Lightweight, stable large-tool concept requires no master model

After two years of testing, partners Advanced Composites Group (ACG, Tulsa, Okla.) and GrafTech (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.

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).

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