Analysis of common faults and troubleshooting methods of injection molds!


Author: MULAN -Plastic Molding Manufacturer

The structural form of the injection mold and the mold processing technology will affect the quality of the plastic parts. Sometimes, injection molds also fail during the production process. How do we eliminate them? Today, the mold manufacturer will share with you the common faults and troubleshooting methods of injection molds.

1. The guide post is damaged. The guide post mainly plays a guiding role in the mold to ensure that the forming surfaces of the core and the cavity will 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 cases, when the injection moves, the fixed mold will generate a huge lateral offset force: (1) When the wall thickness of the plastic part is required to be uneven, the material flow passes through the thick wall at high speed, resulting in a high pressure here; (2) The side of the plastic part is asymmetrical, such as a mold with a stepped parting surface, and the back pressure on the opposite sides is not equal. 2. Dynamic and fixed mold offset. For large molds, there are dynamic and fixed mold offsets due to the different filling rates in all directions and the influence of the mold's own weight during mold installation.

In the above cases, the guide post will increase the lateral offset force during the injection process, and the surface of the guide post will become rough and damaged during the mold opening process. In severe cases, the guide post will 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 the parting surface of the mold, one on each side, and the cylindrical key is simple and effective.

The perpendicularity between the guide post hole and the parting surface is very important. During the processing, the movable and fixed molds are aligned and clamped, and then the boring is completed on the boring machine at one time, so as to ensure the concentricity of the movable and fixed mold holes and minimize the verticality error. In addition, the heat treatment hardness of the guide post and guide sleeve must meet the design requirements.

3. The active formwork is bent. When the mold is injected, the molten plastic in the cavity will generate a huge back pressure, generally 600~1000 kg/cm². Sometimes the mold maker does not pay attention to this problem, and often changes the original design size, or replaces the movable template with a low-strength steel plate.

In molds with ejector pins, the mold bends downward during the injection process due to the large span of the valve seats on both sides. Therefore, the movable formwork must use high-quality steel with sufficient thickness. A3 and other low-strength steel plates shall not be used.

When necessary, support columns or support blocks are set under the movable formwork to reduce the thickness of the formwork and increase the bearing capacity. 4. The pusher rod is bent, broken or leaking. The quality of the self-made ejector pin is good, but the processing cost is too high.

Standard parts are commonly used now, and the quality is poor. If the gap between the ejector pin and the hole is too large, material leakage will occur, but if the gap is too small, the ejector pin will expand and block due to the increase in mold temperature during the injection process. To make things even more dangerous, sometimes the ejection rod won't move and breaks when pushed out the usual distance.

Therefore, the exposed ejector pin cannot be reset during the next mold clamping, and the mold is damaged. In order to solve this problem, the ejector pin should be reground, and a 10-15mm matching section should be reserved at the front end of the ejector pin, and the middle part should be ground to a fineness of 0.2mm. After all ejector rods are assembled, the fit clearance must be strictly checked, generally within the range of 0.05~0.08mm, to ensure that the entire ejector rod mechanism can move back and forth freely.

5. The door is difficult to disassemble. During the injection molding process, the sprue is stuck in the sprue sleeve and is not easy to remove. When the mold was opened, the product was cracked and damaged.

In addition, before demoulding, the operator must knock the tip of the copper rod out of the nozzle to loosen it, seriously affecting production efficiency. The main reason for this failure is the poor finish of the taper hole of the gate and the knife marks in the circumferential direction of the inner hole. Secondly, the material is too soft, the small end of the tapered hole is deformed or damaged after a period of use, and the spherical radian of the nozzle is too small, which causes the rivet head of the gate material here.

It is difficult to process the tapered hole of the door cover, so standard parts should be used as much as possible. If you need to process it yourself, you should also make or buy a special reamer. The taper hole should be ground to above Ra0.4.

In addition, a gate pull rod or a gate ejection mechanism must be provided. 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, excessive product shrinkage or uneven shrinkage causing warping and deformation.

On the other hand, the overall or local overheating of the mold makes the mold unable to form normally and stops production. In severe cases, moving parts such as ejector pins are blocked and damaged due to thermal expansion. The design and processing of the cooling system depends on the product shape.

Don't ignore this system because of the complex 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 distance tensioning mechanism fails.

The core-pulling of the fixed mold or part of the secondary demoulding mold usually adopts a fixed-distance tensioning mechanism, such as a swing hook, a buckle, etc. Since these mechanisms are arranged in pairs on both sides of the mold, their actions must be synchronized, that is, the molds are snapped together at the same time, and when the mold is opened to a certain position, the molds are uncoupled at the same time. Once the synchronization is lost, it is bound to cause skewing and damage to the drawing template.

Parts of these mechanisms should have high stiffness and wear resistance. It's also hard to adjust. The mechanism has a short service life.

Try to avoid it and use other mechanisms instead. When the core pulling force is small, the spring can be used to push out the fixed mold. When the core-pulling force is large, the structure of sliding core when the moving mold is retreating can be adopted, and the core-pulling action is completed first, and then the mold is divided.

Large molds can use hydraulic cylinder core pulling. The core-pulling mechanism of the oblique pin slider is damaged. Some molds are limited by the area of ​​the template, and the length of the guide groove is too small.

After the core pulling action is completed, the slider is exposed outside the guide groove. In this way, the slider is easy to tilt in the post-core-pulling stage and the initial stage of mold clamping and reset. Especially in the mold clamping process, the slider is not reset smoothly, resulting in damage to the slider, or even bending damage.

According to experience, after the slider completes the core-pulling action, the remaining length in the chute should not be less than 2/3 of the total length of the guide groove. When designing and manufacturing molds, according to the requirements of specific conditions such as plastic parts quality, batch size, and manufacturing cycle, it can not only meet product requirements, but also has simple and reliable mold structure, easy processing, and low cost. It is a very good mold.

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