Even when the mold halves are exactly centered on top of each other core shift might happen.
Especially long, thin cores are pushed away due to unbalanced filling.
The unbalanced injection of the melt leads to a one-sided pressure on the core.
The movement results in changing wall thickness in the part.
In the area of reduced wall thickness the filling is more difficult than in the are with higher wall thickness.
Weaknesses due to the changed wall thickness lead to problems during the usage of the part.
A balanced flow around the core and a hold on both sides of it helps to reduce the risk of core shift.