A weakness or visible flaw created when two or more flow paths meet during the filling process.
Weld lines can be caused by holes or inserts in the part, multiple injection gates, or variable wall thickness where hesitation or "race tracking" can occur. If the different flow fronts have cooled before meeting, they may not recombine well, causing a weakness in the molded part. A line, notch, and/or color change can also appear.
The quality of the weld line is dependent on the material type, the type and amount of fillers, and the pressure and temperature at the weld line.
Move weld lines to areas where strength is of less importance and visual appearance is less obvious. Weld lines can be moved by changing:
- Gate position and dimension
- Wall thickness
3D meshed parts can also display weld surfaces. This is the surface through the thickness of the part where flow fronts meet. How the weld surface moves in the remaining fill time can also impact on the part quality. A weld surface that is dispersed through the part will result in more structural integrity than where the weld surface freezes as formed.