heat induced camber

[vinman]

I know heat can effect camber height. What I want to know is how much heat difference does it take to create a certain amount of camber and on which side of the laminate?

I have dual blankets and want to play with heat differential to increase or decrease camber.

I'm designing a race ski for my nephew with 12mm camber but my current mold is only 8 mm so I want to use my heat to increase camber by 4mm.

[twizzstyle]

It's going to be REALLY really hard to come up with actual numbers for how much heat differential will give you how much camber change. Way too many variables, you'd have to just do a bunch of layups and experiment to create your own data set.

But the easy answer: More heat on bottom = increased camber. More heat on top = reduced camber.



Here's how you should think about it. The resin cures to a "shape" at the curing temperature. It then shrinks to ambient temperature. It's that difference between what I'll call "curing temp" and ambient temp that causes the contraction. If both the top and bottom are cured at the same temperature, they shrink the same amount, camber doesn't change. If the bottom is cured at a higher temperature, there is a larger difference to ambient temp, so it shrinks more putting the bottom in tension, increasing the camber. Same thing if the top has more heat.

[twizzstyle]

It's going to be REALLY really hard to come up with actual numbers for how much heat differential will give you how much camber change. Way too many variables, you'd have to just do a bunch of layups and experiment to create your own data set.

But the easy answer: More heat on bottom = increased camber. More heat on top = reduced camber.



Here's how you should think about it. The resin cures to a "shape" at the curing temperature. It then shrinks to ambient temperature. It's that difference between what I'll call "curing temp" and ambient temp that causes the contraction. If both the top and bottom are cured at the same temperature, they shrink the same amount, camber doesn't change. If the bottom is cured at a higher temperature, there is a larger difference to ambient temp, so it shrinks more putting the bottom in tension, increasing the camber. Same thing if the top has more heat.

[doughboyshredder]

There's more than just max temp to think about. If the top heats faster than the bottom, or vice verse, you will see different camber than your mold.

I've been playing around with this since day one. I try and bring both top and bottom to temp equally. This is tricky due to the big heat sink, that is the cat track. It helps if you don't start with a cold cat track, so I try and run the press up to temp, and then let it cool off some before loading it.

Now, to your question. If they are brought up to heat equally, as little as 5-7 degrees F will create a pretty significant difference. Of course length has a lot to do with this as well. Generally speaking a 5-7 degree difference on a 170cm laminate will create a variance of about 3-4mm from the mold. I take em out HOT, and if I have more camber than I want, I actually give it a good flex and then decamber the board while it's cooling down.

If you don't bring them up to heat evenly, all bets are off.

[Dr. Delam]

In my set up, I have found that for every 10 degrees F differential in top and bottom heat equals about 1 mm camber change from the mold.

You might find something totally different though. So many variables.

The key is consistency in as many variables that you can control. Keep the thermocouple in the same place. Have your starting room temperature the same. Keep your epoxy at the same starting temp. Temp ramp up the same. Same cat track, etc.

I had a double cat track initially for a single pressing that was a huge heatsink and then I switched to a single. I basically had to guess how much to adjust for.

I even found that different topsheet affected the camber results.

Another issue I discovered is that my skis get more camber in the cold. I have one pair that I built as flat camber and I stuck them in a freezer for a few hours. They gained about 2 mm camber vs. room temp.

Good luck and hopefully you hit it on the first try.

[vinman]

I won't have the heat sink problem with my wooden hockey stick cat track.

Bit this brings me to another question. How quickly are folks ramping their heat up?

I was thinking about using 40 deg F steps beginning at 100 and then increasing every 5 min to hit 180F then holding that for the 15 min Entropy states. I'm using super sap CPM for my next few layups.

I'll control temp differential by turning the individual blankets on/off to keep the heat with 1-2 degF durning my ramp up.

I guess I'm going to have to make a best guess on how much temp difference will give me that extra 4+ mm of camber.

I think I'll put the top heat at something like 165 and the bottom at 180 and see what happens.

Thanks for the help again. This type of info would also be wiki material.

[doughboyshredder]

I won't have the heat sink problem with my wooden hockey stick cat track.

Bit this brings me to another question. How quickly are folks ramping their heat up?

I was thinking about using 40 deg F steps beginning at 100 and then increasing every 5 min to hit 180F then holding that for the 15 min Entropy states. I'm using super sap CPM for my next few layups.

I'll control temp differential by turning the individual blankets on/off to keep the heat with 1-2 degF durning my ramp up.

I guess I'm going to have to make a best guess on how much temp difference will give me that extra 4+ mm of camber.

I think I'll put the top heat at something like 165 and the bottom at 180 and see what happens.

Thanks for the help again. This type of info would also be wiki material.

I manually ramp the bottom up to match the top. I set the top to 175 and let her rip. Takes about 7 or 8 minutes to get up to full temp. I think (haven't actually timed it).

I don't see any benefit in slowly ramping up temps, but I dunno.

[Brazen]

So many ways. I always start heating from the bottom, figuring heat rises and then bring the top into range.

[doughboyshredder]

So many ways. I always start heating from the bottom, figuring heat rises and then bring the top into range.

how close is your camber profile to your mold? After it's finished, and after it's been ridden for a few days.

[Brazen]

God how embarrassing. Insert pull-out testing, deflection testing, stress to failure testing, peel testing, impact testing. I actually have like no idea on camber measurements before and after flex, or even the difference in the way I do it. I have to go back and measure stuff now, and I'll have to get back to you on this haha. I suck!!!

[dgholstein]

So many ways. I always start heating from the bottom, figuring heat rises and then bring the top into range.

Sorry, that's not the case with conduction heating (which this is). Heat rises only in air, because heated air is lighter than colder air.

[Brazen]

Interesting. I've always cooked with the heat UNDER the pan...and I'll be darned but the food was warm all the way through. I guess it must just be kinetic excitation at the molecular level...anyway, I guess I have to turn the pan upside down now and find out for myself. (I don't care, I think I'm hilarious).

[OAC]

I think it's time for a "basic physics" thread!
Where does the heat go?

[Brazen]

Right?!? I'm crying right now from a combination of laughter and horror.

[doughboyshredder]

The heat radiates in all directions evenly. It does rise, but it also goes down. Heated air only rises. I think that's what he means.