Weld lines refer to weaker regions formed by the impingement of two separate flow fronts.
These may originate from multiple gates as shown here. The fibers near an advancing melt front are parallel to the front or perpendicular to the flow direction. This abrupt change in fiber orientation weakens this region. Weld lines may be formed also by splitting and rejoining of flow fronts around inserts. The orientation of the weld line is affected by the position of the insert relative to the gate, as shown on the diagram here.