Making carbon fiber part in silcone mold
Making carbon fiber part in silcone mold
Silicone molds are great because nothing sticks to them and carbon fiber is very difficult to remove from some molds. The problem with a wet lay-up is that the CF and resin won't stick to the mold for curing so many people advise you to try a rigid mold instead.
Rigid molds are hard to make and remove from some parts with more complex curves. This is an easier way for people with less experience, talent and equipment like me:
First, make your mold using a silicone with a shore hardness of at least 60. This is like pencil eraser so it will be firm enough to hold lines if resting on a flat even surface. The silicone I use can also withstand higher temperatures too which is good for people who like prepreg fabric. Most silicone rubbers I have seen have a shore hardness of 25. This won't work as well, especially for larger parts.
Once your mold is made, make a casting of your part using a liquid urethane foam. I used one with a 3lb density which won't add too much weight to your part.
While the female silicone mold won't stick to anything, your urethane foam casting definitely will. Coat the foam with resin. Wait until it is semi dry but still tacky. Then carefully lay the dry carbon fiber fabric onto the foam model until the outside is covered. This method will allow the cf fabric to stick to fairly complex curves that are usually a problem with a more traditional method. Wait until the resin is fully cured so the cf is firmly stuck and won't come off when you wet it again.
Next wet the cf fabric with resin as normal and push the part back into the female silicone mold when you're done. Seal up the mold (if it's two part), let it cure and you will have a light weight carbon fiber part that is easy to remove from the mold. If the mold and part shape allows for this, you can add some weights on top of the mold to create more pressure for curing or seal the mold halves together tightly with screws.
You can obviously add more layers of cf to make it stronger. You can also scrape out some of the foam after if you want the part to be hollow or you want to build up the thickness from the inside to avoid making it bigger.
Some of the foam won't come out as it will be firmly stuck to the resin so it you want to lay cf on the inside, you can use candle wax to cast your model instead of foam. You can then melt the wax out at the end. Just make sure you choose a resin that can withstand wax melting temperatures or your cf part with go floppy and loose it's shape before resolidifying into a deformed version of you part.
I prefer to use a 5.7k twill weave for this. It will conform to curves better than a plain weave and definitely better than thicker fabrics. Try different manufacturers. One of my suppliers has a 5.7k twill that feels slightly rough and almost crunchy. This conforms to curves better than the softer 5.7k I got from another store for some reason.
Another method that works (although not as well) is to use a spray glue on the silicone mold. I have a 3m spray that works temporarily on silicone but the bond is weak enough that the CF can easily be pulled free when you are done. The right spray glue will provide initial tackiness on the silicone but only for a short time. Use a fast setting epoxy resin for this method and stand over it while it cures (20 minutes with my fast resin). Just before the initial cure has finished, there is a stage where the CF is still maliable and slightly tacky but already hardening. At this stage you can push the CF gently into any corners of the mold that it has come away from to make sure it cures in the correct shape. Use gloves for this obviously.
One final method which works well for parts that are two complex for a regular wet lay up with fabric, is to use carbon fiber strands. This process is more like a regular casting but it still produces surprisingly strong and light-weight parts that are solid instead of hollow. I use this method for scope rings, barrel bands for my air rifle, weaver rails etc.
Take some regular cf fabric. Cut it down with scissors into a pile of fine strands. Don't make them too short unless your part has very fine detail. I usually cut 1" strands approx. Cut enough to fill your mold roughly when mixed with resin. Mix the strands with resin and hardener. Use the same resin you use for wet lay ups. Regular casting resin won't work. The mix will instantly turn into a thick black sludge. Use vinyl gloves and mix it with your fingers.
This black batter will be far too thick to pour but you will be able to pull it out almost in one piece of whatever you mixed it in. Again, using vinyl gloves, push the black sludge into your silicone mold firmly until you have reached every side and corner. The opening of the mold should be covered with something smooth. I use window tint film. Smooth peel ply would work too. I place the covered mold entrance down on a flat surface so it will cure flat and smooth. The mold entrance obviously has no silicone mold to shape it so this cf mix would cure uneven with sharp cf spikes sticking out if you don't cure it face down on the flat surface like this.
Let the part cure and pull it out the mold. You may have to tear you mold to get it out if it was a complex shape like scope rings but that's ok. The finished part will be jet black in appearance and it won't have the pretty cf weave appearance or it's strength to weight ratio. It will be stronger and lighter than aluminum though. It will be stronger and lighter than casting resin too. My scope rings would hold my weight and I am 220lb. They survived a 3 story drop without a scratch too! They weight less than 1/3 of the metal originals. Same with my cf weaver rails.