Differences in wall thickness cause changes in the speed of the melt front.
This can lead to air traps, weld lines, e.g..
To get a well balanced, homogenous flow behavior, increasing local wall thickness, called flow leaders, can speed up the flow front in a specific area.
By adding a flow leader the thin wall is increased and the air trap gone.
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This glossary of plastic industry is provided by PLEXPERT Canada Inc.