Carbon/BMI and carbon foam form Invar alternative
For years, Invar steel has been the material of choice for tooling that must endure repeated autoclave cycling in volume part production. Invar's disadvantage is its relatively high density and resulting large mass. As processors move away from hand layup and toward automated fiber and tape placement, there is a critical need for tools that can be transported and repositioned easily. For very large structural parts, however, a mold or mandrel fabricated from Invar can weigh hundreds or thousands of pounds.
Carbon fiber composite tooling is an alternative, but the number of parts that can be pulled is significantly lower, given the less durable epoxy tool surface. A new carbon fiber-based tooling material developed by Nova-Tech Engineering (Lynnwood, Wash.) is designed to overcome those limitations. Integrated Carbon Structure (ICS) tooling uses a carbon foam substructure faced with a carbon fiber/bismaleimide (BMI) prepreg laminate. After autoclave cure, the final mold surface is CNC-machined to match 3-D CAD data. The carbon/BMI has a low coefficient of thermal expansion (CTE) of 1.90×10-6/°F, and the use of carbon foam minimizes the tool's weight and effectively eliminates CTE mismatches between the tool face and the substructure, says Larry Carver, Novatech's manager for composite tooling. The material's high Tg (>400°F/204°C) enables the tools to tolerate a minimum of 500 autoclave cycles. The company uses carbon foam from two sources: CFOAM, from Touchstone Research Laboratory (Tridelphia, W.V.), and GRAFOAM, from GrafTech International (Parma, Ohio). For its carbon/BMI prepreg, Novatech chooses from HexTOOL, supplied by Hexcel (Dublin, Calif.) or Cytec Engineered Material's (Tempe, Ariz.) DURATOOL 450.
Novatech already has built a master source mandrel designed for a large military aircraft tail cone (see photo). The tool, 14 ft in diameter at its base and 17 ft tall (3.7m by 5.2m) weighs in at about 3,000 lb/1,360 kg, two-thirds lighter than the 15,000-lb/6,800-kg Invar version. A second tool, still in the design stage, is a cylindrical mandrel for fuselage production that measures 19 ft in diameter and 50 ft long (5.8m by 15.2m). It would weigh, Carver estimates, about 46,530 lb/21,106 kg, one-fifth that of the Invar mandrel that Alenia Aeronautica (Grottaglie, Italy) uses to fabricate fuselage sections for the The Boeing Co.’s (Seattle, Wash.) 787 Dreamliner. Carver says the ICS mandrel should show high dimensional stability and rigidity: an analysis predicts a maximum deviation under full gravity load of only 0.015 inch/0.38 mm at its midpoint.