At first, steel was the only suitable material for the construction of racing frames. Now, the variety of available composite materials is amazing.
What is technically called carbon fiber "prepreg”, a composite carbon fiber and epoxy resin, is a material that has excellent impact resistance and energy absorption characteristics which are optimal for the construction of bicycles. The density of carbon fibers used in the resin and their degree of permeation, can reinforce the material required to obtain the desired characteristics.
The composite prepreg is worked and provided by vendors such as Toray and Mitsubishi. The frame builder completes the production process by applying heat and pressure to achieve the finished piece. The perfect planning and execution of this second stage makes the difference in the resulting bike frame performances.
To get a complete cure of the frame, the mold must undergo processing in an autoclave between 6 and 7 bar of pressure. This is essential to obtain a surface finish that is free from any porosity.
Before and during each cycle, the two shells of the mold are cleaned of any residual resin that may interfere with the surface during the lamination process.
After removal of the rolls or pieces already cut from storage in a refrigerator at -20°C, the material is allowed to thaw completely at room temperature before removing it from its case.
Forced defrosting creates moisture that would damage polymerization. It may take from 4 to 6 hours to reach the right temperature between 18° and 25°C.
The pieces are then cut with a laser into the desired format.
Each frame size and model has its own computer-controlled design that marks on the piece parts to be cut, the fiber orientation for each part, and the number of layers as well as the orientation of each layer for each area of the frame.
The placement of the first layer in the mold is very important because it is the basis for subsequent orientations.
After careful study of the effect created by the orientation of fibers in different parts of the frame, a rigid frame can be created that is also supple to road vertical bumps or vibrations. For our frames we use sheets with fibers oriented at 0°, 22° and 45°. We use three types of woven carbon filaments: 1K, 3K and unidirectional.
Much care is given to areas with concave curves by both careful planning on the computer and during manual labor on the frame. The prepreg material must be cut and positioned in areas without creating a "bridge" in the concave corners to allow the pressure to adhere to the walls. Misplacement in an area of a small radius can trap air in the area between the fiber and the wall of the mold, preventing correct material pressure. This can result in a curing fail and a subsequent de-lamination of the carbon. To avoid this, in concave areas we do not use large pieces of carbon strips: in our process we use strips of carbon that are 50mm wide, positioned at 45° with a slight overlap.
The moulding tool and the final vacuum bag are vital to the frame production process in an autoclave, they must not leak during the curing process and both the mould and the vacuum bag must be free of moisture.
The retention of solvent or moisture by the model surface can seriously affect tool surface and the final result. Throughout the curing process, the vacuum bag is constantly checked for leaks by monitoring gas flow through the vacuum lines.
If a leak is found in the system the technician must to stop the process and fix the problem.
No air or dust remains inside the mold and the final frame is a solid piece: a real monocoque.