I was just reading another response to the posting re. replacement core for
making flat composite sheets.  One builder was saying that it was Last-a-foam
which is a urethane foam and rather weak.  It was used as replacement for
Clark foam (Gordon Clark stopped production due to environmental limitations
with their process).  It is actually not quite as good as Clark foam
structurally and pound for pound, isn't as good as the cross linked PVC foams
(like Klegecell and Divinycell).  It does shape extremely well, however and
is great for a lot of things, just be carefull if you ever use it as a
structural core!  

I take it that the use of the foam for this core is a non-structural
application.

And as a note of caution NEVER HOT WIRE URETHANE FOAMS.  They release a lot
of nasty chemicals which CAN BE LETHAL!

Dan Newland