Concrete molds ?

[twizzstyle]

when you pour in the concrete its going to want to bulge that board out

ahhhh, yes good point. gravity, thou art a heartless bitch hha

Not just gravity, but hydrostatic pressure. Heartless bitch with a knife.

You can get around that with a stiff enough board (has to be flexible enough to take the profile shape though I suppose), but EVEN then. It'll need some kind of frame work to make it flat. Doesn't matter with the skateboard mold cause the entire deck is concave, and even if the mold comes out off-nominal, who cares? Won't effect performance. I just know even with my routed MDF rib molds I have a bitch of a time getting flat bases (they're never flat, but close enough I have enough p-tex to grind through to make them flat)

[a.badner]

All valid assumptions, but in case this does happen;
- it is extremely cheap, and easy to retry
- the concrete/cement is sandable. ( not a great to fix, but it is doable )

Im not sure if the density really effects the "hydrostatic" pressure, ( as i have study this "art" yet ) but concrete mix is really thick, and i think it will just slop/pile up in there instead of balancing out as water does.

also, original plans were to make a no-camber park ski, or a light camber, so that the the curves don't really compromise the structural strength/integrity of the casting mold.
possible results.

Also, i dont see much to be lost if this does happen, this is relativley cheap experimenting.

I just wonder, "what if this does work better than mdf" have i changed the face of skibuilding? ... lol
Im not entirely sure that you will understand what i mean, for my visual/structural perception is not communicated very well, but please do ask because i am not (yet) an engineer and i do wish for the best

[twizzstyle]

All valid assumptions, but in case this does happen;
- it is extremely cheap, and easy to retry
- the concrete/cement is sandable. ( not a great to fix, but it is doable )

Im not sure if the density really effects the "hydrostatic" pressure, ( as i have study this "art" yet ) but concrete mix is really thick, and i think it will just slop/pile up in there instead of balancing out as water does.

also, original plans were to make a no-camber park ski, or a light camber, so that the the curves don't really compromise the structural strength/integrity of the casting mold.
possible results.

Also, i dont see much to be lost if this does happen, this is relativley cheap experimenting.

I just wonder, "what if this does work better than mdf" have i changed the face of skibuilding? ... lol
Im not entirely sure that you will understand what i mean, for my visual/structural perception is not communicated very well, but please do ask because i am not (yet) an engineer and i do wish for the best

Yeah its cheap, but if it doesn't work, you have a 6ft long giant slab of concrete to dispose of. How do you take care of that? Sledge hammer and a bunch of muscle.

Your base material is 1.3mm thick (+/- a bit maybe). That means you've got 1.3mm or workable error in the mold flatness, if you have anymore, the skis will be ungrindable. It's just going to be tricky, but there are ways to support the mold.

Are you going to be pouring concrete on both sides (to create a top mold as well) like the skateboard, or just the bottom? If you did both sides you wouldn't have to worry about the hydrostatic pressure (but you'd have an extra top mold that you don't really need). The word you're looking for is viscosity, not density, and it doesn't really play into hydrostatic pressure.

I don't think you're going to be changing the face of anything, but don't get ahead of yourself until you've actually made a mold and know for sure. It's better to have low expectations so it's more exciting when things work out

[twizzstyle]

Also, a cambered casting mold (I'm talking the wood sheet that is the form for the concrete) will be WAY more structurally sound than a no-camber mold. You've got your assumption backwards there

Take a piece of paper and hold it vertically over a table. Push on the top, what happens? It buckles. Now roll the paper up and do the same thing, now it's super strong.

science.

[a.badner]

no, im making a concrete top mold too.

viscosity, that's what i meant

but, about the camber vs no camber, no camber would be better for top an bottom mold. I never really worked with concrete on a artistic basis, but i know that it expands a little bit, and having concrete pushing up on a bowed ( cambered ) section while its casting may be a problem... but then again there is a equal force pushing back, nvm i dont know.

but top mold was in the plan for sure, may be not for use, but to make the bottom mold its pretty much necessary.

Shouldnt you be at work right now?

[twizzstyle]

but then again there is a equal force pushing back, nvm i dont know.

You nailed it. I kept thinking you were doing the bottom mold only for some reason. Camber or not it will make no difference.

Good luck! Curious to see how this pans out.

(yes I was at work)

[ru-tang]

Maybe I am not reading all this correctly- but . . .

Beyond the weight and handling of two big concrete molds, I kind of like the simplicity of the idea. If I am reading the idea correctly, then I don't think it will work very well-

Here's what I think you are talking about- making a long skinny piece of concrete that has your ski's profile, basically the bottom half of the mold. You would rely on the profile of the top edge of the mold to pour and then scrape the top after it is poured, and hope that it sets at that profile?

In order to do it, you will need to cast it upside down- so that the concrete sits on the most important part of the mold. Let it set, then turn it right side up.

And yes, you will need rebar, chicken wire might not be strong enough, but rebar is cheap, just a couple #4 bars should be fine. Might not be a bad idea to cast some metal pipes through the mold to act as handles or something.

[fa]

hey badner, thats intresting
beware of the weight as concrete is about 2,5 tons/m^3:
a 7cm thick slab, 180*33 (for 2 skis) will be 100kg
there are concrete additives (plastisizers etc) to increase strength and reduce shrinkage. try to use a very dry mix. the less the water the greater the stregnt. key to achieve a good dry mix is good aggregate gradation. compact the poured concrete well ie by tapping the mold. concrete takes weeks to fully cure. keep it moist in the first couple of days to avoid shrinkage cracks
reinforced it as suggested
good luck if you go for it

[MontuckyMadman]

plasticizers yes and perhaps rubber or plastic balls or just newspaper to try and reduce weight.

[threeninethree]

You might also want to rent a concrete vibrator.

[MadRussian]

besides concrete you can try: topcoat one don't have rocks in it, grout, nonshrinking grout, masonry mortar.

Personally wouldn't even try concrete for mold making. I thought about epoxy mortar instead.

[a.badner]

ok little update,

i bought four bags of this

http://www.homehardware.ca/en/rec/index ... lastisizer

it has plasticizers in it. not as cheap i as i thought it would be. :s but when mixed with water, it should be enough for two molds.

also, i bought 10m of 8 gauge chicken wire. believe it or not it was in the concrete section, the guy workin in the store said that most people use it for concrete anyway so they put it there. AND i bought four rebar rods. two for top two for bottom

ALL of this for only $56 dollars. but i did get the concrete for cheaper because it was on sale, so next time it might be not that inexpensive.

today i spent building a box and lining it. next weekend i might pour some crete.


BTW

great personal message MM ;)