Abstract:
A flask and tool assembly includes a forming tool insert that is housed within an insulated flask having four vertical sides. An insulation enclosure surrounds the flask and the flask contains a heater plate for contacting the bottom of the forming tool insert. The tops of the sides of the flask provide surfaces for gripping bottom edges of a sheet metal blank to be formed. A cover for the tool-containing flask provides complementary gripping surfaces for top edges of the sheet metal blank and defines a chamber above the sheet metal blank for pressurized forming gas. A peripheral seal underlies the sheet metal blank and bridges a space between the outer side surfaces of the forming tool insert and the inner surfaces of the sides of the flask, thereby obviating the need to provide a precision fit between the flask and forming tool insert.

Description:
TECHNICAL FIELD 
     The present invention generally pertains to hot-gas blow-forming of metal alloy sheet blanks into articles of complex curvature such as automotive body panels. More specifically, this invention pertains to a heated flask tool assembly and a related perimeter seal for use with hot-gas blow-forming operations such as super-plastic-forming (SPF) or quick-plastic-forming (QPF). 
     BACKGROUND OF THE INVENTION 
     Automotive body panels are typically produced by forming low carbon steel or aluminum alloy sheet stock into desired panel shapes, often by conventional room temperature processes such as stamping. Such body panels, however, can also be produced hot gas blow-forming processes, such as SPF. Compared to conventional stamping processes, SPF processes are capable of producing more complex panel shapes from a single sheet of material. SPF processes involve complex integrally heated presses and low material deformation rates that yield cycle times typically between 20 and 60 minutes. Such relatively long cycle times are incompatible with automotive production rates. Also, because SPF heat sources are remotely located from SPF forming tool surfaces, SPF processes do not provide a high degree of temperature control at the workpiece. 
     Therefore, QPF processes were developed to reduce the cycle time of SPF and to provide better temperature control closer to forming tool surfaces by attaching insulation to, and embedding heating elements within, the forming tools themselves. Providing insulation and heating elements in each forming tool, however, requires a lead time to produce QPF forming tools and increases the costs thereof. Such investment costs are recoverable by suitable production volumes. With lower volume production runs, however, internally or integrally heated hot forming tools may be too expensive. 
     Thus, there is a need for a hot blow-form tooling apparatus that avoids the expense and lead times associated with producing integrally heated tooling, and avoids the long cycle times and lack of localized temperature control of SPF processes. 
     SUMMARY OF THE INVENTION 
     The present invention provides low-cost hot blow-forming tooling that is particularly adapted for the shaping of prototype sheet metal parts or sheet metal parts in low volume production. A forming surface for the sheet metal is machined in the top of one or more blocks of tool steel or other suitable tool material. A flask for holding the forming tool is provided wherein the flask has four vertical sides. An insulation enclosure may be provided to partially surround and thereby insulate the flask. The flask contains a heater plate for contacting the bottom of the forming tool block, and the tops of the sides of the flask provide surfaces for gripping bottom edges of a sheet metal blank to be formed. A cover for the tool-containing flask provides a chamber for pressurized forming gas and complementary gripping surfaces for top edges of the sheet metal blank. An insulation enclosure may be provided to partially surround and thereby insulate the cover. The present invention also uses a peripheral seal to underlie the sheet metal blank and bridge a space between the outer side surfaces of the tool block and the inner surfaces of the sides of the flask, thereby obviating the need to provide a precision fit between the various tool components. 
