1. Difficulty in gate stripping
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 product may be damaged by cracks. In addition, the operator must use the tip of the copper rod to knock it out from the nozzle to loosen it before demoulding, which seriously affects production efficiency.
The main reasons for this kind of failure are: the smoothness of the gate taper hole is poor, and there are knife marks in the circumferential direction of the inner hole; secondly, 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 The rivet head is created 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, a gate pull rod or a gate ejection mechanism must be installed.
2. Large mold dynamic and fixed mold offset
Large molds have different filling rates in all directions and are affected by the mold's own weight during mold loading, resulting in dynamic and fixed mold offsets. In the above situations, the lateral offset force will be added to the guide pillar during injection, and 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 are added to the parting surface of the mold, one on each side. The simplest and most effective one is to use cylindrical keys. The perpendicularity between the guide post hole and the parting surface is crucial. During processing, the movable and fixed molds are aligned and clamped, and then boring is completed in one go on the boring machine. This ensures the concentricity of the movable and fixed mold holes and minimizes the verticality error. 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, during injection, the movable and fixed molds will generate huge lateral deflection forces. 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, generating greater pressure here; the sides of the plastic part are asymmetrical, such as a mold with a stepped parting surface, and the opposite sides are subjected to reaction. The pressure is not equal.
4.Dynamic template bending
When the mold is being injected, the molten plastic in the mold cavity generates huge back pressure, generally 600-1000 kg/cm. Mold makers sometimes do not pay attention to this problem, often changing the original design dimensions, or replacing the movable template with low-strength steel plates. In molds that use ejector pins to push materials, due to the large span of the seats on both sides, the template will bend downward during injection. Therefore, the movable formwork must be made of 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 rod is bent, broken or leaking material
The quality of the self-made ejector is better, but 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 gap is too small, the ejector pin will expand and become stuck due to the increase in mold temperature during injection. What is even more dangerous is that sometimes the ejector pin cannot be pushed out and breaks after being pushed out for a certain distance. As a result, when the mold is closed next time, the exposed ejector pin cannot be reset and the die is damaged.
In order to solve this problem, the ejector pin should be reground, leaving a 10-15 mm matching section at the front end of the ejector pin, and grinding the middle part smaller by 0.2 mm. After all ejector pins are assembled, their matching clearance must be strictly checked, generally within 0.05-0.08 mm, to ensure that the entire ejector mechanism can move forward and backward freely.
6. Poor cooling or water channel leakage
The cooling effect of the mold directly affects the quality and production efficiency of the product. For example, if the cooling is poor, the product will shrink greatly, or the shrinkage will be uneven, causing defects such as warping and deformation; on the other hand, the entire or partial overheating of the mold will prevent the mold from forming normally and stop production. In severe cases, the ejector pin and other movable parts may become stuck due to thermal expansion. And 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, the cooling issue must be fully considered.
7. The slider is tilted and the reset is not smooth.
In some molds, due to the limited area of the template, the length of the guide groove is too small, and the slider is exposed outside the guide groove after the core pulling action is completed. In this way, the slider is easily tilted in the post-core pulling stage and the initial stage of mold closing and reset, especially during closing. When 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 tensioning mechanism fails
Fixed-distance tensioning mechanisms such as swing hooks and buckles are generally used in fixed mold core pulling or some secondary demoulding molds. Since such mechanisms are set up in pairs on both sides of the mold, their actions must be synchronized. That is, the mold is closed and buckled at the same time, and the mold is opened to a certain position and decoupled at the same time. Once the synchronization is lost, the template of the pulled mold will be skewed and damaged. The parts of these mechanisms must have high rigidity and wear resistance, and are difficult to adjust. The life of the mechanism is short, so avoid using them as much as possible. You can use other mechanisms instead.
When the core-pulling force is relatively small, a spring can be used to push out the fixed mold; when the core-pulling force is relatively large, a structure can be adopted in which the core slides when the movable mold retreats, and the core-pulling action is completed first and then the mold is separated; On large molds, hydraulic cylinders can be used to pull the core.
The oblique pin slider core-pulling mechanism is damaged. The most common problems with this mechanism are that the processing is not in place and the materials used are too small. There are two main problems: the oblique pin inclination angle A is large. The advantage is that it can produce larger molding strokes in a shorter mold opening stroke. Core pulling distance. However, if the inclination angle A is too large, when the extraction force F is a certain value, the bending force P=F/COSA on the oblique pin will become larger during the core pulling process, 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 frictional resistance when the slider slides, easily causing slippage. Not smooth, the guide groove is worn. According to experience, the inclination angle A should not be greater than 25 degrees.
9. Poor exhaust in the injection mold
Gas is often produced in injection molds. What causes it?
(1) Air existing in the pouring system and mold cavity;
(2) Some raw materials contain moisture that has not been removed by drying, which will vaporize into water vapor at high temperatures;
(3) Because the temperature is too high during injection molding, some unstable plastics will decompose and produce gas;
(4) Gases generated by volatilization of certain additives in plastic raw materials or chemical reactions with each other.
At the same time, the cause of poor exhaust also needs to be found out as soon as possible. Poor exhaust of injection molds will bring a series of hazards to the quality of plastic parts and many other aspects, mainly as follows:
(1) During the injection molding process, the melt will replace the gas in the cavity. If the gas is not discharged in time, it will cause difficulty in filling the melt, resulting in insufficient injection volume to fill the cavity;
(2) Unsmooth gas removal will form high pressure in the mold cavity, and penetrate into the interior of the plastic under a certain degree of compression, causing quality defects such as cavities, pores, loose tissue, and silver streaks;
(3) Because the gas is highly compressed, the temperature in the mold cavity rises sharply, which in turn causes the surrounding melt to decompose and burn, causing local carbonization and scorching of the plastic parts. It mainly appears at the confluence of two melts and at the gate flange;
(4) Poor gas removal results in different melt speeds entering each cavity. Therefore, flow marks and fusion marks are easily formed, and the mechanical properties of the plastic parts are reduced;
(5) Due to the obstruction of gas in the cavity, the mold filling speed will be reduced, the molding cycle will be affected, and the molding efficiency will be reduced.
In plastic parts, the main distribution of bubbles is:
(1) Bubbles generated by air accumulated in the mold cavity are often distributed in the parts opposite to the gate;
(2) Bubbles produced by decomposition or chemical reaction in plastic raw materials are distributed along the thickness of the plastic part;
(3) The bubbles generated by the vaporization of residual water in the plastic raw materials are irregularly distributed throughout the entire plastic part.
