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Reasons for cracking of plastic shell

Release time:2022-03-24

The cracking of plastic injection products includes filiform cracks, microcracks, white tops and cracks on the surface of plastic shell, as well as the trauma crisis caused by part sticking and runner sticking. According to the cracking time, it can be divided into demoulding cracking and application cracking. It is mainly caused by the following reasons:

1. Processing

(1) Too much processing pressure, too fast speed, more filling materials, and too long injection pressure holding time will cause excessive internal stress and cracking.

(2) Adjust the mold opening speed and pressure to prevent demoulding cracking caused by rapid and strong tensile parts.

(3) Properly increase the mold temperature to make the parts easy to demould, and appropriately reduce the material temperature to prevent decomposition.

(4) Prevent cracking due to weld lines and plastic degradation, thereby reducing mechanical strength.

(5) Use release agent properly, and pay attention to often eliminate aerosols and other substances attached to the surface of the mold.

(6) The residual stress of the plastic shell can be eliminated by annealing heat treatment immediately after forming, so as to reduce the occurrence of cracks.


2. Mold aspect

(1) The ejection should be balanced, such as the number and cross-sectional area of ejector pins should be sufficient, the demoulding slope should be sufficient, and the surface of the cavity should be smooth enough to prevent the residual stress concentration of ejection from cracking under external forces.

(2) The plastic shell structure 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) Try to use less metal inserts to prevent the internal stress from increasing due to the different shrinkage of the plastic shell of the inserts.

(4) The deep bottom plastic shell should be equipped with appropriate demoulding air inlet channels to prevent the formation of vacuum negative pressure.

(5) The sprue is large enough to demould the gate before solidification, which is easy to demould.

(6) The connection between the sprue bushing and the nozzle should prevent the plastic shell from sticking to the fixed mold due to the dragging of cold and hard materials.

3. Material aspect

(1) The content of recycled materials is too high, which leads to the low strength of the plastic shell.

(2) The humidity is too high, which makes some plastics react with water vapor, reduces the strength, and causes the plastic shell to eject and crack.

(3) The material itself is not suitable for the processing environment or the quality is poor, and the plastic shell is polluted, causing cracking.

4. Machine

The plasticizing capacity of the injection molding machine should be appropriate. If the plasticizing capacity is too small and insufficient, it will become brittle without sufficient mixing, and if it is too large, it will degrade.

5. Technical aspects

Improper process selection, such as too high material temperature, too high back pressure, too fast glue melting, etc., leads to the decline of physical properties.

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