Inflatable Rubber Mandrels in Aerospace Construction
Composite Materials in Aerospace
The use of composite materials in the manufacture of Aerospace articles has steadily increased and they now are used in structural components, in addition to wing skin coverings. The development of these applications have required shaped elastomeric tooling that can apply pressure to the internal matrix of a composite structure, while the autoclave applies external pressure and elevated temperature to cure the adhesive resin in the lay-up. In these applications, the shaped rubber mandrel is in direct contact with the resin. The inherent resistance of silicone rubber to elevated temperature and adhesion by epoxy type resin makes it an ideal candidate for the elastomer. Some of the early development efforts used liquid silicone rubber, catalyzed for room temperature vulcanization. The obvious advantage was low cost tooling. The disadvantage was in the life expectancy of the rubber part in service. The RTV silicone was not usually a high strength / high tear resistant polymer. It could be damaged during removal from the fabricated article. Shrinkage of the solid rubber blocks, after several cure cycles, could decrease important dimensions and thereby reduce compaction pressure in the composite structure.
Overcoming Disadvantages
In an effort to overcome these disadvantages, Mosites Rubber Company has produced high strength silicone rubber inflatable mandrels. These shaped bags are hollow; usually with an inflation stem in one end; and if the outside dimensions are critical, the bag can be made inside female tooling to yield crisp detail. For less critical applications, the silicone bag can be layed up on inexpensive male sheet metal tools. One advantage of the female tooling is that it allows for variations in the wall thickness of the rubber bag without altering the outside dimensions of the part. If extra material is necessary at a certain location to reinforce the part, it can be added internally. When placed inside the composite article during its cure cycle, the pressure inside the bag can regulated as desired. In some applications the pressure stem is vented to the autoclave atmosphere to maintain exact internal and external pressure equilibrium. Upon completion of the cure cycle, a vacuum can be used to collapse the mandrel and aid in its removal.
This procedure has been utilized to manufacture helicopter rotor blades; to allow the co-cure of entire wing sections with integral stiffeners and hat section channels; to apply erosion coatings to jet engine turbine blades; and to manufacture composite recreational and sporting equipment such as bicycles, golf clubs and fishing rods.
