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These Injection Mold Failures Can Be Excluded
Jun 05, 2018

Core Tip: The structure and processing quality of injection molds directly affect the quality and production efficiency of plastic parts. There are many kinds of failures in the mold production process. The following are solutions to several common failures.

 

    Sprue feeding is difficult: In the injection molding process, the gate sticks in the sprue bushing and 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 cause of 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. There is also the material is too soft, the deformation of the small end of the tapered hole after use for a period of time, and the arc curvature of the nozzle is too small, so that the gate material will 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. If it is necessary to process it by itself, 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.

 

    Damage to the guide post: The guide post mainly plays a guiding role in the injection 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 the following cases, a large lateral offset force will be generated when the injection moves and the fixed mold: (1) When the wall thickness of the plastic part is not uniform, the velocity of the material flow through the thick wall is large, resulting in a large The pressure; (2) Asymmetrical side of the plastic parts, such as the mold of the stepped parting surface, the opposite pressure on the opposite sides is not equal.

 

    Movable, fixed mold offset: large mold, due to the different charge rate, and the impact of the weight of the mold in the mold, resulting in dynamic, fixed mold offset. 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 most convenient and 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 can be finished on the boring bed once. This can guarantee the concentricity of the moving and fixing holes and minimize the verticality error. In addition, the heat treatment hardness of the guide post and guide sleeve must meet the design requirements.

 

    Bending of the movable platen: When the injection mold is injected, the molten plastic in the mold cavity generates a huge back pressure, generally 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-passenger frame 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 set below the moving plate to reduce the thickness of the plates and increase the bearing capacity.

 

    Pole bending, breaking or leaking material: The quality of the self-made ejector rod is better, which means that the processing cost is too high. Normally, standard parts are used now, and the quality is general. 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.

 

    Poor cooling or water leakage: The cooling effect of the injection mold directly affects the quality of the product and the production efficiency, 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.

 

    Fixed-distance tensioning mechanism failure: Fixed-length tensioning mechanisms such as swing hooks and buckles are generally used in fixed-mold core-pulling or some secondary mold-releasing dies, since such mechanisms are set in pairs on both sides of the mold. The action requirements must be synchronized, that is, at the same time the closure of the mold, 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, and the adjustment is also very difficult. The life span of the mechanism is short, and the use of other mechanisms can be avoided. 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 slide when the moving die retreats, and the structure of the die splitting 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, 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°.

 

    The length of the guide channel is too small: some molds are limited by the area of the template, the length of the guide channel is too small, and the slider exposes the outside of the guide slot after the core pulling operation is completed, so that the slider is inclined to tilt during the core pulling phase and the initial stage of mold clamping reset. In particular, when the mold is closed, the slider does not reset smoothly, causing the slider to be damaged or even broken. 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. In the design and manufacture of molds, according to the plastic parts quality requirements, the size of the batch, the requirements of the manufacturing period and other specific circumstances, not only can meet the requirements of the products, but also the most simple and reliable in the mold structure, easy to process, so that the cost is low. It is the perfect mold.