This is the procedure to make an ogive carbon fiber (CF) nosecone. If you find any steps which are unclear, erroneous, unnecessary, or need improvement, please add an issue for it on this repo.
To form a thick block of foam, laminate boards of insulation foam together with epoxy. Spray adhesive should not be used, since it will delaminate during machining. You may need to apply weight to the foam boards while the epoxy cures. Formular styrofoam is probably the most convenient source for this foam. Use the most dense version available to you.
After allowing the epoxy to cure, secure the block to the table of the CNC mill using VHB tape. When cutting the foam, it's best to have a vacuum hose attached near the cutter, to collect the fluff created by the foam.
Apply two coats of room-temperature curing epoxy to the plug, allowing 24 hours between the coats. 40 minute F-2400 epoxy is recommended.
Note:
Don't mess with the epoxy after it's passed its working time. If you do, it will clump up and form a rough surface.
Don't apply an overly thick coat. This will change the final geometry of the nose cone.
Don't apply an overly thin coat. The point is to form an air-tight seal over the foam.
After the two main coats have been applied, check for any spots which may provide a path for gasses to reach the foam. Later, when the gel coat is applied, any such paths will allow the solvent in the gel coat to reach the foam. If this happens, gas will bubble out of these paths, forming bubbles in the gel coat which must be patched later on. You will get a better nosecone for less work if you seal these paths now. You should look for any pinholes in the seams between the foam blocks or any areas where the coat does not appear to form a smooth, continuous surface. After the second coat has cured for 24 hours, apply epoxy to any such spots as necessary.
After being coated with epoxy, the plug may have rough or sharp protrusions where the epoxy soaked into fuzz. These should be removed by gently running a piece of ~600 grit sandpaper over the plug.
Before beginning the layup, precut all the layers of CF you will need. Remember that you will need a few extra inches on all sides, especially the tip (front) and tail (back). For the LV3 nosecone, this is three bolts of 13 inch by 5 foot bolt of twill weave carbon fiber. You can mitigate almost all of the CF dander by putting masking tape along the lines you wish to cut and cutting along its center.
Note: This carbon fiber "dander" is very fine threads of carbon fiber which can stick to clothes and be inhaled! You should take it seriously and clean it up promptly. ALWAYS wear respiratory protection when cutting dry CF! What people usually think is an individual CF is actually a yarn. (Yarns are grouped into tows which are woven into a fabric.) The individual fibers are nearly invisible. When cutting CF, some fibers inevitably get double-cut, creating very short dusty strands. These can only be seen when illuminated from the side with a bright, directional light.
Using a paper towel, apply a thin paste wax to the plug. Be careful not to apply so much pressure as to remove the wax you're applying (or to crush the foam!). You should just be able to see a difference in the surface finish as you apply the wax.
You can use a heat gun to accelerate the drying of the paste wax. If you do, be careful not to melt the foam. If it melts, it will become darker. So, if you see it darkening, stop heating!
Apply two more coats of wax. These coats should be a little thicker. The surface of the plug should appear slightly lighter with these coats and the surface finish should be matte. Keep in mind that the solvent in the current coat will diffuse into previous coats. If you're slow and heavy-handed in applying the coat, you may dislodge pieces of the previous coat. This is hard to see, so err on the side of quick and gentle.
Using chip brushes, gently apply a layer of gel coat to the plug. Be careful not to apply any gel coat to the unsealed parts of the plug. This will melt the foam and make a mess. Focus on covering the whole surface. On this first coat, it's okay if there are some brush streaks where you can see the plug through the gel coat.
Make sure there is a patch of gel coat far away from the molding surface of the plug (somewhere on the flat) that you can use to test how much it has cured. To test this, poke it with a stir stick or gloved finger. While it's in its working time, it will be a runny liquid. After the working period, it will be thick, very non-newtonian, and show some texture when stirred/brushed. Eventually it will gel, becoming a weak rubber-like layer which leaves a residue when touched. (This is when the next coat should be applied.) As time passes, it will become firmer and leave less residue. Regular gel coat will never fully cure on any surfaces exposed to air. (This is not true of gel coat which has been mixed with wax.)
Allow this first coat to cure for 30 to 40 minutes. This time depends heavily on the ambient temperature. If it's cold, you may need to wait longer. If it's hot, you may need to move onto the next step sooner.
Once the first coat is gelled but still tacky (!), apply a thicker layer of gel coat. On this coat, focus on covering any thin spots in the first coat. If there are any holes in the epoxy seal forming bubbles in the gel coat, be careful not to pop these. Gently paint over them.
While the gel coat is still barely tacky, use chip brushes to gently apply a layer of epoxy (two batches, 60 g each). This should cover the whole gel-coated area and be thicc on the top and near the tip.
Immediately after the previous layer of epoxy, apply the next layer of twill weave carbon fiber. The CF should be held by two people: one standing in front of the tip and another standing behind the tail of the nosecone. Both should make sure the CF is centered on the plug before lowering it onto the previous layer of epoxy. Also make sure there is enough CF at the tail end to make it all the way to the flange.
Starting from the center, smooth the CF over the whole plug. Take care to get the CF into the inside corners of the flange, the tip, and the edges. Go over the CF with rollers to eliminate bubbles. Be careful not to dislodge the CF or apply too much force.
Apply two 60 g batches of epoxy to the CF. Use HDPE scrapers to spread the epoxy over the whole surface. You should notice a slight color difference between wet and dry CF. Leave a little extra epoxy near the tip, tail, and edges
Repeat the steps from the first layer. Apply an extra 60 g of epoxy to ensure it's epoxy-rich.
Using 20 g (per square foot?) twill weave, this creates a mold that's somewhat flexible in torsion. If you're working on a design where this is unacceptable, a three layer mold is fairly rigid.
After at least 48 hours, use plastic wedges to pry under the CF flange. Take care to get the wedge under the CF. It's easy to accidentally get some dry CF in front of the wedge. If this happens, the dry CF will get embedded in the gel coat. (not a big deal) If you're lucky, once you've demolded the flange, air will get into the main pocket of the mold and it will pop off easily. If you're unlucky, it will take quite a bit of prying to remove it.
Cover the edges of the mold with wide masking tape to contain the dry fibers.
The gel coat will have a layer of paste wax still on it. Unless the mold is perfect (it won't be), you'll need to remove that before patching the mold. Pumice soap works well for this. We used the Gojo brand. After completely removing the paste wax, thoroughly rinse and dry the mold.
The defects you're most likely to encounter are bubbles between the gel coat and the CF. These usually get revealed when demolding or when cleaning the mold. You can mostly fill them with gel coat with a wax additive. (Fiberlay sells this.) The wax is necessary to allow the gel coat to cure in the air. (It forms a thin layer on the surface to make the solvent evaporate slower.)
Unfortunately, even the waxy gel coat leaves some uncured spots near the border of the patches. We haven't found a solution for this yet. It's not a big problem though, since it works fine to ignore those tiny uncured spots.
This hasn't been tried yet.