| Shape-adjustable mold, skin and interior-core structures for custom board production -> Monitor Keywords |
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Shape-adjustable mold, skin and interior-core structures for custom board productionRelated Patent Categories: Plastic And Nonmetallic Article Shaping Or Treating: Processes, Mechanical Shaping Or Molding To Form Or Reform Shaped Article, To Produce Composite, Plural Part Or Multilayered Article, Shaping Material And Uniting To A Preform, Preform Embedded In Or Surrounded By Shaped Material, Electrical Component Encapsulating, With Component Positioning Procedure Or Incorporation Of Article Positioning MeansShape-adjustable mold, skin and interior-core structures for custom board production description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070145638, Shape-adjustable mold, skin and interior-core structures for custom board production. Brief Patent Description - Full Patent Description - Patent Application Claims
RELATED APPLICATION [0001] This application is a continuation-in-part of U.S. Pat. No. 6,623,323 issued Sep. 23, 2003, which claims priority from U.S. provisional application 60/176,136 filed Jan. 14, 2000. BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] This invention relates to methods and apparatus used in the design and manufacture of surfboards, sailboards or similar aquatic boards, referred to generically herein as "board" or "boards." [0004] 2. Description of the Related Art [0005] Surfboards and sailboards are similar in shape and basic structure--the board typically has a high strength exterior skin covering that protects and is supported by very low-density material in the interior core; in construction, moldable plastic is used for the compound curvatures and sharp trailing edge contours conducive to a low-drag hydrodynamic shape; the board's primary strength is usually derived from a woven fabric, made from high-strength glass, carbon or aramid fiber, that is imbedded in the plastic to form a fiber-reinforced plastic or plastic composite skin. [0006] The composite skin, which is very thin, can be reinforced with specially manufactured high-density PVC sheet foam, end-grain balsa or honeycomb core materials to form a "structural sandwich" or "cored composite." The stiff, lightweight core material, used as a substrate to separate the high-strength composite layers on either side, creates a fundamentally different structure--the sandwiched core delivers the stiffness and rigidity of much thicker material, but at a fraction of the weight, and provides the impact resistance and compressive strength the very thin layers of plastic composite lack. [0007] The composite core materials and reinforcing fabrics impart a high degree of stiffness and very high strength but, unlike the weaker plastics and foamed plastics with which they are combined, have a limited capacity to conform to compound curvature (i.e., a surface that curves in two directions at once). Where the curvature is severe, divisions are necessary to prevent structural defects such as wrinkles in the reinforcing fabric, or the breakage and/or failure of the core material to conform to the required shape. Since a break in the continuity of either material causes a large reduction in strength, the placement of a division--usually referred to as a joint or seam--is critical to the overall structural integrity of the board. [0008] Currently, with molded methods of production, or in custom "one-off" manufacture (i.e., when the board is fabricated by hand), a joint or seam is required to accommodate the sharp curvature at the board's perimeter edge or "rail"--this division creates a number of seemingly unrelated but very serious problems, which increase manufacturing costs and seriously compromise the board's potential strength. [0009] In prior art molded manufacture, for example, the sharp curvature around the board's perimeter edge compounds a number of very basic drawbacks inherent in the mold's concave configuration itself--the structural problems and high manufacturing costs that result make the concave female mold of the prior art fundamentally unsuited for board production. The problems begin with the mold's inward curving surface: when the reinforcing fabric is saturated by hand, the resin naturally tends to flow out of the fiber and pools in the concave cavity of the mold; the mold's sharp edge contours then create a dam that makes it very difficult for the squeegee to completely remove the excess--the result is a weak, heavy resin-rich skin. In areas of severe concave curvature, wrinkles in the reinforcing fabric easily occur, and are difficult or impossible to remove--pulling the fabric taut tends to lift it from the surface of the mold; pushing on the fabric is analogous to pushing on a string, and causes wrinkles to (re)appear. [0010] To minimize the above problems, in the prior art the mold is divided into top and bottom halves; with