, the lateral offset force will be applied to the guide post during injection. When the mold is opened, the surface of the guide post is pulled and damaged. In severe cases, the guide post is bent or cut off, and it is impossible to open the mold. In order to solve the above problem, a high-strength positioning key is added on each side of the mold parting surface. The most convenient and effective method is to use a cylindrical key. The verticality between the guide post hole and the parting surface is very important. During the machining, it is used to move. After the clamping position of the fixed mold is clamped, the boring is completed once on the boring bed so that the concentricity of the moving and fixing holes can be guaranteed. Minimize the squareness error. In addition, the heat treatment hardness of the guide post and guide sleeve must meet the design requirements.
4. Move the template to bend. When the mold is injected, the molten plastic in the mold cavity generates a huge back pressure, generally 600 to 1000 kg/cm. Moldmakers sometimes do not pay attention to this problem, often changing the original design size, or replace the moving plate with a low-strength steel plate. In a die with a tipping material, the two-passenger frame has a large span, causing the template to bend under injection. Therefore, the moving plate must be made of high-quality steel, and must have sufficient thickness. It is not possible to cut low-strength steel plates such as A3. When necessary, support columns or support blocks should be set below the moving plate to reduce the thickness of the plates and increase the bearing capacity.
5. The mast is bent, broken or leaking. The quality of the self-made ejector rod is better, that is, the processing cost is too high, and now standard parts are generally used and the quality is poor. If the clearance between the crowbar and the hole is too large, leakage occurs, but if the clearance is too small, the ram swells and seizes due to the increase in mold temperature during the injection. What is even more dangerous is that sometimes the top bar is broken at its normal distance from the top and the result is that the exposed top bar cannot be reset and hit the die at the next clamping. In order to solve this problem, the top bar is reground, leaving a 10 to 15 mm mating section at the front end of the jack, with the center section being 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 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 defects such as warping surface deformation. On the other hand, the whole or partial overheating of the mold may cause the mold to not be properly formed and be discontinued. In severe cases, the movable parts such as the ejector rods may be blocked by thermal expansion. The design of the cooling system depends on the shape of the product. The system should not be omitted due to the complexity of the mold structure or processing difficulties. In particular, large and medium-sized molds must fully consider the cooling problem.
7. Fixed distance tension mechanism failure. Fixed-length tensioning mechanisms such as hooks and buckles are generally used for fixed-mold core-pulling or some secondary mold-releasing dies. As such mechanisms are set in pairs on both sides of the mold, their motion requirements must be synchronized. Simultaneous mold clamping at the same time, open the mold to a certain position at the same time unhooking. Once the synchronization is lost, it will inevitably cause the template of the die to be worn out and be damaged. The parts of these mechanisms must have high rigidity and wear resistance, and the adjustment is also very difficult. The life span of the mechanism is short, and the use of other mechanisms can be avoided. In the case where the pulling force is relatively small, the spring can be used to push the fixed die, and when the core pulling force is relatively large, the core can slide when the moving die retreats, and the structure of the die splitting can be completed after the core pulling operation is performed. Hydraulic cylinders can be used to pull cores on the mold. The oblique pin slider core-pulling mechanism is damaged. The most common problems with this type of organization are that the processing is not in place and the materials used are too small. There are mainly two problems.
The inclination angle A of the oblique pin is large, and the advantage is that a relatively large core pitch can be produced within a short opening stroke. However, when a large inclination angle A is adopted, when the drawing force F is a certain value, the bending force received by the oblique pin in the core pulling process is P=F/COSA, and the oblique pin deformation and oblique hole wear are likely to occur. At the same time, the oblique pin generates an upward thrust force on the slider N=FTGA, which increases the positive pressure of the slider on the guide surface in the guide groove, thereby increasing the frictional resistance when the slider slides. It is easy to slip and the guide groove is worn. According to experience, dip A should not be greater than 25
8. Some molds are limited by the template area, guide channel length is too small, the slider exposed after the core pulling action out of the guide slot, so that in the core pulling stage and mold clamping reset are likely to cause the slider tilt, especially in When the mold is closed, the slider will not be reset smoothly, causing the slider to be damaged or even broken by bending. According to experience, after the slider completes the core pulling operation, the length left in the chute shall not be less than 2/3 of the total length of the guide chute.
9. Finally, when designing and manufacturing molds, according to the 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 requirements of the products, but also be the most simple and reliable in the mold structure, and easy to process, making the cost Low, the production of such a mold is excellent.
