Plastic is an extremely generic name, sometimes called “polymers” or “resins”. The name plastics is usually reserved for cheaper consumer goods That are recyclable, but it’s a blurred line. The resins for bike frames used are a thermoset type which cannot be recycled. Their are literally millions of formulations and as different as flavors of soup. (Polymer) Soup is actually a great analogy.

The role of the resins is to protect the fibers and transfer the loads to the fibers. The resins themselves are relatively weak but in a liquid processing state wet out the fibers for a strong bond. The fibers have the majority of the attractive engineering properties we want. Carbon fibers being the most common for their high tensile properties and strength to weight ratio. The continuous fiber strands are also key, short fibers don’t have the aspect ratio to take advantage of the material properties for high structural applications.

The design and manufacture of these fiber-resin Composites is high level engineering stuff, essentially Aerospace level. These are not simple boat hulls. The directionality of the layers (lamina) are highly controlled and specifically designed for your strength and flex in different locations on your bike frame. A LOT can go wrong, which is why I would never trust a cheap no-name brand. These things are expensive for a reason And quality matters. Joe’s auto repair shop can’t just weld one or fabricate one up.

I got my Engineering degree in composites. I fondly recall Laminate Analysis being one of the most difficult classes ever as computers back in the early 90s weren’t what they are today for analysis. Yes, I calculated things by hand involving large matrices mathematics that still gives me nightmares 25 years later. Today’s computers handle the analysis much better... god bless technology. But computers don’t make the design decisions. Quality manufacturing has come a long way too.