Camber Loss

twizzstyle · Post by **twizzstyle** » Sun Feb 05, 2012 5:48 pm

Heating from the bottom only will increase camber (increased from the mold shape) regardless of the core material used. Heating from the top only does the opposite.

SkiTheNortheast · Post by **SkiTheNortheast** » Sun Feb 05, 2012 5:56 pm

so the only way to create camber is with heat? an unheated, or top and bottom heated mold with camber won't leave the ski with camber?

twizzstyle · Post by **twizzstyle** » Mon Feb 06, 2012 7:27 am

If you don't heat, or heat evenly from both sides, you should get pretty dang close to the exact mold shape (you might get some relaxation from the core, but the composites will overcome most of that).

SkiTheNortheast · Post by **SkiTheNortheast** » Mon Feb 06, 2012 7:49 am

oh so you only really need to heat with bamboo to get camber?
because this thread put me under the impression that you couldn't get any camber without heat, i.e. the ski will just relax to a flat shape after pressing

twizzstyle · Post by **twizzstyle** » Mon Feb 06, 2012 10:53 am

Bamboo is pretty stiff and has a pretty uniform grain, but it shouldn't be different from any other stiff wood. All wood or bamboo will want to relax to some degree.

There's lots of variables, a big one is the epoxy you're using. Only way to really know what results you'll get with the combination of the core material you're using, and the epoxy you're using, is to build some test pieces and see what you get. If you get the camber you wanted, great, if not, then exaggerate your mold, or add heat. It's hard to precisely predict what your results will be until you get at least one test piece pressed, because so many things can differ from person to person.

SkiTheNortheast · Post by **SkiTheNortheast** » Mon Feb 06, 2012 10:58 am

alright I get it now, thanks for the info

looks like I'm going to be doing some experimenting

bullion mine · Post by **bullion mine** » Wed Mar 28, 2012 11:18 am

I have been dealing with some camber issues also. My bottom mold is such that I should have about 7mm camber/ski (14mm for both). When I pull the skis out of press I end up with about 7-9mm for both skis not 14mm.
I heat top and bottom @ 150 *f and pess @ 50 psi for 90 minutes and then let cool down under pressure overnight.
Poplar/basswood core 2.5 - 12 - 2.5. 20oz FG top/bottom. supersap epoxy.
Is that normal to lose that much camber with top and bottom heat? I read an old thread about this but it was unclear about the amount of heat they were using and if it was top and bottom. I understand that limiting heat on the top or bottom can effect camber but won't that affect the bonding of my epoxy. I would like to use heat top and bottom for best bonding. Any suggestions?
Would you recommend 150*F bottom 125*f top. I hate to tweek the heat when the real solution is something I am not thinking of. My press deflects about 6mm in the center but mostly on the top I-Beams 4mm and 2mm on the bottom, this should only increase my camber a little right?

PTTR · Post by **PTTR** » Wed Mar 28, 2012 11:57 am

On my last pair (my second) I heated from the bottom on a flat mold and came out with 16mm of camber! I use PRO SET epoxy ( WEST SYSTEM? ).

Anyway, I took them out hot after 4 hours at 80C (1 hour to heat up, 4 at 80. It is a slow set epoxy). Had to rush home and came back 2 days later. They still had 16 mm of camber. I cut them clean and sanded the bases and all of that. Everything was hard, no extra ordinary epoxy filling the sandpaper. when the skis where finished I put them back in the press with heat form above and pressed them flat for 1 hour. And they stay flat!? I have skied on them for 2 days and and still no change. I don't get it and niether does the epoxy man I bought it from but he did say that 80 c (175 f) was VERY high. He advised me to never go over 65 c (150 f).

twizzstyle · Post by **twizzstyle** » Wed Mar 28, 2012 12:33 pm

bullion mine wrote:My press deflects about 6mm in the center but mostly on the top I-Beams 4mm and 2mm on the bottom, this should only increase my camber a little right?

Any deflection (i.e. expansion) in your press frame will only decrease camber. You are bending the mold in a reverse-camber fashion.

How are you measuring that 4mm and 2mm compared to top and bottom? That is certainly part of your problem, although a small part (2mm out of your 7mm deficit).

Are your composite layers oriented symmetrically about the core? If the bottom layer of triax has the longitudinal fibers facing out - towards the base - then the top layer should have the longitudinal fibers facing up, towards the top sheet. Or vice versa.

