Originally Posted by piloto
It's injection molded, from a CFRP (i.e. some type of moldable resin, could be a type of ABS for instance, with carbon fiber strands mixed in it). The raw material (i.e. the pellets) have the glass or carbon fibers chopped into them already, so once they heat up and flow into the cavity, you get a finished part with some fiber content for added strength. The more fiber (to a point), the stronger, but also more brittle as it limits the plastic range of the material. Sorry for the poor explanation, didn't realize how tired I was until I started typing.
They are most definitely not CFRP. The 6 series fenders for instance are made of an engineered polymer called Noryl GTX which is a blend of polyamide and a polyphenylene ether polymer. No filler is used.
I can't imagine the 3 series fenders would be significantly different from those developed for the e63 6 series.