Airplane manufacturers are under increasing pressure to produce lightweight, strong, and durable aircraft at the lowest cost for manufacture and lifecycle maintenance. An airplane or helicopter must have sufficient structural strength to withstand stresses during flight, while being as light as possible to maximize the performance of the aircraft. To address these concerns, aircraft manufacturers have increasingly used fiber-reinforced resin matrix composites.
Fiber-reinforced resin matrix composites provide improved strength, fatigue resistance, stiffness, and strength-to-weight ratio by incorporating strong, stiff, carbon fibers into a softer, more ductile resin matrix. The resin matrix material transmits forces to the fibers and provides ductility and toughness, while the fibers carry most of the applied force. Unidirectional continuous fibers can produce anisotropic properties, while woven fabrics produce quasi-isotropic properties. Honeycomb core is often sandwiched between composite sheets to provide stiff honeycomb core sandwich panels having the highest specific strength.
To form a honeycomb core sandwich panel, prior art methods used a lay-up mandrel. A composite outer skin, usually a lay-up of prepreg sheets, was laid against the upper surface of the lay-up mandrel, and a honeycomb core was laid over the outer composite skin. A composite inner skin, also usually a lay-up of prepreg sheets, was then arranged over the honeycomb core, and the three layers were bagged and cured so as to form the honeycomb core sandwich panel.
Tolerances at the outer mold line (i.e., the outer side of the composite outer skin) of honeycomb core sandwich panels formed by this method were near exact because the outer mold line was formed against the upper surface of the lay-up mandrel. However, this method did not provide index control for the inner mold line (i.e., bagside) of the composite inner skin. Inexact tolerances at the inner mold line made locating and attaching details on the inner surface of the honeycomb core sandwich panel difficult. Another problem encountered in formation of the honeycomb core sandwich panels was crushing of the honeycomb core caused by the application of high pressure and vacuum during curing.
Multistage curing was one prior art process for avoiding core crush. Multistage curing consisted of bonding the honeycomb core sandwich panel together one element at a time and curing or pre-curing the assembly after each element was added. Although crushing of the honeycomb core was generally avoided, higher cost and risks were associated with this type of processing. For example, exposing the bonded assembly to multiple high-temperature cure cycles induced various degrees of shrinkage in the honeycomb core sandwich panel. Thermal expansion between the lay-up mandrel and the honeycomb core sandwich panel, as well as between the skins and the honeycomb core, also caused problems. The resultant stresses were proven to cause disbonding in the previously-cured bond interfaces.
In some prior art processes, tooling, for example, a caul plate, was used along the inner mold line of the composite inner skin. The caul plate helped to prevent crushing of the honeycomb core and helped to maintain tolerances at the inner mold line. Caul plates were especially desirable when honeycomb core sandwich panel was to be formed by co-curing the three layers of the honeycomb core sandwich panel, because core crush was most likely to occur during this type of process.
While co-curing with a caul plate was a preferred option for formation of a honeycomb core sandwich panel, fabricating a caul plate with conventional tooling methods was costly. Prior art caul plate fabrication required a physical model. In one prior art method, a part model was fabricated by bonding together epoxy or polyurethane modeling boards. The surfaces of the bonded epoxy and polyurethane modeling boards were then machined to match the inner mold line of the desired part. This process was costly and laborious.
Another type of model used in formation of caul plates was a pre-formed cured part made of the same or similar details as the final part to be formed. The pre-formed part was filled and sanded to achieve an acceptable model surface. Forming a model from a pre-formed part in this manner was labor intensive. Moreover, after the pre-formed part was used as a model, it could no longer be used for its intended purpose because the filling, sanding, and curing required to transform the part into a model and form a caul plate on the model caused the part to lose its integrity.
There exists a need for more efficient, less costly method of forming inner mold line tooling such as a caul plate. Preferably, the method can be used to form a caul plate that is to be used in formation of a honeycomb core sandwich panel.