Thermal treatment and particle size effects on coalescence in extrusion-based additive manufacturing

Transverse and out-of-plane mechanical properties of objects fabricated by fused filament fabrication, an additive manufacturing technique, are typically 20-50% lower than bulk properties. A major contributor to this knockdown is poor raster-toraster and interlayer bond strength driven by incomplete polymer coalescence. The two major steps in the coalescence process responsible for bond strength development are neck growth and interfacial diffusion. The goal of this work is to explore the effects of temperature, time, and particle size on coalescence of ABS polymer in a controlled environment to identify appropriate thermal history to target during the printing process for enhanced raster-to-raster and interlayer bond strength. Particle size was found to have a significant impact on neck growth where doubling the size reduced the normalized neck growth by 58%. Diffusion was qualitatively assessed by tracking absorbance of red pigment from a red colored ABS particle into the neighboring white ABS particle at 550 nm. Smaller particle size held at elevated temperature for longer time had pigment diffusion across longer distances from the neck region. Results suggest that smaller diameter rasters promote faster and more complete coalescence.