When at least two melt fronts meet, a weld line occurs.
Weld lines display on one hand a reduction of the mechanical strength, on the other hand a beauty error in the part.
The lower the melt front temperatures, the more difficult it is for the melt fronts to merge.
The smaller the meeting angle (melt fronts meet head-on), the more critical.
For fiber filled materials, a weld line changes the orientation of the fibers, that they will orient perpendicular to the flow direction.
To reduce weld lines it is possible to work with sequential injection or overflow cavities.