Manufacturing Fiber Reinforced Composites (FRP) is fundamentally different than metal or plastics. Metals are typically cast into molds directly from the liquid state or CNC machined from bulk stock. Plastics can be cast, injected, or thermo-formed. However, high performance composites must be formed using operations that can maintain specific fiber orientations. The primary routes for forming complex, hollow structures in FRP all require tooling that supports the fibers and resin during lay-up and curing, but it must also be removable. Conventional tooling for hollow composites consists of inflatable bladders, plaster, or eutectic salts. This technology has changed little over the last two decades.
Inflatable bladders in use in the composites industry are either elastic or thermoplastic materials that can be deformed into a complex shape. Deformation is accomplished by vacuum or pressurized fluid. Although reusable, they wear out after ~2 to 30 pulls. In addition, these materials have high thermal expansion values. Removal can be difficult depending on the material properties of the bladder material and on part geometry. Further, bladders require specialized tooling to produce not only the bladder but also an external tool for the shape of the final part.
Plaster based mandrels are used extensively in the composites industry. Plaster has the advantages of being low cost and readily cast into complex shapes. However, plaster mandrels need to be dried thoroughly and their surfaces are porous. Porous surfaces are undesirable because resin infiltration can occur during a composite lay-up. In addition, the coefficient of thermal expansion (CTE) of plaster is relatively large (~25 ppm/°C). For complex parts, plaster removal is non-trivial. Very aggressive mechanical forces must be used to break the plaster from the part, which can damage the part during the removal process, leading to scrap or loss.
Eutectic salts are another tooling material used to form complex composite geometries and internal passageways. Common salts consist of alkali nitrates such as sodium nitrate, and potassium nitrate. The chief advantage of eutectic salts is their water solubility which enables removal from complex parts. However, eutectic salts also have several properties that make them difficult to work with. This includes high processing temperatures (casting is done at high temperature ~260°C), high densities, corrosive waste streams, high shrinkage on solidification (~20%), and slow washout times (on the order of days).
While foams are sometimes used as “leave in” mandrels since they can provide stiffening or dampening characteristics, polymer foams can be removed with solvents or by bead blasting. Aggressive solvents can potentially damage the composite matrix. Bead blasting can also produce undesirable composite erosion.