Carbon Fiber Fabrication: 3 Common Processes Explained

Fabricating parts with carbon fiber isn’t a single process endeavor. In fact, there are multiple processes to choose from. A lot of what we hear today involves 3D printing and complex overbraiding. Both are great processes, but sometimes you just want to do things the old school way. That’s what this post is about.

Three of the oldest and most common fabrication processes are open molding, closed molding, and vacuum bag molding. A large percentage of carbon fiber parts produced around the world today are still fabricated using one of these three methods. If you are doing fabricating at home, you probably use one of them as well.

Below are descriptions of each of the three processes. All have one thing in common: they are manual processes. Unlike 3D printing, braiding and pultrusion, working with molds (a.k.a. tools) is manual work.

Open Mold Fabrication

This first fabrication process is the easiest of the three. It begins with creating a mold in the same shape as the finished part. A variety of materials can be used here, including various metals. Molds can also be made of high-density foam, fiberglass, and even clay or plaster. The only real concern is that the chosen material be able to withstand high-heat curing.

With the mold ready to go, the fabricator places layers of carbon fiber fabric into the mold. After each layer, the fabric is coated with epoxy resin. Layers are continually added until the required depth is achieved. The fabricator uses a roller to make one final pass, ensuring that all the fabric has been impregnated. Then the mold is put in an autoclave to cure under high heat.

Once cured, the part can be removed from the mold. You are left with a part that may need extra finishing prior to use. The outside face should require minimum work if the mold was carefully crafted.

Closed Mold Fabrication

The closed mold method combines the basics of open mold fabrication with fundamental principles of casting. For this process, you need to mold pieces that, when joined, create a single piece. Each side of the mold is layered with carbon fiber fabric or some other reinforcement, then the two halves of the mold are joined.

Next, a thermal resin is forced into the mold via a vacuum or pressure pump. The added pressure does a better job of fully impregnating the fabric prior to curing. And while this method requires specialized equipment, it produces less waste. It also offers greater consistency and repeatability.

Curing is also identical. The entire mold is put in an autoclave and subjected to high heat for a set amount of time. Afterward, the mold can be opened and the part removed for finishing.

Vacuum Bag Molding

Last but not least is vacuum bag molding. This is a process that starts with an open mold and carbon fiber fabric that comes from the supplier already impregnated with resin (otherwise known as a prepreg). The fabric is layered in the mold until the desired thickness has been achieved.

Next, a bag is placed over the mold and sealed to create a vacuum. All the air is removed via a pump, thus forcing even distribution of the resin throughout the entire fabric stack. Very precise fabric-to-resin ratios can be achieved this way. Once all the air has been removed, the assembly is sent to an autoclave to cure.

Now you know three of the most common processes for creating carbon fiber parts with a manual layup. Maybe you want to try one of them yourself.

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