     In a representative assembly of a flask and forming tool block, the flask is insulated by an insulation enclosure and is carried on a flat rectangular base plate for mounting to a press platen. The flask is suitably rectangular wherein the four vertical sides of the flask provide top surfaces for supporting an overlying sheet metal blank. If necessary, one or more spacer rails may be mounted within the flask to provide height adjustment for a particular forming tool block. A heater plate is mounted atop the spacer rails within the flask for heating a forming tool to a suitable superplastic forming temperature. The heater plate includes embedded electrical resistance heating elements connected to a suitable controlled electrical power source. A die or forming tool block, selected for the current forming operations, is mounted atop the heater plate within the flask for to provide a forming surface for a sheet metal blank. A sheet metal blank is placed on the top surfaces of the sides of the flask so that it overlies the forming surface of the forming tool in the flask. Then, an insulated cover for the flask is positioned above the sheet metal blank. The periphery of the cover includes a seal bead having a binder surface for gripping the sheet metal blank to the top surfaces of the flask. The cover also defines a pressure chamber over the sheet metal blank to receive a pressurized gas to blow form the sheet metal blank against the machined surface of the inserted forming tool. The flask is sized to accommodate different forming tools for different low production sheet metal forming operations. Usually there is a peripheral gap or void is defined between an exterior periphery of the form tool insert and an interior periphery of the flask. Therefore, a peripheral seal is provided to bridge such a gap or void. 
     In contrast to the prior art, the press itself and the major sub-elements of the press are not integrally heated. Likewise, the forming tools themselves are not integrally heated or insulated. Rather, the investment expense and lead time required to provide such insulation and heating elements are borne by the dedicated heated and insulated tool enclosures of the present invention. Thus, the expense and lead time associated with such auxiliary apparatus can be eliminated from each individual set of forming tools that are swapped in and out of the reusable containers. 
     Moreover, the peripheral seal is provided to bridge the peripheral gap between the machined forming tool block and the sides of the flask thereby preventing the sheet metal blank from being pushed down into the peripheral gap when the upper chamber is pressurized. The peripheral seal also eliminates the need to provide a precision fit between the form tool insert and flask, and enables “hot” tooling to be swapped out for “cold” tooling without having to wait for the hot tooling to cool. Accordingly, the peripheral seal eliminates machining costs and increases production throughput. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the invention will become apparent upon reading the detailed description in combination with the accompanying drawings, in which: 
         FIG. 1  illustrates an exploded perspective view of a flask tool assembly including a heater plate and a thermally-compensating perimeter seal according to an embodiment of the present invention; 
         FIG. 2  illustrates a partially broken-away cross-sectional view of a portion of the flask tool assembly of  FIG. 1 , showing the perimeter seal bridging a gap or void between a form tool insert and a flask; 
         FIG. 3  illustrates a partially broken-away cross-sectional view of a portion of the flask assembly of  FIG. 2 , showing a sheet metal blank trapped between a cover and the flask and extended over the form tool insert and perimeter seal; 
         FIG. 4  illustrates a partially broken-away cross-sectional view of a portion of the flask tool assembly of  FIG. 3 , showing the sheet metal blank blow-formed against the form tool insert and perimeter seal; and 
         FIG. 5  illustrates a partially broken-away cross-sectional view of a portion of a flask tool assembly according to another embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring specifically to the Figures, there is illustrated in  FIG. 1  a flask tool assembly  10  in exploded view for use with hot-gas blow-forming processes such as superplastic forming (SPF), quick-plastic-forming (QPF), or the like. The flask tool assembly  10  includes a base plate  12  for supporting the assembly, a four-sided housing or flask  14  mounted atop the base plate  12 , an insulation enclosure  15  laterally or partially surrounding the flask  14 , a pair of spacer rails  16  mounted atop the base plate  12  within the flask  14  to provide height adjustment, a heater plate  18  mounted atop the spacer rails  16  within the flask  14  for heating the assembly, a form tool insert  20  mounted atop the heater plate  18  within the flask  14  for forming a sheet metal blank, and a cover  22  positioned atop the assembly to define a pressure chamber. The flask tool assembly  10  and components thereof are shown as generally rectangular in shape, but may be any shape that is necessitated by the desired forming process and final part shape. 
