Meaning
Structural failure of moulded plastic parts under sustained load often occurs at the junction where two separate melt fronts meet and fuse during the injection process. This localized deformation is known as weld line creep and it represents the accelerated time-dependent strain that occurs at these molecular fusion boundaries. The mechanical strength of this region is always lower than that of the continuous polymer matrix.
It does not apply to areas of the part that do not contain weld lines.
Structural Weakness
Molecular chains across a knit line are poorly entangled because the polymer melt has cooled when the fronts meet. This incomplete entanglement makes the region highly susceptible to weld line creep when subjected to continuous stress, causing the weak interface to slowly separate.
Moulding Parameter
Injection moulding conditions dictate the quality of the fusion and the resulting resistance to long-term strain. High melt temperatures, fast injection speeds, and high holding pressures help maximize molecular diffusion across the knit line. If the mould temperature is too low, the polymer chains freeze too quickly upon contact, which results in a highly distinct weld line that will creep rapidly under any applied force.
Design Mitigation
Engineers must position gates carefully to locate these fusion lines in areas of the part that experience minimal stress. When a knit line cannot be avoided in a load-bearing region, the designer must apply a lower allowable stress value to account for the increased risk of weld line creep over the service life of the product. This design practice prevents premature structural failure and extends the durability of the assembly, reducing warranty claims and ensuring that the plastic part can withstand continuous assembly stresses without cracking.
