The present work investigates, by both mathematical modelling and experiment, the contribution of bonding between the filaments to the strength of the parts manufactured by fused deposition modelling (FDM). A mathematical model for neck growth between cylindrical filaments is derived based on viscous sintering for cylindrical filaments. Theoretical ultimate tensile load is determined from the layer thickness information and predicted average final neck size between filaments. The experimental ultimate tensile load is obtained by conducting a simple tension test with two different build directions. Agreement between theoretical and experimental ultimate tensile load and scanning electron microscope (SEM) photomicrographs of the fracture surface indicate that the strength of the FDM part is primarily due to intra-layer bonding, interlayer bonding and neck growth between filaments. It also indicates that the total time and heat available to the filaments are just sufficient to grow necks but not enough to fully coalesce.

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