Bonding ply to XPS foam for secondary structures.

It looks like the "first mate" has approved me building a flybridge for my wee powercat so I'm moving forward before she get's a change of heart . . . . .
I need to keep things as light as possible and am thinking now to use XPS foam sandwiched between two skins of marine ply - probably 5/16" external / 3/16" internal. I may then sheathe the final product with Dynal or polyester cloth and all tertiary components will be chopped strand over 30mm XPS
What advice can anyone give about a suitable adhesive please? Ideally something that gives me more than one shot at lining things up and that doesn't require sustained pressure while curing.
Thanks in advance!

 

Epoxy, or epoxy, or epoxy.

Alternatively you could use epoxy, but I would stick with epoxy.


Kinda get where I am goin here....

 

Hmmm - I wonder if epoxy would work . . . . . ?
Cheers Stumble

 

Nope I would go with epoxy instead.

 

I would add that you don't need, nor do you want mat in any of this laminate, if using epoxy. The obvious and hands down choice is epoxy, but it doesn't have to be.

8 mm plywood on the outside and 4 mm on the inside, over foam sounds like you just don't need the foam. I'm not sure how big your cat is, by 12 mm of plywood doesn't need any foam.

If looking for light and interested in foam sandwich construction, consider 3 mm plywood on both sides of 30 mm closed cell foam. This is half the plywood thickness (and most importantly half the weight), but if lightly sheathed, quite strong, certainly enough for a flybridge. A Dynel or Xynole sheathing will add considerably to the weight (3 or more times than that of conventional 'glass cloth), so unless you expect to use it as a driving range back stop, where you need a high level of abrasion protection, reconsider and go with a conventional 'glass cloth, say 10 ounce (340 GSM) at most, with 2 (68 GSM) or 3 ounce (102 GSM) being just fine.

Epoxy is the only adhesive that meets or exceeds all of your requirements. It requires very little clamping pressure, just enough to insure contact. You can have as little or as much working time as you like, depending on hardener used, it's incredibly strong, truly waterproof and you can bond the sheathing down with it too. No other goo in a can can match this level of preformance.

 

Making any kind of a structure thats above deck level needs to be light and strong . When a boat starts to rock in one of those confused seas the more weight up high the more the rock and roll motion . mono or cat still rocks and rolls ! Epoxy yes and vac bag it together !!

 

Just another of my little posts saying PAR nailed it!

 

Not if you're a stoned pirate . . .

 

I like the idea of 3mm skins thanks - should be very light and durable and if I bond one side then score the reverse of the foam I should be able to form a gently curved combing for the front. Oh yes, and I think I may use epoxy . . . . . .
Thanks guys!

 

If you're hoping for a curved end result, you're best off laminating the plywood on in the curve, rather then attempting to bend the plywood/foam sandwich afterward. Bending the pre-sandwiched panel will likely not come out well, testing the peel strength of the foam. Make a simple jig to bend the panel over, place the inner plywood skin in place, then glue the foam to it, followed by gluing the exterior skin. A modest curve probably will not need to have the plywood glue side scored. 3 mm will take quite substantial bend, but intend on about 10% "spring back". This simply means when you make you curved gluing jig and it has say a 1.2 meter radius (as an example), make the curve on the jig 1.8 meter radius, so it will "spring back" to 1.2 when you release it from the gluing jig.

 

Thanks again PAR. I was thinking to work in reverse of that so I glue the outer skin to the foam then score the inner side of the foam to achieve the curve. I figured that if I then glue the inner skin to that, the epoxy would fill the score voids to set the curve and because the inner skin is now in tension, this would create a form of bridge beam structure to hold the shape and as such it should have very high deformation resistance. Guess some test runs are in order. At least using XPS the materials cost isn't as bad as some of the more advanced marine foams!

 

let us know how it all goes, i got a whole pack of XPS foam in the garage that i havnt had the chance to play with yet...

 

See this link where I've posted some work I did with XPS Foam and 1/8" thick Meranti skins.

http://www.boatdesign.net/forums/fi...-building/extruded-polystyrene-xps-24213.html

Use 3M Fastbond 30 water based contact cement to bond the skins to the foam. It will not degrade the foam like a solvent based contact cement. It also eliminates the need for clamping or vacuum bagging the panel.

It is easy to make a curved panel this way. Make a form, lay the lower skin on the form, bond the foam to the lower skin, then bond the upper skin to the foam. You do not need to score the XPS foam if curving in one direction, it bends rather easily. But once bonded to the skins, it is very stiff and light. I can stand on the cabin or cuddy roofs of my Tolman and there is almost no deflection.

 

PL Premium works well too, although if I were using it as a core I would want 100% coverage which would be difficult to do with PL. Epoxy has been correctly recommended.

 

You can get very consistent, thin coverage with PL Premium if you use a notched trowel. But the difficulty is keeping the surfaces in contact during the cure.

Of all the options I've tried, water based contact cement like 3M Fastbond 30 gives the best result.