Webbing
There is a molding phenomenon called webbing that can occur in corners and other locations that rapidly distort the parison when the mold closes. As the core and cavity mold pieces close onto the parison, the parison is rapidly transformed from a tube or bag shape into a functional configuration. As the core is pushing the parison into the cavity it is possible for the opposite sides of the parison to touch before the air is injected to form the part. When this happens, the plastic welds together inside the parison and when the part is blown, the weld resists separation. The result is either a part with a very thin, weak section all around the web or, if the nearby material tears when the part is blown, no part whatsoever.

Webbing is more pronounced in deep cavities with corresponding deep cores. However, certain configurations can make the parison collapse or fold back on itself to cause webbing when the mold closes. It is a good idea to ask for a simulation test on deep parts that might produce webbing.