During injection molding, molten plastic in the mold cavity may encounter insert holes, areas of discontinuous flow, or areas where the flow is interrupted, causing multiple streams to converge. Because these streams cannot fully fuse, linear weld lines are formed. Additionally, gate jetting during mold filling can also generate weld lines, resulting in poor strength and other properties at the weld lines. The main causes are analyzed as follows:
1. Processing Aspects:
(1) Injection pressure and speed are too low, and barrel and mold temperatures are too low, causing the molten material entering the mold to cool prematurely, resulting in weld lines.
(2) Injection pressure and speed are too high, causing jetting and weld lines.
(3) Increase the rotation speed and back pressure to reduce the viscosity and increase the density of the plastic.
(4) Ensure the plastic is well-dried, use less recycled material, and avoid excessive or poor-quality mold release agents, which can also cause weld lines.
(5) Reduce the clamping force to facilitate venting.
2. Mold-related issues:
(1) Too many gates in the same cavity should be reduced or arranged symmetrically, or placed as close as possible to the weld joint.
(2) Poor venting at the weld joint should be addressed by installing a venting system.
(3) Excessively large runners or improper gating system dimensions should be addressed by minimizing melt flow around insert holes when setting gates, or minimizing the use of inserts.
(4) Excessive variations in wall thickness, or excessively thin walls, should be addressed by ensuring uniform wall thickness in the part.
(5) If necessary, a weld well should be installed at the weld joint to separate the weld joint from the part.
3. Plastic-related issues:
(1) For plastics with poor flowability or heat sensitivity, appropriate lubricants and stabilizers should be added.
(2) If the plastic contains many impurities, a higher quality plastic should be used if necessary.
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