PU foam VS Core-Cell or other?

[Danielsan]

hi there,

What about the use of PU foam instead of Core-Cell or other for the creation of sandwiched hull.

I found out that PU has >90% closed cells and has about same tensile strength and compressive strength as Core-Cell.

Could I assume that I can use it for creating a sandwich hull?

What am I missing? The price is half the price of Core Cell?

Greetings,

[mmd]

I'd look at compatibility with the resins you plan on using before comitting to the stuff. It's ruin your whole day to find out that the truckload of foam you just bought is gonna be melted by the resin you plan to use.

[michael-compwes]

'PU'??--gotta link?...or further description?

[Danielsan]

PU -> Poly Urethane

Not much info, but I know it is widely used ad boyancy foam for filling up spaces.

otherwise:

density vary from 35 to 300kg/m3
thermal cond. 0.019 to 0.070W/m.K
Impact resist >220 to >4500 KPa
tensile strength >300 to 4000 KPa
closed Cells >90%
Water absorption <2 vol-%

don't know about other specs, will be searching on the net some more and put it here.

greetz,

[nero]

How big is your project?

[Danielsan]

It is rather small, it has become a runabout LOA7.00m Beam 2.10 an total surface area of about 40m2

[yokebutt]

PU foam will tolerate styrenated resins, but, it's very brittle stuff. I couldn't recommend it for coring in good concience unless it is of high density, wich mitigates the benefits of sandwich construction severely. You might want to look into the Divinycell and Airex/Herex products, or even balsa, for price differences.

Yoke.

[Danielsan]

PU foam will tolerate styrenated resins, but, it's very brittle stuff. I couldn't recommend it for coring in good concience unless it is of high density, wich mitigates the benefits of sandwich construction severely. You might want to look into the Divinycell and Airex/Herex products, or even balsa, for price differences.

I was thinking about the density of 80 - 100 Kg/m3 (or more?)

I know Etap yachting uses PU, but they have a hull in hull principle filled with this PU in between, so the hulls are allready cured and the hollows are filled with liquid PU curing and (bonding) to the cured hull parts.

The concept (build up) is different but the materials are the same

as for price I don't know for balsa or herex, but density 80 in 20mm kosts around 50EUR/m2 and PU is around 19EUR/m2 hence the difference.

could be good to do some minor testing? laminating some flat strokes with the diff materials and doing some bending and slamming to check for early failures?

[SamSam]

Originally Posted by Danielsan

PU foam will tolerate styrenated resins, but, it's very brittle stuff. I couldn't recommend it for coring in good concience unless it is of high density, wich mitigates the benefits of sandwich construction severely. You might want to look into the Divinycell and Airex/Herex products, or even balsa, for price differences.

I was thinking about the density of 80 - 100 Kg/m3 (or more?)

I know Etap yachting uses PU, but they have a hull in hull principle filled with this PU in between, so the hulls are allready cured and the hollows are filled with liquid PU curing and (bonding) to the cured hull parts.

The concept (build up) is different but the materials are the same

as for price I don't know for balsa or herex, but density 80 in 20mm kosts around 50EUR/m2 and PU is around 19EUR/m2 hence the difference.

could be good to do some minor testing? laminating some flat strokes with the diff materials and doing some bending and slamming to check for early failures?

Here is how Carolina Skiffs build their boats with polyester resins..put in this patent #, 4495884, at this site, then hit IMAGES..
http://patft.uspto.gov/netahtml/srchnum.htm
Their patent expired in Jan. so it is now public domain and anyone can use the process, even commercially. Here is who makes the foam they use..
http://www.elliottfoam.com/index.html
Here is the kind of foam I used with polyester resin to make a boat just like CS does..
http://www.pima.org/
I got it from a commercial insulation place for 1/2 the price of an equilivant amount of foam from Home Depot and probably 1/20 th. the price of anything labeled 'MARINE'. It comes in various thicknesses up to 8" or so in various densities and is laced through and through with fiberglass strands like they put in concrete nowadays. Sam

[Raggi_Thor]

Interesting overview from a Norwegian college
http://ansatte.hin.no/am/studenter/modmatr/pensum.pdf

[yokebutt]

Daniel,

A couple of possible alternatives. (I prefer working around problems rather than full frontal attack)

First thought, if the boat you want is mostly developable surfaces, you could build a quick-and-dirty female mold with melamine-coated particle board or masonite. That would allow you to build the part with balsa core.

Second thought, if you're worried about cores, you can put a bunch of little stringers on the inside. One method is to wax up solid bungee-cord, put it in place, and laminate over it. Then, when you pull it out it contracts and releases.

Lastly, please use epoxy resin.

Yoke.

[dog]

pu foam had poor shear properties, use divinycell or core cell and epoxy

[JR-Shine]

Originally Posted by dog

pu foam had poor shear properties, use divinycell or core cell and epoxy

Yes. PU should not be used if you are serious. If PU was OK for a hull core, don’t you think boat manufactures would use it? There other places I would consider using PU foam, but not in the hull.

[War Whoop]

Another thing PU foam besides being the WRONG material to use has no Peel Strength until you reach the density of Granite with it!

[zerogara]

Originally Posted by yokebutt says:

A couple of possible alternatives. (I prefer working around problems rather than full frontal attack)
..........
Second thought, if you're worried about cores, you can put a bunch of little stringers on the inside. One method is to wax up solid bungee-cord, put it in place, and laminate over it. Then, when you pull it out it contracts and releases.

Yoke.

I too believe in stringers and solid laminates and I haven't tried bunjee cords but all sorts of other things to make hollow stringers on molds and moldings, but I usually end up kicking myself for when the outside (good side) ends up with a little warp/wave and destroys the otherwise perfect panel. On decks I don't care, I actually think it is helping, but on the hull bottom and especially verticaly to movement it is disastrous. I've tried all tricks I can read and think about and to some extend it still happens.

This is for a light weight (as possible) sailing dinghy, and my attempts not to use a core, which I believe is the easy way out of stringers and reducing cost /weight of manufacturing.

KoZ