1. Foam Core
2. Inserts
3. Face Sheets
4. Motor Housings.

Foam core serves a dual purpose: it provides shear stiffness and it defines the shape of the final part. Because of the strength requirements and the weight restrictions, polymethacrylimide (PMI) foam will be chosen over other foam cores such as polyvinylchloride (PVC), polyurethane, or polystyrene. Polymethacrylimide, when heat treated, is capable of withstanding 100 psi at 250oF, which is required for manufacturing the device links. Two-inch thick sheets of polymethacrylimide will be bonded together with an epoxy adhesive until the desired thickness is reached. Different densities of foam core will be used for different areas of the links as dictated by the local stresses in those areas. Once bonded, the foam blank will be machined to the required shape and hand-sanded to the proper contours.
Carbon-epoxy inserts are laid-up on an aluminum mandrel to control the critical surfaces (i.e., the connections to the motor housings). This eliminates any costly carbon machining. An intermediate modulus carbon pre-impregnated fabric (pre-preg) is chosen for its balance of strength and stiffness. One ply of E-glass is used on surfaces of the insert that contact aluminum to prevent any chance of galvanic corrosion. Once cured, the inserts are potted into the foam core using structural adhesive (Hysol EA9394). This two-part epoxy cures at room temperature, but has a maximum operating temperature of 350oF which is well above the operating temperature of the enclosed motors.
The face sheets are multi-directional fibers with a 50 MPSI modulus which are pre-impregnated using a 250oF curing epoxy. It has an average thickness of 0.060", and a maximum thickness of 0.120" in several high-stress areas. In areas where the face sheets overlap the carbon inserts, a co-curing film adhesive is used to promote the bond. As each ply is laid-up, they are debulked at 29 inches Hg for 1 hour at 100oF to eliminate any voids between plies. Once all of the plies are consolidated, the part is jigged, vacuum bagged, and autoclaved at 250oF and 100 psi for the final cure. The last step to complete the project is bonding the aluminum motor housings to the carbon inserts. Because of the poor bonding properties of aluminum oxide, the surface of the housings is etched with chromic acid and bonded within 16 hours to retain full bond strength. The housings are potted into the carbon inserts using Hysol EA9394 adhesive, completing the part.