Part Details Review
| Material Difference |
Weld lines affect yield, tensile, fatigue, impact, and elongation properties of a part to different degrees. Moreover, weld line strength can vary drastically from material to material.
Welds with good tensile strength retention may not fare well under impact or fatigue loading and vice versa. Consider the kind of loading that the part will be subjected to and then evaluate which material will perform better in that environment.
A comparison of different thermoplastic materials shows that butt welds in polysulphones and polycarbonates can retain 95% to 100% of the tensile strength under ideal conditions. Polystyrene is at the lower end around 60% to 65%. SAN, PPS, ABS and Polypropylene fall in between, at around 80% to 85%.
Contrary to what seems obvious, addition of fiber reinforcement to the thermoplastic materials does not affect the weld line properties positively. The extent of property loss at the weld line is most significant with long fibers, less with short fibers and lesser with the milled fibers and the beads. Also, the strength retention decreases as the fiber concentration in the material is increased. Tensile strength retention of 10% glass fiber filled polycarbonate is 86% as compared to almost 99% of unfilled material. A 30% fiber loading brings it down to about 64%.
Unfavorable fiber orientation at the weld zone, caused by fountain-like flow behavior of the impinging flow fronts, is mostly responsible for low property retention. Fountain flow at the weld zone causes the fibers to turn at right angles to the normal orientation and renders them parallel to the melt front (see figure).