the relatively flat and shallow surface the fabric is easily aligned and much of the excess resin can be successfully removed--the placement of the part-line, however, is in the worst position possible: at the board's exposed perimeter edge. Because the division of the mold also breaks the continuity of the high-strength fiber, the mold-seam on the finished board has only a fraction of the strength of material where the fiber is fully intact. The design of the joint is then compromised by the limitations of the mold's concave surface. The mold-seam is far stronger when reinforcement is applied to the interior of the joint--the inside surface of the joint, however, becomes completely inaccessible once the mold is closed. The mold-seam is therefore typically reinforced after the board is removed from the mold; this adds weight to the already resin-rich skin, and sufficient rework to negate much of the labor-saving advantage. [0011] The difficulty molding the board's interior foam core then raises production costs further still. Because the expansion of plastic foam involves heat (e.g., polyurethane foams undergo an exothermic reaction; steam is required to expand EPS "bead" foams), there is both an expansion and a very slight cooling contraction cycle in the molding of the foam--the slight cooling contraction makes it very difficult to pre-mold the board's interior foam core to sufficiently tight tolerances to eliminate potential voids between it and the interior surface of the closed mold and, when the expansion of the foam occurs in the mold, the cooling contraction begins before the foam has fully hardened, which often causes poor adhesion or an inconsistent skin-to-interior core bond. [0012] To reduce the problem, in the prior art the foam is contained in an extremely strong mold and the very high outward pressure generated by the foam's expansion is used to compress the foam against the interior surface of the mold to enhance adhesion and attain an adequate skin-to-interior core bond. Drawbacks include the high cost of the mold (the mold typically has steel reinforcing jigs attached and is held in a hydraulic press or by other mechanical means to prevent buckling, separating or failure under the high pressure of the expansion) and, because of the compression of the foam against the surface of the mold, the higher density of the foam and added weight,. [0013] The additional problem is that the plastic composite is thin and bendable, and the resin generally shrinks between five and six percent as it cures. The direction of shrinkage is primarily into the fiber and against the surface of the mold, where it is held in place by the perfect vacuum that develops as the resin hardens and cures. Because the two halves must eventually meet at a precise point around the perimeter, the mold functions to stabilize the laminate, and prevents distortion or shrinkage of the resin from creating a mismatch between the board's two opposing sides. The skin must then be fully cured and receive the support of additional material (ordinarily provided by the bond between the two opposing sides and the board's interior core) before it can be removed from the mold. The order of application is a major problem: the fact that the least stable and longest curing material (i.e., the composite skin) is applied to the mold first, and quickly curing foam(s) or pre-molded interior core structures are added later, lengthens the mold-cycle and causes very slow production. [0014] a. Molded Methods of Production [0015] With excess weight, high-capital costs, and lack of any competitive advantage in terms of price, the molded fiberglass skin/polyurethane foam core surfboards manufactured in the early nineteen-sixties, soon after the introduction of polyurethane foam, were derisively referred to as "pop-outs" due to their structural inferiority. The commercial production of molded hollow boards was attempted in the early nineteen-seventies, but was also very brief-absent the interior foam core, the lack of an effective joint between the board's top and bottom sides (see, e.g., U.S. Pat. No. 3,514,798 to Ellis) caused the mold-seam at the perimeter to split open with relatively modest impact; with higher impact often detaching the skin from the interior support structure, the damage was difficult or impossible to repair. [0016] Reviewing prior art clearly shows the structural defects and compromises caused by the concave configuration of prior art female molds. U.S. Pat. No. 3,802,010 to Smith, for example, suggests that the mold-seam at the board's perimeter can be eliminated by dividing a conventional female mold into right and left halves, and laying the saturated fiberglass fabric into the mold in a single sheet. According to the invention, the centerline division means that there are no joints along either side or rail where the board is subject to the greatest beating during use. [0017] What is completely ignored is the