Cracking, including surface cracking, microcracking, whitening, cracking, and due to sticking of parts or runners, or traumatic crisis, cracking and cracking according to the cracking time. Description: The injection molded part breaks when it is ejected or is easily broken or cracked during processing. analyse as below: pvc connectors
1. Injection molding machine injection molding machine plasticizing capacity to be appropriate, too small plasticization can not fully mix and become brittle, will degrade when too large.
2, mold:
(1) Ejection should be balanced. For example, the number of ejector pins and the cross-sectional area should be sufficient. The draft should be sufficient, and the cavity surface should be sufficiently smooth so as to prevent cracking due to the concentration of residual stress caused by the external force. pvc connectors
(2) The structure of the parts shall not be too thin, and the transitional part shall be rounded as far as possible to avoid stress concentration caused by sharp corners and chamfers.
(3) Minimize the use of metal inserts to prevent internal stress from increasing due to the difference in shrinkage between inserts and parts.
(4) Proper mold release air inlets shall be provided for deep-bottom parts to prevent the formation of vacuum negative pressure.
(5) The sprue of the main channel is sufficient enough for the ambassador to be demolded in the future, so that it is easy to demold.
(6) The joining of the main runner bushing to the nozzle should prevent the chilling material from dragging and sticking the workpiece to the fixed mold.
(7) The runners and gates are too small, excessive shear rates occur in the mold fill, full round runners are used and runners and gate sizes are increased to provide acceptable shear rates during the mold fill phase. pvc connectors
3, process aspects:
(1) The injection pressure is too high, the speed is too fast, the more the filling, the injection, and the holding pressure for too long, will cause the internal stress is too large and cracking.
(2) Adjust the mold opening speed and pressure to prevent the mold from cracking due to rapid and strong drawing.
(3) Appropriately increase the mold temperature, limit the speed of the cooling body flowing through the mold, make the product easy to demoulding, plastic degradation in the shot tank, causing the molecular structure of the plastic to rupture, drop the temperature of the material cylinder in all districts, Lower the pressure back and use an exhausted shotgun cylinder to ensure that the drain hole is operating correctly and that each well is set to the correct temperature. pvc connectors
(4) Prevention of cracking due to welding marks, degradation of the plastic due to plastic degradation.
(5) Appropriate use of mold release agents, pay attention to the elimination of aerosols and other substances that adhere to the mold surface.
(6) The residual stress of the part can be reduced by reducing the internal stress by annealing heat treatment immediately after molding.
(7) The melt temperature is too low. Increase the temperature in the rear zone and the nozzle on the shot tank. Reduce the screw speed or high speed to obtain the correct screw surface speed.
(8) The filling speed of the mold is too slow to increase the injection speed; a stable dunnage is maintained on the injection molding machine.
(9) The pressure of the injection molded part is not released properly or is not treated. The injected plastic part is annealed. If it is a nylon plastic product, it is immersed in warm water. pvc connectors
4, materials:
(1) The content of recycled materials is too high, resulting in too low strength of the parts and reducing the amount of recycled materials mixed with new plastic.
(2) Excessive humidity causes chemical reactions between some plastics and water vapor, reducing strength and causing cracking.
(3) The material itself is not suitable for the processing environment or the quality is poor, and the contamination will cause cracking, heterogeneous plastic impurities, check the impurities in the plastic; thoroughly clean the shot tank; remove the hopper or material loader and thoroughly clean
Some molded parts do not have partial crushing, but only have fine cracking on the surface. According to the degree and appearance difference, the more severe cracks or cracks are called, and the minor ones are called micro cracks or cracks. The cracks look like cracks. It resembles, but in essence, it means two kinds of differences, that is, cracks are not void-like defects, because the added stress of the polymer itself is arranged in parallel, so if it is heated, it can return without cracks. In this way, cracks and cracks can be distinguished in this way. Microcracks occur not only after molding but also after placement or contact with solvent vapors and the like. ABS and high-impact polystyrene do not produce this phenomenon at all, but they are expressed in the form of a push rod pushing the top position, and the use of hot air heating can eliminate this phenomenon of whitening. There are two reasons for the above phenomenon. Whitening refers to the fact that the stress is white. From the process of the stress yield curve on the graph to the cracking whiteness, it is necessary to use energy. After whitening, cracks will occur as soon as a little energy is added. pvc connectors
1, demoulding is not smooth
The draft angle of the mold during molding is not enough, and it becomes the draft angle of the draft, or the mold cavity is rough and poorly polished. Therefore, it is difficult to demold the workpiece, so that the pushing force is too large, the stress is generated, and the workpiece is damaged. Or white. This phenomenon is more likely to occur when the main sprue is poorly polished so that the part adheres to the static or dynamic mold and the sidewalls are convex and concave and hard demoulding is used. In short, when this kind of defect occurs, firstly the mold must be polished, and at the same time when the draft angle is increased, an ejector pin is added near the cracked part of the molded part, so that the part is not bent and can be demoulded reasonably. For polymethyl methacrylate (PMMA) molded parts, because the plastic itself is more brittle and the surface requires luster, most of them are formed by chrome plating. However, electroplating has the property of being hard to be plated on the flat surface, but it is easy to place it on the corners; therefore, it becomes an inverted draft angle, so special attention must be paid to it. In addition, because polycarbonate (PC), PVC and other materials easily stick to the chrome plating layer of the mold, especially in the corner as an inverted cone, so pay attention. Polycarbonate (PC) is prone to cracks at the mold inserts. In this case, it is better to add reinforcing glass fibers to the material. A product with residual stress due to the presence of solvents, oils, and medicines will crack (hereinafter referred to as stress cracks), and a small amount of oil will have a lot of cracking effects after being applied. The demoulding slope should be sufficient, the release pins must be balanced, the product should not be designed with acute angles, and the thickness difference of the products should be avoided as much as possible. As long as you carefully observe the location of the crack, you can determine the cause. pvc connectors
2, overfilled
Due to excessive concerns about the formation of shrinkage cavities, as a result, too much molten material is injected into the mold cavity, resulting in a great strain inside the molded article. At this time, the shrinkage becomes very small, and not only is it easy to crack, but also it is easy to cause micro-cracking when the inside is changed for a period of time. To eliminate the cracking of the filling can increase the temperature of the melt, reduce the injection pressure, and increase the temperature of the mold, but as long as the melt is easy to inject into the mold cavity. According to the appearance of molded articles and other related reasons, when molding must be carried out by overfilling, in order to prevent microcracks from occurring, post-processing (such as heat treatment) should be performed after molding, which is effective for eliminating internal strain.
3, insufficient cooling
When finished hard at the end, the parts are ejected, and sometimes the top sheet is cracked or white. This defect can be eliminated by sufficiently cooling or changing the cooling mode of the mold itself. pvc connectors
4, cracking around the insert
When the insertion insert is molded, the stress is significantly concentrated around the insert due to the shrinkage of the plastic. Although this force can firmly hold the insert, when the stress is too large, the plastic around the insert often cracks. When metal parts are embedded in the injection molding, stress is most likely to occur, and cracks are easily generated after a period of time, which is extremely harmful. This is mainly due to the fact that the difference in thermal expansion coefficient between the metal and the resin causes stress, and that, over time, cracks should be generated beyond the strength of the gradually deteriorating resin material. To reduce the cracking around the insert, it is effective to preheat the insert or minimize the shrinkage difference, and the effect of plasticizing treatment is more obvious (ie, when the metal insert is preheated, the residual stress during forming can be alleviated. The same principle is used. Annealing after molding instead of insert heating is also a method). pvc connectors
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