Ski Builders Forum

Alright, So I'm a newbie at this game and would greatly appreciate some help.

I've pulled two skis now and both have gone flat when I pulled them off the mold. The tips are right but the camber dissapears. The mold has 10 mm of camber but both skis have come out with flat bottoms.

We are doing thinds a little differently than the how to guide. We have been vacuum bagging them instead of using a press, and we have been baking them at 120 F. The laminant has been a poplar core with 2 layers of +\- 45 200gsm carbon on the top and bottom. I work in a boat building shop and this is our standard procedure for the epoxy I've been using. I used Vectorlam blue to do a rough laminant comparison and the 2 layers of carbon should be comparable to the 744 gsm triaxial glass. I'm using carbon because we have a role of it in the basement.

The rest I'm pretty sure follows the guide but could be missing something.

I've been getting good seals on the bags so I should have close to 14 psi which should be plenty of compression. The first one I did the core was 12 mm thick under foot with a linear taper down to 2 mm in the tips. It came out super stiff and with no camber. I figured the core must have been too stiff and the straight bottom of the core dominated the curve of the carbon, driving the ski back to flat. So in my next attempt I went for the other end of the core thickness window and used a core that was 8mm under foot tapering to 2mm, using the same carbon skins. This one is super soft but still has no camber.

So I am more than a little confused. I think my next move will be to press the soft one back into the mold with some uni along the top and see if that does anything. But If anyone has any advice or can figure out where I have been going wrong I would greatly appreciate the advice.

Thanks

Oliver

where is your heat source top or bottom?

Is your epoxy for a compression molded application?

Perhaps your double layer of carbon is to ridged? I don't really know

I read in the HowTo guide that they had experinced a loss of approx 10mm of camber compare to the mold during the laminate process. I took that in calculation when I made my mold, it's 20mm. I'm using maple and pine(fir). They went "back" 10mm. So I got a 10 mm camber... Just what I wanted. ( Ithink) I will try flat (no camber) next time. Need to experiment a little..;-)

Oliver,
I think the wood in your core is relaxing back to its default shape, even in your softer skis.
You wrote that the carbon is +/- 45. Does this mean there is no carbon running along the length of your skis? Your idea to re-vacuum with unidirectional carbon or glass might solve your problem.
You might also consider making a mold with more camber and see what happens.
10mm is not a huge amount of camber so it could be overcome by the wood's desire to be straight/flat. If your epoxy is a room temperature type of epoxy it may be relaxing even though you used a heater to accelerate curing. Try leaving the ski in the vacuum mold through the entire cool down and then some.
From what I've read, if you heat from the bottom only the metal in the edges expands and then contracts by a larger amount than the other materials in your ski (metal does this anyway). This can help camber to increase. This would be a less than exact method of increasing camber.
Are you using a topsheet? If not, you are missing another layer that could help to maintain camber.
Good luck and post results on how you fix the problem!

As long as you're not using any fibers in 0 degree direction you will loose the camber regardless of the amount build into the mold. The woodcore creates forces to get back into it's original shape and there is nothing that could take the forces.....

Your laminate is just able to take torsional forces. The triax you compare it to has some amount of fibers in 0° direction.

Thanks for the help. Forgot about bent beams springing back. But I did add a layer of uni directional in and made a pair with the mold as is and skied on them last weekend. As luck would have it we got a pretty good helping of snow (I'm an east coaster so 8 inches is a lot) and my zero camber fatties were in their prime conditions.

I'm reworking the mold now, but was psyched with how good the skis were. I'm actually really kind of shocked how easy this actually is. With all the marketing crap the ski industry spouts you would think designing and building skis was rocket science. I only started thinking about this a couple of months ago and after only a couple of miss steps I was able to produce a perfectly serviceable pair of skis, it kind of blows my mind. But then again Peter Harken was wise in saying "Any asshole can make one of something."