Re: 3-d Sculpture Modeling Programs - Sculptris
You need to burn PLA out for longer than wax - it doesn't liquefy and run out of the mold, so it needs more time. More oxygen in the kiln helps too. You can usually blow out a mold after burnout to remove any loose ash that remains.
Somehow I pictured all the unbound sand running out of a closed mold like sand in an hourglass - but it might take some pre-planning to make that happen.