fact that the board's outline around the perimeter is roughly twenty percent longer than the straight line along the axis of symmetry--if the part-line is placed at the shortest distance between the nose and tail of the board, the fiberglass must elongate a total of ten percent per side to cover the perimeter of the mold, while maintaining its original length at the center. Since fiberglass is not elastic, the fabric must be carefully cut and trimmed to conform to the shape of the mold, or the ten percent that is excess will appear as folds and wrinkles in direct proportion to the differential in length. [0018] The mold's deep internal cavity and lack of access makes it impossible to accurately trim and create an overlapping joint in the fabric at the perimeter of the mold, however, and also prevents the defects from being properly repaired. The sharp folds in the reinforcing fabric create voids if subsequent layers are applied on top--this precludes the possibility of adding fiberglass layers or the use of any composite core material at all, or using these materials to create a bonding/reinforcing flange between the two opposing sides. The two sides are therefore joined by pouring a very thick layer of adhesive into a concave depression in the foam core, creating a very weak and heavy mold-seam between the opposing sides. The invention suggests trading the well-known structural problems caused by the relatively shallow concave cavity of the prior art female mold, for the much larger defects of a very deep one. [0019] The closely related U.S. Pat. No. 4,383,955 to Rubio et al. specifically identifies a number of the more obvious problems outlined above, and teaches a conventional solution: to improve access, the right- and left-hand mold configuration to Smith is given an extra division that turns it into quarters--with the four relatively flat mold surfaces, the fiberglass fabric can be successfully applied to the mold without wrinkles; moreover, with the accessible mold surface ordinary steps such as polishing, prepping and applying release agents to the mold become possible so that the board can subsequently be removed. From a structural or fabrication standpoint, however, there is no improvement at all--the extra division adds a mold-seam at the perimeter of the board, and neither disclosure addresses any of the well-known problems involved in molding the board's interior foam core. [0020] In both inventions, the liquid pre-foam is poured into the mold cavity and allowed to rise parallel to the board's width. The foaming reaction of the polyurethane resin is deceptively simple, however, because when complete, even the mixing cup appears to make a perfectly acceptable mold. The hidden problem is that the expansion of the foam occurs before the resin begins to harden--because the mold configuration causes considerable upward movement during expansion, the foam's cellular structure tends to be destroyed against the interior surface of the board/mold, and released blowing agent or gas is concentrated in the same area area; this leaves large voids directly beneath the surface of the fiberglass skin, and little or no skin-to-interior core bond. [0021] In the disclosure to Rubio, et al. the voids are identified, but the inventors incorrectly attribute the "soft spots" (i.e., the voids beneath the skin) to the expansion and contraction cycle of the foam (the soft spots are described as areas where the foam has "pulled away" from the fiberglass skin). They therefore suggest using a baffle to contain the expansion of the foam to compress it against the interior surface of the skin--a partial step towards the prior art method of containing the foam in a high-strength mold and hydraulic press. Neither invention has seen production, since prior art problems of molding the board's interior foam core, the length of the mold-cycle, and the weak or inadequate joint between the board's two opposing sides are neither noted nor addressed. In known methods of sailboard production, the latter two problems are "solved" by either foaming the resin matrix or eliminating the reinforcing fiber in the skin; both solutions therefore entail a major reduction in strength. [0022] In low-cost methods of sailboard production, for example, blow-molding or rotationally molding techniques are used to blow or melt a thermoplastic resin to the surface of the mold; although this produces a continuous one-piece skin, production is relegated to beginner and entry-level sailboards due to the excess weight/inadequate strength caused by the lack of any composite material at all. The added drawback is that the interior foam core must be formed by injecting liquid pre-foam into the interior cavity of the closed mold, which involves the production problems outlined above. Continue reading about Shape-adjustable mold, skin and interior-core structures for custom board production... 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