Fabrication of Carbon Fiber Parts

One important process in my career field is the process of manufacturing parts using carbon fiber.  It is a fairly intricate process with many different ways to do it, but it typically consists of four main steps:

Mold Preparation

Before making a composite part, you will need a mold to lay the carbon fiber on.  Depending on the geometry of a part this can range from a simple bent sheet of aluminum, to a complex CNC machined piece of tooling board.  Making the mold can be considered a separate process of its own, so I will assume that the mold has already been designed and machined on a CNC mill.  At this point it will need to be sanded to smooth out the surface, primed, and painted.  Personally, I think the mold should be polished after this to further smooth the surface.  This step is not always done, but it can help to make parts with a much nicer finish, which are also easier to remove from the mold.   

Once you have a polished mold, it will need to be released so that you will be able to remove the composite part after it is complete.  This can be done by applying a cleaning solution to the mold, followed by several coats of a releasing agent.  After this is done, the layup can begin.  

Layup

While there are many different methods such as wet layups and resin infusion, I will assume that you are using pre-preg, which is carbon fiber fabric that has already been impregnated with resin to hold it all together.  These sheets of carbon can be laid on top of the mold and carefully pressed into the surface, precisely following all curves and contours of the mold.  It is important to make sure that no voids or air bubbles are present, and to minimize overlapping different pieces of carbon.  The required structural integrity of a part determines how many layers should be used, and in what direction the fibers of each layer should be oriented.  

Smaller parts are often made using whatever pieces of carbon are available- typically a combination of scrap pieces from previous layups.  One extra step I would take to improve this process is to cut out properly sized pieces ahead of time, which makes the layup much easier and minimizes the number of pieces which need to be used, making the part stronger and lighter.  This can be done by simply using paper to lay over the mold and trace out the shapes that will be needed, and then using this as a template to cut out pieces of carbon fiber in the correct orientation.

Curing

Pre-preg at room temperature is a flexible, sticky fabric which is easy to form into the shape of the mold, but not very strong.  The resin needs to be heated in an oven in order for the part to cure, gaining its strength and rigidity.  It is very important that there are no air bubbles in the resin, so the part will need to be depressurized in a vacuum bag.  This consists of covering the part in a thin plastic sheet known as peel ply, which is then wrapped by cotton breather material to allow air to flow out of the bag.  This is finally placed onto a thicker piece of bagging material, which is closed and sealed together using a very thick, sticky type of tape.

A bag fitting with a hose connector is inserted, and connected to a vacuum pump which sucks the air out of the bag.  The part is then put into a large oven while still connected to the vacuum.  The cooking temperature and time are dependent on the materials being used.

Finish

After the part comes out of the oven, it may take some effort to get it off of the mold.  It should carefully be pried off with plastic pry tools, to avoid damaging the part or mold.  Once it is off it will typically need to be sanded down to eliminate any bumps or creases from the bag, as well as just to smooth out the surface and prepare it for clear coat.  The part may also need to be trimmed, typically using a rotary cutting tool.  Finally, clear coat can be sprayed onto the part, and once this dries the part will be complete!

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