Additive manufactured sandwich structures for enhanced mechanical and thermal performance of PLA/carbon fiber composites

Fused filament fabrication often utilizes polylactic acid (PLA) due to its biodegradability and ability to be easily processed, but PLA has some restrictions on its structural reliability because of the weak bonding between layers. To improve the stiffness of PLA, it has been observed that adding short-carbon fibers (CF) increase the stiffness of the material but can reduce the tensile strength and quality of the printed parts because of the agglomeration, voids, and pull-out of short-carbon-fibers due to the increased CF content within the PLA. In order to increase interlayer adhesion and reduce the formation of defects in CF-PLA, this study presents a dual-extruder sandwich-printing method, where the CF-PLA is confined to the layers specified within the build. The sandwich build-up comprised 15%, 35%, 55%, 75%, and 95% CF–PLA layer fractions, where the percentage denotes the proportion of total printed layers fabricated using CF–PLA, and were evaluated through tensile, compression, flexural, and Shore-D hardness testing, supported by thermal analysis testing (Thermogravimetric analysis (TGA), differential thermogravimetric (DTG), and differential scanning calorimetry), and microstructural and spectroscopic evaluations (Scanning Electron Microscopy (SEM), Raman spectroscopy, Fourier Transform Infrared (FTIR), and surface profilometry). Sandwich builds with high fiber (75–95% CF) content achieved as much as a 24% increase in tensile strength over the fully fiber reinforced CF-PLA while maintaining the modulus in a high range, indicating effective transfer of loads between layers and reduced defect sensitivity. The 55% sandwich achieved the best flexural strength (up to 18.8% higher than fully CF–PLA), whereas the 15% sandwich provided the highest compressive strength (12.8% higher than neat PLA). Thermally, the 75% sandwich exhibited the greatest stability, reaching a peak degradation temperature of 389.11 °C. Overall, sandwich printing offers a material-efficient route to tune mechanical and thermal performance through layer-level reinforcement design.