Using a carrier polymer that can be easily printed, like PLA, means that some of the properties of the additives which could not normally be easily 3D printed can be enjoyed.
The ratio of carrier polymer to additive will vary between manufacturers. An important consideration when selecting a composite 3D printing filament is that the higher the percentage of additive, the more abrasive the filament will be. This can lead to damaging the nozzle of your 3D printer. For high ratio composites this can only be avoided using a hardened nozzle.
You should also consider how much post-processing you want to carry out on your composite 3D print. To get the desired finish, some composite filaments will require extensive processing with rock tumblers and polishing creams whereas other composite filaments will require little or no processing to achieve the desired finish.
Iron or ferrous 3D printing filaments share many of the properties of iron that are not commonly found in the 3D printing world. Firstly, iron filaments are magnetic although the strength of the magnetism will depend largely on the percentage of iron in the filament. Like pure iron, iron filaments can also rust which can give a pleasing, natural look to your 3D printed product.