1. Field of Invention
This invention relates generally to a structural laminate in which an end-grain balsa core panel is sandwiched between facing skins, and more particularly to a laminate of this type in which edges of three core panels are bevelled and in which the facing skins fully cover these edges as well as the opposing faces of the core panel.
2. Status of Prior Art
Balsa has outstanding properties unique in the field of lumber, for on the average it weighs less than nine pounds per cubic foot, this being 40% less than the lightest North American species. Its cell structure affords a combination of high rigidity and compressive and tensile strength that is superior to any composite or synthetic material of equal or higher density. Balsa is dimensionally stable and may be processed by standard woodworking techniques.
It is known that end-grain balsa wood, because of its high compressive strength, is capable of supporting far greater loads than flat-grain material of the same density, and that low-density balsa in the end-grain direction will support greater loads than flat-grained material of higher density. The cellular structure of balsa is such that the number of cells per cubic foot is extremely high, the wall thickness of each cell being quite thin. The cells are effectively independent of each other, each cell being comparable to an independent column or fiber. The cells of balsa wood are substantially parallel to each other.
Structural sandwich laminates can be created by bonding thin facings or skins to balsa wood panels which function as a core. Thus the Kohn et al. U.S. Pat. No. 3,325,037 and the Lippay U.S. Pat. No. 3,298,892 disclose structural sandwich laminates whose core is formed of end-grain balsa. The resultant laminates having a remarkably high strength-to-weight ratio as well as excellent thermal and acoustic insulation properties.
Of particular prior art interest is the Kohn U.S. Pat. No. 4,343,846 which discloses a structural laminate in which an end-grain panel is laminated to composite facing sheets formed by synthetic plastic films reinforced by glass fibers.
End-grain balsa-core sandwich laminates are widely used in transportation and handling equipment, such as for floors of railroad cars, shipping containers, cargo pallets, bulkheads, doors and reefer bodies, as well as in a variety of other applications. These laminates are also employed for structural insulation in aircraft applications, in housing and in boating.
While it is known to use thin metal facings or plastic skins in structural laminates having an end-grain balsa core, in recent years it has become the practice to enhance the strength of the laminate by means of composite facing skins reinforced by carbon or graphite fibers. Also in use are facing skins made of Kelvar, a synthetic plastic of exceptionally high strength.
For many installations, such as in automotive flooring applications, it is essential that the facing skins laminated to the end grain balsa core not only cover opposing faces of the panel, but also its edges so that these edges are protectively shielded and not exposed.
Composite facing skins which incorporate relatively stiff reinforcing fibers, though flexible, cannot be sharply bent. Should, therefore, one seek to bend a composite facing skin placed on a face of a core panel so as to conform to a right-angle edge of this panel, the skin cannot be fully bent. As a consequence, an air pocket will be created between the edge and the bent skin which introduces an unacceptably weak point in the structural laminate.
It therefore becomes desireable to bevel the edges of the end-grain balsa core panel to impart a gentle slope thereto. This slope makes it possible to laminate a facing skin which cannot be sharply bent yet can be made to fully conform to the gentle slope of the core panel and leave no air pocket.
Though one may bevel the edges of the panel by means of a router machine having a rapidly revolving spindle and a cutter, the use of this machine for this purpose has serious drawbacks. A milling operation of this sort is difficult to perform and is time consuming, thereby adding substantially to the cost of producing the structural laminate. Moreover, milling of the end grain balsa core edges disintegrates the wood and yields balsa waste, giving rise to pollution problems in the production plant.