I was discussing a processing issue with a technician and encountered this question…
The cause of the poor weld line was mold temperature, but I don’t really understand why since the melt flows from the center and should be hot when the flow fronts meet.
This is a true, but the strength and appearance of the weld line is often dependent on the ability of the two flow fronts to interact after meeting. When the flow fronts meet, the polymer near the mold surface will stop flowing and interacting faster when the mold surface temperature is lower. This can cause the weld line to freeze before the intersection of the two flow fronts makes contact with the mold surface, thus reducing the appearance.