1.  Field of the Invention
This invention relates to a fluid-filled bladder suitable for footwear applications.  More particularly, the invention is a fluid-filled bladder having a core that includes strands of fusing agent to secure the core to outer layers of the bladder. 
2.  Description of Background Art
Footwear is divided into two general parts, an upper and a sole.  The upper is designed to comfortably receive the foot and the sole provides traction, protection, and a durable wear-resistant surface.  The considerable forces generated during athletic activities require that the sole of footwear attenuate ground reaction forces and absorb energy, thereby providing protection to the foot and leg.  Accordingly, the sole of athletic footwear typically has a layered structure that includes a comfort-enhancing insole, a resilient midsole, and a ground-contacting outsole that provides both durability and traction. 
Midsoles are traditionally formed of conventional foam materials such as ethylene vinyl acetate or polyurethane that compress resiliently under an applied load to attenuate ground reaction forces and absorb energy.  Conventional foam materials are resiliently compressible, in part, due to the inclusion of a plurality of open or closed cells that define an inner volume substantially displaced by gas.  That is, the foam includes bubbles formed in the material that enclose gas.  After repeated compressions, however, the cell structure deteriorates, thereby resulting in decreased compressibility of the foam.  Thus, the force attenuation and energy absorption characteristics of the midsole may decrease over the lifespan of the footwear. 
One way to overcome the drawbacks of utilizing conventional foam materials is disclosed in U.S. Pat.  No.  4,183,156 to Rudy, hereby incorporated by reference, in which cushioning is provided by inflatable inserts formed of elastomeric materials.  The inserts include a plurality of tubular chambers that extend substantially longitudinally throughout the length of the footwear.  The chambers are in fluid communication with each other and jointly extend across the width of the footwear.  U.S. Pat.  No.  4,219,945 to Rudy, hereby incorporated by reference, discloses an inflated insert encapsulated in a foam material.  The combination of the insert and the encapsulating material functions as a midsole.  An upper is attached to the upper surface of the encapsulating material and an outsole or tread member is affixed to the lower surface. 
Such bladders are generally formed of an elastomeric material and are structured to have an upper or lower surface that encloses one or more chambers therebetween.  The chambers are pressurized above ambient pressure by inserting a nozzle or needle connected to a fluid pressure source into a fill inlet formed in the bladder.  After the chambers are pressurized, the fill inlet is sealed, for example, by welding, and the nozzle is removed. 
Bladders of this type have been manufactured by the prior art two-film technique in which two separate sheets of elastomeric film are formed having the overall peripheral shape of the bladder.  The sheets are welded together along the periphery to form a bladder having upper, lower, and side surfaces, and the sheets are welded together at predetermined interior areas to give the bladder a desired configuration.  That is, the interior welds provide the bladder with chambers having a predetermined shape and size at desired locations. 
Bladders have also been manufactured by the prior art blow-molding technique, wherein a liquefied elastomeric material is placed in a mold having the desired overall shape and configuration of the bladder.  The mold has an opening at one location through which pressurized air is provided.  The pressurized air forces the liquefied elastomeric material against the inner surfaces of the mold and causes the material to harden in the mold, thereby forming a bladder with the desired shape and configuration. 
Another type of prior art bladder used in soles of footwear is disclosed in U.S. Pat.  Nos.  4,906,502 and 5,083,361, both to Rudy, and both hereby incorporated by reference.  This type of bladder is formed as a fluid pressurized and inflated structure that comprises a hermetically sealed outer barrier layer which is securely fused substantially over the entire outer surfaces of a double-walled fabric core.  The double-walled fabric core is comprised of first and second outer fabric layers that are normally spaced apart from one another at a predetermined distance.  Connecting or drop yarns, potentially in the form of multi-filament yarns having many individual fibers, extend internally between the proximal or facing surfaces of the respective fabric layers.  The filaments of the drop yarns form tensile restraining means and are anchored to the respective fabric layers.  A suitable method of manufacturing the double walled fabric structure is double needle bar Raschel knitting. 
U.S. Pat.  Nos.  5,993,585 and 6,119,371, both issued to Goodwin et al.,  and both hereby incorporated by reference, disclose a bladder utilizing a double-walled fabric core, as with the '502 and '361 patents, but without a peripheral seam located midway between the upper and lower surfaces of the bladder.  Instead, the seam is located adjacent to the upper surface of the bladder.  Advantages in this design include removal of the seam from the area of maximum sidewall flexing and increased visibility of the interior of the bladder, including the connecting yarns.  The process utilized to form a bladder of this type involves the formation of a shell, which includes a lower surface and a sidewall, with a mold.  A double-walled fabric core is placed on top of a covering sheet, and the shell, following removal from the mold, is placed over the covering sheet and core.  The assembled shell, covering sheet, and core are then moved to a lamination station where radio frequency energy fuses opposite sides of the core to the shell and covering sheet and fuses a periphery of the shell to the covering sheet.  The bladder is then pressurized by inserting a fluid so as to place the connecting yarns in tension. 
Another type of prior art bladder used in soles of footwear is disclosed in U.S. Pat.  No.  4,874,640 to Donzis, and U.S. Pat.  Nos.  5,741,568 and 6,127,010 to Rudy, all of which are hereby incorporated by reference.  This type of bladder utilizes a compressible insert encapsulated within an elastomeric barrier member.  In the '640 patent, the elastomeric barrier member is generally impermeable to air and generally filled with a fluid that can be pressurized.  The insert includes a compression, impact absorbing foam core glued or fused on its external surfaces to the elastomeric barrier member.  The internal foam cushioning member can be reinforced by including filaments, fibers, or fabrics within the foam core.  The '568 and '101 patents suggest various materials can be used for the compressible insert, including cotton, rubber, foam, horsehair, plastic mesh, etc.  A preferred material is identified as at least partially open cell, flexible foam, such as polyurethane or ethylene-vinyl acetate.  Filaments are utilized to enhance the connection between the compressible insert and the barrier member, with portions of the filaments being imbedded in both the compressible insert and the barrier material.  Examples of suitable filaments are listed as polyesters, polyethylene terephthalate, polyamides, nylons, fiberglass, carbon, glass, silk, cotton, wool, urethane, aramid, Dacron, cellulose, rayon, copra, acetate, polyvinyl alcohol, polyacrylics, and mixtures thereof. 
While the cushioning benefits of bladders in articles of footwear are well documented, the prior art methods of manufacturing bladders utilizing a double-walled fabric core have made them costly and time consuming to manufacture.  In particular, the double-walled fabric core is typically secured within the bladder by attaching a layer of thermally activated fusing agent to the outer surfaces of the core, and then heating the bladder components to cause the fusing agent to melt, thereby securing the core the outer layers of the bladder.  In practice, it is time consuming to add the fusing agent to the outer surfaces of the core and requires additional manufacturing steps, thereby increasing overall cost.  Accordingly, the art requires a simple, cost effective method for securing a double-walled fabric core within a bladder.  In addition to other benefits that will become apparent from the following disclosure, the present invention fulfills this need.