Carbon ski troubles

[NiekvG]

Hi,
I'm new here. I'm trying to build a set of carbon touring ski's. I have a pneumatic press (I-beams with bladders) with silicone heating mats on top and bottom. I've been having some troubles, maybe someone here can help me?

I'm having some problems with getting ski's out of the press with the right amount of camber. I was pressing at 100C between aluminum mould plates. My camber mould has 5mm camber, but the ski's came out with 30mm camber. I know changed to a different epoxy system and tried curing at 60C. Now my camber is 10mm, so 5mm to much.

My layup is as follows:
-0,3mm nylon topsheet
-160g flat tow 0/90 carbon
-2x 100g ud carbon
-300g biax carbon
-Bcomb balsa core
-300g biax carbon
-3x 100g ud carbon
-1,2mm base

Does anyone have an idea what could cause this excessive camber and how it could be remedied. I've read some post about differential temperature curing, but most are about glass fibre ski's. Would the statement for glass be the other way around for carbon? Does anyone have experience in this area? Thanks.
Niek

[chrismp]

Do you measure and control the temperature for your top and bottom heat blanket separately?

I also noticed that your layup is slightly asymmetric on the top and bottom of the core, which might also have an impact.

[NiekvG]

Yes, I measure and control the bottom and top seperatly. I measure the temperature on the top and bottom mould plates. I also placed some sacraficial thermocouples inside a testski to compare the measurements to see if the temperature on the mould matches the temperature of the top and bottom laminate, which it does.

I agree that a assymetrical layup might have some effect. But from what I've read in another thread, it is thought that more carbon on the bottom would make a ski with less camber because it would have more expanding force during the cool down (with negative CTE and all).

[vinman]

heating issue? Bad tthermoscouple? Uneven heat? Bad blanket?

[24Dave]

I use carbon in sometimes asymmetrical layups and have not had issues if I cure the resin at about 60C or 140F for a :40 minute cure. There are a lot of factors though with your resin type. Oops, I see you have tried lower temperature cures already.

[mikic1]

One potential downside of carbon fiber is its negative coefficient of thermal expansion. This property means that when the material is heated, it will actually shrink.
Since you have assymetric layup, I would suggest you either
1) Change your mold to compensate for this 'springback' factor. However check your skis at minus tempetratures first perhaps and measure camber.
2) Press at room temperature or closer to it...
3) Change to symmetric layup.

Hope you get it fixed.
M