Meaning
Tensile capability of the region where two or more melt fronts meet and fuse during the moulding process defines the value. In injection moulded parts, weld line strength determines whether the component can withstand mechanical loading at its weakest points. This value is typically lower than the bulk strength of the uninterrupted polymer matrix.
Moulding Parameter
Melt temperature and injection speed are the primary variables that determine how well the flow fronts fuse. High melt temperatures and rapid fill rates allow the polymer chains at the front of each flow to interdiffuse across the boundary. If the melt is too cold when the fronts meet, they will not mix well, resulting in a weak joint that can easily fail.
Reinforcement Effect
Fiber reinforcement can significantly reduce the strength of the fusion zone. When the melt fronts meet, the glass fibers align parallel to the boundary rather than crossing it. This lack of fiber bridging across the weld line makes the joint much weaker than the rest of the fiber-reinforced part, leading to brittle failures.
Part Design
Gate placement is the main tool used to locate weld lines in low-stress areas. This prevents weak zones from coinciding with high mechanical load.
