A die assembly for forming a composite gap filler (58, 60, 64), comprising: a first die having a first central axis, about which first central axis the first die is rotatable, wherein the first die comprises: a first portion which extends along the first central axis and has a first curved surface having a radius which changes as the first portion extends about the first central axis; and a first planar surface portion which extends from a first end of the first curved surface and a second planar surface portion which extends from a second end of the first curved surface; a second die has a second central axis, about which second central axis the second die is rotatable, wherein the first central axis is coaxial with the second central axis wherein the second die comprises: a second portion which extends along the second central axis and has a second curved surface having a radius which changes as the second portion extends about the second central axis; and a third planar surface portion which extends from a first end of the second curved surface and a fourth planar surface portion which extends from a second end of the second curved surface; and a third die has a third central axis, about which third central axis the third die is rotatable, and defines a third wall member which extends about the third central axis and changes in width dimension as the third wall member extends about the third central axis, wherein with the first planar surface portion and the third planar surface portion abutting one another and with the third wall member abutting the second planar surface portion and the fourth planar surface portion, a closed gap is formed. a first die having a first central axis, about which first central axis the first die is rotatable, wherein the first die comprises: a first portion which extends along the first central axis and has a first curved surface having a radius which changes as the first portion extends about the first central axis; and a first planar surface portion which extends from a first end of the first curved surface and a second planar surface portion which extends from a second end of the first curved surface; a first portion which extends along the first central axis and has a first curved surface having a radius which changes as the first portion extends about the first central axis; and a first planar surface portion which extends from a first end of the first curved surface and a second planar surface portion which extends from a second end of the first curved surface; a second die has a second central axis, about which second central axis the second die is rotatable, wherein the first central axis is coaxial with the second central axis wherein the second die comprises: a second portion which extends along the second central axis and has a second curved surface having a radius which changes as the second portion extends about the second central axis; and a third planar surface portion which extends from a first end of the second curved surface and a fourth planar surface portion which extends from a second end of the second curved surface; and a second portion which extends along the second central axis and has a second curved surface having a radius which changes as the second portion extends about the second central axis; and a third planar surface portion which extends from a first end of the second curved surface and a fourth planar surface portion which extends from a second end of the second curved surface; and a third die has a third central axis, about which third central axis the third die is rotatable, and defines a third wall member which extends about the third central axis and changes in width dimension as the third wall member extends about the third central axis, wherein with the first planar surface portion and the third planar surface portion abutting one another and with the third wall member abutting the second planar surface portion and the fourth planar surface portion, a closed gap is formed.