foam and/or honeycomb

[BigG]

I would like to ask you what would be the best way to work with honeycomb and/or foam. Would it be possible to get the camber in it by using a heated press or should it be routed with camber?

Geoff

[davide]

Both are very soft (before lamination), softer than wood, so they will easily follow the mould camber, even at room temperature.
Regarding kevlar honeycomb, it seems it is quite difficult to machine...

I am planning to make super light skis using balsa end grain as core. It is easy to machine and it is cheaper and stronger than Airex.

[BigG]

The balsa end grain option looks very promising. A ski core would be about 100 to 200 g based on the information I was able to find here:
http://www.balsasales.co.uk/technical_info.html

[collin]

I've been thinking about balsa too. But why are you guys thinking about end grain balsa, instead of having the grain run the usual way?

[collin]

bump.
Silly software locked the thread. So guys, why endgrain?

[littleKam]

i think burton used to (or maybe still does) use end grain balsa in some of their cores. not too sure so don't quote me on that.

[G-man]

I was doing a patent search a couple of evenings ago and I ran across a page where some one had patented, or was attempting to patent, the use of end grain balsa wood as a ski core material. I thought that I saved the page, but I can't find it at the moment (it was late and I was quite groggy). I did find another page a few minutes ago that gives a lot of information regarding the beneficial properties of end grain balsa as a core material. The information may or may not be very helpful, but it does demonstrate that one can find a lot of great information on the patent office site. Heres the page:

http://patft.uspto.gov/netacgi/nph-Pars ... rain+balsa"

Wow, I hope that the link works. I'll test it first.

Okay, I tested it in 'Preview', and it worked.

Cheers,

G-man

[davide]

In a true sandwich construction, the core material must be stiff in both compression and tension perpendicular to the flat face, and have a high shear modulus. The stiffness in the longitudinal direction is much less important, because it will come from the top and bottom carbon fiber skins.
The highest compressive, tensile and shear modulus are found in the end grain panels, see here: http://www.baltek.com/data/tables/selec ... a_new.html
The other popular core material is Airex C70, the one used by Goode and former DB, but it is more expensive and have lower mechanical properties than balsa (and it is a bad smelling plastic, and not a nice warm wood…) : http://www.baltek.com/data/tables/airex ... a_new.html

The balsa core will be also much easier to machine then usual wood, and it would be fast to machine both taper and sidecut: then the core could be wrapped in a carbon fiber sock, to get a torsion box construction…

I hate the way patent are written. It is extremely difficult to understand. Anyway it seems it concern a method that could be eventually used to make a cap construction.

This is a kite made with end grain balsa: http://www.pure-boards.de/technologie.htm

[collin]

Yeah Burton is/was doing that. I can't find the patent I was looking at, but one where they're talking about "non-orthogonal" grain directions in cores is #6105991. But I'm pretty sure that Burton was using end-grain balsa mixed with vertical grain harder woods. And http://www.pat2pdf.org/ is quite useful, since the uspto site is too hard to use.


davide-

What you said about compression/tension strength perpendicular to the plane of the ski and shear strength makes sense. But wouldn't those properties of balsa be the same for any wood? If so, why does no one (besides Burton) use this grain orientation? Or am I wrong about that? Is the end-grain going to be flexible enough for tips? Or do you have to do a large tip spacer?

As for the carbon sleeve torsion box, I was thinking the exact same thing.
What weights of carbon were you thinking about? I haven't seen triax sleeves, just biax and uni. These are the options I was thinking about:

-Just a biax sleeve (torsionally stiff, maybe too soft)
-Biax sleeve + uni sleeve (maybe too expensive)
-Biax sleeve + uni fabric in sandwich construction (but I'm a little worried about getting the sidwalls on -after- the sleeve, but this is my favorite option)
-Biax sleeve + uni fabic in the core like the DB's (this is why I was originally thinking about vertical lam balsa cores, though I guess there's no reason you couldn't do this with end grain)

But some of these options would involve multiple step layups (probably?) and other issues... I really just need to sit down and do some math to see how stiff things would end up. Sorry for the thread drift...

