Composite materials are one of the most revolutionary products invented in the late 20th century.
What you can touch on the outside is resin, in our case epoxy: a sort of plastic tougher than most of the plastics we are accustomed to see. What really sets apart these products from anything else you can meet in the day to day life, are the threads you can see on the inside which are fibers of carbon crystals.
Carbon is a chemical element which can have an amorphous structure, devoid of mechanical properties, like coal, but it can also have crystal molecular structures, like diamonds or like the fibers we are dealing with. Carbon fibers have extraordinary mechanical properties: a tensile strength 2 or 3 times that of the best steels, a stiffness similar to steel with a weight less than one quarter. They can be produced only in very thin lines which are then joined in thicker threads and woven in fabrics of various thickness and weave styles. To use those fabrics we must lock them in a stable shape, and here comes epoxy resin. This kind of plastic is particularly good in wetting fabrics and has itself very good mechanical properties.
What’s really difficult is to reach a uniform resin content in the fabrics, avoiding both areas with resin excesses, which would add extra weight, and areas with low resin content, which would allow fibers to move independently, compromising the structure. To this end the solution is to use industrial quality pre-preg fabrics, where the resin content is strictly controlled and uniformly applied. The layers of carbon fabric must then be applied along the directions of the structural loads, following a lamination plan defined in the course the design and analysis phase.
In particular, we must absolutely avoid the air bubbles which form during the early manufacturing phases and this is not at all easy if you imagine big layers of fabric, wet with a sticky material, put one above the other.
The best method to take out air bubbles from the laminate is autoclave curing. An autoclave is a big pressurized oven. Internal pressure can reach 10 bars (147 lb/sq in: just think that your car tyres barely reach 2 bars…) and the typical working temperature is 120° C (250° F), necessary to cure the resin, transforming the sticky paste in a tough, solid structure, called matrix. There are resins with a lower curing temperature, but the best products need 120° C.Thus, the composite structure is put, with its mould, inside a plastic bag, which has been emptied of all the air, to get near vacuum, and take out the biggest air bubbles. The product inside its bag is finally placed into an autoclave, at over 4 bars (60 lb/sq in), to totally press each layer against the next one without air in the middle so that the structure becomes continuous.
Many widespread and inexpensive products are built with less advanced and cheaper techniques. They often are manually wet out with resin and cured without an autoclave so they must have a greater thickness and weight to comply with the structural requirements. These products are often filled and sanded in order to obtain a smooth surface over the texture of the fabrics and the overlaps. Complex objects are made by gluing together simpler parts which are separately laminated and cured. This practice increases weight, worsens the surface and often decreases structural properties. This practice allows the use of moulds which are smaller and easier to produce with obvious commercial advantages.The final painting covers any sign of fillings and secondary bondings.
In EXIT ENGINEERING we use secondary bondings only if strictly necessary and if it’s required by unavoidable design constraints, or in some cases, when the structural advantage of secondary bonding exceeds its drawbacks.
All our products are built in one piece, without secondary bondings. Even if we can paint them on request, in their original transparent finishing, right from the autoclave, they show a perfect lamination quality, aesthetically pleasing too. To reach these results, we use complex moulds, made with several smaller moulds working together, and employ only very experienced laminators.
The reward is a line of products which are very light, strong and whose quality is totally consistent on the whole series.