TY - BOOK

T1 - Development of Ceramic Feedstocks for Fused Filament Fabrication

AU - Cano Cano, Santiago

N1 - no embargo

PY - 2020

Y1 - 2020

N2 - The Fused Filament Fabrication (FFF) of ceramics is a promising technique for the low-cost production of components with complex geometry. The process relays in a thermoplastic binder highly filled with ceramic powder to produce feedstock filaments. Filaments are used to shape parts in the same machines as for FFF of plastics. The binders are removed in the debinding step, and the final parts are obtained after sintering. The use of filaments requires of strong, flexible, and low viscous feedstocks, which is a challenge due to the high content of ceramic powder (≥45 vol.%). Different FFF feedstocks have been reported in the literature. However, these formulations use a single step of thermal debinding, which requires long thermal cycles to avoid defects. On the contrary, most of the commercially available FFF feedstocks are debound in two steps. A major fraction of the binder is dissolved, creating an open-pore structure to remove the remaining polymers. Currently little information about the binders required for the two-step debinding is publicly available, which hinders the development of new formulations. The purpose of this thesis was to develop a better understanding of the binders required for the FFF and solvent debinding of ceramics. The properties required for FFF of ceramics and metals were compiled and combined with the requirements for solvent debinding known from the powder injection moulding process. A novel zirconia feedstock was developed using as criteria the morphology, rheological and mechanical properties, and the solvent debinding performance. Two soluble components were necessary for the production of flexible and low viscous feedstocks with no debinding defects. The feedstock morphology had a strong influence on the debinding behaviour since the inhomogeneity leads to defects. Based on these results, a formulation composed of high density polyethylene grafted with acrylic acid, stearic acid, paraffin wax and styrene-ethylene/butylene-styrene copolymer was developed. The fractions of the components had to be adjusted to ensure an adequate combination of properties. Incorporating stearic acid reduced the viscosity and facilitates the FFF shaping, but it reduced the strength and flexibility of the filaments. The powder-binder interfacial interactions and adhesion when the binder was grafted with polar groups were also investigated. The improved adhesion for the grafted binder resulted in better powder dispersion and high mechanical properties. The effect of the processing parameters on the solvent debinding performance was determined for a zirconia feedstock with a binder composed of a grafted polyolefin as backbone and thermoplastic elastomer as soluble binder. Increasing the temperature of the solvent increased the debinding rate but also the defects; incorporating stearic acid in the feedstock reduced the debinding defects; on the contrary, using a swelling inhibitor had little effect. Finally, the effect of the FFF parameters on the defects and the mechanical properties of the final parts was determined. The changes in the feedstock during shaping, the variability on the filament diameter and the orientation of the extruded roads affected the size and orientation of defects. The FFF defects determined the bending properties of the sintered parts. Therefore, the loads applied to the ceramic parts during their service have to be considered during the FFF. The results of this PhD thesis provide a first insight into the binders required for the combination of FFF and solvent debinding. Moreover, the influence of the processing parameters on the processability and final properties were determined. These results can facilitate the development of new binder formulations, improve the understanding of the phenomena occurring during processing ceramics by FFF, and provide new perspectives in the processing of ceramics by other conventional and

AB - The Fused Filament Fabrication (FFF) of ceramics is a promising technique for the low-cost production of components with complex geometry. The process relays in a thermoplastic binder highly filled with ceramic powder to produce feedstock filaments. Filaments are used to shape parts in the same machines as for FFF of plastics. The binders are removed in the debinding step, and the final parts are obtained after sintering. The use of filaments requires of strong, flexible, and low viscous feedstocks, which is a challenge due to the high content of ceramic powder (≥45 vol.%). Different FFF feedstocks have been reported in the literature. However, these formulations use a single step of thermal debinding, which requires long thermal cycles to avoid defects. On the contrary, most of the commercially available FFF feedstocks are debound in two steps. A major fraction of the binder is dissolved, creating an open-pore structure to remove the remaining polymers. Currently little information about the binders required for the two-step debinding is publicly available, which hinders the development of new formulations. The purpose of this thesis was to develop a better understanding of the binders required for the FFF and solvent debinding of ceramics. The properties required for FFF of ceramics and metals were compiled and combined with the requirements for solvent debinding known from the powder injection moulding process. A novel zirconia feedstock was developed using as criteria the morphology, rheological and mechanical properties, and the solvent debinding performance. Two soluble components were necessary for the production of flexible and low viscous feedstocks with no debinding defects. The feedstock morphology had a strong influence on the debinding behaviour since the inhomogeneity leads to defects. Based on these results, a formulation composed of high density polyethylene grafted with acrylic acid, stearic acid, paraffin wax and styrene-ethylene/butylene-styrene copolymer was developed. The fractions of the components had to be adjusted to ensure an adequate combination of properties. Incorporating stearic acid reduced the viscosity and facilitates the FFF shaping, but it reduced the strength and flexibility of the filaments. The powder-binder interfacial interactions and adhesion when the binder was grafted with polar groups were also investigated. The improved adhesion for the grafted binder resulted in better powder dispersion and high mechanical properties. The effect of the processing parameters on the solvent debinding performance was determined for a zirconia feedstock with a binder composed of a grafted polyolefin as backbone and thermoplastic elastomer as soluble binder. Increasing the temperature of the solvent increased the debinding rate but also the defects; incorporating stearic acid in the feedstock reduced the debinding defects; on the contrary, using a swelling inhibitor had little effect. Finally, the effect of the FFF parameters on the defects and the mechanical properties of the final parts was determined. The changes in the feedstock during shaping, the variability on the filament diameter and the orientation of the extruded roads affected the size and orientation of defects. The FFF defects determined the bending properties of the sintered parts. Therefore, the loads applied to the ceramic parts during their service have to be considered during the FFF. The results of this PhD thesis provide a first insight into the binders required for the combination of FFF and solvent debinding. Moreover, the influence of the processing parameters on the processability and final properties were determined. These results can facilitate the development of new binder formulations, improve the understanding of the phenomena occurring during processing ceramics by FFF, and provide new perspectives in the processing of ceramics by other conventional and

KW - Fused filament fabrication

KW - feedstock

KW - highly-filled polymers

KW - solvent debinding

KW - ceramics

KW - zirconia

KW - grafted polymers

KW - infill orientation

KW - Fused filament fabrication

KW - Feedstock

KW - hochgefüllte Polymere

KW - lösungsmittelbasiertes Entbindern

KW - Keramik

KW - Zirkoniumdioxid

KW - gepfropfte Polymere

KW - Infill Orientierung

M3 - Doctoral Thesis

ER -