Forming a three dimensional fiber truss from a fiber slurry

Details Forming a three dimensional fiber truss from a fiber slurry Forming a three dimensional fiber truss from a fiber slurry

2000-04-26

2002-09-17

Silbaugh, Jan H. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Removal of liquid component or carrier through porous mold...

C264S087000, C425S084000, C425S085000, C162S218000, C162S221000, C162S224000, C162S227000

Reexamination Certificate

active

06451235

ABSTRACT:

FILED OF THE INVENTION
This invention relates broadly to the production of three-dimensional structural components that make up the structural cores of lightweight composite sandwich panels and other load-bearing structures that have a high level of strength relative to structure weight. More specifically, the invention discloses a method and apparatus for forming from a fiber slurry a lightweight, fiber truss having three-dimensional features that reinforce the truss and that reinforce composite structures containing the truss.
BACKGROUND OF THE INVENTION
A three-dimensional fiber truss is defined herein as a fibrous element that has a strategically engineered three-dimensional shape designed to create, either stand-alone or in combination with other elements, a structural framework for load-bearing purposes. Typically, the wall thickness of a three-dimensional fiber truss will be small compared to the overall height of the three-dimensional features making up the fiber truss. With relatively thin walls, fiber mass is minimized relative to the strength and rigidity that may be attained in products that incorporate the fiber truss.
In many panel applications, the three-dimensional structure of the fiber truss consists of a series of hollow protrusions from a truss base. In this form, the fiber truss is bonded to exterior skins or to other fiber trusses. The composite structure formed in this way completes the three-dimensional truss work of the fiber truss element, and produces a product having a high level of strength and rigidity. A composite structure in which the fiber truss functions as a structural core between exterior skins is commonly known as a “sandwich panel.” Because the flat sheets that form the exterior skins of the panels carry most of the stresses during bending, these composite sandwich panels are sometimes referred to as “stressed-skin panels”.
Fiber trusses can be manufactured using a wide variety of methods and a diverse selection of materials, including metal filaments, wound fiber composites, folded paper sheets, and wood fiber compositions. The present invention is practiced using a narrowed class of materials composed of fibers that may be suspended and randomly distributed in a carrier fluid. Fibers of this type may be derived from various lignocellulosic materials as well as a wide range of synthetic materials. Different fiber types may be mixed together in the carrier fluid and/or a number of chemical additives may be applied to the fiber or fiber blend to achieve specific properties. The fiber trusses described in U.S. Pat. Nos. 5,900,304; 4,495,237; and 4,348,442 are examples of fiber trusses that may be produced using the disclosed method and apparatus.
When the fiber truss or core element is sandwiched between skins to complete the three-dimensional truss work, the resulting composite sandwich panel behaves in a manner analogous to a common I-beam, which is a very efficient engineering structure. By analogy, the fiber truss or core element corresponds to the central rib of the I-beam and the skins correspond to the top and bottom flanges of the I-beam. In bending an I-beam parallel to the central rib, one flange of the I-beam undergoes compression, while the other flange is in tension. In an analogous manner, bending of a composite panel places one of the skins in compression while the other skin is held in tension.
Numerous applications for sandwich panels exist in construction, furniture, material handling and packaging industries. The most common sandwich panel is the corrugated panel used extensively in light-weight box construction. In this case, the structural core is formed by bending thin paper sheets into an undulating or corrugated pattern. The corrugated cores are glued between paper skins to form the familiar corrugated panels. Numerous other lightweight structural cores and sandwich panels have been disclosed in the prior art, but only a few of these have been successfully commercialized. The honeycomb core is one of the more successful of these other cores.
The relative success of the honeycomb core and the corrugated core derive from the relative simplicity of their structural shapes and formation methods. There has been a need to define a correspondingly simple and cost-competitive method for constructing more complex structural core shapes which have improved structural properties. The present invention focuses upon such a method for the manufacture of a wide range of lightweight structural cores, and other load-bearing structures, using both natural plant fibers and synthetic or man-made fibers.
In the initial step of the method, the fibers are mixed in a carrier fluid to form a random or quasi-random fiber distribution. The mixture comprising fiber and carrier fluid will be referred to as a “fiber slurry” in this disclosure. Slurry formation is followed by a step in which the carrier fluid is almost completely discharged from the slurry and the retained fiber is concentrated and compacted in a wet-pressing operation, forming a pre-form fiber truss. Finally, the pre-form fiber truss is dried to form a finished fiber truss.
While the disclosed invention emphasizes the formation of fiber trusses used to construct core elements used in composite panels, the invention also can be used to form a wide range of molded fibrous products and components, including egg cartons, produce trays, conformal packaging, molded packaging inserts, and molded pallets. Well known conventional fiber-molding processes used to make these types of products begin by extracting fiber from a slurry and depositing the fiber onto a single forming mold using vacuum suction through a single porous mold containing an overlay of screen elements. Occasionally, air pressure is applied to the slurry either alone or in combination with vacuum extraction to increase the pressure differential which drives the carrier fluid through the porous mold and screen elements.
The equipment used for these conventional forming processes is fragile and expensive. For the more commonly utilized vacuum-only forming process, the pressure differential exerted across the porous mold and screen elements is limited to a maximum level equal to ambient air pressure. This low pressure differential results in comparatively slow carrier fluid removal and fiber formation. Fiber formation slows down substantially as product thickness increases.
In both vacuum-only processes and conventional gaseous-pressure-forming methods, slurry fluid-discharge and fiber deposition forces are applied across only a single forming surface. Single-surface fiber formation is not only a slow process, but fiber tends to be lumpy and wall thickness erratic. These conventional processes generally yield wet pre-forms having low solids content and high water content. High water content results in a fragile pre-form that is difficult to handle. The need to remove large quantities of water per pound of fiber leads to inefficiencies in energy usage and high costs in drying the wet pre-forms.
In the prior art, several improved methods are disclosed for forming from fiber slurries fiber trusses and other fiber structures, but these prior methods are still relatively complicated and expensive. For example, Setterholn and Hunt in U.S. Pat. No. 4,702,870 describe a method for forming specific three-dimensional structural components from wood fiber. Their method and apparatus require the use of a fragile resilient mold insert to form three-dimensional features in the finished fiber product. The use of resilient mold elements results in slow drying of the fiber, long production cycles, and maintenance problems associated with the fragility of the resilient mold elements. While the product formed according to the method of Setterholm and Hunt has excellent mechanical properties, the aforementioned problems associated with the resilient mold elements make the product expensive and limit the scope of applications. Nonetheless, the product has attracted considerable commercial interest and has had some degree of marketing success.

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