Successful application of PP Honeycomb sandwich structures hinges on design. Key considerations include:
- Core-to-Skin Matching (“Core-to-Face Ratio”): The ratio of core thickness to skin thickness needs optimization. A core that’s too thin offers little stiffness gain; too thick may cause skin buckling. Typically, the core constitutes 70%-90% of the total thickness. Skins must have sufficient stiffness and strength to handle in-plane loads and local concentrated forces.
- Joint Design: This is often the most challenging and critical aspect. It is absolutely forbidden to bolt or fasten directly into an untreated honeycomb core area, as this will crush the core immediately.
- Potting/Insert Method: At connection points, holes are pre-cut in the core, and solid plastic blocks, wood blocks, or metal inserts are embedded and bonded with adhesive. Fasteners are then attached to these inserts.
- Filling Method: Holes are drilled at connection points and filled with a high-strength casting resin (e.g., epoxy), which cures to form a solid “plug.” Threads can then be tapped into the plug.
- Edge Connections: Typically use edge banding or aluminum angles, transferring load to the skins via adhesive and minimal mechanical fasteners.
- Preventing Skin Wrinkling and Core Shear Failure: Through calculation and simulation, ensure that under working loads, compressive stress on skins is below their critical buckling stress, and shear stress on the core is below its shear strength. This requires appropriate selection of core density and skin thickness.
- Moisture Sealing Design: If used in environments possibly exposed to moisture, all exposed honeycomb cell edges must be reliably sealed to prevent water ingress leading to debonding or performance loss.
- Accounting for Coefficient of Thermal Expansion (CTE) Mismatch: Skins (e.g., aluminum) and PP core have different CTEs. In environments with large temperature swings, design should allow for thermal stress relief or select skin materials with matching CTEs (e.g., composites).
- Design for Manufacturing: Consider adhesive flow and fill, breather paths for vacuum bagging, temperature/pressure curves for hot pressing, etc., to ensure manufacturability. Close collaboration with material suppliers and manufacturers during the design phase is ideal.
