hydraulic ski press

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bigmoq
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hydraulic ski press

Post by bigmoq »

Hi everyone!

My friend(who own a kitchen company) and i just started to make our own ski. We have access to c.n.c machine and also to one very big hydraulic press. We have already done one nice pair of ski using the cnc to profile the wood core and the shape of the ski. To press the ski we used one big hydraulic press but doesn't use the hydraulic. We simply used the press to contain the up and down mold for the ski camber and for the pressure we used the bladder with the carpet of small tubing(too apply equal pressure).

My question is: If we have perfect mold(done by cnc) could we use only the hydraulic pressure of the press to press the ski. No bladder, no tubing, no heating blanket, only the pressure of the hydraulic press to squeeze all the material together .

Also, is it very important to use the heating blanket or is it find to let the epoxy cure by itself.

Thank you.

mattman
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Post by mattman »

Plenty of epoxies will cure just fine at room temp...just have to let them press a bit longer. Anyway, I would say that you would be fine if your mold is really this perfect. Just to be on the safe side, why not include a thin (say 1/8" or 3/16" thick) sheet of soft silicone on top of your layup. This will even out the pressure in any areas that arent exactly the thickness you planned on. There are too many variables to assume that your layup will be exactly a given thickness. For example, most triax glass has little ripples or irregularities in the glass. One rouge tow of glass and topsheet isnt perfectly flat. A thin 3/16" soft buffer will prevent you from getting resin wells in imperfect areas.

burny
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Post by burny »

bigmoq: I've done my skis with the kind of press you describe, with an upper and an under mold. both were cnc made. it worked pretty good, however, I used a 1cm sheet of rubber to equal everything.

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falls
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Post by falls »

Sorry to dredge an old thread. With this hydraulic pressing method people are saying the top and bottom moulds have to be perfect. The bottom mould I understand is like any other bottom mould. Should the top mould match the bottom mould exactly or should it be contoured to the predicted top profile of the layed up ski using CAD?
If I make a top mould that exactly matches the bottom will a strip of rubber between the layup and top mould be ok or do I need to try and cut the top mould to match the core profile and ski length?
Burny if you could tell me what you did with your moulds using the veneer press I would really appreciate it. Thanks

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nate
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Post by nate »

You would need to account for the thickness of your ski layup (and any rubber) when you make your top mold. For example at the tips where your ski core is profiled down to 2mm you would have the top mold closer to your bottom mold as opposed to the middle of the ski where it's more like 10-12mm.

knightsofnii
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Post by knightsofnii »

you have to make sure your material thicknesses are pretty much exact too.

For instance the transition points between core and tip fill need to be exact or else any difference and the mold wont squeeze down. That's also why the rubber sheet is a good suggestion.

And if youre putting in inserts, make sure they dont stick out above the core, this will stop the mold from clamping down all the way.
Doug

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falls
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Post by falls »

Thanks for the tips. Sounding a bit difficult without a margin for error

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falls
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Post by falls »

Does the rubber act like a miniature fire hose in this situation and absorb error in the top mould? If I made the top and bottom moulds exactly meet then used a 20mm thickness of rubber wouldn't it compress to 18mm at the tip and 8mm at the centre (given 2mm ski tip and 12mm core centre thickness) and exert force the length of the ski? The force might be uneven though. Wishful thinking?

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nate
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Post by nate »

I think the force would come out unevenly.

To be honest I think an interesting take on the problem would be to combine your plans. Instead of just using your hydraulic press to contain your molds and inflating your fire hose for the pressure, use it to give the downward pressure as you were thinking. But don't abandon the fire hose. Use it instead of rubber. Just fill it up some with some water or air and then use your hydraulic press to squeeze it together.

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falls
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Post by falls »

I think you are right Nate. I talked to my Dad and he said that over linear slopes or flat areas the force would be even, but the force around a curve is going to be variable. If you had a ski tip that made up 1/4 of a circle the pressing force at its tip would I think actually be zero and it would probably try and shear the tip down the mould. It makes sense. He also said you could work out the forces around the bend using calculus and determining areas under the curve, but that sounds a bit too high school.
I think I might make some trials around the tip curve with smaller amounts of material as I figure this is the place where the problems may lie. I am hoping that if my tips and tails don't have too tight a radius then although the pressure might not be as great there compared to the middle it might still be adequate for my needs.

I read about the fire hose filled with water and using in that case clamping pressure to increase the pressure within the hose. It seems a good idea. I just added to this post and dont have a fire hose as yet, but think maybe I should get one and combine the two ideas and also then if I lose access to the hydraulic press I am on my way to having what I need for a pneumatic press.

Thanks for your help.

knightsofnii
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Post by knightsofnii »

with pneumatic, you could go beyond that 90degree mark and still get pressure, if you built it right, not that you'd want to or ever need to, unless you were building some really wierd skis. Air bladder/hose will inflate and squeeze to whatever is in its way. But if you use it as a simple pressure equalizer with a clamp down hydraulic, it should be fine. I'd put the cat track under it to get the even side to side pressure that the airbag has trouble providing, unless you want cap or a light amount of 3D work, there's ways of using a wide air bladder to do that.
Doug

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falls
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Post by falls »

thanks
i bought a router and jig saw last weekend and will order some ski materials this week.
will post and let you know how things go.

with pneumatic, you could go beyond that 90degree mark and still get pressure, if you built it right, not that you'd want to or ever need to, unless you were building some really wierd skis.
going past 90 degrees would be real elf shoe technology (is it 4frnt that call their rockers that?)

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