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The Analysis of Injection Mold Failure

The structure and processing quality of the injection mold directly affect the quality and production efficiency of plastic products. This article makes a brief analysis of some of the most common failures in mold production.

1. The gate is difficult to break away from the material

During the injection molding process, the gate sticks to the sprue sleeve and is not easy to come out. When the mold is opened, the product is cracked and damaged. In addition, the operator must knock it out from the nozzle with the tip of the copper rod. After loosening it, the demoulding can be done, which seriously affects the production efficiency.

The main reason for this failure is the poor finish of the gate taper hole. Besides, there are knife marks in the circumferential direction of the inner hole. The second is that the material is too soft, the small end of the taper hole is deformed or damaged after a period of use. Moreover, the curvature of the nozzle spherical surface is too small, causing the gate material to produce riveting heads here. The taper hole of the sprue bush is difficult to process. So standard parts should be used as much as possible. If you need to process it yourself, you should also make your own or buy a special reamer. The taper hole needs to be ground to less than Ra0.4. In addition, you must set a gate pull rod or gate ejection mechanism.

2. Guide post 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 collide with each other under any circumstances. The guide post cannot be used as a force-bearing part or a positioning part. In the following situations, the moving and fixed molds will produce huge lateral offset force during injection:

(1) When the wall thickness of the plastic part is not uniform, the material flow passes through the thick wall at a high speed, and a large pressure is generated here;

(2) The side of the plastic part is asymmetrical, such as a stepped parting surface mold, the opposite pressures on the two opposite sides are not equal.

3. Moving and fixed mold deviate

For large molds, due to the different filling rates in all directions and the influence of the mold’s dead weight when it’s installed, the moving and fixed molds will deviate. In these cases, the lateral deflection force will be added to the guide post during injection. Besides, the surface of the guide post will be roughed and damaged when the mold is opened. In severe cases, the guide post may be bent or cut off, and the mold cannot even be opened.

In order to solve the above problems, a high-strength positioning key is added on each side of the mold parting surface. The most simple and effective way is to use a cylindrical key. The perpendicularity between the guide hole and the parting surface is very important. In the processing, the moving and fixed molds are aligned and clamped, and the boring is completed at one time on the boring machine. This ensures the concentricity of the moving and fixed mold holes and minimizes the verticality error. In addition, the heat treatment hardness of the guide post and guide sleeve must meet the design requirements.

4. Movable template bending

When the mold is injected, the molten plastic in the cavity produces a huge back pressure, generally 600 ~ 1000 kg/cm². Mold makers sometimes don’t pay attention to this problem, often change the original design size, or replace the movable template with a low-strength steel plate. In the mold that uses ejector pins to eject the material, due to the large span between the two sides of the seat, the template is bent during injection. Therefore, the movable formwork must be made of high-quality steel, with sufficient thickness. And the low-strength steel plates such as A3 should not be used. When necessary, support columns or support blocks should be installed under the movable formwork. It can reduce the thickness of the formwork and increase the load-bearing capacity.

5. The ejector rod is bent, broken or leaking

The self-made ejector rods are of better quality, but the processing cost is too high. Nowadays, standard parts whose quality is average are usually used. If the gap between the ejector pin and the hole is too large, leakage will occur. But if the gap is too small, the ejector pin will become stuck due to the increase in mold temperature during injection. What’s worst, sometimes the ejector rod will not move and be broken when it is ejected for a general distance. As a result, the exposed ejector rod cannot be reset during the next mold clamping and will damage the die.

In order to solve this problem, the ejector rod was re-ground. And the mating section of 10~15 mm long was retained at the front end of the ejector rod. While the middle part was ground down by 0.2 mm. After all ejector rods are assembled, the fit clearance must be strictly checked, generally within 0.05~0.08 mm. It’s 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, resulting in defects such as warping deformation. On the other hand, the whole or part of the mold is overheated. So the mold cannot be formed normally and the production is stopped. In severe cases, the movable parts such as the ejector pin will be thermally expanded and become damaged. The design and processing of the cooling system depends on the shape of the product. Don’t omit this system because of the complex mold structure or processing difficulties. Especially for large and medium-sized molds, we must fully consider the cooling problem.

7. The fixed-distance tensioning mechanism fails

Swing hooks, buckles and other fixed-distance tensioning mechanisms are generally used in fixed mold core pulling or some secondary demolding molds. Because such mechanisms are set in pairs on both sides of the mold, their actions must be synchronized. That is, the mold is closed at the same time, and it’s released at the same time when opened to a certain position. Once it’s out of synchronization, it will inevitably cause the template of the drawn die to be skewed and damaged. The parts of these mechanisms must have higher rigidity and wear resistance. And it’s difficult to make some adjustment. Also, the life of the mechanism is short. We should avoid using it as much as possible and using other mechanisms.

When the core pulling force is relatively small, we can use the spring to push out the fixed mold. When the core pulling force is relatively large, we can slide the core when the movable mold is retracted. Complete the core pulling action first and then divide the mold. Next, use the hydraulic cylinder to pull the core on the mold.

The more common problems of this kind of mechanism are mostly inadequate processing and too less materials.

The two main problems are as follows:  

If the oblique pin inclination angle A is large, it can produce a larger core pulling in a shorter mold opening stroke. However, if the inclination angle A is too large, when the extraction force F reaches a certain value, the bending force P=F/COSA of the inclined pin during the core pulling process is larger. What’s worst, the inclined pin deformation and the inclined hole wear are prone to occur.

At the same time, the upward thrust N=FTGA generated by the oblique 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 frictional resistance of the slider when sliding. 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. The length of the guide groove is too small

Some molds are limited by the template area, so the guide groove length is too small. The slider will be exposed outside the guide groove after the core pulling action is completed. This will easily cause the slider to tilt during the core pulling stage and the initial stage of mold resetting, especially during mold clamping. At this time, the sliding block is not reset smoothly, causing damage to the sliding block and even make it broken under bending. According to experience, after the slider completes the core-pulling action, the length left in the chute should not be less than 2/3 of the full length of the guide groove.

When designing and manufacturing molds, we should base on the requirements of the plastic parts quality, the size of the batch, the requirements of the manufacturing period, etc.. It’s also necessary to meet the requirements of the product. And mold structure should be the most simple and reliable, easy to process, and low cost. Then it will be the most perfect mold.

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