In areas with thick walls and across from ribs, a “hole” in the inside might appear.
Mass accumulation leads to a higher volumetric shrinkage, which is not insufficiently adjusted by packing pressure.
The melt is pulled to the surface during cooling, that a hollow remains in the inside.
CT-Scan and cutting the part help to detect these errors.
Small wall thickness and higher packing pressure in the critical area reduce the risk of voids.

Void in Plastic Industry