Rolf,

I have never fabricated an obturator for a patient, but I have seen several of them, all of which are used by patients with cleft palates. Their function is the same as for post cancer surgery. They have all had a mound on the top that fits into and closes off the openning. Although I'm not familiar with the term "bung" you piqued my interest so I "Googled" it and found the following:

"A bung is an apparatus used to seal a container, such as a bottle, tube or barrel. Unlike a lid which encloses a container from the outside without displacing the inner volume, a bung is partially inserted inside the container to act as a seal."

This definition seems to fit the bill and it appears that it's function with an obturator would be the same. An obturator without a bung would be like a lid and one with a bung would seal it much better.

Good luck with your new oburator.

Jerry