•  Thermoplastics—good insulating properties, good  energy absorption and/or redirection, smooth  cell walls, moisture and chemical resistance, are  environmentally compatible, aesthetically pleasing,  and have a relatively low cost. •  Aluminum—best strength-to-weight ratio and  energy absorption, has good heat transfer properties,  electromagnetic shielding properties, has smooth, thin  cell walls, is machinable, and has a relatively low cost. •  Steel—good heat transfer properties, electromagnetic  shielding properties, and heat resistant. •  Specialty metals (titanium)—relatively high strength- to-weight ratio, good heat transfer properties, chemical  resistance, and heat resistant to very high temperatures. •  Aramid paper—flame resistant, fire retardant, good  insulating properties, low dielectric properties, and  good formability. •  Fiberglass—tailorable shear properties by layup, low  dielectric properties, good insulating properties, and  good formability. •  Carbon—good dimensional stability and retention,  high-temperature property retention, high stiffness,  very low coefficient of thermal expansion, tailorable  thermal conductivity, relatively high shear modulus,  and very expensive. •  Ceramics—heat resistant to very high temperatures,  good insulating properties, is available in very small  cell sizes, and very expensive.  [Figure 7-19]  Honeycomb core cells for aerospace applications are usually  hexagonal. The cells are made by bonding stacked sheets  at special locations. The stacked sheets are expanded to  form hexagons. The direction parallel to the sheets is called ribbon direction. Bisected hexagonal core has another sheet of material cutting  across each hexagon. Bisected hexagonal honeycomb is  stiffer and stronger than hexagonal core. Overexpanded core  is made by expanding the sheets more than is needed to make  hexagons. The cells of overexpanded core are rectangular.  Overexpanded core is flexible perpendicular to the ribbon  direction and is used in panels with simple curves. Bell- shaped core, or flexicore, has curved cell walls, that make it  flexible in all directions. Bell-shaped core is used in panels  with complex curves. Honeycomb core is available with different cell sizes.  Small sizes provide better support for sandwich face sheets.  Honeycomb is also available in different densities. Higher  density core is stronger and stiffer than lower density core.  [Figure 7-20]  Foam cores are used on homebuilts and lighter aircraft to  give strength and shape to wing tips, flight controls, fuselage  sections, wings, and wing ribs. Foam cores are not commonly  used on commercial type aircraft. Foams are typically heavier  than honeycomb and not as strong. A variety of foams can  be used as core material including:  •  Polystyrene (better known as styrofoam)—aircraft  grade styrofoam with a tightly closed cell structure  and no voids between cells; high compressive strength  and good resistance to water penetration; can be cut  with a hot wire to make airfoil shapes.  Foam Figure 7-20.  Honeycomb density. 7-12 Hexagonal Honeycomb Core Overexpanded Core Flexicore