ok, so I was on the right track when I first suggested that rectangular slabs thick enough to accommodate rocker were being supplied. I also said "or maybe as Sharky alluded to, they are thermoformed - needs a high temp to avoid
springback, doesn't sound cheap. "
So Evonik say it can be thermoformed at low or high temp (200c). At low temp they say "
however, the panel must be held in form during subsequent processing."
As a side note when I was a hobbyist in California, I obtained one of the last blue Dow XPS rectangular slabs produced for surfboards thick enough to accommodate rocker. You know this, but for others XPS is the generic name for the foam XTR uses. I hot wirecut it with two masonite rocker templates pinned to either side and by the time it was outlined and further shaped down, I would have ended up throwing more than half the volume away - terrible waste. I've also glued two thinner sheets of pink XPS/EPS together to make a thicker one which is why I gave
@stringcheese 's glue up suggestion the thumbs up. There would be cosmetic issues from the glue line with a non sandwich lam.
Nevertheless, you gave my post the thumbs down. I am not necessarily expecting an answer and enjoying wondering if it is because:
a) I've guessed the wrong foam - its not PMI.
b) The reason for discontinuation is not due to the manufacturers reluctance to produce slabs of suitable dimensions.
c) or you are not impressed with the promotional material I shared from Varial
I got a theory on the double springback in your part 1. The foam would have insulating properties so without prolonged exposure to a hot oven only the outer layer of the sheet would become thermoelastic and the outer layers would be holding in place a tensioned inner core. So once the thermoformed outer layers are machined off the inner layer is free to spring back to its original shape.
Regarding your PVC stringer suggestion - custom stringers and bending the polyurethane when glueing up was how Clark and others produced custom rockers wasn't it? It must have worked, although I always wondered that when it got shaped down the perimeter would want to curl back up a little, so if say a rocker increase was supplied then when machined the hull might concave a little?
I enjoyed your insider tales, thanks for sharing.