A weld line on plastic parts
can cause structural problems and be visibly unacceptable. Weld lines occur
when two or more flow paths meet during the filling process. If the two flow
fronts have cooled before meeting and don't interdiffuse well, they can cause
a weakness in the molded part. A line, notch and / or color change can appear.
Weld and meld lines can be caused by holes or inserts in the part, multiple
gates, or variable wall thickness where hesitation or race tracking occurs.
A
meld line is a weakness or visible flaw created when two or more flows meet
and converge when filling the part. A meld line differs from a weld line by
the angle at which the flows meet. Weld lines meet at higher angles (head on)
and tend to be of lower quality than meld lines. When flows meet as meld lines
they flow in the same direction. However, there is still little mixing of the
flow fronts. As a result, there may be a line, notch and /or color change at
the meld line. The quality of the meld line is dependent on the material type,
the type and amount of fillers, and the pressure and temperature at the meld
line.
