eddgarpv

Houdini Indie will turn into Houdini Apprentice until you renew your indie license.

There it is, just search “shell thickness” in that link and you'll see it:

Enabling this option interprets the input geometry as a shell around its surface of the given thickness (as opposed to a filled volume).

I assume you're using H17?

I'm keyframing the “Shell Thickness” parameter from the FLIP Source node (It works fine in my case, probably can help you too).