Different foam products

[StandUpGuy]

I have seen a build thread where a guy was using white standard styrofoam. Is this effective as a core material to make a stress skin panel? Is there a hazzard to using this material? I know when it burns there are several toxic gases produced. Not that I like my chances in a burning 4'x8' box anyhow!

[StandUpGuy]

I just found this article on the the flammabilty of different foam products.




http://www.scaletree.com/foamsafety.html

[StandUpGuy]

from the article it looks like white foam if caught on fire would quickly fill a trailer with thick choking smoke.

[StandUpGuy]

What kind of foam did you use?

[StandUpGuy]

A second ago you said don't worry about it. I do not have any white foam to ditch.

Yes I had not thought of using white foam then I saw it used here. Nobody said anything about its use. I was thinking because nobody said anything that it was OK. It is cheaper so I considered it. Clearly the blue and the pink are engineered for the construction industry to not burst into flames.

[RockyMountainTeardrops]

I have seen a build thread where a guy was using white standard styrofoam. Is this effective as a core material to make a stress skin panel?

What kind of skin are you talking about?
Fiberglass cloth with epoxy resin?
If so, the answer is "no"
The shear strength of the styro is not enough to do any good.
A point impact will cause the styro to crush, but the composite skin will rebound. You will get delaminated area that can spread.
1/4" plywood should give you enough impact resistance to keep a delamination from spreading.
Regardless........the pink or blue styro would be a better bet.

And I think that being worried about the fire and smoke hazard is very smart.
If you are diligent with your electrical wiring, and cover the styro (like a true stressed skin) you shouldn't have much to worry about.

Lee

[StandUpGuy]

I have seen a build thread where a guy was using white standard styrofoam. Is this effective as a core material to make a stress skin panel?

What kind of skin are you talking about?
Fiberglass cloth with epoxy resin?
If so, the answer is "no"
The shear strength of the styro is not enough to do any good.
A point impact will cause the styro to crush, but the composite skin will rebound. You will get delaminated area that can spread.
1/4" plywood should give you enough impact resistance to keep a delamination from spreading.
Regardless........the pink or blue styro would be a better bet.

And I think that being worried about the fire and smoke hazard is very smart.
If you are diligent with your electrical wiring, and cover the styro (like a true stressed skin) you shouldn't have much to worry about.

Lee

well I am looking at 1/4" plywood skin in and out. I was wondering exactly what you state. The white styro is very crumbly and I thought perhaps it would not be as good strength wise.

[Pottercounty]

There's always something out there that'll kill ya......
I must live on the edge...

[RockyMountainTeardrops]

]well I am looking at 1/4" plywood skin in and out. I was wondering exactly what you state. The white styro is very crumbly and I thought perhaps it would not be as good strength wise.

The stressed skin is only as strong as:
1. The bond between the skins and the core (delamination of the skin from the core)
2. the strength of the core itself. (core shear)


White styro is lacking in both qualities.

Last night I bonded two layers of 10oz fiberglass cloth to blue (Dow) styrofoam with epoxy resin.
This morning it took very little effort to peal the cloth off of the styro. (low shear strength...there was a thin layer of blue styro still stuck to the cloth)

All that being said, I think you can use 1/4" plywood and get a good build, but I don't think the weight savings over a traditional 3/4" framing build will make it worth it.
Not to mention the additional work needed to add blocking for the doors and galley cabinets.

Lee

[Ageless]

Well, I just have to chime in . . . .

Lee; did you bond up under pressure? Several test pieces I made up using white foam became very strong. These were laid up using waxed aluminum plates clamped together. Possibly that is what caused you bond failure?

BTW, anything that is going to burn inside a trailer is gonna give of toxins. Your polyester finish, your enamel paint, . . .

[RockyMountainTeardrops]

Lee; did you bond up under pressure? Several test pieces I made up using white foam became very strong. These were laid up using waxed aluminum plates clamped together. Possibly that is what caused you bond failure?

Yep, stacked approx 70lbs on a 5" x 20" test strip. (not a lot compared to vacumm, but plenty to ensure a good bond)

It wasn't a bonding failure...
When I pulled the skin off, there was a nice thin layer of blue styro stuck to it.

That's what leads me to say it was a core shear failure.

I think it's plenty strong enough for a TD,....but I don't feel like the core (styro) is tough enough with a light, thin layer of fiberglass to give a good, long lasting TD. (if I bump a tree, I want a scratch. not a hole)
1/4" plywood would work, but now you haven't saved much weight over traditional.

It's the same argument for tanks.....armor makes them heavy and slow, but the crew gets home alive. No armor makes them fast....but the crew doesn't last long.

I know that eaglesdare is building a foam TD with fabric and Titebond glue in the Construction Tips forum....I'm very interested in seeing how that comes out.

Lee

[pete42]

Lee I built an all fiberglass airplane well almost all of it.
the white coffee cup styrofoam does not like resin/epoxy it tends to melt and is hard to cut to shape.
The foam we used came in a 2'X8' block if I remember correctly.
the test piece that everone had to build was about 12 inches long and 2 inches wide bottom flat top and top edges rounded over
the fiberglass we used was bi-directional cut on a bias.
we laid the foam on wax paper mixed up a small batch of epoxy laid out the bottom layers of cloth wetted them laid the foam on the wet fiberglass draped
the top with layers of fiberglass wetted them out overlapped onto the bottom fiberglass
ended up with a piece of fiberglass about 14 inches long by 3 inches wide.
I took it to work and had people try to break it some stood on it between chairs others tried breaking it over their knee I still have it some where.
You are right the foam does not have much sheare strength it is used only for the shape you want.
remember the fellow who used cardboard and fiberglass for his TD? it was so strong he showed someone sitting on the roof.
if you have read this far sorry for such a long post

pete

[RockyMountainTeardrops]

I would bet that you had a urethane foam, or something like it.
Typical pink or blue is polystyrene. (not sure if you can get a 2'x8' block of polystyrene)

Discussion about polystyrene foam in boat building

My biggest worry about the pink/blue is a "point impact" and what that would do to the foam.

I'm currently kicking around the idea of using two layers of 10oz woven, one layer of 2mm Coremat, 1" blue/pink, then another two layers of 10oz.

My thinking is the Coremat will give some toughness to the outer surface without adding a million lbs to the sandwich.

Lee

[GPW]

QUOTE: " My biggest worry about the pink/blue is a "point impact" and what that would do to the foam. "
What kind of impact ??? Punching it , kicking it , running into it with a truck ???

[StandUpGuy]

Point impact aside, why use the white styro? It would seem it has not the strength of the blue foam and the white stuff burns like gasoline. I think it is about $3 cheaper than the blue/pink per sheet. I am very economically based but this seems like $3 well spent.