Enhancing the Performance of FFF-Printed Parts: A Review of Reinforcement and Modification Strategies for Thermoplastic Polymers

The technology of 3D printing has become one of the most effective methods of creating various parts, such as those used for fast prototyping. The most important aspect of 3D printing is the selection and application of the appropriate material, also known as filament. The current review concerns mainly the description of the mechanical and physical properties of the different filaments and the possibilities of improving those properties. The review begins with a short description of the development of 3D printing technology. Next, the basic characteristics of thermoplastics used in the fused filament fabrication (FFF) are discussed, namely polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), and polyethylene terephthalate glycol (PETG). According to modern concepts, the printed parts can be reinforced with the use of different kinds of fibers, namely synthetic fibers (carbon, glass, aramid) or natural fibers (wood, flax, hemp, jute). Thus, the impact of such a reinforcement on the performance of FFF composites is also presented. The current review, unlike other works, primarily addresses the problem of the aging of parts made from the thermoplastics above. Environmental conditions, including UV radiation, can drastically reduce the physical and mechanical properties of printed elements. Moreover, the current review contains a detailed discussion about the influence of the different fibers on the final mechanical properties of the printed elements. Generally, the synthetic fibers improve the mechanical performance, with documented increases in tensile modulus reaching, for instance, 700% for carbon-fiber-reinforced ABS or over 15-fold for continuous aramid composites, enabling their use in functional, load-bearing components. In contrast, the natural ones could even decrease the stiffness and strength (e.g., wood–plastic composites), or, as in the case of flax, significantly increase stiffness (by 88–121%) while offering a sustainable, lightweight alternative for non-structural applications.