Method for forming shaped preform

A method for shaping a fibrous preform comprising:

• (a) providing a substantially flat fibrous preform, said fibrous preform comprising of an assembly of fibrous materials bonded to each other by a resin binder;
• (b) providing an upper flexible diaphragm and a lower flexible diaphragm, said diaphragms being formed of an elastomeric material and are impermeable to gas;
• (c) providing a housing with a mold positioned therein, said mold having a non-planar molding surface;
• (d) holding the fibrous preform between the upper and lower diaphragms in an air-tight manner by creating a sealed pocket between the diaphragms;
• (e) positioning the diaphragms with the preform there between over the housing so as to define a sealed chamber bounded by the lower diaphragm and the housing, and such that the lower diaphragm is positioned above the molding surface;
• (f) removing air from between the diaphragms to establish a vacuum pressure of less than 950 mbar and less than the pressure in the housing;
• (g) heating the fibrous preform to a temperature above the softening point of the resin binder;
• (h) creating a vacuum inside the sealed chamber between the lower diaphragm and the housing by removing air at a rate of 1 mbar/15 mins or faster until a vacuum pressure of 950 mbar or below is reached, while heating is maintained, whereby the diaphragms with the preform there between are pulled toward the molding surface and eventually conform thereto, thereby forming a shaped preform;
• (i) reducing the vacuum pressure between the diaphragms to 10 mbar or below;
• (j) cooling the shaped preform to a temperature that is below the softening temperature of the resin binder;
• (k) relieving the vacuum from between the diaphragms;
• (l) removing the upper diaphragm from the cooled preform while maintaining vacuum inside the sealed chamber between the lower diaphragm and the housing;
• and
• (m) removing the cooled, shaped preform from the lower diaphragm.