Kiteforum.com

zfennell wrote:zob,
forgive me, if i have misunderstood you construction methods.
All of your boards appear to have plywood on the bottom and foam on the top ( with perhaps a bit of glass on top of the foam)
But you need to locate it between layers of strong stuff. specifically, the top layer of reinforcement should be as strong (or stronger) than the bottom skin.

If kite-stuff.org wasn't down for a couple of months now, I would simply give you a link from my report of my first 4 boards I did at the same time, but basicly that's the story. Ply for the bottom (2 X 2,5 mm), foam on the top (8-12 mm), carbon pieces under heels and everything laminated with glass.
There are a lot of how-to's, 100's of different constructions and everything ''works''. Everyone reports just about driving characteristics, but usually no one gives any feed back on long term durability of the construction.
My intention with first 4 boards was to experiment. For instance how does the vacuum bag wrap different edges (sharp, chamfered, rounded), how would x-laminating the plys cut at different angle affect the flex, different core thickness, different shapes and sizes. And learned a lot!
But I didn’t really think about construction failure. Obviously I am over-confident

You can see the second construction, if you look at first picture I posted in this tread. I replaced the foam with poplar ply, but not completely. There is still 6mm under the ply, and around the edge of this upper part of the core and around the second bottom layer ply, which is smaller (for progressive flex). This two foam edges are for cosmetic purposes only. And are the main f***-ups in this construction, because in the middle of the board this foam goes from the edge to upper ply (about 30 mm) and the glass has only this crappy material to bond to in the area which flexes a lot. And here the delamination happened (15 cm wide), and I suspect the upper core layer also delaminated in that area, because concave is most distinctive there and the upper ply wants to be flat (if you know what I mean!?).

Now I am replacing foam edges with wooden ones and routed the core foam layer for 15 mm and applied 30 mm wood stripes (15 mm come out of poplar core, to sand it down as previous) and I also applied 2 20mm stripes longitudinal and 1 across (in the middle). So there is direct wood to wood connection between the upper and bottom core parts, but the board looks the same. Hopefully it won’t be to stiff now. Foam is now just a filler for middle core layer.

I wish this board will last at least one year Now I will do extensive tests on this one, before I start to modify the old ones if the construction turns out to be OK.

If you use 9mm corecell core and 20oz eglass each side it will last forever.
I think you will find it to be the most cost effective way to make a board. Plywood is not all that cheap. The corecell for a board is only $20.

Stan

zob wrote:.......This two foam edges are for cosmetic purposes only. And are the main f***-ups in this construction, because in the middle of the board this foam goes from the edge to upper ply (about 30 mm) and the glass has only this crappy material to bond to in the area which flexes a lot. And here the delamination happened (15 cm wide), and I suspect the upper core layer also delaminated in that area, because concave is most distinctive there and the upper ply wants to be flat (if you know what I mean!?)....
.

I think i get it.
but if the net result is 5mm plywood / 8 mm foam/ 3-5 mm plywood you should know that the section of foam around the perimeter of the board was not simply cosmetic. It did not fail because it has crappy bond properties. it failed because it had crappy shear properties.

the top and bottom sheets of plywood are very strong
when you bend them they try to slide past each other. the foam is needed to resist this shearing motion. The glass around the rails/foam edges also provides much of the shear strength of the sandwich, but i fear you have treated this as a cosmetic element as well.
think about this:
drill a hole thru the middle of your board just large enough to fit your finger.
put your finger in the hole and try to bend the board.
the relative motion of top and bottom sheet will try to shear (cut off) your finger if the core cannot carry the necessary shear stress.

adding the wood stringers that you have suggested will solve the shear problem.
however, as you have also suggested the foam is no longer required at all
(you've now designed a hollow board )
i suspect that if you used 3-4 layer of 2 mm plywood with no additional core and no stringers the board would also last a long time.
that would also reduce the overall thickness and give you back the flex you are looking for..

or you could just try it stan's way.

regards,
-bill

Hello,

EPS = Expanded polystyren. This is a moulded foam of small EPS balls. Depending on the density of the EPS foam and the ball size you will have very different mechanical properies, just like every other foams. The normal EPS we all know from food packeging, transportation boxes etc. are low density, down to 10kg/m^3, but you can it up 100kg/m^3 and the the hardness and pull out strength is like pine wood

XPS = extruded polystyren, comes in alot of different colors and som densities. The structure is like a "normal" pvc, PE, PU etc foams. Fine small closed cells. I can't remember about the mechanical properties. But so fare I haven't seen this foam used for sandwich construction s in other technical issues, which means it is properly not the best mechanical properties.

Kind regards from Denmark
Lars