layup symmetry confusion

General Composites Discussions Composites Talk
#1

So i am building a CF/glass skimboard. Since I am new to composites the balanced layup has got me confused. So far I have tested shape and rocker not worried about orientation and did 6k ±45 carbon fiber on both sides also 3in strips of uni down both rails in between carbon and core, using epoxy and post cure in the car. It kept its shape just fine little if no warping. Now I am making my real board and want to make it right. Here is what I am getting on the right way to do it.

4 layers total

0/90
±45
core
±45
0/90

8 layers total

2 layers 0/90
2 layers ±45
core
2 layers ± 45
2 layers 0/90

So here is where I am confused I want to use 6 or 5 layers of thickness to reach desired weight stiffness. 4 layers to much flex and 8 layers to heavy and stiff.
can I get creative and do something like this and not have to hide it under a towel out of sunlight for fear of it warping on me?

0/90 carbon
± 45 glass
0/90 uni strips
core
±45 carbon
±45 glass
0/90 carbon

#2

I my experience a balanced laminate is something mostly associated with pre pregs and less of a problem with wet layups. I can imagine a single skin layup may have warping issues in the sun on a hot day but a sandwich construction should be pretty stable. Boat builders make yacht hulls with unbalanced sandwich construction all the time, that’s normal!

Balanced is more like this or any variation of it that is symmetrical about the centre line/core.

0/90 carbon
± 45 glass
0/90 uni strips
core
0/90 uni strips
±45 glass
0/90 carbon

#3

but boat hulls have a lot more stiffness from their curvature, where a foam skim is only curved in one direction. Glass shrinks more then carbon too :slight_smile:

a good postcure on the mould itself helps a lot too. If there is a tension from the different layers, it will show when you release it.

#4

Yeah boats have a lot of curvature but sandwich construction is still pretty stable. If it’s a concern then just don’t mix carbon and glass. All carbon or all glass and any unbalanced laminate should be fine.

Do a test part to prove what works or doesn’t, they’re not a big expensive part so not great loss if it f##ks up!

Do one that is all glass with the black fibreglass fake carbon??

#5

Good thoughts guys thanks for the input. Susho what do I look for when you say tension will show when you release it. I had texture or print through on the mold side that wasn’t on the mold could that be tension?

#6

The core will resist some of the springback, but depending upon type/thickness this may or may not be a problem.

If you want a truly flat/balanced board you’ll need to keep your lay up balanced and symetric. Prepregs do suffer more due to the temerature effects as any internal strain will be locked in at temperature. But wet lay will still shrink to a lesser extent - throwing more material into the mould typically makes the problem worse (more material equals more shrinkage).

Print through is unlikely to be from the ply stacking sequence - but can be from your resin choice, rate of cure etc…

Also, as you skim board is going to have curvature you can use the stacking sequence to increase or decrease the curvature. As 0’s don’t contract as much as 45’s. But best to keep the layup balanced and symetric in my opinion.