CF application
Roffa -
Since you're interested in protecting your car's underbody, I might guess you're interested in rallying. Right on.
I can give you some of the manufacturing steps, but you will need to get your hands on the raw materials first - and if you take any piece of advice from me, make damn sure you get CF with a thermoplastic matrix. This is for two main reasons: One - thermoplastics can be remolded if your parts don't cure with the desired finish or don't laminate properly. Two - when finished, you will have the strongest CF material available.
Back to the raw materials. You can buy material in a number of ways from their producers. You can purchase the raw CF (very messy and a serious breathing hazard) and add the matrix (usually in powder form) during the molding cycle. The best and definately easiest way to get the CF is with the matrix in a pre-impregnated form; better known as pre-preg. The only drawback to pre-preg is its very high cost. You will find from CF producers a CF thread count - usually between 3K and 12K. The more fibers per square centimeter, the stronger it will be. Now you'll want to choose the matrix itself for your application.
For the underbody of a car, I would recommend a high-temp thermoplastic like PEEK (polyether-ether-ketone). It has a sustained service temperature above 600 degrees F - perfect for protecting the hot underside of a 4cyl boxer engine.
Once you have the materials, you will need a male/female mold cavity that can be heated and held above the thermoplastic melt point (PEEK happens to be 680 degress F). And once at operating temperature (about 700 degrees F), thin parts (1mm-5mm thick) take about 10 minutes to fully cure. Thicker parts might take as long as 30 minutes. Once the heat cycle is completed, make sure to cool below the glass-transition temperature (completely hardened) - which for PEEK is about 380 degrees F - before trying to remove your part.
Most molds I've used have been made out of aluminum or steel. This mold will also need to be held in a compression mold press at a pressure somewhere between 500 and 2000psi (depending on the part dimensions and thickness - more surface area, more pressure). And finally, don't forget to get a hold of a good solvent-based mold release agent for your tool. After all, nothing is worse than spending all that time getting material and molding a part just to have it stuck inside its mold cavity.
Long winded, huh? Not really - just some friendly advice to another WRX enthusiast. Besides, Scandinavians need to stick together. One last piece of advice - when you're in a rally, make sure you have the right pace notes!