HOW TO: Make a Carbon Fiber Part (Part 5: mold prep and cutting CF)

HOW TO: Vacuum formed Carbon Fiber Part for $200 (Part 1)
HOW TO: Make a Carbon Fiber Part (Part 2: making a clay mold)
HOW TO: Make a Carbon Fiber Part (Part 3: making a fiberglass mold)
HOW TO: Make a Carbon Fiber Part (Part 4: making the budget vacuum bag)
HOW TO: Make a Carbon Fiber Part (Part 5: mold prep and cutting CF)

HOW TO: Make a Carbon Fiber Part (Part 6: vacuum bagging CF in mold)
HOW TO: Make a Carbon Fiber Part (Part 7: two part piece and final steps)

So now we move to the step of cutting out some Carbon fiber cloth for your project. The easiest way to measure how much you need is to use an old shirt or piece of fleece. Using a large enough piece of material, press it into the the details and crevasses of the mold. When you make a part mold make sure to be mindful of the limitations of vacuum bagging to make highly complex parts. To narrow or deep contours for the vacuum and material to conform to will result in a flawed part. This part will be a two piece part for that reason.

After the cloth is adequately representing what a piece of carbon fiber will contour to in the mold cut the excess off this substitute fabric as you are going to be using it as a template to cut out your Carbon fiber... obviously leave some excess for good measure.

The reason you want to use a substitute material for measuring is because the more you handle dry carbon fiber cloth the more the weave will distort. The strands pull apart and bunch up and cause what's called "crimp." Crimp not only translates to a flawed pattern, it's also not as structurally as strong.

If you are using two different weaves or types of Carbon fiber than obviously the first layer you cut out should be the pattern you want showing if you have a negative mold like this. Also take note of the orientation or "grain" of the fabric weave (like wood grain) as you cut the carbon fiber piece to place in the mold. Do you want the pattern to be horizontal, vertical, or diagonal? 

after cutting out the piece you want you can tape the edge of the CF on the roll to prevent distortion while the roll is stored... not everyone does that, so it's up to you.

So I used one layer of 3k 2x2 twill and two layers of 12k plain weave. Please don't be a poser and use just one layer of CF for the outside layer and then use fiberglass for the subsequent layers... if you're that cheap of a person then safe yourself the time and just buy some "carbon like" vinyl sticker and cover the part you're trying to make. For a part like this you'd save maybe $5 by doing that. 

After you get it cut out you need to weigh it in order to later calculate how much resin to mix up. This is a cosmetic part that I'm not worried about structural integrity or minor weave flaws in so I obviously didn't care that folding CF isn't a good idea if you want perfection. To avoid folding for large pieces of CF you want to weigh than you'll need a large enough piece of clear acrylic or equivalent to place on the scale first. (don't forget to zero it out)... then place your CF on the clear plastic... you can view the read out through the clear plastic. A 2"x2" sheet can be bought at most hardware stores for $5-$10. Or if you're budget savy you can borrow the glass shelf out of your fridge or the 3"x3" grate out of your oven for free

When you cut out the layers of carbon fiber most fabricators pay attention to detail and cut out the layers as such the weaves alternate in different directions every other layer. I figured: "why not?"

After you cut out all the layers of CF you need to make your alternate "peel ply" that'll serve as a porous barrier between the CF and the breather cloth that'll be soaking up the excess resin. Without a "peel ply" type layer the breather cloth would permanently cure to the CF.

To emulate what is peel ply I simply took cooking wax paper (not parchment paper, tin foil, etc) and used a wire wheel or even a wire grill brush and poked into the wax paper. Being careful to only create the thousands of pin holes, not tears through which the CF could bond to the breather cloth through.

Before you mix up your resin you'll want your mold to have the mold release on it. "Mold Release" is very expensive... so are releasable coatings that you can put on your parts. I learned from youtube that Carnuaba Car Wax is a perfectly acceptable alternative... problem was the guy in the how to video didn't explain how to apply it right. So the correct way is to use the sponge that comes with it and apply a layer to the mold. Let it air dry completely... don't slop on the layer or feel the need to rub it in etc. Just nice even smooth layer like you would if you were to be applying it to wax your car... after the layer dries then apply about 2 more layers in the same way each making sure not to distort or smear the thin layer below it. And do not remove it (like waxing a car).

(left) before (middle and right) after one layer