A simple, EPA-friendly approach solves a long-standing problem in heat-formed composite manufacturing.
Honeycomb structures are commonly employed as load- and force-bearing structures as they are structurally strong and lightweight. These structures include many aircraft and spacecraft surfaces, including aircraft wings and fuselages, spacecraft pressure vessels, and heat-shield materials. Many other processes in other areas of transportation and defense, as well as the pharmaceutical and construction industries, employ pressure vessels with similar heat-formed composite structures.
Manufacturing processes for heat-molded composite honeycomb structures commence with the placement of preimpregnated composite layups over metal mandrels. To prevent permanent bonding between the composite layup and the metal mandrels, an agent, known as a mold release agent, is used. Mold release agents allow the molded composite material to be removed from mandrels after a heat-forming process. Without a specific removal process, mold release agents may continue to adhere to the surface of the composite material, thereby affecting the bonding of other materials that may come into contact with the composite surface in later stages of processing.
Mold release agents have a unique chemistry that, upon heating, requires a unique chemical method for removal. Prior art includes immersion solvent cleaning (trichloroethylene, hexane, limonene), vapor degreasing, and plasma cleaning. Many of the solvent and vapor degreasing techniques, such as Freon and trichloroethylene, can no longer be used per EPA standards. Plasma cleaning has limited use in structures that have a deep channel or high aspect ratio, as the plasma penetrates diffusively and thus, may result in incomplete cleaning for the full depth of the structure.
A constituent common to commercially available household cleaning agents is employed for the removal of mold release agents common to the manufacturing of heat-formed composite materials. The reliability of the solvent has been proven by the longevity and reliability of commercial household cleaners. At the time of this reporting, no one has attempted using constituent for this purpose. The material to be cleaned is immersed in the solution, vertically removed so that the solution is allowed to drain along cell walls and into a solvent bath, and then placed on a compressed airflow table for drying. Non-destructive evaluation techniques [see “Non-Destructive Evaluation of Materials via Ultraviolet Spectroscopy” (GSC-15338), NASA Tech Briefs, Vol. 32, No. 6 (June 2008) p. 81] are employed to detect mold release residue and evaluate the degree of cleanliness. The cleaning process is repeated until non-destructive evaluation shows minimal detection limits for mold release residue.
This work was done by Diane Pugel of Goddard Space Flight Center. GSC-15902-1