     The mounting plate or base plate  12  may be a separate component from the flask  14  or may be integrated therewith as a single component. In any case, the base plate  12  is preferably composed of P20 steel or the like, is generally rectangular, and for exemplary purposes measures about 141″ along its sides, 85″ along its ends, and 4″ in thickness. The base plate  12  also includes a top surface  24  thereon for supporting the flask  14 . Preferably, the base plate  12  includes on the order of about 30 load posts (not shown) that are evenly distributed about, and extend upwardly from, the top surface  24  of the base plate  12 . The load posts are preferably spool-shaped, composed of Inconel®, and are about 2-3″ in diameter and about 6″ in height. Four press mounting slots (not shown) may also be formed along each side of the base plate  12  and two material handling brackets (not shown) may also be provided on each side of the base plate  12 . Accordingly, the base plate  12  provides an attachment means and rigid support for the flask tool assembly  10  to a press bed (not shown). The base plate  12  also provides a means for material handling of the flask tool assembly  10  by way of the material handling brackets. 
     The housing, container, or flask  14  is a four-sided structure composed preferably of P20 steel plates, or the like, and is open at a top end  28  thereof. The flask  14  includes a base plate (not shown), two ends  30 ,  32 , and two sides  34 ,  36 . As such, the flask  14  is generally rectangular in shape and for exemplary purposes measures about 131″ along its sides  34 ,  36 , about 75″ along its ends  30 ,  32 , and about 31″ in height. The base preferably includes three evenly spaced locating pins (not shown) centrally disposed along the length of the base that provide alignment for the form tool insert  20 . The sides  34 ,  36  may also include access holes (not shown) therethrough. The flask  14  may be mounted to the tops of the load posts of the base plate  12  and fastened to the base plate such as by tension rods (not shown) or by any known attachment method such as by welding, attaching with fasteners, or the like. The flask  14  includes opposed ends  30 ,  32  and opposed sides  34 ,  36  that are fastened together by fasteners such as bolts, cap screws, or the like. Each of the ends  30 ,  32  and sides  34 ,  36  include top surfaces  30   t ,  32   t ,  34   t ,  36   t , which are machined and collectively define a top surface of the flask  14 . Also, the ends  30 ,  32  and sides  34 ,  36  collectively define an interior of the flask  14  and an interior periphery of the flask  14 . The ends  30 ,  32  and sides  34 ,  36  also include internal side surfaces  30   i ,  32   i ,  34   i ,  36   i  that collectively define an interior surface of the flask  14 . Accordingly, the flask  14  provides a support perimeter or binder ring function for supporting the sheet metal blank, and locates and supports the form tool insert  20 . 
     The insulation enclosure  15  is only partially shown and in broken out form, for ease of illustration. It is to be understood that the insulation enclosure  15  actually surrounds the ends and sides  30 ,  32  and  34 ,  36  of the flask  14  as well as the base  26  of the flask  14 . The insulation enclosure  15  is generally rectangular in shape and is constructed of one base member, two sides, and two ends (not shown). The insulation enclosure  15  measures about 131″ along its sides, about 85″ along its ends, and about 11″ in height. The base member includes clearance holes (not shown) therethrough that correspond in quantity and location to the quantity and locations of the load posts of the base plate  12 , such that the load posts pass therethrough to provide support to the flask  14 . Thus, neither the base nor the sides or ends are composed of load bearing insulation. Rather, each of the members or panels of the insulation enclosure  15  may be an assembly constructed of an outer shell composed of, for example, 304 stainless steel, and an internal insulation material composed of, for example, Cer-Wool RT available from Premier Refractories and Chemicals, Inc. of King of Prussia, Pa., or the like. Accordingly, the insulation enclosure  15  insulates the flask  14  to keep heat in and protect the shop environment outside of the hot flask  14 . 
     The shims or spacer rails  16  are rectangular shaped blocks preferably composed of P20 steel, or the like. The spacer rails  16  are mounted and fastened to the top surface  24  of the base plate  12  within the confines or interior of the flask  14 . The spacer rails  16  provide height adjustment for the assembly so as to accommodate form tool inserts of varying thicknesses. Accordingly, the spacer rails  16  may be provided in a variety of thicknesses and may be omitted altogether if unnecessary. 
