Analysis of Causes of Cracking of Injection Molded Products Cracking, including surface filiform cracks, micro-cracks, whitening, cracking, and the risk of trauma caused by the sticking of parts and runners, according to the cracking time, it is divided into demoulding cracking and application cracking. The main reasons are as follows:
1. Processing:
(1) Excessive processing pressure, too fast speed, more filling, too long injection and 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 cracking due to weld marks and degradation of plastics, resulting in lower mechanical strength.
(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.
2. Mould:
(1) The ejection should be balanced, such as the number of ejector pins, the cross-sectional area should be sufficient, the demolding slope should be sufficient, and the cavity surface should be smooth enough to prevent the residual stress 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 adopt arc transition as far 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 demolding air inlets should be provided for deep-bottomed parts to prevent the formation of vacuum negative pressure.
(5) The main runner is sufficient to allow the gate material to be demolded when it is cured in the future, so that it is easy to demold.
(6) The sprue bushing and the nozzle shall be joined to prevent the drag of the chilled material from sticking to the fixed mold.
3. Material aspect:
(1) The content of recycled materials is too high, resulting in too 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 processing environment or the quality is not good, and pollution will cause cracking.
4. For machines: The plasticizing capacity of the injection molding machine should be appropriate. If it is too small, it will be brittle due to insufficient plasticization, and it will degrade if it is too large. Cause analysis of bubbles in injection molded products The gas in the bubble (vacuum bubble) is very thin and belongs to the vacuum bubble. Generally speaking, if bubbles are found at the moment of mold opening, it is a gas interference problem. The formation of vacuum bubbles is due to insufficient plastic injection or low pressure. Under the action of the rapid cooling of the mold, the fuel at the corner of the cavity is pulled, resulting in volume loss. Solution:
(1) Increase the injection energy: pressure, speed, time and material volume, and increase the back pressure to make the filling full.
(2) Increase material temperature and flow smoothly. Lower the material temperature to reduce shrinkage, and appropriately increase the mold temperature, especially the local mold temperature at the part where the vacuum bubble is formed.
(3) Set the gate in the thick part of the part to improve the flow conditions of the nozzle, runner and gate, and reduce the consumption of pressure.
(4) Improve mold exhaust condition.
Cause analysis of warpage and deformation of injection molded products The deformation, bending, and twisting of injection molded products are mainly due to the fact that the shrinkage rate in the flow direction is larger than that in the vertical direction during plastic molding, which causes the parts to be warped due to different shrinkage rates in all directions. There are large internal stresses inside the parts that cause warpage, which are all manifestations of deformation caused by high stress orientation. Therefore, fundamentally speaking, the mold design determines the warpage tendency of the part. It is very difficult to suppress this tendency by changing the molding conditions. The final solution to the problem must start with the mold design and improvement.
Post time: Nov-01-2021