Abstract:
A hollow metal door using hollow structural sections with closed cross section, along with the method for making same, is described herein.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Hollow metal doors are typically constructed of steel stiffeners or a laminated core sandwiched between relatively thin face sheets of carbon steel. The steel stiffeners are usually open-walled thin sections: channel, Z-shapes, hat-shapes, truss sections, or similar members, positioned vertically and attached to the steel face sheets by spot welding. Laminated cores employ either honeycomb, polyurethane, polyisocyanurate, or polystyrene that are laminated to the face sheets using structural adhesive. Certain self-adhesive polyurethane cores can also be foamed-into the cavity between the face sheets. Hollow metal doors are both strong and lightweight, making them desirable for use in a wide variety of architectural applications as security and fire doors in commercial, residential, industrial, and detention settings. 
         [0002]    There are four types of steel commonly used in hollow metal door manufacturing. Cold-rolled steel is the most commonly used material for the majority of commercial hollow metal door components. Hot-rolled steel is used in hardware reinforcements. Zinc-coated steel sheets can be either galvannealed or galvanized and are used to prevent corrosion. Stainless steel is also used for corrosion protection, and for aesthetic purposes. 
         [0003]    The current methods for constructing hollow metal doors are detailed in the Hollow Metal Manufacturers&#39; Association (HMMA) standard 802-07  Manufacturing of Hollow Metal Doors and Frames . The four primary steps involved in manufacturing the steel face sheets and stiffeners are shearing, blanking, brake forming, and welding. The hollow metal door fabricator&#39;s raw material usually consists of flat steel sheets. These sheets are cut to exact sizes using a shearing machine. The sheared steel then has desired openings blanked, using a punch press, turret press, or a laser cutting machine. The blanked steel is then brake formed into the desired shape using a press brake. 
         [0004]    There are now typically at least two steel face sheet portions, to which steel stiffeners are attached by either spot or projection welding. Other components, such as hinge supports and closer reinforcements, are also spot- or projection-welded to the face sheets. This assembly process is cumbersome and time consuming, because numerous components have to be welded into place, and also because parts that do not fit exactly must be modified to fit. Most conventional hollow metal doors that with steel stiffeners spot welded in this fashion require over 600 spot welds in addition to the MIG welding required to assemble the two steel face sheet portions. 
         [0005]    Once these two face sheet portions and attached stiffeners, hinge supports, and closer reinforcements are completed, they are then welded together by a continuous weld along the edges. Any hat channels used as stiffeners must also be welded together at this time. End channels are welded into place at each end. All pre-drilled screw holes in these assemblies usually must be re-drilled and re-tapped due to deformation that occurred during bending, welding, and assembly. Finally, the assembled door must be finished and painted before being shipped to the customer. Finishing usually involves a significant amount of grinding and polishing because of the numerous welds on the surface of the door. 
         [0006]    A standard commercially-available 3′0″×7′0″ hollow metal door and frame contains at least 70 pieces and takes approximately 8 to 10 hours to produce. The lead time required to produce the component parts can be significant. For example, the lead time required to produce a typical order of 100 hollow metal doors can be on the order of 8-12 weeks. The assembly process for most hollow metal doors is best suited for an elaborate assembly line so that all of the pieces can be added efficiently. Costs to manufacture are therefore higher because of the large amount of parts and labor required to produce these doors. 
         [0007]    Moreover, current assembly lines make it almost impossible for most manufacturers to produce a flat door. Usually the door may bow in a variety of directions. This is because conventional doors are manufactured on a series of assembly line roller tables passing through a number of spot welding stations. As spot welders apply pressure to the product, the component parts of the door flex because of the heat and pressure. This often results in permanent warping of the door after the weld is complete. If this warping is significant enough, rework may have to occur before the door is acceptable to the customer. 
         [0008]    What is needed is a hollow metal door design that reduces the number of parts and welding required, without sacrificing these doors&#39; light weight and strength. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The hollow metal door described herein has an internal structure comprised of pre-formed thin-walled closed sections, commonly known as hollow structural sections, instead of thin-walled open section steel stiffeners in conventional channel, Z-shaped, hat-shaped, or truss sections. Using pre-formed hollow structural sections in lieu of thin-walled open section stiffeners significantly reduces the amount of time and materials to assemble hollow metal doors and provides weight and strength characteristics that are equal to or better than conventional doors. This hollow metal door design uses approximately 30 individual components instead of 70, and each door can be built in approximately 2 hours, instead of the 8 to 10 hours it typically takes to assemble conventional hollow metal doors. In addition, hollow metal doors produced with hollow structural sections are substantially free from bowing and warping, because spot welding is not used and the door is able to be assembled in one manufacturing station. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows the inner core and a hollow structural section. 
           [0011]      FIG. 2  shows the outer core. 
