Polystyrene foam comes as a "uniform" extruded product, the blue or
pink foam commonly seen and on which I've done my compressed foam tests,
and it also comes as expanded polystyrene as seen in foam coolers, cups,
etc.  Also known as bead board. This is manufactured from granules that
are expanded with heat, usually steam to fill an confined space.  The
typical bead board one sees in the lumber yard is 1.5 pounds or less per
cubic foot. It can be had at higher densities, which clearly have far
superior properties to what we unusually see.

    I visited a local manufacturer of this product the other day, and
requested a sample of 4 PCF foam.  Unfortunately they rarely manufacture
it, and had none, but offered me a sample of 3 PCF foam.  I went out
onto the factory floor where a technician was running an CNC hot wire
cutter making slabs with cut outs for hot water heat under a floor, and
he gave me a sample of what he thought was 3 PCF..... Unfortunately when
I weighed and measured it, I found it to be 2 PCF.... I was dubious at
the time as it didn't seem to have the compression strength I would
expect.   Oh well.... I'll have to try again.

    It is a fairly impressive product for what it is, showing good
integrity, but it is far too thick for me to make bend strength
comparisons without hot wire cutting it, and I have not built a hot wire
cutter yet, though I do have quite a bit of wire that would be suitable.

    The 4PCF product is special order only, and costs $1500 per slab,
which is 4x8x30"(thick), and is hotwired to your needs. That would be 60
sheets 1/2" thick at approximately $25 per sheet. Not bad for a higher
density foam.  The added benefit of course being that one could have
sheets cut to various thicknesses in the process. Unfortunately I have
no way to get my hands on a sample of the 4pcf product.

    The piece I have has a rough grippy surface on the side that faced
the mold, and a smooth slick surface on the hotwired side. Clearly the
die side would have better adhesion than the hotwire side.  You can
break a chunk out with a thumbnail..... but not easily.  It has
significant internal integrity, and isn't crumbly like much of that type
of foam is.  The beads are clearly distinct, but seem to have a fair
bond between them.   I would not consider the 2PCF foam as a suitable
core material by any means. It is clearly inferior to the comparable
blue and pink XPS (extruded polystyrene).   The 4PCF product might be a
different story.... I don't know.

    When I get a chance to build a hot wire setup, I'll experiment with
compressing this foam as I did the blue XPS.   I consider compressed XPS
as a potentially useful construction foam due to it's developed
resilience and flexibility.  In many situations these properties are
better than rigidity.  If the core can give with the skin, it is not
going to shear away from it / delaminate, nearly as easily.

                                H.W.