Heat affecting camber

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Skierguy
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Heat affecting camber

Post by Skierguy » Wed Mar 24, 2010 9:41 pm

When using a heated press, how does the rate at which heat is ramped up affect the camber of a ski? Specifically when applied in a single side application. I have seen scattered information around the board about this topic, but thought I would try and consolidate some answers.
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satch
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Post by satch » Thu Jun 07, 2012 1:29 am

Hi everyone!

After lurking for a long time and reading a lot through the topics it's time for a post. I know this thread is rather old but it hasn't been answered and i want to know the exact same thing.
I want to build a mold for vacuum pressing with built in heater. I am aware of the fact that one sided heating affects camber/rocker. Does anyone have some numbers on how much the camber distortion is influenced by the heating rate? What are your heating rates when using heat from one side and from both sides? More than 1°C/min ?

I'll try to post a build thread as soon as I start the actual building.

Cheers,
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vinman
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Post by vinman » Thu Jun 07, 2012 2:28 am

I don't have info on rate of heat rise. But bottom heat increases camber top only would decrease it or a heat differential with the bottom hotter that the top will increase camber.

I heat both sides and I try to keep the temp differential less than 5 deg differen as the heat ramps up.

I do not control ramp up, i can only control temp difference with separate on-off switches for my blankets.

I general it take about 5 min or less to go from room temp to 160-180F depending on what epoxy I using.
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satch
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Post by satch » Thu Jun 07, 2012 3:24 am

RT->~80°C (Yes, I hate the imperial units) is a rate of more than 10°C/min. That is really fast. I guess it's no wonder you would get a distorted camber/rocker with heating just one side. The bottom laminate is probably set before the top reaches the given temperature. I'm aiming for a maximum temperature of about 70°C. Think I'd be aiming for a heating rate of about 1-2°C/min. Yes, it's goig to take much longer but I think it would be worth waiting if the ski isn't distorted afterwards. Not aiming for large scale production.
If anyone else has some input or maybe even tried different heating rates and their outcome I'd be more than glad.
Any chance you know a source for a programmable temperature controller in europe?

Cheers

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falls
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Post by falls » Thu Jun 07, 2012 5:09 am

I heat from top only - raise from room temp quickly to 35C, then to 80C in 30 minutes. Go from 10mm camber in the mold to about 4-5mm in the final product. Also creates a bit more reverse camber in the tips.
When flat camber mold is used I get some reverse camber when doing the same heating from the top.

I think there is more to it than heat rate. Amount of pressure, actual temperature and the rate of heating and cooling all play a part.
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twizzstyle
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Post by twizzstyle » Thu Jun 07, 2012 8:30 am

Yeah there is a LOT at play here, so it's hard to come up with any numbers, and in my opinion not worth the effort deriving the equations to calculate it. Collecting some emperical data is more appropriate (build a few pairs with different methods, and note what your results are).

But in general, as others have already said, more heat on bottom gives you more camber. The faster the rate on the bottom, the more camber.

For sake of discussion, if you heated infinitely slowly, both the top and bottom would be the same temperate and you'd get no change. If you heat really really fast from the bottom, the bottom comes up to temp quickly while it still takes a while for the heat to transfer all the way through to the top, so you get large camber changes.

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MontuckyMadman
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Post by MontuckyMadman » Thu Jun 07, 2012 10:42 am

i can increase or decrease camber/rocker by a full cm with a 15-20 degree variance in temp top and bottom while the ramp happens. These are only 120V/1400 watt blankets so 30 min to temp.
sammer wrote: I'm still a tang on top guy.

satch
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Post by satch » Fri Jun 08, 2012 4:41 am

Thanks everyone!

I want to build a backcountry ski with about 145/120/135. 350mm of nose rocker and 250mm of a light tailrocker. running length 1260. Was aiming for a slight camber in the middle to get some grip on the occasional onpiste run. I think I'll just build the mold flat in the middle(easier anyway) and try to get the camber from the heating.
I'll try to ramp it up slower and will probably only go to about 60°C. Won't be starting till mid july though. Just planing everything. Don't like it to do all that stuff on the fly.

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MontuckyMadman
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Post by MontuckyMadman » Fri Jun 08, 2012 9:01 am

also something to keep ion mind.
Heating different top and bottom will change the base shape potentially and you could end up with convex base heating from bottom and conversely the other way.
sammer wrote: I'm still a tang on top guy.

Toddh77
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Post by Toddh77 » Tue Oct 29, 2013 11:41 am

MM, I've been searching the forum to find some answers about getting convex bases. I just pulled a pair from the press, and they were more convex than ever. Ground it out, but there's almost nothing left to the base now. Not the first time this has happened, but I can't seem to pinpoint the exact problem.

Do you have any tips for getting flatter bases while heating from both sides? Using heat has been both a blessing and a nightmare all in one.

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MontuckyMadman
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Post by MontuckyMadman » Tue Oct 29, 2013 2:07 pm

The main thing will be the depth of the rabbit for the edge recess. No rabbet or not deep enough this will be the result. Shoot for at least .7mm
sammer wrote: I'm still a tang on top guy.

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tufty
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Post by tufty » Wed Oct 30, 2013 12:10 am

Surely not rabbeting would tend to give concave bases, not convex?

Toddh77
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Post by Toddh77 » Wed Oct 30, 2013 2:56 pm

i've been rabbeting the bases, not even quite as deep as the edge, and I'm still getting convex bases. I was wondering if it has to do with the heat. From my mold up, I have silicone heat blanket, aluminum sheet, cassette with skis inside, silicone blanket, aluminum sheet.

At first I was wondering if the compression of the silicone blanket was causing it to be convex. I have a sample in the press right now without bottom heat to find out.

Any other suggestions would be appreciated. This is driving me nuts.

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Dr. Delam
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Post by Dr. Delam » Wed Oct 30, 2013 3:06 pm

There are quite a few variables that come into play when trying to achieve perfect flatness.

What I have found is that if my top heat is greater than bottom I am more likely to get convex bases. If I leave the ski in the press to cool under pressure I am more likely to get a convex base. The ones I have pulled out hot went concave.

Are you using a cat track? What is your pressure set up like?

Toddh77
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Post by Toddh77 » Thu Oct 31, 2013 11:28 am

I've been using a cat track, and i've been trying both top and bottom heat. My results have been better with only top heat, but getting my skis flat is difficult. I'm pressing the skis at 45PSI and ramping the heat up to 160 degrees which is what my epoxy calls for. I let the skis cool in the press to room temp which is 70degrees.
My most common problem is convex (high center) bases. Sometimes its better than others, but I haven't been able to put my finger on the exact reason why. Bottom heat only, or heating from both sides seems to be worse. I've never had concave (railed) skis come out of the press.

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