     The heater plate  18  is a rectangular shaped structure, preferably composed of P20 steel or the like, and measures for example about 118″ in length, about 45″ in width, and about 3″ in thickness. The heater plate  18  mounts atop the base of the flask  14  and includes through holes or slots (not shown) to permit the locating pins of the flask  14  to pass therethrough. The heater plate  18  also includes electrical heating elements  19  embedded therein for warming the form tool insert  20  to suitable temperatures for superplastic forming. It is known to embed electrical heating elements within SPF dies and within SPF press platens. It is also known to embed such heating elements within QPF forming tools. Uniquely, however, the present invention involves embedding the electrical heating elements  19  within the separate or intermediate heater plate  18 , wherein the apparatus is operated in accordance with known QPF principles such as that disclosed in U.S. Pat. No. 6,253,588, which is assigned to the assignee hereof and which is incorporated by reference herein. The heater plate  18  is mounted directly atop the spacer rails  16  within the interior of the flask  14 . The heater plate  18  thus provides a housing for the heater elements  19  and, therefore, a heat source that is internal to the flask tool assembly  10  and may be used repeatedly from one form tool insert to the next. 
     The die, product detail load, or form tool insert  20  is a generally two-piece rectangular structure having opposed ends  38  (only one end shown) and sides  40  (only one side shown) and may be composed of steel, titanium, aluminum, refractory materials, or the like. Preferably, the form tool insert  20  is composed of two sub-assemblies  20   a ,  20   b  retained together by a tie plate (not shown) beneath the form tool insert  20 . The form tool insert  20  is preferably composed of P20 steel or the like, and measures for example about 120″ in length, 64″ in width, and about 24″ in overall height. Vent channels (not shown) may be suitably provided within the form tool insert  20 . Each of the opposed ends  38  and sides  40  collectively define an exterior periphery of the form tool insert  20 . The form tool insert  20  is mounted directly atop the heater plate  18  and is preferably fastened thereto or to the base plate  12 . The form tool insert  20  includes a forming surface  42  thereon that replicates the desired final part configuration to be formed and further includes a marginal or addendum surface  44  thereon around the forming surface  42 . A perimeter or peripheral seal  46  is fastened to the addendum surface  44  around the circumference of the form tool insert  20  and measures for example about 123″ in length, 67″ in width, 1.5″ in overall height, and about 0.1875″ in thickness. Accordingly, the form tool insert  20  and seal  46  provide part definition surfaces for forming a component and provide a means for bridging a void between the form tool insert  20  and flask  14 . 
     The pressure lid or cover  22  is a rectangular-shaped component that is preferably composed of P20 steel or the like. The cover  22  is preferably mounted to an upper platen of the press (not shown), such as by tension rods and hardware (not shown) or the like. Accordingly, the cover  22  is positioned above the form tool insert  20 . Preferably, the cover  22  is heated, such as by embedding heating elements (not shown) therein. The cover  22  includes an integral seal bead  48  that projects from an underside surface  50  and circumscribes the cover  22  about a peripheral margin thereof. The seal bead  48  includes a machined surface  52  that is adapted for surface contact with the top surface  24  of the flask  14  to trap a sheet metal blank (not shown) therebetween. Accordingly, the machined surface  52  and top surface  24  act as binder surfaces for binding the edges or peripheral margin of the sheet metal blank when the cover  22  occupies a lowered position. In such position, the cover  22  also defines a pressure chamber above the top surface of the sheet metal blank for blow-forming the sheet metal blank against the form tool insert  20 . 
