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How to solve the problem of cracking of injection molding parts

Author:Editor ComeFrom:Internet Date:2017-12-15 14:46:14 Hits:1010
     Cracking is a common defect in plastic products, and the main reason is due to stress and deformation. The stress and deformation produced mainly by residual stress, external stress and external environment.
     1. the crack caused by residual stress
The residual stress is mainly due to the following three cases, that is, overfilling, release of mould and metal inlay. As a fissures produced in the case of overfilling, the solution is mainly to start with the following aspects:
     (1) because of the minimum pressure loss of straight gate, if the crack is mostly located near the gate, it can be considered to switch to multi point distribution gate, side gate and handle gate.
      (2) under the premise of ensuring that the resin is not decomposed or deteriorated, increasing resin temperature can reduce melt viscosity and improve fluidity, and at the same time, reduce injection pressure to reduce stress.
      (3) in general, the stress is easily produced when the temperature is low, and the temperature should be raised properly. But when the injection speed is high, even if the mold temperature is lower, the stress can be reduced.
      (4) the stress can be produced by the injection and the long holding time, and it is better to shorten or carry out the Th holding pressure switching.
      (5) non crystalline resins, such as AS resin, ABS resin and PMMA resin, are easier to produce residual stresses than crystalline resins such as polyethylene and polyformaldehyde, which should be paid attention to.
      2.crack caused by external stress
     The external stress here is mainly due to the unreasonable design of the stress concentration, especially at the sharp corner.
      3.the crack caused by the external environment
      Chemical drugs, water absorption caused by water degradation, as well as the excessive use of recycled materials will cause deterioration of physical properties and crack.
        When ejection is launched, the release force is too small due to the small release angle, the mold glue and the rough die, which causes stress. Sometimes it even produces whitening or cracking around the bar. The reason can be determined by carefully observing the location of the crack.
        When inserting metal parts at the same time of injection molding, it is the most easy to produce stress, and it is easy to produce crack after a period of time, and it is very harmful. This is mainly due to the great difference of thermal expansion coefficient between metal and resin, and stress. And over time, the stress is more than the strength of deteriorated resin. In order to prevent the resulting cracking, as an experience, the wall thickness is 7 "and the outer diameter of the embedded metal parts.
        The general polystyrene is not suitable for adding inlay, but the inlay has the smallest effect on the nylon. Because the thermal expansion coefficient of glass fiber reinforced resin is smaller, it is more suitable for the embedded parts.
         Before molding, the metal insert is preheated, and it has good effect.