When you want to reinforce strength, to create strong but lightweight parts due to their strength-to-stiffness-to-weight ratio, continuous fibre reinforced polymer composites (FRP) are used.
What is fibre-reinforced filament fabrication?
This 3D printing process is known as:
CFF-Continuous Filament Fabrication - Fabrication with continuous fibre.
FFF+CF-Fused Filament Fabrication.

This method makes it possible to multiply the strength of a nylon part by up to 20 times compared to the same part made of ABS.
For example, the mechanical properties of 606T6 aluminium can sometimes be matched, but with a significantly lower weight.
Fibre reinforcements are usually made of carbon, glass and/or Kevlar, depending on the mechanical properties required by the part’s application.
The result of the combination with fibre improves the performance of final technical part applications.
Its use is just as reliable and valid as machined parts.
The only difference is that machining usually requires more machine time.
The combination of polymers and fibres has allowed additive manufacturing to become the most feared rival to traditional manufacturing processes in short production runs, by achieving substantial reductions in manufacturing times and considerably lower costs.

Examples of materials used: PA (polyamide)+33% carbon fibre.
Flame retardants (FR):
PA FR PA+33%CF FR
Electrically conductive materials: graphite.

HOW DOES THE FFF + CF PROCESS WORK?

Fused filament fabrication (FFF or FDM) is a 3D printing process that uses a continuous filament of a thermoplastic material.
Filament is fed from a large spool, through a heated and moving printer extrusion head. The melted material exits the printhead nozzle and is deposited on the growing 3D model work. The printhead moves, controlled by a computer, to define the printed shape. As it is a fibre reinforcement (FFF+CF), there are two nozzles, one extruding the filament and the other the fibre.
 
TECHNICAL SPECIFICATIONS:
330 x 270 x 200 mm
Layer height: 0.05 – 0.25 mm
Tolerances = ± 0.08 mm