Discussion on the causes of frequent problems in injection molds

1. It is difficult to remove material from the gate

During the injection molding process, the gate is stuck in the gate sleeve and is not easy to come out. When opening the mold, the finished product was damaged by cracks. In addition, the operator must use the top end of the copper rod to knock it out from the nozzle to loosen it before demoulding, which seriously affects the production efficiency.

The main factors of this defect are poor brightness of the gate taper hole and 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, and the spherical curvature of the nozzle is too small, causing the gate material to form a rivet here. The taper hole of the sprue sleeve 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 purchase a special reamer. The tapered hole needs to be ground to Ra0.4 or above. In addition, it is necessary to set up the gate pull rod or gate ejection.

2. Large mold moving and fixed mold offset

Large molds may have dynamic and fixed mold offsets due to different filling rates in each direction and the influence of the mold's own weight during mold loading. In the above situations, the lateral deflection force will be added to the guide pillar during injection. The surface of the guide pillar will be roughened and damaged during mold opening. In severe cases, the guide pillar will be bent or cut off, and the mold may even be unable to be opened.

In order to solve the above problems, high-strength positioning keys, one on each side of the mold parting surface, are added. The simplest and most effective one is to use cylindrical keys. The straightness of the guide pillar hole and the parting surface is crucial. During processing, the movable and fixed molds are aligned and clamped, and then boring is completed on the boring machine at one time. This ensures the concentricity of the movable and fixed mold holes, and Minimize straightness errors. In addition, the heat treatment hardness of the guide posts and guide bushes must meet the design requirements.

3. Damage to guide pillar

The guide pillar mainly plays a guiding role in the mold to ensure that the molding surface of the core and the cavity do not collide with each other under any circumstances. The guide pillar cannot be used as a force-bearing part or a positioning part.

In several cases, infinite lateral deflection forces will occur in the moving and fixed molds during injection. When the wall thickness of the plastic part is required to be uneven, the material flow passes through the thick wall at a high speed and generates greater pressure here; the sides of the plastic part are asymmetrical, such as the pressure on the opposite sides of the mold with a stepped parting surface. The back pressures are not equal.

4. Moving template twists and turns

When the mold is injected, the molten plastic in the mold cavity generates infinite back pressure, usually 600~1000 kg/cm. Mold makers sometimes do not pay attention to this problem, and often change the original design specifications, or replace the movable template with low-strength steel plates. In molds that use ejector pins to push materials, due to the large span of the two seats, the template will bend downward during injection.

Therefore, the movable formwork must use high-quality steel with sufficient thickness. Low-strength steel plates such as A3 must not be used. When necessary, support columns or support blocks should be set under the movable formwork to reduce the thickness of the formwork and improve the load-bearing capacity.

5. The ejector pin is bent, cracked or leaking material
The quality of homemade ejector pins is better, but the processing cost is too high. Nowadays, standard parts are usually used, and the quality is worse. If the gap between the ejector pin and the hole is too large, material leakage will occur. However, if the gap is too small, the ejector pin will expand and get stuck due to the increase in mold temperature during injection.
What's more dangerous is that sometimes the ejector pin cannot be pushed out and breaks after being pushed out for a short distance. When the mold is closed once, the exposed ejector pin cannot be reset and the die is damaged. In order to solve this problem, the ejector pin was reground and a 10~15 mm cooperation section was retained at the front end of the ejector pin, and the base part was ground 0.2 mm smaller. After all ejector pins are installed, the cooperation gap must be strictly checked, usually within 0.05~0.08 mm, to ensure that the entire ejector mechanism can move forward and retreat freely.

 6. Poor cooling or water channel leakage
The cooling effect of the mold directly affects the quality and production efficiency of the finished product, such as poor cooling, large shrinkage of the finished product, or uneven shrinkage resulting in warping and deformation. On the other hand, the whole or part of the mold will be overheated, which will prevent the mold from forming normally and stop production. In severe cases, the ejector pin and other moving parts will be stuck due to thermal expansion and be damaged.
The design and processing of the cooling system depend on the shape of the product. Do not omit this system because the mold structure is complex or the processing is difficult. Especially for large and medium-sized molds, cooling issues must be fully considered.

