Hi all,
I am about to build a winsurf board and I have a question for all the experienced members in this topic here. I am wondering what you think about using a material like EPDM for the sandwich layer instead of something like Divinycell foam.
Thanks in advance for all your responses.
Greetings from Slovenia,
Rumba

Is the EPDM you are thinking about elastic? If so, I think that would remove a lot of the stability gains you get from the sandwich layer. But if it's a rigid foam with similar mechanical properties to d-cell, and bonds well with epoxy, maybe it could work. Would be interesting to hear if anyone has used it.

I don't think it would be a good idea to have something soft and elastic under the laminate.

It will offer no structural benefits and make the skin more prone to delaminate.

You want something that is impact resistant and less flexible than the core but also bonds ok to the core and laminate.

You mean the rubber??!!! you wouldn't get resin to stick well enough for a start. then, as others have said above it's just way too elastic

apart from all those negatives, it looks like it will be very heavy. 1gm/cm^3 I think translates into an awful lot of kg/m^3,
remind me how many cm^3 in a m^3, my gut tells me it's 100^3, but my gut can be wrong.
Convert units, says that's 1000kg/m^3. the foam we use is typically 80kg/m^3.

For sandwich skin you want a light and stiff material for core, heavy and elastic epdm is not good.
If it's first board build you can go simple without sandwich skin, just multiple layers of glass (and carbon) without vacbag can allow you to build a good Diy board. Many board are buid like that: rrd etech, starboard starlite, tabou mte, exocet ast, jp es, goya wood tech... glass and a bit of thin 0,6 wood deck patch, cheap way to add some stiffness to fight against foot dents. If you want to go the best harder vacuum bag way buy real pvc core and real vacuum system.

What I have found, is that core material needs to be stiff or porous. Porous cork or that clothy stuff, is heavy when it soaks resin. I think cork could be good under feet pads because it ends up like concrete, so you don't need as much glass or carbon under the foot pads. Probably still heavier than extra layers over PVC. I've made a total cork board, heavy and indestructible. It will never die and in 3000 years will be dug up and still be in perfect condition. Little known fact, I believe they found one in King Tutts chamber. PVC has good soak to stiffness ratio. A bit soaky , but not too much. I've looked into a lot of fancy core materials and believe PVC to be the best. I did like the idea of double-sided honeycomb laminate. It weighed nothing, non-porous and I did feel like a rocket scientist touching it, but it wouldn't bend much, so could only be used on the bottom. PVC is cheapest, lightest and has the qualities we want. Then there is PVC and PVC. Green, yellow. Not same, same. I use both kinds, each 80 kg. One is more bendy than the other. Bendier for top of board. I can't tell if there is a difference in performance or absorption. Earlier, I've used a denser EVA on the rails. It's nice and bendy for going around sharp rails and it sticks well to resin. But it is still heavy compared to PVC.
I'm getting better making my boards without bagging. But the techniques I need to use are very time consuming and more complex than bagging. I overlap the rails at every stage. Although very tough, they are also 20% heavier than equivalent production boards. I'm working on improving that. My boards need to be tough because I'm heavy and don't use board bags Use PVC

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Imax1 said..
Little known fact, I believe they found one in King Tutts chamber.

No it was millions of chocolate balls

It was the Pharoah Rocher.

Yes cork can be an alternative sandwich material with appropriate technic and fiber schedule give nice durable results. I use it for many years. 2mm thick 200kg/m3, microsphere epoxy filler, glass under, carbon over.