           [0012]      FIG. 3  shows the combined inner and outer core sections. 
           [0013]      FIG. 4  shows an exploded view of the invention, including the combined inner and outer core section and outer shell components. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    The door design described herein consists of an inner and outer core comprised of hollow structural sections, which fit into an outer shell made of a plurality of bent metal sheets. In the preferred embodiment, only two bent metal sheets are required. The inner core is nested within the inner dimension of the outer core. The hollow structural sections forming both the inner core and outer core are cut to specified lengths and joined together into the desired configuration, typically by MIG welding. The configuration of the inner and outer cores is designed to accommodate the majority of hardware on the market, (i.e. hinges, locks, handles, closers, etc.). One skilled in the art will appreciate that door design elements such as the window size and location, food pass location, lock type, or hinge and handle location can be easily changed without using significantly more parts by simply welding the hollow structural sections in the desired configuration. Once the inner and outer cores have been separately assembled, the inner core is joined to the outer core using MIG welding to form a combined core. 
         [0015]      FIG. 1  shows the inner core,  1 , formed from hollow structural sections,  2 . These hollow structural sections,  2 , are tubes with closed cross sections. Rectilinear cross sections are typically used, although in certain applications, other circular and other closed cross sections may be appropriate. The metal used in the hollow structural sections will depend on the requirements of the particular application, and are selected using engineering design techniques known in the art. For hollow metal doors used in correctional facilities, the hollow structural sections are typically comprised of ASTM A500-grade steel tubing. These sections are cut to pre-determined lengths depending on the desired design of the door. The sections are then MIG welded into the desired inner core configuration that accommodates the desired hardware such as windows, food pass doors, locking mechanisms, closing mechanisms, or other desired features and hardware. A person skilled in the art will appreciate that there are an infinite number of such configurations. A typical inner core design requires approximately 70-80 MIG welds, including welds used to join the inner core and the outer core, although the number of welds will obviously vary with the design. 
         [0016]    Similarly, hollow structural sections MIG welded together form outer core  3  as shown in  FIG. 2 . The outer core is sized so that the inner core fits exactly within the inner perimeter of the outer core. Cutouts  4  for hardware such as hinges can be cut into the outer core using a laser cutter or punch either before or after the hollow structural sections are assembled into the outer core. Once the inner core  1  and outer core  3  are finished, the inner core is welded to the outer core using MIG welds, to form combined core  5  as shown in  FIG. 3 . 
         [0017]    It was found that using hollow structural sections in this manner solves the problem of warped doors and frames, because MIG welding is used exclusively instead of spot welding, so that the door can be manufactured at a single flat assembly station in one location instead of having to move the in-progress door through many spot welding stations. This saves time and also prevents warping of the door because the parts are all kept flat during assembly. This also allows heat from the MIG welding process to dissipate over an extended area so that minimum finishing is required for the final product. Using hollow structural sections also means that redundant reinforcements, dust covers, and other hardware that are normally spot-welded to the structure are eliminated. In addition, it also eliminates the bowing and warping that is inherent in many of the current doors and frames being produced. 
         [0018]    The outer shell  6  surrounds the combined core. The outer shell is formed using two or more flat metal sheets. In one embodiment shown in  FIG. 4 , sheets  6   a  and  6   b  are cut using a laser cutter or mechanical sheet metal punch to accommodate windows, food pass doors, locking mechanisms, closing mechanisms or other desired features and hardware. Then sheet  6   a  is formed using a brake press into an elongated C-shaped channel as shown in  FIG. 4  whose inside dimensions conform to the outer dimensions of combined core  5 . One end of sheet  6   a  is left open so that combined core  5  can be easily inserted. Once inserted, combined core  5  is welded into place using MIG welding. A second sheet  6   b  has cutouts similar to sheet  6   a . Once combined core  5  has been inserted into sheet  6   a , sheet  6   b  is then welded onto sheet  6   a , forming outer shell  6 . 
         [0019]    Once outer shell  6  has been sealed around combined core  5 , the hollow metal door is now ready for finishing. Finishing takes significantly less time in the presently described invention. Prior art doors usually have exterior welds on all six sides of the door, while the present invention only has exterior welds on one side, where sheet  6   a  is attached to sheet  6   b . This significantly reduced the amount of grinding required to finish the door. 
         [0020]    The door design using hollow structural sections described above requires less time and materials, and is therefore less expensive and time-consuming to manufacture. Using hollow structural sections also alleviates the problem of warping and bowing during manufacture. 
         [0021]    The foregoing examples and embodiments should be construed as explanatory in nature and should in no way limit the scope of the present invention. Words used in the foregoing paragraphs are thus words of description and illustration, not limitation. Although the present invention has been described herein with reference to specific means, materials, and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally-equivalent structures, methods, and uses as are within the scope of the appended claims.