Postby **Johnny Rotten** » Sat Apr 12, 2014 3:22 am

To build on what zfennell and billie wrote and to expand on resin choice

I want to invite "the rule of mixtures" to the party

I care not WHAT epoxy I use (at least for stiffness) however I care obsessively HOW MUCH epoxy is in my part.

I suggest others should do the same.

The stiffness of the part is dictated by the modulus of the composite Ec.

The modulus of carbon fiber (Ef) is somewhere around 33,000,000 psi or up to 50,000,000 psi for TRUE high modulus carbon. (some scumbags will try to sell basic 33,000,000 psi carbon as "high modulus"...... know what you're paying for)

The modulus of Epoxy is somewhere around 400,000 psi for an average resin

Now when you put these two together. You get the following stiffness.

Ec= fiberpart+ resinpart

Ec= Vf x Ef + (1-Vf) x Em

where Vf = the volume fraction of fiber...

As I've built parts I've hit Vf's from 30% to 55%

So let's work through the numbers so this makes sense

@ 30% Vf

Ec = 0.3*33,000,000 + 0.7 * 400,000

= 9,900,000 + 280,000

= 10,180,000

The first thing you should notice here is the resin contributed almost nothing to the overall modulus.....less than 2%....i.e. resin doesn't matter much for stiffness, use whatever resin can give you a better Vf, doesn't poison you and is easy to work with.

So what about build quality

@55% Vf

Ec = 0.55*33,000,000 + 0.45*400,000

= 18,150,000 + 180,000

= 18,330,000

Almost 2 x the modulus based on build quality ALONE

What this means ISN'T that a part which has 1 layer of carbon at 60% vf is 2x stiffer than a part with 1 layer of carbon at 30% Vf.

they will have similar stiffnesses, in fact the 30% Vf will be (mildly) stiffer, cuz it's thicker and has more goo. but it's way heavier and takes up a lot of space

Modulus compares materials of the same thickness so a given thickness of a 60% Vf part will be ~ 2x stiffer than the same thickness of a 30% Vf part.

In other words I can throw in 2 layers of the same carbon at 60% Vf, in the same space as a single 30% Vf part.

So when you want to make a Thin part that is as stiff as possible, (like a hydrofoil mast) you better know

1) What is your Vf

2) how can you make it better.

In general

worry more about

Vf, Moment of inertia, fiber direction, actual fiber modulus

worry less about

weave type, epoxy type, core type

Tkettlepoint: 60tons sounds like a proper force, but please consider how to press the part or design some means to get the high tech resin OUT of your mold. I think you may have done this with the side channels but I'm not sure how you're pressing it to know if this will work or not.

It appears you have the tools to make the most amazing parts possible and you're well on your way to making them....keep at it.