one more foam question

[TWsup]

 I have resources and skills to design a board in 3d cad and have a local contact that can mill blank on cnc machine.  I think I can even make a hollow foam board. we talked about it today we will mill a 14' board in 4 pieces.

problem is that I have problems with getting low density foam. I can get Divinycell, a higher density foam in thin dimensions for deck and fin reinforcement, but the 1.5 lb stuff is only available in blocks of EPS building insulation foam. it's the ' beady" stuff. Will this work ?  or should I glue 50 mm sheets of XPS together. ( this is higher density, bit I'm planning on hollow board ).

if I can make a hollow board how thick should foam wall be and what density ? I'm thinking to use low density foam and 50 mm wall, then inlay higher density or wood sandwich on deck and around fin.
I'll glass inside with 155 g glass and then glass outside with 2x 155g glass. could add full plywood stringer too.

[Dwight (DW)]

Lots of race boards are built from 1 lb EPS building construction foam. When you use divinycell, most use the 1 lb, not 1.5.

[Ucycle]

i think most hollow board use hd foam for their sandwich construction.  the problem with low density sandwich foam core is it have low compression strength so it will buckle.  I said keep things simple for your first board and just build a eps foam core board instead of hollow board.
 Most hollow board is construct in a female mold to compress all the fiber and hd foam down.  you can construct a hollow board without mold by glassing the shaped blank and let it cure.  then vacuum on your thin hd foam and the outside layer of glass/veneer. after everything cure, you can melt out the eps foam with acetone, the acetone wont melt the hd foam.  But you have to do some testing to figure out the amount of glass needed for a 14' hollow sandwich board. 

Also talk to your cnc guy, make sure he can machine those type of foam, he probably know which foam will work.

[PonoBill]

the problem with dissolving foam is that the resin that made the foam is the source of the weight, unless you can flush that out you won't have gained anything except for releasing some air, CO2 or N2. It would take a lot of acetone to flush the resin.

Hollow boards are made in a female mold with a layer of composite, a layer of HD foam for the center of the sandwich, and a layer of composite inside, all vacuum bagged. Then generally a lot of precisely sized stringers and supports made of PVC foam, with the lap section for the rail being cast separately and glued on place to form a strong interlocked edge. Not a simple thing.

The idea of hollowing out the EPS for your first board is WAY over complicated, and might result in half a pound of weight savings at most--you won't feel it. By the time you get done glassing the inside and adding stringers you will have far more than eliminated any weight advantage.

[TeeJay]

Yes you can use the 1.5 lb insulation foam.  If you're only making one or two I'd skip the cad and cnc and hand cut your close tolerance blank from the glued up slab.  Here is one I did this summer with 2" thick insulation sheets.

[TWsup]

ok. got it. forget the hollow board idea and just do a solid EPS block. i get Bills point about the weight savings.

next step is to really think it through: do really want to sand and finish a 14 ft board ?

I'll post some images of my CAD design to get design feedback. see where this goes...
my cnc guy is willing to do it for me at cost if I assist. machine is new and he's interested in showcasing what it can do. his shop is only 2km away.

[Celeste]

the problem with dissolving foam is that the resin that made the foam is the source of the weight, unless you can flush that out you won't have gained anything except for releasing some air, CO2 or N2. It would take a lot of acetone to flush the resin.

Hollow boards are made in a female mold with a layer of composite, a layer of HD foam for the center of the sandwich, and a layer of composite inside, all vacuum bagged. Then generally a lot of precisely sized stringers and supports made of PVC foam, with the lap section for the rail being cast separately and glued on place to form a strong interlocked edge. Not a simple thing.

The idea of hollowing out the EPS for your first board is WAY over complicated, and might result in half a pound of weight savings at most--you won't feel it. By the time you get done glassing the inside and adding stringers you will have far more than eliminated any weight advantage.

Except.  I am trying to convince a maker of a water soluble packing foam that he wants to sell me a reasonable amount of 4" sheet because it will be the next big thing and he is one of only a handful of producers, so is sitting pretty to expand his business.

Building this way is going to require some internal design rethinks, so there will be a learning precess, but if it is worth building molds to make conventional hollow boards, then putting the effort into learning how to build hollow with removable foam core should bring the hollow advantage to smaller volume production  

[punasurf]

TeeJay, that board is sick.  But is it only 8'?  I'm interested in some kind of downwinder, maybe 14' or so, but that means gluing up length too.

[TeeJay]

TeeJay, that board is sick.  But is it only 8'?  I'm interested in some kind of downwinder, maybe 14' or so, but that means gluing up length too.

It is a 12'6"  - With a full 8' long sheet paired with one cut off at 5' then reverse the seam each layer = 13' and cut down from there.

[blackeye]

I was going to suggest using the shaped foam as a male mold and build up the board around it, then cut it in two and release the lay-up from the form.  Lay in half-round composite stringers and bond the two pieces back together.  If carefully done, the form could be re-used.   

But maybe TWsup had this in mind:  Use the CNC machine to form the outer shape AND scoop out the insides.  Then the four pieces could be re-assembled to form a hollow foam blank as if it were an egg cut in quarters then its shell miraculously put back together again.  Then do a layup as if for a hollow board ie with core sandwiched between fibres.

Get the CNC to route-in half-round grooves where we want stringers to be, then lay up the fibres there first ie before the hull and deck. 

Vacuum bagging this would be a problem unless air pressure could be gently increased in the middle - a big lightweight air bladder?

[blackeye]

TeeJay, that board is sick.  But is it only 8'?  I'm interested in some kind of downwinder, maybe 14' or so, but that means gluing up length too.

It is a 12'6"  - With a full 8' long sheet paired with one cut off at 5' then reverse the seam each layer = 13' and cut down from there.

Teejay - can you post more details / pics of that build?  It looks awesome.  Maybe in another thread?

[TWsup]

I was thinking of cnc milling  both sides and laying up inside on 2 pieces to form one half then glue the two halves together using stringers and bulkheads. glass outside and double up at rail/ bond joint to get good bond.

other way is to mill female mold in low density foam and glass inside of that. then v-bag onto the laminate higher density foam and inside glass layer. I would make 2 half forms each from 2 blocks. only have to mill one side and easy to control when I have block of foam around cavity. then I destroy the female mold and clean up outer laminate. I could then glass on extra layers of glass around rail to bond halves. would use stringer and bulkheads as needed. ( that would be a bit of a fiddle).

all that said I think Bill is still right. go solid on first build.

[punasurf]

When Clark Foam went under, I built a mold and poured foam.  Trust me, it isn't worth the hassle.  Just buy some foam and shape it.  One real good thing about solid foam too, if it starts taking on water, you won't sink miles from shore.