Bicycle frames are often made from fiber reinforced composites. To create a completely integral or seamless bike involves pre-pregs are draped in a mould and the different parts are interconnected in a curing step. By using thermoset composite materials the manufacturing process is made less complex. Once the curing is finished the shape of the parts is irreversibly set.
However, there are some disadvantages to thermoset materials including poor recyclability and repairability and/or a need for a complex and thus costly mould for each part.
Furthermore, the joints are known to have a negative impact on the performance of the product, in terms of stiffness and/or strength, leading to cracking or failure of the part at the joints under heavy loads.
An alternative joining method involves the use of thermoplastic materials.
Rein4ced invented a method to manufacture bicycle frames by joining two hollow tube parts, made from a thermoplastic fiber reinforced composite, by positioning the first shell part adjacent to the second one across a seam. By inserting an end portion of the first tube part into the second one an overlap is formed between both parts. The fusing is done by heating the first hollow tube part and the fiber reinforced shell parts at the overlap to a temperature above a softening temperature of the first and second thermoplastic fiber composite; The joint is consolidated by pressure to form a fused interconnect between the hollow tube parts.
This method enables to manufacture a monocoque frame which is light and stiff and can be used to manufacture other bicycle parts as well. It is preferable to use fibers with a high specific stiffness, such as carbon and steel fibers or combination of both. Suitable polymers include but are not limited to; polypropylene, polyethylene, polyethylene terephthalate, polyamide (nylon), polyetheretherketone, polyetherketoneketone, polyetherimide, polyphenylene sulphide, (poly)acrylonitrile-butadiene- styrene (ABS), polylactic acid (PLA), polycarbonate (PC), and polyaryletherketon (PAEK).
Both thermoplastic fiber reinforced composites may be of the same nature or may be different but are preferably of the same nature. Using composites of the same nature, e.g. the same type of polymer, was found to improve joint strength. Using composites of the different nature can result in small visual disturbances, presumably due to a different behavior in response to varying temperature exposures.