Fused Filament Fabrication (FFF) is one of the most competitive additive manufacturing technologies. It is based on the layer-by-layer deposition of a melt mass passing through a heating head and nozzle. The main advantage of FFF is the design flexibility to produce complex geometries and internal porous structures without supports with high material efficiency, saving weight, energy, and raw material. Its application to the ceramic field has been developed by formulating composites with a high amount of ceramic particles embedded into a thermoplastic matrix. In this way, the 3D printing challenge of the free-mold fabrication of ceramic parts with complex geometries can be faced.      
 
Sinterable filament requires at least a particle concentration of 50 vol.% to provide dense components. The solid load of inorganic particles and their dispersion within the filament are key factors to obtain uniform flow during printing, homogeneous printed samples, controlled shrinkage during sintering, and dense components. Colloidal procedure allows reaching high particle dispersion using submicronic powders that enhance sintering, as well as, print resolution. Colloidal feedstocks show high-quality particle dispersion compared to feedstocks produced by thermal mixing, then better control of sintering shrinkage is expected.
The incorporation of a high amount of ceramic particles stiffs the filament making it so brittle to print and handle. The choice of the thermoplastic matrix composition determines the flexibility of the filament required to spool and handle it. The use of biomass thermoplastics, such as PLA, turns printing eco-efficient, limiting debinding to a thermal step and, contributing to the zero footprint and the circular economy of the process since waste is biodegradable and compostable. PLA filaments are usually very fragile, tending to break easily. To face this, PCL can be added to bring higher flexibility to the filament.
 
In this work, sinterable filaments of 1.75 mm diameter containing 46 vol.% of submicron-sized Al2O3 were produced in collaboration with colloidal filament producer COLFEED4Print. A two-component binder system, consisting of PLA and PCL was used. Miscibility, interaction, and compatibility of PLA/PCL mixtures were analyzed by rheological measurements and FTIR test.
Oscillatory rheological measurements were carried out to study the flow curve of filaments simulating the shears and stress applied in the printing process in order to determine the work window that ensures homogeneous deposition of ceramic particles during printing. Thermal analysis (ATD-TG) and dilatometric analysis were carried out to design the debinding and sintering cycle. Printed and sintered scaffolds were also evaluated by SEM to analyze the adhesion between layers and the sample densification. Debinding and sintering process was performed in a thermal step. The addition of PCL to PLA matrix improves the flexibility of filament and the printing quality compared to PLA matrix. Samples were sintered at 1450ºC achieving relative density above 91%.