Lightweight, stable large-tool concept requires no master model

After two years of testing, partners Advanced Composites Group (ACG, Tulsa, Okla.) and GrafTech International (Parma, Ohio) are commercializing the ACG GRAFOAM FPA-20 tooling system, a new concept in dimensionally stable, low-mass, high-temperature-compatible composite tooling. The system’s foundation is carbon foam. GrafTech carbonizes a proprietary polymer resin in a graphitization furnace to create blocks of the finely textured, closed-cell foam in a range of densities. The resulting isotropic foam has no ordered crystalline structure and is, therefore, nonconductive. GRAFOAM exhibits a coefficient of thermal expansion (CTE) compatible with that of carbon fiber (2.3 x 10^-6/°C), and its thermal resistance exceeds composite processing requirements. Further, it can be easily and rapidly machined to near-finished size and then fully encapsulated in ACG’s proprietary, patented, “compliant interface technology” and a tooling laminate skin to form a tool without a master model. The tooling laminate is typically formed from ACG’s trademarked Low Temperature Moulding (LTM) prepreg, but ACG’s medium- (MTM) and high- (HTM) temperature tooling laminate ranges, including high-temperature bismaleimide (BMI) prepregs, can be used as well. The prepreg can be cured directly onto the foam tool body in a single operation, without the need for a heat-resistant intermediate splash tool. The final tool profile is created in a second machining operation (trials showed that machining results in no loss of tool vacuum integrity). This overcomes problems encountered with the master model approach, such as shrinkage, spring-in/spring-out, and pattern/laminate CTE mismatch, says ACG. Design changes can be accommodated with additional local lamination and subsequent remachining. Tools are self-supporting and suitable for tape placement and filament winding, where low weight is a major benefit when manipulating extremely large tools.

ACG reports that large GRAFOAM FPA-20 development tools recently manufactured for a major North American aircraft manufacturer achieved dimensional accuracy of 0.2 mm/0.008 inch on particularly complex surface profiles, with the vast majority falling within ±0.1 mm (±0.004 inch).