1, difficult to discharge the gate
In the injection molding process, the gate is stuck in the sprue bushing and it is not easy to escape. When the mold is opened, the product is cracked. In addition, the operator must use the tip of the copper rod to knock out from the nozzle, make it loose and release it, which will seriously affect the production efficiency.
The main reason for this failure is the poor finish of the tapered hole in the gate, and the tool hole in the circumferential direction of the inner hole. The second is that the material is too soft. After a period of use, the small end of the tapered hole is deformed or damaged, and the arc of the nozzle surface is too small, causing the gate material to produce a rivet at this point. The taper hole of the sprue bushing is more difficult to process. Standard parts should be used as far as possible. For self-processing, special reamers should be made or purchased. Taper holes need to be ground to below Ra0.4. In addition, a gate puller or gate ejector must be provided.
2. Damage to guide posts
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 touch each other in any case, and the guide post cannot be used as a force member or a positioning member. In the following cases, a large lateral offset force will be generated when the injection is performed and the mold is fixed:
(1) When the wall thickness of the plastic part is not uniform, the velocity of the material flow through the thick wall is large, and a large pressure is generated there;
(2) The side of the plastic part is asymmetrical, such as the mold of the stepped parting surface, the opposing pressures on the opposite sides are not equal.
3. Movable, fixed mode offset
Large-size molds have dynamic and fixed mold offsets due to different loading rates of the anisotropic material and the influence of the mold's own weight during mold mounting. In these cases, the lateral offset force will be applied to the guide post during injection. When the mold is opened, the surface of the guide post is pulled and damaged. In severe cases, the guide post is bent or cut off, and it is impossible to open the mold.
In order to solve the above problems, a high-strength positioning key is added on each side of the mold parting surface. The simplest and most effective method is to use a cylindrical key. The verticality of the guide post hole and the parting surface is crucial. In the processing, it adopts motion. After the clamping position of the fixed mold is clamped, it is finished on the trampoline one time. This can ensure the concentricity of the moving and fixed hole and minimize the verticality error. In addition, the heat treatment hardness of the guide post and guide sleeve must meet the design requirements.
4. Dynamic template bending
When the mold is injected, the molten plastic in the mold cavity generates a tremendous back pressure, typically 600 to 1000 kg/cm2. Moldmakers sometimes do not pay attention to this problem, often changing the original design size, or replace the moving plate with a low-strength steel plate. In a die with a tipping material, the two-side seat has a large span, causing the template to bend under injection. Therefore, the moving plate must be made of high-quality steel, and must have sufficient thickness. It is not possible to cut low-strength steel plates such as A3. When necessary, support columns or support blocks should be installed below the moving plate to reduce the thickness of the plates and increase the bearing capacity.
5. Pole bending, fracture or leakage
The quality of the self-made ejector rod is better, which means that the processing cost is too high. Normally, standard parts are usually used and the quality is generally good. If the clearance between the crowbar and the hole is too large, leakage occurs, but if the clearance is too small, the ram swells and seizes due to the increase in mold temperature during the injection. What is even more dangerous is that sometimes the top bar is broken at its normal distance from the top and the result is that the exposed top bar cannot be reset and hit the die at the next clamping.
In order to solve this problem, the top bar is reground, leaving a 10 to 15 mm mating section at the front end of the jack, with the center section being 0.2 mm. After assembly, all ejector rods must be strictly checked for fit clearance, generally within 0.05 to 0.08 mm, to ensure that the entire ejector mechanism can move freely.
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, large shrinkage of the product, or non-uniform shrinkage and warping deformation. On the other hand, the whole or partial overheating of the mold may cause the mold to not be properly formed and be discontinued. In severe cases, the movable parts such as the ejector rods may be blocked by thermal expansion. The design of the cooling system depends on the shape of the product. The system should not be omitted due to the complexity of the mold structure or processing difficulties. In particular, large and medium-sized molds must fully consider the cooling problem.
7. Fixed distance tension mechanism failure
Fixed-length tensioning mechanisms such as hooks and buckles are generally used for fixed-mold core-pulling or some secondary mold-releasing dies. As such mechanisms are set in pairs on both sides of the mold, their motion requirements must be synchronized. Simultaneous mold clamping at the same time, open the mold to a certain position at the same time unhooking. Once the synchronization is lost, it will inevitably cause the template of the die to be worn out and be damaged. The parts of these mechanisms must have high rigidity and wear resistance, the adjustment is also very difficult, the life span of the mechanism is short, and the use of other mechanisms can be avoided as far as possible. In the case where the pulling force is relatively small, the spring can be used to push the fixed die, and when the core pulling force is relatively large, the core can be slipped when the movable die retreats, and the structure of the split die can be completed after the core pulling operation is performed. Hydraulic cylinders can be used to pull cores on the mold. The oblique pin slider core-pulling mechanism is damaged.
The most common problems with this type of mechanism are that the processing is not in place and the material used is too small. There are mainly two problems: the large inclination angle A of the oblique pin, and the advantage is that a relatively large core can be produced within a short opening stroke. distance. However, when a large inclination angle A is adopted, when the drawing force F is a certain value, the bending force received by the oblique pin in the core pulling process is P=F/COSA, and the oblique pin deformation and oblique hole wear are likely to occur. At the same time, the oblique pin generates an upward thrust force on the slider N=FTGA, which increases the positive pressure of the slider on the guide surface in the guide groove, thereby increasing the frictional resistance when the slider slides. It is easy to slip and the guide groove is worn. According to experience, the angle of inclination A should not exceed 25°.
8. The guide slot length is too small
Some molds are limited by the area of the template, the length of the guide groove is too small, and the slider exposes the outside of the guide groove after the core pulling operation is completed, so that the slider tilt is easily caused in the post-pulling core stage and the initial mold clamping reset stage, especially in the mold clamping. When the slider is not smoothly reset, the slider is damaged and even bent. According to experience, the length of stay in the chute should not be less than 2/3 of the total length of the chute after the slider has completed the core pulling action.
