Extruded Polystyrene (XPS)

Not sure about the unbacked wood venier thing, but I made up some test panels that were very hard to break. And peel testing showed only foam stuck to the adhesive, no bare wood, indicating that there was an excellent bond between the luan skin and the foam.

XPS foam gets a bad reputation for this type of use due to its low shear strength. But many others have used it successfully by making sure that all foam edges are filled or butting up against another solid, putting the foam almost entirely in compression.

 

Wood veneers are very thin decorative woods supplied with paper backing, wood backing and raw. http://www.veneersupplies.com/

The 3M waterbased contact adhesive would water damage the raw veneer. Plywood is not affected.

Regards,

P

 

I have made some test panels using 1/2" XPS (Blue Board from home building supplier) with two 6 Oz Glass cloth on each side. The only failure was the test panel where I did not pull the thin membrane film off the XPS. I also used a bread knife to cut an epoxied scarf joint. Everything worked quite well. I am preparing to embark on a similar path where I will use unbacked veneer, over the glass, for cosmetics. I plan on shooting a clear PU top coat for UV protection.

 

Great site that. I went through their web site, and found no reference to 'unbacked' veneer, though there is plenty mention of 'raw' veneer, which are single 'peels' (no cross grain backing) and 'backed' veneer.

This may be fine for drum kits and guitars, but I would be worried could be very unsatisfactory in a marine environment, epoxy and varnish notwithstanding.

Marine grade ply with epoxy and varnish is a high maintenance surface on its own. Still, a lot of small boats spend most of their life under cover.

 

It seems the blind are leading the blind on this set of ventures. Compressive strength is only one concern. A much bigger and more commonly encountered difficulty is sheer strength at the bond line, which is quite low with XPS, regardless of adhesive used.

My point about the blind leading the blind simply means, if you're going to conduct tests, then test what's necessary, not just a small segment of the physical properties list.

 

Here's what a section of the 1" XPS foam with 1/8" doorskins looks like.

I've got this on the cuddy and cabin roofs for my Tolman Jumbo.

Although these will not normally see any load on them at all, there is very little flex in the cuddy roof when a person stands on it.

 

This type of foam may just be the next best thing for lightweight interior fabrication or perhaps even light structural purposes... This foam can be found in many flavours upto 0.7Mpa shear strength with a density of only 32-55kg/m3...

It doesnt absorb water - less than 1%, the higher densities even less than 0.05%,
Its lightweight and inexpensive, 1/10th-1/5th the cost of PVC foam cores...
Epoxy bonds well to it, styrene based resins cannot be used with it however...
Its easy to work/shape,
Has excellent insulation properties...

How many people are already using this material and is there any further feedback on its benefits or limitations?

 

I've used this sandwich construction for the cuddy and cabin roofs on my Tolman Jumbo, it works very well. Very light, strong, and stiff.

The key is using a water based contact cement.

 

Why is that the key?

 

i think its just easier and simpler to use contact cement, although i would rather have piece of mind and use epoxy to know its water proof and going to last...

What im thinking is using this stuff structually aswell... For example, im thinking laminating some okume plywood on both sides of a thicker piece of XPS foam, then glass both sides for a structural panel. This reduces weight from using just ply alone, and less cost then using PVC foam especially when you need a very thick panel. The loads are smallest in the center of the sandwich, so the highest density XPS foams should be adequate in the center of the sandwich.

It appears some builders are already using this stuff as a core for decks (multimarine) and simply laminating the topside with thin ply to spread the point loading problem. Both sides are then glassed obviously... The trinado guy is convinced enough to use 10mm of it as a core for his trimaran main hull... although it seems he is careful to brace the edges of it with wooden core and solid glass/single skin laminate under his main connective structural bulkheads and other high stress areas such as the transom, bow, keel and sheer... quite intelligent use of it in otherwords...

Im thinking this stuff could also be used as a core material for composite hull stiffeners and bulkheads of small and even medium size yachts, simply make the core thicker - its lighter and 1/10th the price so who gives a damn if you make 30mm bulkheads?

anyone else have any ideas?

 

I'm seriously considering using it with a glass skin for non structural walls, cabin soles, cabinetry and settees and stuff since I can't find a more suitable material. Does anyone know where to get it with some thin skins already bonded on, so i can just put it in place and tape it in?

 

Because it allows for an instant bond between the foam and the thin luan plywood skin. Other adhesives require clamping for a long period of time until the adhesive is cured. This is not practical for large, curved surfaces like a roof or cabin top.

The thicker the foam, the stronger the structure. The skin thickness really does not matter. I used 1/8" Luan with 1" XPS foam, it has no problem supporting ~300 lbs on the cuddy roof. We use it like a sun deck!

 

vacuum pressure is your friend... once you have a small vac pump it makes everything with resin so easy, i dont know what i would do without it these days...

another option to bonding ply on the outside of XPS, would be to bond a thinner layer of PVC foam either side of it before laminating the sandwich skins... why? so save on weight and cost when making thicker structural panels...

Does anyone know if XPS can be thermoformed or bent with heat applied in a similar fashion to PVC foam? The guy building the trinado seems to have done this somehow...?

 

(Emphasis added)

Tensile stresses are smallest in the center. Shear stresses due to bending are maximum in the center.

 

I'll repeat the blind are leading the blind again . . . Speak what you know folks or add a "qualifier" such as "I really don't know anything about the structural, durability or physical properties of this setup, I'm just guessing it'll work", type of thing.

Go for it Cat, I think a water based, whatever skin you want, over low shear strength core, is just the ticket . . .