Splitting and rejoining of flow fronts due to presence of inserts. This diagram shows the flow splitting up before the insert and rejoining after it. At sufficiently high flow rates, instead of flowing uniformly, the polymer may shoot out of the gate until it impinges against the mold boundary and is compressed against the opposite wall. Backfilling fills in the rest of the mold. This phenomenon creates many weld lines.
Weld lines tend to weaken an injection molded part.
The weld line factor describes the weakening caused by the presence of a weld line. This graph shows how weld line factor is affected by fiber concentration. Increasing the fiber concentration decreases the weld line factor. The reason weld line factor drops with the addition of fibers is found in fiber orientation. Fibers at the flow front are oriented parallel to the front.