Feman Tooling Co.,Limited
Feman Tooling Co.,Limited
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Cause Analysis on the Cracking of Injection Molding Products

Cracking of injection molding mold, including filamentous cracks, microcracks, abnormal whitening on the surface of the parts, and cracks caused by the mold being glued with parts and flow channel. According to the time of cracking, it is divided into demoulding cracking and application cracking. The main reasons are as follows:

1. Processing

(1) If the processing pressure is too high, the speed is too fast, the filler is too much, and the injection and pressure maintaining time is too long, the internal stress will be too powerful and crack will be caused.

(2) Adjust the mold opening speed and pressure to prevent demoulding cracks caused by parts being rapidly and strongly pulled.

(3) Properly raise the temperature of the die to make the parts easy to demould and properly lower the material temperature to prevent decomposition.

(4) Prevent cracking due to the decrease of mechanical strength caused by weld line and plastic degradation.

(5) Appropriately use release agent and frequent eliminate fog and other substances attached to the die surface.

(6) Lower residual stress of the part can be eliminated by annealing immediately after forming to reduce cracks.

2. Mold

(1) Ejection should be balanced including sufficient number of ejectors, sufficient cross-section area, sufficient mould release slope and smooth cavity surface, so as to prevent concentrated residual stress that leads to cracks.

(2) The part structure should not be too thin. It is recommended to use arc transition as far as possible to avoid stress concentration caused by sharp corners and chamfers.

(3) Try not use metal inserts to prevent internal stress increasing due to the different shrinkage rates between inserts and parts.

(4) For deep bottom parts, ejection inlet passage shall be provided to prevent vacuum negative pressure.

(5) The main channel is big enough to allow the castings to be easily demoulded during solidification.

(6) The main channel bushing should be connected with the nozzle head to prevent the cold and hard material from dragging and making the piece stick to the fixed model.

3. Material

(1) High content of recycled material results in low strength of the part.

(2) With excessive humidity, plastic may react with water vapor, which reduces strength and leads to ejection cracking.

(3) Unsuitable material, poor processing environment and contamination all will cause cracking of injection molding mold.

4. Machine

The plasticizing capacity of injection moulding machine should be appropriate. Material can not be mixed completely and becomes brittle in machine with small plasticizing capacity. However, overlarge plasticizing capacity will make plastic degrade.