Cracking is a common defect in the manufacture of plastic products by plastic product manufacturers. The main cause is cracking due to stress and deformation. There are mainly residual stress, external stress and stress deformation caused by the external environment.
Cracks caused by residual stress
Residual stress is mainly caused by the following three conditions, namely overfilling, ejection from the mold and metal inserts. As the cracks generated in the case of overfilling, the solutions can be started in the following aspects:
(1) Since the pressure loss of the direct gate is the smallest, if the cracks are mainly generated near the direct gate, the multi-point distributed point gate, side gate, and shank gate method can be considered instead.
(2) On the premise of ensuring that the resin does not decompose and not deteriorate, appropriately increasing the resin temperature can reduce the melt viscosity and improve the fluidity, and at the same time, the injection pressure can be reduced to reduce stress.
(3) In general, stress is easy to occur when the mold temperature is low, and the temperature should be appropriately increased. However, when the injection speed is high, even if the mold temperature is lower, the generation of stress can be reduced.
(4) If the injection and dwell time is too long, stress will be generated. It is better to shorten it properly or perform th dwell pressure switch.
(5) Non-crystalline resins, such as AS resins, ABS resins, and PMMA resins, are more likely to cause residual stress than crystalline resins, such as polyethylene and polyoxymethylene, which should be taken into consideration.
When ejecting from the mold, the ejection force is too large and the stress is generated due to the small mold release gradient, rough mold glue and convex mold, and sometimes even whitening or cracking occurs around the ejection rod. Just look closely at the location of the crack to determine the cause.
When inserting metal parts at the same time as injection molding, stress is most likely to occur, and cracks easily occur after a period of time, which is extremely harmful. This is mainly due to the large difference in thermal expansion coefficient between metal and resin, which generates stress, and with time, the stress exceeds the strength of the resin material that is gradually degraded and cracks occur. In order to prevent the resulting cracking, as a rule of thumb, the wall thickness is 7 ”and the outer diameter of the embedded metal
General-purpose polystyrene is basically unsuitable for inserts, and the impact of inserts on nylon is minimal. Since the glass fiber reinforced resin material has a small thermal expansion coefficient, it is more suitable for inserts.
In addition, pre-heating the metal insert before molding has a better effect.