Glass plates are used as cure plates in a modified process for the fabrication of flat composite-material (matrix/fiber) panels. In the unmodified previous version of the process the cure plates were made of aluminum.
A cure plate provides a smooth, flat surface on which a polymer resin and reinforcing carbon fibers are laid up. The layup is then vacuum-bagged on the cure plate, and the polymer is cured by heating the entire assembly in an autoclave.
To be useful as cure plates, aluminum plates must be machined and polished to the required flatness and smoothness. Both the cost of material and the cost of finishing are considerable. In addition, because aluminum is soft, it is easily scratched; hence, when an aluminum cure plate is used repeatedly, costly resurfacing is necessary to maintain an acceptable cure surface.
In the modified process, an aluminum plate is used as a substrate for curing, but no attempt is made to maintain its surface finish. Instead, a glass plate is simply laid on the aluminum plate, so that the glass surface becomes the cure surface. The glass plate should be at least 1/4 in. (6.4 mm) thick, and the glass should be of a tempered or heat-resistant type.
A polymer resin and fibers are laid up on the glass plate. A vacuum bag that extends past the edges of the glass plate is placed over the layup, and is then sealed to the aluminum substrate (see figure). Then the bag is evacuated, and the resin is cured in the traditional way.
Glass cure plates offer several advantages over aluminum ones:
- Glass plates as supplied by manufacturers are adequately flat and smooth; no additional surface finishing is necessary.
- The cost of a glass plate is only about 5 percent of the cost of an aluminum plate of similar size and surface quality.
- Glass is expendable and can be replaced economically if damaged, whereas aluminum must be resurfaced.
- Being harder than aluminum, glass is less vulnerable to scratching and gouging.
- The CTE (coefficient of thermal expansion) of glass matches the CTE of composite materials more closely than does the CTE of aluminum. The cure cycle, therefore, subjects the composite to less stress and risk of damage.
This work was done by James E. Baker of Goddard Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Manufacturing & Prototyping category.