How to solve the common failures of injection molds?

An injection mold is a tool for producing plastic products; it is also a tool for giving plastic products a complete structure and precise dimensions. The structural form of the injection mold and the quality of the mold processing directly affect the quality of plastic products and production efficiency. The most common and frequent causes of injection mold failure and its elimination methods are described below:

1. Difficulty in the sprue release

In the injection molding process, the sprue is stuck in the sprue sleeve, and it is not easy to get out. When the mold is opened, the product will be cracked and damaged. In addition, the operator must use the tip of the copper rod to knock out from the nozzle to make it loose before demoulding, which seriously affects the production efficiency. The main reason for this failure is the poor finish of the conical hole of the gate and the knife marks in the circumference of the inner hole. Secondly, the material is too soft, the small end of the cone hole is deformed or damaged after using for a period of time, and the arc of the nozzle sphere is too small, resulting in the rivet head of the sprue material here.

The tapered hole of the sprue is difficult to process, so standard parts should be used as much as possible, if you need to process it by yourself, you should also make your own or buy a special reamer. The tapered hole should be ground to Ra0.4 or more. In addition, a sprue puller or sprue ejector must be installed.

2. Guide pillar damage

The guide pillar is mainly used as a guide in the mold to ensure that the molding surface of the core and cavity do not touch each other under any circumstance, and should not be used as a force or positioning part.


In the following cases, the dynamic and fixed mold will produce a huge lateral offset force during injection: (1). When the wall thickness of the plastic part is not uniform, the material flow through the thick wall at a large rate, resulting in a large pressure here; (2). The side of the plastic part is not symmetrical, such as the step-shaped parting surface of the mold on both sides of the opposite side of the counter-pressure is not equal.

3. Large mold

Because of the different charging rates in each direction, as well as the influence of the self-weight of the mold when loading, resulting in dynamic and fixed mold offset. In the above cases, the lateral offset force will be added to the guide column during injection, and the surface of the guide column will be stretched and damaged when the mold is opened, and in serious cases, the guide column will be bent or cut off, and even unable to open the mold.

In order to solve the above problems in the mold parting surface to set up a high-strength positioning key on each of the four sides, the most simple and effective is the use of cylindrical keys. The perpendicularity of the guide pillar hole and the parting surface is very important. In the processing is used to move, fixed die alignment after clamping, in the boring machine once boring, so as to ensure the concentricity of the moving, fixed die hole, and make the verticality of the minimum error. In addition, the heat treatment hardness of the guide pillar and guide bush must meet the design requirements.

4. Bending of moving mold plate

When the mold is injected, the molten plastic in the mold cavity generates huge backpressure, generally in 600~1000kg/cm. Mold makers sometimes do not pay attention to this problem, often changing the original design size, or the dynamic template with low strength steel plate instead, in the mold with the top bar top material, due to the large span of both sides of the seat, resulting in the injection of the template down bending.

Therefore, the moving template must use high-quality steel, to have enough thickness, do not use A3 and other low strength steel plates, in case of necessity, should be set up under the moving template support column or support block, in order to reduce the thickness of the template, improve the bearing capacity.

5. Top bar bending, fracture, or leakage of material

Homemade top bar quality is better, is too high processing costs, now generally use standard parts, poor quality. If the gap between the top bar and the hole is too large, there will be leakage, but if the gap is too small, the top bar will expand and jam during injection due to the increase in mold temperature.

More dangerous is that sometimes the top bar is jacked out of the general distance on the top can not be broken, the result in the next time the mold when the section of the exposed top bar can not be reset and hit the concave mold. In order to solve this problem, the top bar is resharpened, the front end of the top bar retains 10~15mm of the mating section, and the middle part is ground down by 0.2mm. All the top bars in the assembly must be strictly checked with the clearance, generally in 0.05 ~ 0.08 mm, to ensure that the entire ejector mechanism can enter and exit freely.

6. Poor cooling or water leakage

The cooling effect of the mold directly affects the quality and productivity of the products, such as poor cooling, the products shrinking large, or shrinkage is not uniform and warped surface deformation and other defects. On the other hand, the overall or local overheating of the mold, so that the mold can not be normal molding and stop production, and in serious cases, the top bar and other moving parts of the thermal expansion of the jammed and damaged.

The design of the cooling system, processing to the shape of the product, do not because of the complex structure of the mold or processing difficulties and omit this system, especially large and medium-sized mold must be fully considered cooling problems.

7. Fixed distance tensioning mechanism failure

Pendulum hook, buckle, and other fixed distance tensioning mechanism is generally used in the fixed mold core or some secondary demoulding mold, because this kind of mechanism is set in pairs on both sides of the mold, its action requirements must be synchronized, that is, close the mold at the same time buckle, open the mold to a certain position at the same time off the hook.

Once the loss of synchronization, will inevitably cause the template of the pulled mold skewed and damaged, the parts of these institutions to have high rigidity and wear resistance, adjustment is also very difficult, the institutional life is short, try to avoid using, can be replaced by other institutions.

In the case of relatively small pumping force can be used to push out the spring method of fixed die, in the case of relatively large pumping force can be used when the dynamic die backward core sliding, first complete the pumping action and then the structure of the mold, in large molds can be used hydraulic cylinder pumping core. Inclined pin slider type core extraction mechanism damage. This mechanism is more often the fault is not in place on the processing and too small with the material, mainly the following two problems.

The advantage of a large inclination angle A of the slanted pin is that it can produce a large core extraction distance within a short mold opening stroke. However, if the angle of inclination A is too large when the extraction force F is a certain value, the bending force P=F/COSA in the process of core extraction will be bigger, and the pin will be easily deformed and the hole will be worn.

At the same time, the slanted pin on the slider to produce upward thrust N = FTGA is also greater, this force makes the slider on the guide groove guide surface pressure increase, thus increasing the slider sliding friction resistance. It is easy to cause the sliding is not smooth, guide groove wear. According to experience, the angle of inclination A should not be greater than 25

8. Some molds are limited by the area of the template

Guide slot length is too small, the slider in the core action is completed outside the guide slot, so in the core after the stage and the initial stage of the mold reset are easy to cause the slider tilt, especially in the mold, the slider reset is not smooth, so the slider damage, and even bending damage. According to experience, the length of the slider should not be less than 2/3 of the full length of the guide slot after the completion of the core extraction action.

9. Finally, in the design

When manufacturing the mold, should be based on the requirements of the quality of the plastic parts, the size of the batch, the requirements of the manufacturing period, and other specific circumstances, both to meet the product requirements, in the mold structure and the most simple and reliable, easy to process, so that the cost is low, the production of such a mold is excellent.