Acrylic products generally crack during injection molding, so what kind of solution should we take when cracking occurs? The main reason for this is due to stress deformation. There are mainly residual stresses, external stresses, and stress deformations caused by the external environment.
Crack caused by residual stress
Residual stress is mainly caused by the following three conditions, namely, overfilling, demolding, and metal inserts. As a crack generated in the case of excessive filling, the solution can be mainly started in the following aspects:
Since the pressure loss of the sprue is minimal, if the crack is most likely to occur near the sprue, consider using a multi-point distribution gate, side gate, and shank gate.
Under the premise of ensuring that the resin does not decompose or degrade, appropriately increasing the resin temperature can lower the melt viscosity, improve the fluidity, and also reduce the injection pressure to reduce the stress.
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.
Excessive injection and holding time will also generate stress, and it will be better to shorten it appropriately or to perform Th holding pressure switching.
Non-crystalline resins, such as AS resins, ABS resins, PMMA resins, etc., are more likely to cause residual stress than crystalline resins such as polyethylene, polyoxymethylene, etc., 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 differ greatly in 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 a rule of thumb, the wall thickness 7" and the outer diameter of the embedded metal part of the general-purpose polystyrene are basically unsuitable for the insert, and the insert has the least effect on the nylon. The resin material has a small coefficient of thermal expansion and is suitable for the insert. In addition, preheating the metal insert before molding also has a good effect.
Crack caused by external stress
The external stress here is mainly due to the unreasonable design of the stress concentration, especially at the sharp corners. It can take R/7" from 0.5 to 0.7.
Crack caused by external environment
Chemicals, water degradation due to moisture absorption, and excessive use of recycled materials can degrade physical properties and cause cracks.
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