Analysis Of The Cause Of Cracking Of Injection Molding Parts

Analysis of the causes of cracks in injection molded parts

Cracking includes filamentous cracks, micro-cracks, whitening, cracking, and traumatic crisis caused by the sticking of the workpiece and the runner sticking to the mold. According to the cracking time, it is divided into demoulding cracking and application cracking. The main reasons for being hungry are as follows:

Processing aspects: 1. Excessive processing pressure, too fast speed, too much filling, and too long injection pressure holding time will cause excessive internal stress and cracking

2. Adjust the speed and pressure of mold opening to prevent quick and strong drawing of the part from demoulding and cracking

3. Properly increase the mold temperature to make the part easy to demould, and appropriately lower the material temperature to prevent decomposition.

4. Prevent hesitating weld marks, plastic degradation causes lower mechanical strength and cracks

5. Appropriate use of mold release agents, and pay attention to frequently removing substances such as aerosol attached to the mold surface

6. The residual stress of the part can be annealed and heat treated immediately after forming to eliminate the internal stress and reduce the generation of cracks.

Mould aspect:

1. The ejection must be balanced, such as the number of ejector pins, the cross-sectional area must be sufficient, the demolding slope must be sufficient, and the cavity surface must be smooth enough, so as to prevent the residual stress of the ejector from being concentrated and cracking due to external force.

2. The structure of the part should not be too thin, and the transition part should use arc transition as much as possible to avoid sharp corners and chamfering that cause stress concentration

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. For the deep-bottomed parts, due to the design of appropriate demolding air inlets to prevent the formation of vacuum negative pressure

5. The main runner is large enough to allow the gate material to be demolded when it has time to solidify, so that it is easy to demold

6. The sprue bushing and the nozzle joint should prevent the chilled material from dragging the column and making the parts stick to the fixed mold

Material aspect:

1. The content of recycled materials is too high, causing the strength of the parts to be too low

2. Excessive humidity causes some plastics to react chemically with water vapor, which reduces the strength and causes ejection cracks.

3. The material itself is not suitable for the processing environment or the quality is not good, and the pollution will cause cracking

4. For the machine: the plasticizing capacity of the injection molding machine should be appropriate. If it is too small, it will become brittle due to insufficient plasticization, and it will degrade when it is too large.