The structural form of injection mold and the quality of mold processing directly affect the quality and production efficiency of plastic parts. The most common in the production of injection molds and plastic products, the most common mold failures and their main reasons are as follows:
1, the gate is difficult to remove.
During the injection molding process, the gate sticks to the sprue bushing and is not easy to come out. When the mold is opened, the product is cracked. In addition, the operator must use the tip of the copper bar to knock out from the nozzle, so that it can be released after being loosened. Affects production efficiency. The main cause of this failure is the poor smoothness of the taper hole and the knife mark in the circumferential direction of the inner hole. The second is that the material is too soft, the small end of the tapered hole is deformed or damaged after a period of use, and the spherical curvature of the nozzle is too small, causing the pouring The grooving is produced here. The taper hole of the sprue bushing is difficult to process. 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 should be ground to Ra0.4 or above. In addition, the gate pull rod or gate ejection mechanism must be set.
2, guide column 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 touch each other under any circumstances, and the guide post cannot be used as the force receiving member or the positioning member. In the following cases, the injection is performed. When moving, the fixed mold will produce a huge lateral offset force:
(1) When the wall thickness of the plastic part is not uniform, the flow rate of the material 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 stepped surface mold, the opposite The back pressure on both sides is not equal.
3, large mold, due to the different filling rate, and the mold weight is affected by the mold, resulting in dynamic, fixed mode 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 will be pulled and damaged. In severe cases, the guide post will be bent or cut, and even the mold cannot be opened. In order to solve the above problems, The high-strength positioning key is added on each side of the mold parting surface. The most convenient and effective is to use the cylindrical key. The verticality of the guide post hole and the parting surface is very important. In the processing, the moving, fixed-mode alignment position is adopted. After clamping, it is finished once on the trampoline, which can ensure the concentricity of the moving and fixed hole, and minimize the verticality error. In addition, the heat treatment hardness of the guide post and the guide bush must meet the design requirements.
4, moving template bending.
When the mold is injected, the molten plastic in the cavity generates a huge back pressure, generally 600 ~ 1000 kg / cm 2. Mold makers sometimes do not pay attention to this problem, often change the original design size, or replace the moving template with low-strength steel In the mold with the top rod topping, due to the large span of the two sides, the template is bent down during the injection. Therefore, the moving template must be made of high-quality steel, and it must have sufficient thickness to cut the low-strength steel plate such as A3. When the support column or support block should be set under the moving template, the thickness of the template can be reduced to improve the carrying capacity.
5, the ram is bent, broken or missing.
The quality of the self-made ejector is better, that is, the processing cost is too high. Now the standard parts are generally used, and the quality is poor. If the gap between the ejector pin and the hole is too large, the leakage occurs, but if the gap is too small, the mold is used during the injection. When the temperature rises, the ejector rod expands and is stuck. More dangerously, sometimes the ejector rod is pulled out at a normal distance and is broken at the top. As a result, the ejector rod exposed during the next clamping cannot be reset and the die is broken. In order to solve this problem, the ejector rod is re-grinded, and the 10-15 mm mating section is retained at the front end of the ejector pin, and the middle section is ground 0.2 mm. All the ejector pins must be strictly checked for fit clearance after assembly, generally at 0.05. Within ~0.08 mm, it is necessary to ensure that the entire ejection mechanism can move forward and backward.
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 warping surface. On the other hand, the mold is completely or partially overheated, so that the mold cannot be formed normally and the production is stopped. In severe cases, the moving parts such as the ejector pin are stuck and damaged. The design of the cooling system depends on the shape of the product. Do not omit the system because of the complicated structure of the mold or the difficulty of processing. Especially for large and medium-sized molds, it must be fully considered. Cooling problem.
7, the distance tensioning mechanism is out of order.
The fixed distance tensioning mechanism such as the swing hook and the buckle is generally used in the mold of the fixed mold core or some secondary demoulding. Since the mechanism is arranged in pairs on both sides of the mold, the action requirements must be synchronized, that is, When the mold is clamped at the same time, 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 is bound to be damaged and damaged. The parts of these mechanisms have high rigidity and wear resistance, and the adjustment is also very difficult. Short life, try to avoid use, can use other mechanisms. In the case of relatively small pumping force, the spring can be used to push the fixed mold. When the core pulling force is relatively large, the core can be used when the moving mold is retracted. The structure of the splitting machine is completed after the core pulling operation, and the hydraulic cylinder can be used for core pulling on the large mold. The oblique pin type sliding core mechanism is damaged. Most of the problems that occur in this mechanism are not processed and used. Too small, there are two main problems: The inclination angle A is large, and the advantage is that a relatively large core distance can be generated in a shorter mold opening stroke. However, an excessive inclination angle A is adopted, and when the extraction force F is constant When the value is used, the skew pin is subjected to the core pulling process. The bending force P=F/COSA is also larger, which is prone to skew pin deformation and oblique hole wear. At the same time, the oblique pin produces an upward thrust on the slider. N=FTGA is also larger. This force causes the slider to be in the guide groove. The positive pressure of the guide surface increases, which increases the frictional resistance when the slider slides. It is easy to cause the sliding to be unsmooth and the guide groove is worn. According to experience, the inclination angle A should not be greater than 25°.
8, the length of the mold changes.
Some molds are limited by the area of the stencil, and 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 inclined in the post-core pulling stage and the initial stage of the mold clamping reset, especially in the mold clamping. When the slider is not reset properly, the slider is damaged, and even the bending is broken. According to experience, the length of the slider after the core pulling operation is not less than 2/3 of the total length of the guide groove.
9, Finally, when designing and manufacturing molds, it should meet the requirements of the products according to the requirements of the quality of the plastic parts, the size of the batch, the requirements of the manufacturing period, etc., and it is the easiest and most reliable in the mold structure, easy to process, and low in cost. This is the perfect mold.