7. The slider is tilted and the reset is not smooth.
Some molds are limited by the template area and the length of the guide groove is too small. The slider is exposed outside the guide groove after the core pulling action is completed. This easily causes the slider to tilt during the post-core pulling period and the initial mold closing reset period, especially during closing. When injection molding, the slider does not reset smoothly, causing the slider to be damaged or even damaged due to 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 entire length of the guide groove.

8. The fixed distance tightening arrangement fails
Fixed-distance tightening devices such as swing hooks and buckles are usually used in fixed mold core pulling or some secondary demoulding molds. Since these devices are arranged in pairs on both sides of the mold, their action requirements must be synchronized. , that is, the mold is buckled together when the mold is closed, and the mold is unhooked at the same time when the mold is opened to a certain position.
Once synchronization is lost, the template of the pulled mold will inevitably be tilted and damaged. The parts of these devices must have high stiffness and wear resistance, and adjustment is also very difficult. The device life is short, so avoid using it as much as possible. You can use another one. arrange.
When the core-pulling force is relatively small, the spring can be used to push out the fixed mold. When the core-pulling force is relatively large, the core can be slid when the movable mold retreats, and the core-pulling action is completed first and then the mold is separated. Hydraulic cylinder core pulling can be used on large molds. The oblique pin slider core-pulling mechanism is damaged.
The most common shortcomings of this kind of design are that the processing is not in place and the materials used are too small. There are two main problems:
The advantage of the large inclination angle A of the bevel pin is that it can produce a relatively large core pulling distance in a short mold opening stroke. However, if the inclination angle A is too large, when the extraction force F is a certain value, the bending force P=F/COSA encountered by the oblique pin during the core pulling process will become larger, and the deformation of the oblique pin and the wear of the oblique hole will easily occur.
At the same time, the upward thrust N=FTGA generated by the oblique pin on the slider is greater. This force increases the positive pressure of the slider on the guide surface in the guide groove, thereby increasing the resistance when the slider slides. 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.

9. Poor exhaust in the injection mold

Gas often occurs in injection molds. What causes this?
The air existing in the pouring system and mold cavity; some materials are rich in moisture that has not been removed by drying, which will vaporize into water vapor at high temperatures; because the temperature is too high during injection molding, some plastics with unstable properties will produce Gases are generated by decomposition; gases generated by the evaporation of certain additives in plastic materials or chemical reactions with each other.
At the same time, the cause of poor exhaust also needs to be found out quickly. Poor exhaust of the injection mold will bring a series of damage to the quality of plastic parts and many other aspects. The main manifestation is: during the injection molding process, the melt will replace the gas in the cavity. If the gas is not discharged in time, a melt will be formed. Filling is difficult, resulting in insufficient injection volume and failure to fill the cavity; poorly cleared air will form high pressure in the cavity, and enter the interior of the plastic at a certain degree of compression, forming voids, pores, tissue sparseness, and silver streaks, etc. Quality shortcomings;
Because the gas is highly compressed, the temperature in the mold cavity rises sharply, which in turn causes the surrounding melt to decompose and bake, causing some carbonization and scorching of the plastic parts. It mainly appears at the confluence of two melts and at the gate flange; the removal of gas is not smooth, causing the melt speed entering each cavity to be different, so it is easy to form movement marks and fusion marks, and make the plastic parts The mechanical properties are reduced; due to the obstruction of gas in the cavity, the mold filling speed will be reduced, the molding cycle will be affected, and the tax efficiency will be reduced.
The distribution of bubbles in plastic parts, the bubbles generated by the accumulation of air in the mold cavity, are often distributed at the position opposite to the gate; the bubbles generated by decomposition or chemical reaction in the plastic material are distributed along the thickness of the plastic part; in the plastic material The bubbles generated by the vaporization of residual water are scattered irregularly on the entire plastic part.