Cracking often occurs in the molding process of plastic molds, which is a more common defect of plastic products. The main reason for this is due to stress and deformation. First, there are residual stress, external stress, and stress deformation that occurs in the external environment. Let's talk in detail about the causes of cracking and the corresponding solutions:
First, the crack caused by residual stress:
Residual stress is mainly caused by the following three conditions, namely overfilling, ejection and metal inserts. As the cracking that occurs under the condition of excess filling, the solution can be started in the following aspects:
1. Because the pressure loss of the direct gate is small, if the cracking occurs mainly near the direct gate, you can consider using the multi-point distributed point gate, side gate and shank gate method;
2. On the premise of ensuring that the resin does not decompose and not deteriorate, properly increasing the resin temperature can reduce the melt viscosity and improve the fluidity. It can also reduce the injection pressure to reduce stress;
3. In normal conditions, stress occurs simply when the mold temperature is low, and the temperature should be appropriately increased, but when the injection speed is high, even if the mold temperature is lower, the stress can be reduced;
4. Stress will also occur when the injection and holding time is too long. It is better to shorten it properly or perform th holding pressure switching.
5. Non-crystalline resins, such as AS resins, ABS resins, PMMA resins, and other crystalline resins, such as polyethylene and polyformaldehyde, simply cause residual stress, which should be noted;
a. When the mold is ejected, the ejection force is too large and the stress is generated because of the small mold release gradient and rough mold glue and convex mold. Sometimes, whitening or cracking occurs around the ejection rod. As long as you carefully observe where the cracking occurs, you can determine the cause;
b. When metal parts are embedded together with injection molding, stresses occur relatively easily, and cracking occurs simply after passing through for a period of time, which is extremely harmful: This is mainly because the thermal expansion coefficients of metals and resins are very different, and stresses occur, Over time, stress exceeds the strength of the gradually degraded resin material and cracks occur.
c. General-purpose polystyrene is basically not suitable for adding inlays, and inlays have less influence on nylon. Because the coefficient of thermal expansion of glass fiber reinforced resin materials is small, it is more suitable for inserts;
d. Pre-heating the metal insert before forming also has a good effect;
Second, the crack caused by external stress:
The external stress here is mainly due to unreasonable planning and stress concentration, especially at sharp corners.
Third, the crack caused by the external environment:
Chemicals, water degradation caused by moisture absorption, and excessive use of recycled materials will degrade physical properties and cause cracking.