If that's good, then you may just have to tweak your heat a bit until you find a sweet spot. Maybe go 10 deg hotter on the bottom, 10 deg cooler on top, and see what you get.

MontuckyMadman · Post by **MontuckyMadman** » Wed Mar 28, 2012 5:28 pm

the super-saps call for a 180F cure not 150. read the data sheet.
no one cures under pressure to cold.
the laminate is cured after its time at temp aside from the 5% post cure you get.
There is a post cure schedule recommended as well.

Pull it hot 150F ish and put it on a camber rack.

I bet your bases are concave as well, curing under pressure to cold will do this.

bullion mine · Post by **bullion mine** » Thu Mar 29, 2012 8:47 am

Thank you for the replies. Twizz. I never paid attention to the fibers on the fiberglass. I think I can see what you are saying and I will try to orient them as such. Basically I lay the FG with the long fibers facing up on both layers. I measured the deflection in my I beams with a string line and with a tape measure before and after pressure.

MM thanks for the reply about the temps and cure. I thought 180*f seemed really hot but I will try it out. Entopy also recommends only 20 minutes at 180*f but that seemed a little short. Are you also saying that I should release the pressure after my 90 minute heat cycle and my PID's read @ 150*f and let it cool down the rest of the way without any pressure? I am guessing at my formula of 50psi 150*f for 90 minutes. I have read posts about this subject and the answers are usually too many variables to recommend a psi / temp. / time for pressing but if you guys had any suggestions about how I might tweak any of these variables (with heat top and bottom) for better results I would appreciate it. Yes I have had some problems with concave in bases and edge tabs dimpling bases. MM you also said a camber rack? Is that a 2 dowel rack like I have seen on this site and if so does it matter where the 2 dowels are placed for best results.

I wish I could show pictures to explain my scenarios and to help others but I cannot figure out how to put pics on these posts. I have a MAC desktop if anyone could help with this. I tried to drag, cut and paste, but all that transfers are a bunch of letters about the picture and not the picture itself. I feel stupid asking about this but I have tried several times and spent lots of time trying to add a picture and can't figure it out.

twizzstyle · Post by **twizzstyle** » Thu Mar 29, 2012 9:30 am

You need to upload your photos to some web-hosting service first. Check out photobucket.com, it's free. Once the photos are uploaded there, you can post the URL on here with the IMG tag and they'll show up in your post.

You should trust the recommendations from the epoxy manufacturer, they will know best. I'm using Entropy as well, and for a few pairs I was doing 180 deg (only from one side) for about 30 minutes, then pulling them hot.

For the last two pairs I brought the temp down to I think 150deg (I was having base convexity issues, so it was an experiment) and let them cook in the press for about two hours. Pulled them hot as well.

MontuckyMadman · Post by **MontuckyMadman** » Thu Mar 29, 2012 9:55 am

things to keep in mind. follow the directions from entropy.
I do cook for about 40 min.
you have to rabbet the core for the edge recess.
The bases are concave because you held them under pressure till cold.
yes pull and put on a rack that mimics the camber you are shooting for.
when hot the ski is slightly malleable so be careful.
dont torque it all up.

bullion mine · Post by **bullion mine** » Thu Mar 29, 2012 10:17 am

I just spoke to Bob at Entropy (he is the new Desi - who is in France now) and he said to cook at 180*f for 16 to 20 minutes and then pull from press and put on a rack or surface that mimics the camber (like you said MM). Would leaving it on the mold to cool with no pressure be a good idea. He also said you should be able to work the ski's in 1 to 2 hours (cutting flash / router sidewalls). I am not sure I would start over handling the ski's this soon.
I started rabbiting my core for edges and it helped but I also got a little con-caved trough where the width of the rabbit was wider than the edge. I hope changing the way I am pressing will help with this.

vinman · Post by **vinman** » Thu Mar 29, 2012 12:05 pm

Using symetrical lay up, 45s away from core. When I use entropy I press to 50 psi without heat for 10 min to spread the epoxy, then I'm cooking at 180 pn both sides for 30 min or until the epoxy that has been squeezed out is hard and then Cool in the mold without pressure to 140 deg, after this pull them out and place the laminate on its side, prevents camber loss since gravity is not effecting it in that plane. cool for 1 hr or so before flashing them out. So far cambers match mold almost perfect with this method.

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Camber Loss