The nine most common failures of injection molds

1 Difficulty in gate discharge

During the injection molding process, the gate is stuck in the gate sleeve and is not easy to come off. When the mold is opened, crack damage occurs. In addition, the operator must knock out the nozzle with the tip of a copper rod to loosen it before demoulding, which seriously affects production efficiency. The main reason for this failure is the poor finish of the taper hole of the gate and the presence of 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 curvature of the nozzle is too small, which causes the gate material to produce a rivet head here. The taper hole of the gate 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 or buy a special reamer. Taper holes need to be ground to Ra0.4 or more. In addition, a gate pull rod or gate ejection must be provided.

2 Large mold dynamic fixed mold offset

Large molds have different filling rates in different directions. If you want to learn UG programming and receive learning materials, group 496610960 can help you and move the mold and fix the mold when the mold is loaded. In the above cases, the lateral offset force will be added to the guide post during injection, and the surface of the guide post will be fuzzed and damaged when the mold is opened. In severe cases, the guide post will be bent or cut off, and even the mold cannot be opened.

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

3  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 touch each other under any circumstances, and the guide post cannot be used as a force member or a positioning member.

In several cases, moving and fixing the mold during injection will generate a huge lateral offset force. When the wall thickness of plastic parts is required to be uneven, the velocity of the material passing through the thick wall is large, and a large pressure is generated here; the sides of the plastic parts are asymmetric, such as the counter pressure on the opposite sides of the mold with a stepped parting surface not equal.

4  Bending of moving template

When the mold is injected, the molten plastic in the mold cavity generates a huge back pressure, which is generally 600 ~ 1000 kg / cm. The mold maker sometimes does not pay attention to this problem, often changes the original design size, or replaces the movable template with a low-strength steel plate. In a mold that uses ejector pins, the template has a large bending span during injection, which causes the template to bend down during injection. Therefore, the moving formwork must be made of high-quality steel, and it must have sufficient thickness. Low-strength steel plates such as A3 must not be used. When necessary, support columns or blocks should be set under the moving formwork to reduce the thickness of the formwork and improve the carrying capacity.

5 The jack is bent, broken or leaking

The quality of the self-made ejector is better, that is, the processing cost is too high. Nowadays, standard parts are generally used, and the quality is worse. If the gap between the ejector pin and the hole is too large, material leakage will occur, but if the clearance is too small, the ejector pin will swell due to the rise of the mold temperature during injection.

What is more dangerous is that sometimes the ejector pin is ejected at a normal distance and breaks. As a result, the exposed ejector pin cannot be reset when the next mold is closed, and the concave mold is damaged. In order to solve this problem, the ejector rod was reground, and a fitting section of 10-15 mm was retained at the front end of the ejector rod, and the middle part was milled 0.2 mm smaller. After the assembly of all ejectors, the fit clearance must be strictly checked, generally within 0.05 ~ 0.08 mm, to ensure that the entire ejection mechanism can move forward and backward freely.

6 Poor cooling or water leaks

The cooling effect of the mold directly affects the quality and production efficiency of the product, such as defects such as poor cooling, large shrinkage of the product, or uneven shrinkage and deformation of the warped surface. On the other hand, the mold is overheated in whole or in part, making the mold unable to be molded normally and stopping production. In severe cases, the movable parts such as the ejector pin are thermally expanded and stuck and damaged. The design and processing of the cooling system depends on the shape of the product. Do not omit this system because of the complex mold structure or processing difficulties, especially for large and medium-sized molds. Full consideration must be given to the cooling problem.

7 The slider is tilted, the reset is not smooth

Some molds are limited by the area of ​​the template and the length of the guide groove is too small. The slider is exposed outside the guide groove after the core pulling operation is completed. This makes it easy to cause the slider to tilt during the core pulling stage and the initial stage of mold clamping reset, especially in the mold clamping. When the slider is not reset smoothly, the slider is damaged or even damaged by bending. According to experience, after the slider completes the core pulling action, the length remaining in the slide groove should not be less than 2/3 of the total length of the guide groove.

8 Failure of the fixed tensioning mechanism

Fixed distance tensioning mechanisms such as pendulum hooks and buckles are generally used in fixed mold core pulling or some secondary demoulding molds. Because these mechanisms are arranged in pairs on both sides of the mold, their actions must be synchronized, that is, The mold is buckled at the same time, and the mold is released to a certain position and unhooked at the same time. Once the synchronization is lost, the template of the drawn mold will inevitably be skewed and damaged. The parts of these mechanisms must have high rigidity and abrasion resistance. It is also difficult to adjust and the life of the mechanism is short. Avoid using it as much as possible and use other mechanisms.

In the case of relatively small core-pulling force, a spring can be used to push the fixed mold. In the case of relatively large core-pulling force, the core can slide when the movable mold is retracted, and the structure of the core-pulling is completed after the core-pulling action is completed. The hydraulic cylinder can be used for core pulling on the mold. The oblique pin slider core pulling mechanism is damaged. The most common faults of this mechanism are that the processing is not in place and the material used is too small. There are two main problems.

 
The oblique pin inclination angle A is large, and the advantage is that a large core pulling distance can be generated in a short mold opening stroke. However, if the inclination angle A is too large, when the pulling force F is a certain value, the bending force P = F / COSA that the oblique pin receives during the core drawing process is larger, and the oblique pin deformation and oblique hole wear are likely to occur. At the same time, the oblique pin produces an upward thrust N = FTGA on the slider. 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. Easy to cause sliding irregularities, guide groove wear. According to experience, the inclination angle A should not be greater than 25.

9 Poor ventilation in injection molds

Gas is often generated in injection molds. What causes it? Air stored in the casting system and mold cavity; some raw materials contain moisture that has not been dried out, and they will vaporize into water vapor at high temperatures. When the temperature is too high, some unstable plastics will decompose and produce gas; some additives in plastic raw materials are volatilized or gases generated by chemical reactions with each other.

At the same time, the cause of poor venting needs to be found out as soon as possible. Poor exhaust of the injection mold will bring a series of hazards to the quality of the plastic parts and other aspects. The main performance is that the melt will replace the gas in the cavity during the injection process. If the gas is not discharged in time, it will cause the melt Filling is difficult, resulting in insufficient injection volume to fill the cavity; poorly excluded gas will form high pressure in the cavity and penetrate into the plastic with a certain degree of compression, causing voids, pores, sparse tissue, and silver streaks, etc. Quality defects.