[davide]

What you said about compression/tension strength perpendicular to the plane of the ski and shear strength makes sense. But wouldn't those properties of balsa be the same for any wood?

Yes, it is the same for any wood, but the other woods are too heavy, so it does not worth to use them as end grain core. The goal is not to have the highest shear modulus of the world, but to have it large enough so the ski will not break. With a standard wood used as end grain, some mechanical properties will be overdimensioned.
The best would be to match the mechanical properties of the skin materials with those of the core, taking into account the thickness of the ski and calculating the forces acting on the whole structure...

Is the end-grain going to be flexible enough for tips? Or do you have to do a large tip spacer?.

I hope so, I do not want to use a tip spacer. Maybe I will have to pre-bend the core with hot water, as I do usually.


Regarding the sleeves, first I will make some tests with glass sleeves (cheaper).
K2 is using 38° in their torsion box: they use a braiding machine to wrap the core with glass threads: http://www.k2skis.com/tech/tech_iframe. ... s&TechID=4

I have found only biax sleeves, and as the sleeves can be stretched, the angle can vary. I think that something around 35° is a good compromise to achive high longitudinal and torsional stiffness; as you said, it is also possible to add some UD fabrics, just in case.

[Bucky Mullet]

In my latest order from acg, I had them throw in a sample of spider foam. The stuff seems great, asymetric flex (ie torsionally rigid), cheap and light as hell. Unfortunately they only manufacture sheets that are 60" long, but expressed willingness to make longer sheets if there was enough interest. Has anyone else played with this stuff (60" is long enough for a snowboard)? and would anyone be into trying to get a group buy together (I don't know what the minimum is, but I'll get on it)
B

[mattman]

you might want to reconsider using spyder foam...from my research that foam will crush at 65psi. many presses are well over that. there are plenty of foams out there that are nearing 200psi before crushing. i would like to use a honeycomb, but cant seem to find any that are over 1/2" thick.

[plywood]

my supplier also just has honeycomb with maximum 10mm thickness. but you could put two pieces of honeycomb together, just take one sheet, glue a thin piece of wood (1mm plywood for example) on top of it and then the secon sheet of honeycomb on the wood. i made some testings in a smaller scale and this works great.
but i wouldn`t recommend skis made out of full honeycomb. first of all they would be horribly expensive. and on my small testingsheets i gained the impression that honeycomb is very sensitive to partial pressure. so if someone would step with the edges of a ski on your honeycomb model it would crush the small cells.
to avoid this effect you need a relatively thick layer of fibreglass on top and bottom of the honeycomb. and this would result in a very stiff, unrideable ski i suppose.
so my opinion is, that honeycomb just makes sence in combination with wood: some honeycombinserts in the woodcore to reduce weight.

[Bucky Mullet]

you might want to reconsider using spyder foam...from my research that foam will crush at 65psi. many presses are well over that. there are plenty of foams out there that are nearing 200psi before crushing. i would like to use a honeycomb, but cant seem to find any that are over 1/2" thick.

Mattman,
Thanks for the info. I'm using vacuum bagging for my press, so I'm stuck at < 14.7 psi ( I live at altitude, so less pressure) I've been toying with the idea of building the core like a ship's hull, using wood ribs and two stringers, or to build a truss system out of carbon fibre where the voids are filled with the foam. I know it sounds complicated, but hey...
My main worry about crushing the foam is not from the press, but from my fat ass running them into some bumps.

[BigG]

I think you want to make a torsion box ski.

In this case I think they wrap a light material into a epoxy impregnated fiber. I think the torsional stifness can be compared with the torsional stifness of a tube or maybe better an ellips tube.

I've been reading a lot about foam use in laminations and there some main things you have to keep in mind.

As plywood is writing, foam and honeycomb cores are very impact sensitive. If you use a foam, better use a PVC foam. Some of those PVC foams have a decent impact resistance and have some flexibility.

You also have to think about the bindings. Binding screws need something strong to be screwed into. You probably will have to add something into the lamination when you use a PVC foam or you will have to inject some of the honeycomb cells with epoxy when you use honeycomb.
Injection/addition = weight

I hope this helps

I have still some ongoing research.
I will share it when I make my own conclusions.