Last evening, I modified my fixture, and did another sample of
compressed foam.  In this case it was compressed to 25% of it's original
thickness under 200 deg temp.   That is boiling water temp at this
elevation.   On cooling, it rebounded to 5/8" thickness, compared to
3/4" thickness when compressed to 50%.  Properties were very similar,
needless to say.   The rubbery flexibility under compression was
similar, to the previous sample, and very different from the stiff
brittle foam I began with.   I laminated woven fiberglass cloth to each
face, using the only resin I have, which is T88.    This was a light
weight fabric, I believe 3.7 oz (125gsm).  This was not nearly enough to
create the stiffness needed. It is still very flexible. One thing is
obvious.  An impact load on the glass skin will deform the foam, but the
foam will rebound rather than taking on a permanent deformation.
    I previously did a compression test using a higher temperature
(pressure cooker), which I could not measure, but available information
suggests to be about 250F.  In this case, the nature of the foam changed
completely into more of a rigid plastic, retaining none of it's positive
properties, and becoming brittle.   There probably is a more optimal
temperature than 200 deg.  Even sea level boiling point of 212 might be
enough to make a difference.

    I am really amazed at the change of character that can be achieved
in this product by compressing at this moderate temperature, even to 75%
of it's original thickness.   It becomes a totally different foam,
almost unrecognizable as being the same product.  The surface only
heating seems to yield the best results, but that is a very subjective
assessment.

                                                    H.W.

On 07/07/2018 05:58 PM, StoneTool wrote:
> Today I built a simple fixture to give me a consistent process. It
> just consisted of a piece of steel plate with a 1.5" (51mm) standing
> rim into which another steel block fit.  This was to contain the foam
> while it was being compressed.  The foam I used was 1" (25.4 mm) thick
> blue XPS.  The top block had stops so that it would rest 1/2" from the
> lower plate.  With the foam inserted and squeezed, it could not spread
> as it was squeezed to half it's original thickness.   The metal parts
> were heated in hot water to 200F, the foam inserted, and pressed to
> half it's thickness, then cooled in hot water.   The change in
> properties was impressive. The surface will take a LOT of pressure and
> rebound without permanent deformation.  It's a very different foam
> compared to the original which would permanently deform without a lot
> of pressure.   A sharp object will puncture the tough surface, however
> with a layer of glass over it, there will not be sharp pressure
> points, loads will be distributed out.  For example, I can apply a lot
> of thumb pressure and it will recover, but an unsharpened pencil will
> penetrate fairly easily.  The foam has far more bending resistance,
> the tough outer skin preventing it from snapping until a fairly
> extreme deformation,   I pulled it around a progressively tighter
> radius, and failure occurred at what I would estimate to be an 9"
> radius.   That's pretty darn impressive compared to original.
>
>
>     When I removed the foam from the jig, it was in a pillow shape.  
> 1/2" at the edges, and thicker toward the center.  Size was 3" by
> 6".   The sides were heated along with the top and bottom surfaces, so
> they had formed the tough skin, and were restraining the foam from
> expansion to it's original thickness.  I cut away the sides, and the
> foam rebounded to approximately 3/4" thickness uniformly.  This was
> the thickness of the test block. The foam was at about 80F when
> inserted, and the only compression appears to have been the outer
> surface, which is what I wanted in this case.   Clearly if I did this
> on full 4:8 sheets, it would be desirable to trim a small amount from
> the outer edge to achieve a uniform thickness.
>
>
>     The next test will involve compressing the foam more to attempt to
> achieve a 50% thickness.   I'll heat the foam and the jig at the same
> time to the same temperature, and use a 1/4" thickness shim, resulting
> in the foam being compressed to 1/4 of it's original thickness initially.
>
>
> H.W.
>