Composite Materials FAQ: Other considerations
Damage and Repair
If a CF or fiberglass part you have is damaged it can be fixed. Fiberglass is much easier to fix than carbon. Fiberglass repair can often be as simple as applying a fiberglass patch to the area, sanding down excess material and repainting. CF can be repaired in the same way, however, if the part has been damaged in a way where any fibers have been broken, you will not be able to repair it to its original structural integrity. Stress is transferred along the strands of carbon fiber and damage to them creates stress concentrations within the material. Economically and aesthetically speaking, it is a better idea to just buy a replacement part.
Aesthetics and Finish
The clean glossy finish on most carbon fiber parts is from the use of either a clear gel coat during part creation or multiple layers of clear coat. A layup without the use of a gel coat will result in small air bubbles on the exterior side of a part. For either process, wet sanding with increasing grit sand paper is required to get that perfectly glossy finish. A layup using only carbon fiber will result in a rough surface. Vacuum bagging and use of an autoclave will reduce the presence of air bubbles and improve surface finish.
Vacuum Bagging and AutoClaves
Most resins including both epoxy and polyester have a high viscosity meaning they are fairly thick. When mixed, air bubbles get trapped within the resin. Air bubbles are also formed when resin is applied to a fabric or mat. Bubbles can get trapped within the weave of a fabric. Creating a vacuum around the part lowers the pressure in the immediate area which causes the trapped air bubbles to expand and ultimately be removed by the vacuum pump. An additional benefit of this process is that now with a vacuum and the bag compress on the part being made, 14.7psi of pressure, atmospheric pressure, is now acting on the part which causes the fabric to better conform to the mold. An autoclave is a pressure oven that works in conjunction with vacuum bagging. Instead of the 14.7psi of pressure, an autoclave will greatly increase that force and apply heat at the same time. The increased force gives a better final surface finish, forces addition air bubbles out of the fabric, and the applied heat will speed cure times. For DIY's though, an autoclave is not necessary and a small vacuum pump and use of a gel coat will give very nice results.
Curing Vs. Drying
The termonology Curing and Drying are missused quite often. Curing refers to the process of a liquid undergoing a chemical reaction to form a solid. Drying refers to the process of a evapoarating a liquid. To become solid all resins must chemical react with a catalyst or a hardener. Therefore when refering to a composite part, you should use the term "Cure."