Spray drying is one of the most broadly applicable and widely used methods of producing amorphous solid dispersions (ASDs). ASDs can improve the oral absorption of poorly water-soluble active pharmaceutical ingredients. Eudragit L100 is an appealing ASD excipient, due to its favorable impact on ASD physical stability and dissolution performance. However, spray drying Eudragit L100 can lead to high-aspect ratio filaments which reduce flowability, density and yield of the resulting powder. This negatively impacts downstream ASD performance and dosage form manufacturability. This work presents a mechanism for filament formation, which results in a particle engineering design space of key processing parameters, defined by a dimensionless parameter for assessing filament formation risk. Specifically, it was found that solution concentration, spray dryer inlet temperature and solvent volatility had the largest impact on controlling filament formation, based on their influence on the relative time scale to atomize versus droplet skinning during drying. (C) 2019 The Authors. Published by Elsevier B.V.