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Analysis of the Causes of Cracking in Injection-Molded Products

Release date:

2022/05/18

Cracking is a common defect in plastic products manufactured via injection molding, primarily caused by stress‑induced deformation. The main sources of such deformation include residual stresses, external stresses, and stresses arising from the external environment.

　　Cracking is a common defect in plastic products manufactured by injection molding, primarily caused by stress‑induced deformation. The main sources of such deformation include residual stresses, external stresses, and stresses arising from the external environment.

　　(1) Cracking caused by residual stress

　　Residual stresses are primarily caused by three factors: overfilling, ejection, and metal inserts. As for cracking resulting from overfilling, the main solutions can be approached from the following aspects:

　　(1) Since the direct sprue incurs relatively low pressure loss, if cracking occurs primarily near the direct sprue, consider switching to multi-point distributed sprues, side gates, or stub‑type gates.

　　(2) Provided that the resin does not decompose or degrade, appropriately increasing the resin temperature can reduce its melt viscosity and enhance its flowability, while also lowering the injection pressure to minimize residual stresses.

　　(3) In general, lower mold temperatures tend to induce residual stresses; therefore, the temperature should be raised appropriately. However, when the injection speed is relatively high, even a lower mold temperature can help reduce stress generation.

　　(4) Excessive injection and holding times can also induce residual stresses; appropriately shortening these durations or implementing multiple pressure‑holding stages can help mitigate this effect.

　　(5) Non-crystalline resins, such as AS resin, ABS resin, and PMMA resin, are more prone to residual stresses than crystalline resins like polyethylene and polyoxymethylene; this should be taken into account.

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