Cracking of plastic injection molding products, including filiform cracks, micro-cracks, white tops, cracking and trauma crisis caused by sticking of parts and runners, can be divided into demoulding cracking and application cracking according to cracking time. There are mainly the following reasons:
1. Processing
(1) Excessive processing pressure, too fast processing speed, more filling materials, too long injection and pressure holding time will cause excessive internal stress and cracking.
(2) Adjust mold opening speed and pressure to prevent demolding cracking caused by fast and strong drawing parts.
(3) Adjust the mold temperature appropriately to make the parts easy to demould, and adjust the material temperature appropriately to prevent decomposition.
(4) Prevent cracking due to lower mechanical strength caused by welding lines and plastic degradation.
(5) Appropriate use of mold release agent, pay attention to often eliminate the aerosol and other substances attached to the mold surface.
(6) Residual stress of parts can be eliminated by annealing heat treatment immediately after forming, and the formation of cracks can be reduced.
2. Mould
(1) the ejection should be balanced, such as the number and cross-sectional area of ejector pins should be sufficient, the demoulding angle should be sufficient, and the cavity surface should be smooth enough, so as to prevent cracking caused by concentrated residual stress of ejection caused by external force.
(2) The structure of the part should not be too thin, and the transition part should adopt arc transition as far as possible to avoid stress concentration caused by sharp corners and chamfers.
(3) Use metal inserts as little as possible to prevent internal stress from increasing due to different shrinkage rates between inserts and parts.
(4) Appropriate demoulding air inlet channels should be set for deep bottom parts to prevent the formation of vacuum negative pressure.
(5) The main runner is enough to make the gate material demould before solidification, so it is easy to demould.
(6) The joint between the main runner bushing and the nozzle should prevent the cold hard material from dragging and making the product stick to the fixed mold.
3. Materials
(1) the content of recycled materials is too high, resulting in low strength of parts.
(2) Excessive humidity causes chemical reaction between some plastics and water vapor, which reduces strength and causes ejection cracking.
(3) The material itself is not suitable for the processing environment or the quality is poor, and it will cause cracking if it is polluted.
4. Machine
Plasticizing capacity of injection molding machine should be appropriate. If it is too small, it will become brittle if it is not fully mixed. If it is too large, it will degrade.
5. Technological aspects
Inappropriate process selection, such as too high material temperature, too high back pressure, too fast glue melting, etc., leads to the decrease of physical properties.
