9 kinds of mold failures and solutions

1. It is difficult to remove the gate

During the injection molding process, the gate sticks into the sprue bushing and does not easily come out. 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 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 a special reamer. The tapered hole needs to be ground to Ra0.4 or higher. In addition, the gate pull rod or gate must be set.

2. Large mold moving mode offset

The large molds are displaced by the different filling rates and by the influence of the mold weight during the mold loading. 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, 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 it is processed, it is clamped by the moving and fixed mold alignment position, and it is finished 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.

3. 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 several cases, dynamic and fixed molds during injection will produce a large lateral offset force. 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; the side of the plastic part is asymmetrical, such as the back pressure of the opposite sides of the mold of the stepped parting surface. not equal.

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. 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. If it has 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. Now, the standard parts are generally used, and the quality is worse. 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 warp surface. On the other hand, the mold is overheated in whole or in part, so that the mold 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. In particular, large and medium-sized molds must fully consider the cooling problem.

7. The slider is tilted and the reset is not smooth.

Some molds are limited by the area of ​​the stencil, the length of the guide groove is too small, and 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 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, 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.

8. The distance tensioning mechanism fails

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 mechanism life 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 movable mold is retracted, and the structure of the splitting after the core pulling operation is completed. 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 this kind of mechanism are that the processing is not in place and the materials 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 is also larger. This force 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 slippage and the guide groove is worn. According to experience, the inclination A should not be greater than 25.

9. Exhaust in the injection mold is not smooth

There are often gas generation in injection molds. What is the result? The air is stored in the casting system and the mold cavity; some raw materials contain moisture that is not removed by drying, and they are vaporized into water vapor at high temperatures; When the temperature is too high, some unstable plastics will decompose to generate gas; some additives in the plastic raw materials volatilize or react with each other to form a gas.

At the same time, the reason for poor exhaustion is also needed to find out as soon as possible. The poor exhaust of the injection mold will bring a series of hazards to the quality of the plastic parts, etc. The main performance is: the melt will replace the gas in the cavity during the injection molding process, and if the gas is not discharged in time, the melt will be caused. Difficult to fill, resulting in insufficient injection volume to fill the cavity; poorly excluded gas will form a high pressure in the cavity, and penetrate into the plastic inside under a certain degree of compression, causing voids, pores, tissue and silver, etc. Quality defects;

Due to the high compression of the gas, the temperature in the cavity rises sharply, which causes decomposition and burning of the surrounding melt, causing local carbonization and charring of the plastic part. It mainly appears in the junction of the two melts, at the flange of the gate; the gas is not properly removed, so that the melt velocity entering each cavity is different, so it is easy to form flow marks and fusion marks, and the mechanics of the plastic parts The performance is reduced; due to the gas barrier in the cavity, the filling speed is reduced, the molding cycle is affected, and the taxation efficiency is lowered.

The distribution of bubbles in the plastic part, the bubbles generated by the accumulation of air in the cavity are often distributed on the opposite part of the gate; the bubbles generated by the decomposition or chemical reaction in the plastic raw material are distributed along the thickness of the plastic part; The bubbles generated by the residual water vaporization are irregularly distributed over the entire plastic part.