What are the reasons for the failure of the injection mould?

Update:09-11-2019
Summary:

The most common in the production of injection molds an […]

The most common in the production of injection molds and plastic products, the most common mold failures and their main reasons are excluded as follows.

1. It is difficult to remove the gate. During the injection molding process, the gate is stuck in the sprue bushing and is not easily peeled off. 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, so that it can be released after being loosened, which seriously affects the production efficiency. The main cause of this failure is the poor smoothness of the gate cone and the presence of tool marks in the circumferential direction of the bore. Secondly, the material is too soft. After a period of use, the small end of the tapered hole is deformed or damaged, and the spherical curvature of the nozzle is too small, so that the gate material produces a rivet head here. The taper hole of the sprue sleeve is difficult to process. Standard parts should be used as much as possible. If you need to process it yourself, you should also make a special reamer. The tapered hole needs to be ground to Ra0.4 or higher. In addition, a gate pull rod or gate ejection mechanism must be provided.

2. The guide column is damaged. The guide post mainly plays a guiding role in the mould 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, when the injection is moving, the fixed mould will generate a huge lateral offset force:

(1) When the wall thickness of the plastic part is not uniform, the velocity of the material flowing through the thick wall is large, and a large pressure is generated here;

(2) The side of the plastic part is asymmetrical. For example, the mold of the stepped parting surface is not equal to the back pressure of the opposite sides.

3, large mold, due to the different filling rate, and the mold is affected by the weight of 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 is pulled, damaged, and the guide post is bent or cut in severe cases, and even the mold cannot be opened. In order to solve the above problem, a high-strength positioning key is added on each side of the mold parting surface, and the most convenient and effective one is to use a cylindrical key. The verticality of the guide post hole and the parting surface is very important. When the machining is carried out, the fixed mold is clamped at the position and clamped once on the trampoline, thus ensuring 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 sleeve must meet the design requirements.

4. The moving template is bent. When the mold is injected, the molten plastic in the cavity generates a huge back pressure, generally 600 to 1000 kg/cm2. Moldmakers sometimes do not pay attention to this problem, often change the original design size, or replace the moving template with a low-strength steel plate. In the mold with the top bar topping, the template is bent down during injection due to the large span of the two sides. The moving template must be made of high-quality steel. It must have sufficient thickness. It is not allowed to use low-strength steel plates such as A3. If necessary, support columns or support blocks should be placed under the moving plate to reduce the thickness of the template and improve the carrying capacity.

5. The top rod is bent, broken or leaking. The quality of the self-made ejector is better, that is, the processing cost is too high, and 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, leakage occurs, but if the gap is too small, the ejector pin expands and is stuck due to an increase in mold temperature at the time of injection. More dangerously, sometimes the ejector pin is pulled out at a normal distance and is broken. As a result, the exposed ejector pin cannot be reset and the die is broken during the next clamping. In order to solve this problem, the ejector rod is re-grinded, and a 10-15 mm mating section is retained at the front end of the ejector pin, and the middle portion is ground by 0.2 mm. All the ejector rods must be strictly checked for the fit clearance after assembly, 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, 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 overheated in whole or in part, so that the mould cannot be formed normally and the production is stopped. In severe cases, the movable parts such as the ejector are thermally expanded and damaged. The design of the cooling system depends on the shape of the product. Do not omit this system because of the complicated structure of the mold or the difficulty of processing. Especially for large and medium-sized molds, the cooling problem must be fully considered.

7. The fixed tension 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, The mold is clamped at the same time, and the mold is opened to a certain position and unhooked at the same time. Once the synchronization is lost, it will inevitably cause the template of the pulled mold to be skewed and damaged. The parts of these mechanisms have higher rigidity and wear resistance, the adjustment is also difficult, the life of the mechanism is short, and the use of other mechanisms can be avoided. In the case where the pumping force is relatively small, the spring can be used to push the fixed mold. When the core pulling force is relatively large, the core can be slid when the moving mold is retracted, and the structure of the splitting is performed after the core pulling operation is performed. A hydraulic cylinder can be used for core pulling on the mold. The skew pin slider core pulling mechanism is damaged. Most of the problems that occur in such institutions are that they are not processed and the materials used are too small. There are two main problems.

The angle of inclination of the skew pin A is large, and the advantage is that a relatively large core distance can be produced in a shorter mold opening stroke. However, when the excessive inclination angle A is adopted, when the extraction 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 skew pin deformation and the oblique hole wear are prone to occur. At the same time, the oblique pin produces an upward thrust 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 cause slipping and the guide groove is worn out. According to experience, the inclination A should not be greater than 25

8. Some molds are limited by the area of ​​the template, the length of the guide groove is too small, and the slider exposes the outside of 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 mould clamping reset, especially in the case of the mold. When the mold is closed, the slider is not reset properly, causing the slider to be damaged or even crushed. According to experience, after the slider is finished, the length left in the chute should not be less than 2/3 of the total length of the guide.

9. Finally, when designing and manufacturing the mould, it should meet the requirements of the product 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 the cost is low. This is the most perfect mold.

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