A common defect in the production of plastic products, the main reason is due to stress deformation. There are mainly residual stresses, external stresses, and stress deformations caused by the external environment.
Residual stress is mainly caused by the following three conditions, namely, overfilling, demolding, and metal inserts. As a crack generated in the case of overfilling, the solution can be mainly achieved in the following aspects:
1. Since the pressure loss of the sprue is the smallest, if the crack is mainly generated near the sprue, consider using a multi-point distribution gate, side gate and shank gate.
2. Under the premise of ensuring that the resin does not decompose or deteriorate, the proper increase of the resin temperature can lower the melt viscosity, improve the fluidity, and also reduce the injection pressure to reduce the stress.
3. Under normal circumstances, 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 stress can be reduced.
4. If the injection and holding time are too long, stress will also be generated, and it is better to shorten it appropriately or to perform Th holding pressure switching.
5. Non-crystalline resins, such as AS resin, ABS resin, PMMA resin and other relatively crystalline resins such as polyethylene, polyoxymethylene, etc., are prone to residual stress and should be noted.
When the demoulding is introduced, the ejection force is too large, the stress is generated due to the small draft of the mold release, the mold type rubber and the convex mold are rough, and sometimes whitening or cracking occurs even around the push rod. Just look carefully at the location of the crack and determine the cause.
When metal parts are embedded at the same time as injection molding, stress is most likely to occur, and it is easy to generate cracks after a certain period of time, which is extremely harmful. This is mainly due to the fact that the thermal expansion coefficients of the metal and the resin are in great difference to generate stress, and over time, the stress exceeds the strength of the gradually degraded resin material to cause cracks. To prevent the resulting cracks, as an experience, the wall thickness is 7″ with the outer diameter of the embedded metal
General purpose polystyrene is essentially unsuitable for use with inserts, while inserts have minimal impact on nylon. Since the glass fiber reinforced resin material has a small coefficient of thermal expansion, it is suitable for an insert.
In addition, preheating the metal insert before molding also has a good effect.