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Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).
(a) A schematic of a two-side assembly convergent nozzle; (b,c) circular filament after passing through the two-side convergent nozzle; (d) the cross section of final pultrusion nozzle; (e) the final perfect circular cross section filament.