Cracking is a common defect in plastic products, which is mainly caused by stress deformation.There are mainly residual stress, external stress and stress deformation caused by external environment.
(-) cracking caused by residual stress
The residual stress is mainly caused by the following three conditions, namely overfilling, demoulding and metal Mosaic.As the cracking caused by overfilling, its solution can be mainly carried out in the following aspects:
(1) as the pressure loss of the straight gate is the smallest, if the cracks mainly occur near the straight gate, the multi-point distributed gate, side gate and shank gate can be adopted.
(2) on the premise of ensuring that the resin does not decompose and deteriorate, properly raising the temperature of the resin can reduce the melt viscosity and improve the fluidity, and also reduce the injection pressure to reduce the stress.
(3) generally, when the mold temperature is low, it is easy to generate stress, and the temperature should be increased appropriately.However, when the injection speed is higher, even if the mold temperature is lower, the stress can be reduced.
(4) too long time of injection and pressure preservation will also generate stress, which will be shortened appropriately or the effect of Th time pressure preservation switching is better.
(5) non-crystalline resins, such AS AS AS resins, ABS resins and PMMA resins, are more likely to generate residual stress than crystalline resins such AS polyethylene and polyformaldehyde, which should be noted.
When the demoulding is launched, because of the small inclination of the demoulding mould, the mould glue and the convex mould are rough, the ejection force is too large and the stress is generated, sometimes even around the ejection bar, the phenomenon of whitening or cracking occurs.The cause can be determined by careful observation of the location of the cracks.
When metal parts are embedded in injection molding, stress is most likely to occur, and cracks tend to occur after a period of time, causing great harm.This is mainly due to the difference in thermal expansion coefficient between metal and resin, and over time, the stress exceeds the strength of the gradually degraded resin material, resulting in cracks.In order to prevent the resulting crack, as an experience, the wall thickness 7 "and the external diameter of the embedded metal parts
General purpose polystyrene is generally not suitable for inlay pieces, and inlay pieces have the least effect on nylon.Because the thermal expansion coefficient of glass fiber reinforced resin material is small, it is more suitable for embedding parts.
In addition, preheating metal inserts before forming also has good effect.
Cracking caused by external stress
The external stress is mainly caused by the unreasonable design, especially at the sharp Angle.It can be seen from figure 2-2 that R / 7 "1 0" is preferable.5 to 0.7.
(3) cracking caused by external environment
Chemical drugs, water degradation due to moisture absorption, and excessive use of recycled materials will degrade the material quality and cause cracks.