     The cover  22  is preferably surrounded by an insulation enclosure (not shown) in similar fashion as the flask  14 . It is to be understood that such an insulation enclosure actually surrounds the ends and sides of the cover  22 , as well as a top portion (not shown) thereof. The insulation enclosure is generally rectangular in shape and is constructed of a top layer, two sides, and two ends (not shown). The insulation enclosure measures about 131″ along its sides, about 85″ along its ends, and about 11″ in height. The top layer includes clearance holes (not shown) therethrough that correspond in quantity and location to a quantity and a plurality of locations of load posts (not shown) that interpose the cover  22  and upper platen of the press, such that the load posts pass therethrough to impart press load to the cover  22 . Thus, neither the top layer nor the sides or ends are composed of load bearing insulation. Rather, each of the members or panels of the insulation enclosure may be an assembly constructed of an outer shell composed of, for example, 304 stainless steel, and an internal insulation material composed of, for example, Cer-Wool RT available from Premier Refractories and Chemicals, Inc. of King of Prussia, Pa., or the like. Accordingly, the insulation enclosure insulates the cover  22  to keep heat in and protect the shop environment outside of the cover  22 . 
     Finally, like the base plate  12 , a mounting plate (not shown) may be provided to establish an attachment means and rigid support for the cover to the upper press platen. The mounting plate also provides a means for material handling of the flask tool assembly  10  by way of material handling brackets (not shown) provided thereon. In other respects, the mounting plate may be identical to the base plate  12 , including clearance holes (not shown) for tension rods (not shown) to pass therethrough. 
     Turning now to  FIGS. 2 through 5 , the structure and function of the peripheral seal  46  will be discussed in more detail.  FIG. 2  illustrates a cross-sectional view of part of the flask tool assembly  10  including the form tool insert  20 , peripheral seal  46 , and flask  14 . Intentionally, a predetermined void or gap  54  is defined between a perimeter edge or exterior surface  56  of the form tool insert  20  and an interior surface  58  of the flask  14 . The gap  54  is predetermined by calculating the relative manufacturing tolerances of the flask  14  and the form tool insert  20  as well as the displacement of the form tool insert  20  with respect to the flask  14  under thermal expansion due to the hot superplastic forming temperatures. A void is undesirable because the sheet metal blank would tend to be blow-formed down into the void, thereby locking the formed part between the part definition portion of the form tool insert  20  and the seal bead portion of the flask  14 . 
     There are at least two solutions to this problem. In one solution, craftsmen can fabricate custom addendum details to fill the void. In another solution, the form die or insert  20  is custom designed and constructed to be precision fit within the surrounding tool or flask  14  so as to minimize or eliminate the void therebetween. Unfortunately, however, a precision fit necessitates precision machining and construction of the form die and surrounding tool, which is cost prohibitive, especially on a prototype or otherwise low-volume production basis. Moreover, a precision fit translates into process downtime. The form die, when heated, expands into an interference fit condition with the surrounding tool and must be permitted to cool for a significant amount of time before the die can be loosened from the surrounding tool and removed therefrom. This is of particular concern for small production runs or prototyping when several die changes may be required per week. 
     The peripheral seal  46  bridges and seals the gap  54  between the form tool insert  20  and the flask  14 . The peripheral seal  46  is mounted flush with the addendum surface  44  of the form tool insert  20  by virtue of a recessed portion  60  of the addendum surface  44  that circumscribes the form tool insert  20 . Accordingly, the peripheral seal  46  is attached to a peripheral margin of the form tool insert  20  by a plurality of button-headed fasteners  62 , such as rivets, bolts, cap screws, or the like. Alternatively, however, it is contemplated that the peripheral seal  46  may be attached to the form tool insert  20  by any other heat-tolerant means including welding, interference fit, or the like. The peripheral seal  46  is thus cantilevered from a fastener end  64  that seals against the recessed portion  60  of the form tool insert  20  to a free end  66  that seals against the interior surface  58  of the flask  14 . The peripheral seal  46  is sized such that an overhang from the exterior surface  56  of the form tool insert  20  to the free end  66 , is greater than the width of the gap  54  at any given point around the form tool insert  20 . Also, the peripheral seal  46  is preferably angled for ease of assembly, wherein the form tool insert  20  with the peripheral seal  46  attached thereto is inserted down into the interior of the flask  14 . 
