Cracking, including filiform cracks, microcracks, top whitening, cracking on the surface of the part, and trauma crisis caused by part sticking and flow channel sticking, is divided into demoulding cracking and application cracking according to the cracking time. There are mainly the following reasons:
1. Processing:
(1) Too much processing pressure, too fast speed, more filling materials, too long injection and pressure holding time will cause excessive internal stress and cracking.
(2) Adjust the mold opening speed and pressure to prevent rapid and forced drawing of the parts from demoulding and cracking.
(3) Properly increase the mold temperature to make the parts easy to demould, and properly reduce the material temperature to prevent decomposition.
(4) Prevent cracking due to weld marks and plastic degradation resulting in lower mechanical strength.
(5) Appropriately use the release agent, and pay attention to frequently eliminate the aerosol and other substances attached to the mold surface.
(6) The residual stress of the workpiece can be eliminated by annealing heat treatment immediately after forming to reduce the generation of cracks.
2. Mold aspect:
(1) Ejection should be balanced, such as the number and cross-sectional area of ejector pins should be sufficient, the draft angle should be sufficient, and the cavity surface should be smooth enough, so as to prevent cracking due to the concentration of ejection residual stress caused by external force.
(2) The structure of the workpiece should not be too thin, and the transition part should adopt a circular arc transition as much as possible to avoid stress concentration caused by sharp corners and chamfers.
(3) Minimize the use of metal inserts to prevent the internal stress from increasing due to the difference in shrinkage between the insert and the workpiece.
(4) Appropriate demoulding air intake channels should be provided for deep bottom parts to prevent the formation of vacuum negative pressure.
(5) The main channel is enough to make the gate material demould when it is not solidified in the future, so it is easy to demould.
(6) The sprue bushing should be connected with the nozzle to prevent the dragging of the chilled hard material to make the part stick to the fixed mold.
3. Materials:
(1) The content of recycled materials is too high, resulting in low strength of the parts.
(2) Excessive humidity will cause some plastics to react chemically with water vapor, reduce the strength and cause ejection cracking.
(3) The material itself is not suitable for the environment being processed or the quality is not good, and it will cause cracking if it is polluted.
4. Machine aspect:
The plasticizing capacity of the injection molding machine should be appropriate. If it is too small, the plasticizing capacity will not be fully mixed and will become brittle. If it is too large, it will degrade.
