This morning I did two more tests, this time using my induction hotplate
and an infrared temp measuring gun to achieve an approximate temp of 220F.

The first sample was compressed with the shim as previously done, but
showed  no desire to rebound.  The result was that 1" foam became 1/4"
foam that was quite dense, but still had the resiliency I had noticed on
previous samples when compressed with strong finger pressure.  It would
quickly recover to it's full 1/4" thickness.

As the foam was not rebounding, I removed the shim, and compressed a
second sample to 1/2" thick.  I made the mistake with this sample of
spraying the surface with cooking spray for a mold release.  Previously
I had used soap.  This was a bad idea, as the oil reacted with the
polystyrene, and the surface was not a stable uniform skin.  Like the
previous sample, it was dimensionally stable, and very resilient to
pressure.

Both samples are more brittle under bending than the lower temp
compressed foam, but more than adequate for something planking a hull. 
Fracture of course happens when the surface ruptures, as with the
uncompressed foam.  All compressed samples have far more bending ability
than the original foam.

    It's unfortunate that I have no local source for Divinicell.
Hopefully in a couple of months I'll be in the area where I can get a
sample to compare properties.

    It is my opinion.... for what it's worth, that with the proper
techniques combined with some common sense, that a sufficiently strong
construction system for boat building can be developed based on ordinary
blue board.  It may be necessary to do the plank technique I've been
toying with to create a web at intervals, but I can live with that. 
Compressing the foam under elevated temperatures would involve building
equipment, and simple though it is in principle, in reality I don't see
boat builders building a hot press and pressing foam.  It's more of a
manufacturing technique, and of course adds cost.   A lamination of
think divinicell over blue XPS might have some potential for
distributing compression and impact loads.  Likewise additional layer(s)
of glass or even a carbon fiber ply might be worth looking at, but the
latter would cost more than using the expensive foam.

                    H.W.

On 07/08/2018 09:19 AM, StoneTool wrote:
> 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.
>>
>