Beyond Reinforcement: A Critical Review of Process–Structure–Property Relationships in Additively Manufactured PLA Composites

Polylactic acid (PLA) is chosen as a material-extrusion additive-manufacturing (AM) feedstock due to its bio-origin, degradability, processability, and design for geometries. The mechanical, thermal, toughness, and functional limitations of neat PLA and its composites often hinder their engineering design applications. This review presents an integrated view of the structural and functional design space of PLA-based composites that combines the attributes of the reinforcement, the development of feedstocks, material-extrusion AM techniques, the resulting printed architecture, material properties, and design requirements, and not just the filler selection as an independent variable. The review covers ceramic and mineral reinforcements, carbonaceous and conductive fillers, biowaste and natural materials, metallic and magnetic phases, biomedical applications, polymer blends, and multifunctional composites. The importance of limiting filler content, filler dispersion, feedstock consistency, flowability, extrusion temperature, feed rate, infill, filament diameter, orientation, bonding, porosity, and anisotropy are discussed. Results show how compositional and processing variations can affect the structural and functional properties and behaviour, crystallinity, rheology, surface roughness, surface chemistry, printing tolerances, and processability, but the traditional direct rankings of properties are rendered invalid by the different systems. The review discusses experimental and computational approaches reported in the literature, alongside durability and life-cycle assessment. Finally, an iterative, application-oriented framework linking material selection, preparation, process settings, structure–property evaluation, and application validation is proposed as a qualitative synthesis of the reviewed evidence.