Differential heating

[mammuth]

Hola!

I try to figure out some parameters to minimize production of junk protos


My molds are overengineered to compensate springback. This works perfectly with glasfibre.

With carbon fibre you can guess, the skis come out of the mold with almost no springback, so i get way to much camber, tip & tail.

I did make some notes long time ago, think this was twizz:

Yes. Heat from the bottom in a flat mold, and you will get more camber. Heat from the top, and you'll get reverse camber (or lose camber with a cambered mold).

If you have top and bottom heat blankets, setting the bottom one hotter will increase camber, and vice versa. The greater the delta between the two temperatures, the more effective it will be.

Montucky did state:

That might make sense but then why do my carbon skis have twice the camber as my glass skis. I have to cook top heavy by 25 degrees f to get normal camber in my carbons.

So far so clear. What puzzles me is the fact that imho it doesnt make sense to just heat up one side more then the other. Key should be to reach glas temp first on the top to get lesser camber. I think the final temp should be the same, just the ramp timing should be offset.

Am i right?

If so, how far should the bottom mat timing lag behind the top mat?

Timing = 25 degrees f = offset?

So if i reach 195 degrees f on top the temperature on bottom needs to be ramped up so i have just 170 degrees f at the same time.

Correct?

Ramp down in the same manner or should i hold top temp till the bottom is finished?


Thanks!

[vinman]

As far as glass goes, if you heat the bottom more than the top you will increase camber. I can see changes with 5-7 deg F differential.

I have a fixed camber mold with mix and match tip and tail molds with a running length board.

Example: If I get 6 mm of camber on on a 183 cm ski, if I shorten the length I might only get 4 mm of camber. If I were to want a full 6mm of camber on that shorter ski I would increase the bottom heat by 5-7 deg F.
i.e. 187F bottom/180F top

This occurs because of the thermal expansion/contraction of the fiberglass. When the top and bottom are heated equally the expand and contract equally when heated/cooled. When the bottom is heated more it expands more and therefore contracts more in relation to the top fiber causing the camber to increase.

For carbon it is opposite. Some types of carbon contract when heated and it may vary from what i have read. Carbon can be tricky to work with and you'll likely need to get some data from the supplier on how much/little thermal expansion/contraction occurs when it is heated during layup.

[mammuth]

Thanks Vin


Maybe i miss something, but does it mean you use the same ramp timing for both sides, just with a different top (bottom) end temperature?

I made some quick sketches, the values are not real values, just out of my memory. Degrees in Celcius.

Here its for carbon to get less camber, so more top heating.

The resin has cure temp of 10 min at 90 deg, 20 min at 80 degrees in this example:

1. Time offset, same temperatures:



2. Temperature offset, same ramp timing. Lower temp side needs to stay longer on cure temp:

[mammuth]

...the more i look on my drawings i think just the 1st drawing makes sense....

but i came up with a 3rd version, same soak temp, lesser cure temperature.

[pmg]

I build nearly only carbon skis (or carbon/flax mix now) and never had that issue - about 50% of the mold's camber is in the final ski. But I use a very slow curing resin to ensure even temperature when resin cures.

Regarding your temperature thing:
IMO the bottom should be heated more to get less camber with carbon - its exactly the opposite to glass fibres, as carbon contracts when warming. (glass, as almost anything, expands when warming).

So if you heat the bottom more than the top, the fibres on the bottom contract more when the resin cures, thus expand more when cooling down, resulting in less camber.

What could be a bit of a problem with this:
In the middle of the ski theres lots of wood insulating the top from the bottom. Close to tip and tail, the insulation gets less and so top and bottom may get quite the same heat until the resin cures. So the overall camber profile will be changed a little bit.

Best regards
Philipp

[mammuth]

I got 86% of the mold camber with carbon and fast cure.During ramp up my pids are max 3 degrees apart.

The top bottom heating for carbon is one Q. Especially cause montucky quoted to cook the top more to reduce camber on carbon. Hmmm...

The fact regarding the thinner tip/tails i had in my mind too, but two more things to consider here. 1. Fast curing (less heating trough the core), 2. Less force (thinner tip bends more easily then the center)

Tom

[mammuth]

Heres more from my notes. These are from different discussions, so no direct relation:

<Montucky>
That might make sense but then why do my carbon skis have twice the camber as my glass skis. I have to cook top heavy by 25 degrees f to get normal camber in my carbons.

..

So the ski had less camber and more rocker.
From my experience its more about what side reaches the resins glass trans temp first. If its the bottom, more camber if its the top, more rocker.
The carbon is definaetly a shinkage deal. Always the most rocker and camber, thats why you have to cure it top heavy.


<MadRussian>
Twizz if I understand correctly. Can amount of camber controlled with only heat blankets? In other words make flat bottom mold and difference in temperature between top and bottom blankets will create camber/rocker. Is this correct assumption?


Yes. Heat from the bottom in a flat mold, and you will get more camber. Heat from the top, and you'll get reverse camber (or lose camber with a cambered mold).

If you have top and bottom heat blankets, setting the bottom one hotter will increase camber, and vice versa. The greater the delta between the two temperatures, the more effective it will be.

This all has to do with the thermal expansion/contraction of the composite. It cures at some shape (the mold shape) at some elevated temperature. Then as it cools it contracts. The amount it contracts is proportional to the delta in temperature from ambient. If it was cured warm, it contracts a little bit to room temperature. If it was cured hot, it contracts a lot back to room temperature. If the bottom contracts more than the top, it pulls the bottom in increasing camber. Make sense?

[mammuth]

nobody?

[MadRussian]

heat differential do not working too well or consistently for me. Even when heating almost exclusively from the bottom I'm getting no more than couple millimeters of camber in zero camber mold… Maybe I shouldn't put carbon fiber next to the core.