The wet cusp effect of plastic mold often occurs when the mold manufacturer designs the mold. The so-called mid-cusp effect mainly refers to the concentration of stress at the cusp when the angle between two or three surfaces is less than 120 degrees. When the stress reaches a certain level, the cusp is the most prone place to crack. Under the action of a continuous stress, the crack expands continuously and finally leads to the fracture or cracking of the finished product; how to avoid the design of the die? What about the crack effect?
On the premise of guaranteeing the normal mold release, injection processing should be as simple as possible and easy to demould, avoiding or reducing various core-pulling mechanisms around the mold as far as possible. This can not only greatly simplify the structure of the mold, but also facilitate the injection molding of the mold and improve the production efficiency.
In the process of making the die, the stress concentration at the corner of the part is avoided as much as possible, and the strength of the part is increased to ensure the stress concentration at the corner of the part, so as to reduce the rupture of the vibration when the product is subjected to re-stress or impact. If the forming conditions are inappropriate or the structure of the part is unreasonable, great internal stress will be generated, which is particularly prone to stress cracking. When the radius of the corner is less than 0.3 times the wall thickness of the part, the stress concentration increases sharply. When the radius is greater than 0.8 times the wall thickness, the stress concentration decreases obviously.
Only fillets can be easily filled and demoulded. For some plastics with poor fluidity or filled plastics, fillers are particularly important in part design, which can not only improve the performance of filling, but also improve the performance of parts.
Round corners are conducive to the manufacture of various dies. In order to improve the strength of dies, various rounded corners are designed and the corresponding parts of the dies should also be rounded corners, which increases the firmness of the dies. When quenching or using, the dies will not crack because of the concentration of stress, thus increasing the strength of the demoulding of the dies.
Round corners are one of the best ways to avoid the sharp corner effect in injection mould design.
In order to meet the different physical and mechanical properties of the machine in use. The main requirements are compact products, strong welding, high strength and low internal stress. But in our daily injection moulding or test process, even if the process parameters are adjusted properly, the products will often appear various phenomena, such as insufficient filling, high internal stress, surface streamline and so on. The main reason is that the exhaust gas of the mould has problems.
When the injection moulds are processed, the gases produced in the moulds mainly come from the following sources and sources: air existing in the cavity and the pouring system, water vapor evaporated from the water contained in the plastic raw materials at the injection temperature, and gas produced by the decomposition of the plastics at high temperatures. At the same time, in order to ensure the complete demoulding and reduce the forming defects of the products in the forming process, we must also pay attention to the design of the system for discharging excess gas from the die.
In the process of injection moulding, if the exhaust of the mould is not good, the gas in the cavity will be compressed to produce a great back pressure, which greatly hinders the normal and rapid filling of the plastic melt and makes injection difficult. At the same time, it will force the pressure to increase continuously, the holding time to increase, the forming cycle to prolong, and the efficiency of the production of injection moulding parts will also continue to decrease. Sometimes corrosive gases form fouling on the surface of the cavity, which reduces the service life of the die under normal use.
In order to keep the fluidity of melted plastics in the optimum state during the filling process of injection moulding machine plastics, it is necessary to increase the temperature and injection pressure of melted plastics, but because the residual stress of such products will increase with the pressure, the warpage around the products and the possibility of plastic cracking will also increase. For example, under normal exhaust conditions, injection speed can be increased, filling and holding pressure can easily reach the best state, without additional increase in the temperature of the barrel and nozzle. In this way, the residual stress of the product is small and the warpage deformation is small.
We also need to consider how to exhaust during injection processing. A good exhaust system will improve the quality of plastic parts.
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