Cracking often occurs in the molding process of plastic molds, which is a common defect in plastic products, 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. Let's talk about the causes of the cracking phenomenon and the corresponding solutions in detail below:
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 generated in the case of overfilling, the solution can be mainly in the following aspects:
1.1 Since the pressure loss of the sprue is the smallest, 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;
1.2 Under 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;
1.3 Under normal circumstances, stress is likely to occur when the mold temperature is low, so the temperature should be increased appropriately, but when the injection speed is high, even if the mold temperature is lower, the stress generation can be reduced;
1.4 Excessive injection and pressure holding time will also produce stress, it is better to shorten it appropriately or perform Th times of pressure holding switch;
1.5 Non-crystalline resins, such as AS resin, ABS resin, PMMA resin, etc., are more likely to produce residual stress than crystalline resins such as polyethylene and polyoxymethylene, so attention should be paid;
a. 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, the cause can be determined;
b. When inserting metal parts during injection molding, stress is most likely to occur, 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, as an experience, the wall thickness is 7" and the outer diameter of the embedded metal is his.
c. General-purpose polystyrene is basically not suitable for adding inserts, and inserts have the least impact on nylon. Because glass fiber reinforced resin material has a small thermal expansion coefficient, it is more suitable for inserts;
d. Preheating the metal inserts before forming 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.