A shear vacuum forming method for producing a three-dimensional preform, said method comprising: (a) providing a molding tool having a length, a web surface with a convex bent section along the length, and two contiguous sidewalls extending from said web surface for forming flanges; (b) placing a preform material on a flexible diaphragm; (c) placing the molding tool over the preform material such that the web surface of the molding tool is in contact with a portion of the preform material; (d) placing at least two movable inner restraints next to each sidewall of the molding tool such that one end of each inner restraint is adjacent to the bent section of the web surface; (f) forming a vacuum sealed chamber defined by the flexible diaphragm and the molding tool, said vacuum sealed chamber enclosing the inner restraints and the preform material; (g) evacuating air from the vacuum sealed chamber until a partial vacuum is reached such that the flexible diaphragm is pulled toward the molding tool; (h) withdrawing each inner restraint along the sidewall of the molding tool in a direction away from the bent section during evacuation of the vacuumed sealed chamber such that there are at least two inner restraints moving in opposite direction along each sidewall of the molding tool, thereby allowing the preform material to come into contact with the sidewalls of the molding tool in a gradual manner; and (i) increasing vacuum pressure after the restraints have been withdrawn and the preform material has conformed to the molding tool, thereby forming a shaped preform, said vacuum pressure being sufficient to consolidate the shaped preform, wherein heating is applied to heat the preform material prior to evacuating air at (g) or after the shaped preform is formed at (i). (a) providing a molding tool having a length, a web surface with a convex bent section along the length, and two contiguous sidewalls extending from said web surface for forming flanges; (b) placing a preform material on a flexible diaphragm; (c) placing the molding tool over the preform material such that the web surface of the molding tool is in contact with a portion of the preform material; (d) placing at least two movable inner restraints next to each sidewall of the molding tool such that one end of each inner restraint is adjacent to the bent section of the web surface; (f) forming a vacuum sealed chamber defined by the flexible diaphragm and the molding tool, said vacuum sealed chamber enclosing the inner restraints and the preform material; (g) evacuating air from the vacuum sealed chamber until a partial vacuum is reached such that the flexible diaphragm is pulled toward the molding tool; (h) withdrawing each inner restraint along the sidewall of the molding tool in a direction away from the bent section during evacuation of the vacuumed sealed chamber such that there are at least two inner restraints moving in opposite direction along each sidewall of the molding tool, thereby allowing the preform material to come into contact with the sidewalls of the molding tool in a gradual manner; and (i) increasing vacuum pressure after the restraints have been withdrawn and the preform material has conformed to the molding tool, thereby forming a shaped preform, said vacuum pressure being sufficient to consolidate the shaped preform,