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What Happened To These Injection Mold Failures
Jun 11, 2018

Core Tips: 1. Difficulties in the discharge from the gate 2. Large mold movements and mold deflection 3. Guide column damage 4. Dynamic plate bending 5. Top rod bent, broken or missing material 6. Poor cooling or water leakage 7. Slider Tilt, reset is not smooth 8. Fixed distance tension mechanism failure 9. Exhaust in the injection mold

 

 1. Difficulties in sprue feeding

 

    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 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. 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 curvature of the nozzle's spherical 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. If it is necessary to process it by itself, special reamers should be made or purchased. Taper holes need to be ground to Ra0.4 or more. In addition, you must set the gate pull bar or the top of the gate.

 

    2. Large-scale die movement

 

    Due to the difference in the rate of the anisotropic charging of the large mold and the impact of the weight of the mold during mold mounting, the movement of the mold and the displacement of the fixed mold are generated. In the above cases, the lateral offset force during injection will be added to the guide post. 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 of utmost importance. In the machining, the clamping is performed after aligning the movable and fixed molds, and the boring is completed once on the boring bed so that the concentricity of the movable and fixed mold holes can be guaranteed. Minimize the squareness error. In addition, the heat treatment hardness of the guide post and guide sleeve must meet the design requirements.

 

    3 guide pillar damage

 

    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 a few cases, the injection mold will generate a large lateral deflection force when the mold is injected. 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 greater pressure; the side of the plastic part is asymmetric, such as the back pressure on the opposite sides of the mold of the stepped parting surface. not equal.

 

    4. Dynamic template bending

 

    When the mold is injected, the molten plastic in the mold cavity generates a huge back pressure, which is generally 600-1000 kg/cm. 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.

 

    5. Top rod bent, broken or missing material

 

    The quality of the self-made ejector rod is better, that is, the processing cost is too high. Now the standard parts are generally used and the quality is worse. 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 uneven shrinkage and defects such as warping surface 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. It is not necessary to dispense with this system because of complicated mold structures or difficult processing. In particular, large and medium-sized molds must fully consider the cooling problem.

 

    7. The slider tilts and the reset is not smooth

 

    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 stage of mold clamping reset, 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.

 

    8. 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, 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 organization are that the processing is not in place and the materials used are too small. There are mainly two problems.

 

    The inclination angle A of the oblique pin is large, and the advantage is that a relatively large core pitch can be produced within a short opening stroke. 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 dip angle A should not exceed 25.

 

    9. The injection mold die exhausted

 

    Gases are often generated in injection molds. What causes this? The air in the casting system and mold cavity; Some raw materials contain moisture that has not been removed by drying, and they vaporize into steam at high temperatures; When the temperature is too high, certain unstable plastics will decompose to generate gas; some of the additives in plastic raw materials volatilize or react with each other to generate gas.

 

    At the same time, the reason for poor exhaust is also needed to find out as soon as possible. The poor exhaust of the injection mold will bring a series of harms to the quality of the plastic parts. 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 melt. Difficult to fill, resulting in insufficient injection volume and can not fill the cavity; the exclusion of poor gas will form a high pressure in the cavity, and infiltrated into the plastic within a certain degree of compression, causing voids, stomata, tissue sparse and silver lines, etc. Defects in quality; As the gas is highly compressed, the temperature in the cavity rises sharply, causing the surrounding melt to decompose and burn, causing partial carbonization and scorching of the plastic parts. It occurs mainly at the junction of the two melts, at the gate flange; the gas is not removed smoothly, and the melt velocity entering the various cavities is different. Therefore, flow marks and weld marks are easy to form, and the mechanics of the plastic parts are made. Reduced performance; due to the obstruction of gas in the cavity, the filling rate will be reduced, affecting the molding cycle, and reducing the taxation efficiency.

 

    The distribution of air bubbles in plastic parts and the bubbles generated by the accumulation of air in the mold cavity are often distributed on the parts opposite to the gate; the bubbles generated by decomposition or chemical reaction in the plastic raw materials are distributed along the thickness of the plastic parts; plastic raw materials Air bubbles produced by gasification of residual water are irregularly distributed throughout the plastic part.