Analyze the causes of cracks in Plastic injection mold products
Jul 30, 2021
In the process of producing injection molded products, plastic molds are prone to some defects, such as cracking, which is a relatively common problem. Why is there a cracking phenomenon? Today I will give you an answer.
Cracking is a common defect in plastic products of injection molding manufacturers, and the main cause is due to stress and deformation. There are mainly residual stress, external stress and stress deformation caused by the external environment.
1 Cracks caused by residual stress
The residual stress is mainly caused by the following three conditions, namely, overfilling, ejection from the mold, and metal inserts. As the cracks produced in the case of overfilling, the solution can be mainly in the following aspects:
(1) Since the pressure loss of the sprue is small, if the cracks are mainly generated near the sprue, you can consider switching to the multi-point distribution point gate, side gate and shank gate method.
(2) On the premise of ensuring that the resin does not decompose or deteriorate, appropriately increasing the resin temperature can reduce the melt viscosity and improve the fluidity, and at the same time, it can also reduce the injection pressure to reduce the stress.
(3) Under normal circumstances, the mold temperature is easy to produce stress when the mold temperature is low, so the temperature should be increased appropriately. But when the injection speed is higher, even if the mold temperature is lower, the stress generation can be reduced.
(4) Excessive injection and holding pressure time will also produce stress, shorten it appropriately or perform several holding pressure switching effects
(5) Non-crystalline resins, such as AS resins, ABS resins, PMMA resins, etc. are more likely to produce residual stress than crystalline resins such as polyethylene and polyoxymethylene, so attention should be paid.
When demolding and pushing out, due to the small demolding inclination, the mold rubber and the roughness of the punch, the pushing force is too large, causing stress, and sometimes even whitening or cracking occurs around the pushing rod. As long as you carefully observe the location of the cracks, you can determine the cause.
When inserting metal parts during injection molding, stress is relatively easy to produce, and it is easy to produce cracks after a period of time, which is extremely harmful. This is mainly due to the large difference between the thermal expansion coefficients of metal and resin, which produces stress, and over time, the stress exceeds the strength of the gradually degraded resin material to cause cracks. In order to prevent the resulting cracks, Dongguan Machke injection molding manufacturer, as an experience, the wall thickness of 7" and the outer diameter of the embedded metal parts are basically not suitable for inserts, and the impact of inserts on nylon Relatively small. Because glass fiber reinforced resin material has a small thermal expansion coefficient, it is more suitable for inserts.
In addition, preheating the metal insert before molding also has a good effect.
2 Cracks caused by external stress
The external stress here is mainly due to stress concentration caused by unreasonable design, especially at sharp corners.
3 Cracks caused by the external environment
Chemicals, water degradation caused by moisture absorption, and excessive use of recycled materials will deteriorate physical properties and cause cracks.
