Bonding PP Honeycomb with face sheets (skins) to form a sandwich panel is a critical step in its application. Common composite processes include:
- Wet Lay-up (Hand Lay-up / Vacuum Infusion): The honeycomb core is placed in a mold, and fiberglass or carbon fiber fabric is laid by hand on its top and bottom surfaces. Resin (e.g., unsaturated polyester, epoxy, or vinyl ester) is then applied, and rollers or vacuum bag pressure is used to saturate the fabric and fill the interface between the core and skins before curing. This method requires low equipment investment and is suitable for low-volume, complex-curvature products.
- Prepreg Hot Pressing / Autoclave Molding: Using pre-impregnated fiber prepreg as skins, they are laid up with the honeycomb core and placed in a hot press or autoclave for curing under heat and pressure. This method produces high-quality, consistent products and is commonly used in aerospace and high-performance sports equipment.
- Continuous Flat Panel Composite Production Line: For high-volume, flat products (e.g., building wall panels, truck floors), continuous production lines are used. Adhesive (film or liquid) is applied to unrolled top and bottom skin materials (e.g., aluminum, prepainted steel, PVC sheets), PP honeycomb core is fed in between, and the assembly is continuously heated and pressed through hot rollers or platen presses for curing.
- Adhesive Film Hot Pressing: Solid adhesive films (e.g., epoxy, polyurethane films) are used as the bonding medium. The core, film, and skins are stacked in order and placed in a hot press, bonding under heat and pressure at the film’s melting temperature.
Adhesive selection is critical and must be compatible with both the PP surface and the skin material. Common choices include:
- Epoxy Adhesives: Good overall performance, high strength, heat resistant, often the first choice for high-performance applications.
- Polyurethane (PU) Adhesives: Good toughness, high peel strength, suitable for applications with thermal expansion/contraction or impact requirements.
- Modified Acrylate Adhesives: Fast curing, good adhesion to low-surface-energy plastics like PP.
- Specialty Thermoplastic Adhesive Films: Facilitate automated production, clean and environmentally friendly.
Before bonding, corona treatment or flame treatment is typically applied to the PP honeycomb surface to increase its surface energy, enhancing adhesive wettability and final bond strength.