     From the above, neither the flask  14  nor the form tool insert  20  need be precision machined for fitting together and the form tool insert  20  need not be precisely centered within the flask  14 , because the peripheral seal  46  absorbs or “takes up” any unbalanced dimensional slack therebetween. In other words, the gap  54  may be greater on one end or side of the form tool insert  20  than another, and the peripheral seal  46  will variably deflect in accordance with such dimensional variation to provide a uniform seal with the flask  14  around the form tool insert  20 . 
       FIG. 3  depicts a particular stage in the forming process wherein a sheet metal blank W has been loaded to the flask tool assembly such that a bottom surface Wb thereof sits atop the top surface  68  of the flask  14  and above the form tool insert  20 . The cover  22  has subsequently been lowered such that the machined surface  52  of the seal bead  48  engages a top surface Wt of the sheet metal blank W so as to bind the sheet metal blank W about its peripheral margin in preparation for the forming step.  FIG. 3  also depicts the outward thermal expansion of the form tool insert  20  relative to the flask  14 , such that the gap  54  of  FIG. 2  has been reduced in width to yield a smaller gap  54 ′ shown in FIG.  3 . Accordingly, under the outward thermal displacement of the form tool insert  20 , the peripheral seal  46  has deflected upwardly so as to take up the thermal expansion and keep the gap  54 ′ sealed. 
       FIG. 4  depicts the forming stage in the process wherein a pressurized gas has been introduced, preferably through the cover  22 , and into a chamber  70  defined between the sheet metal blank W and the cover  22 , so as to blow-form the sheet metal blank W against the form tool insert  20 . Accordingly, the sheet metal blank W closely follows the contours of the forming surface  42 , the addendum surface  44 , the button-headed fastener  62 , and the peripheral seal  46 . The peripheral seal  46  thus prevents the sheet metal blank W from being blow-formed down into the gap  54 ′ between the form tool insert  20  and the flask  14 . In other words, without the peripheral seal  46  in place as shown, a marginal portion of the sheet metal blank W would be formed down into the gap  54 ′ and would thereby pull material in a direction away from the forming surface  42  and possibly lock the finished part onto the form tool insert  20 . 
     In  FIGS. 2 through 4 , the forming surface  42 , addendum surface  44 , and peripheral seal  46  are all disposed beneath a plane defined by the top surface  68  of the flask  14 . In contrast,  FIG. 5  illustrates a flask tool assembly  110  having the addendum surface  44  disposed slightly above a plane defined by the top surface  68  of the flask  14 . This difference in height may be designed in and enabled by providing thicker spacer rails (not shown). Accordingly, a peripheral seal  146  is provided in a generally flat condition to bridge and seal the gap  154  between the form tool insert  20  and flask  14 . Here, a free end  166  of the peripheral seal  146  seals against the top surface  68  of the flask  14 , instead of the interior surface  58  thereof. 
     With all of the embodiments described above, the present invention provides a variety of advantages. First, the press, major sub-elements thereof, and the forming tooling are unheated and uninsulated. In other words, the press and tooling are not integrally heated or insulated in the sense that heating elements are not embedded therein and insulation is not attached thereto. Rather, a dedicated heater plate is integrally heated and positioned within a dedicated insulated flask. Similarly, a dedicated heated and insulated cover is provided. The combination of the heater plate, insulated flask, and heated and insulated cover enables forming cycle times that are similar to that of QPF, but avoids the costs and long lead times of providing QPF tooling. This is because such apparatus can be reused to accommodate a wide multitude of forming tools. In other words, the investment expense and lead time of providing heating elements in close proximity to the forming tool surfaces can be borne by a single, dedicated heated and insulated flask and cover. Thus, such auxiliary apparatus and expense thereof can be eliminated from each of the multitudes of forming tools that are swapped in and out of the flask. 
     Second, the peripheral seal prevents a sheet metal blank from being blow-formed down into a void or gap between a forming tool and a flask and enables a tooling designer to accommodate a predictable range in widths of the gap. More specifically, the peripheral seal eliminates the need to provide a precision fit between tooling components, thereby enabling use of looser dimensional tolerances in the manufacture of the components. Looser tolerances yield reduced machining operations and time, which leads to reduced manufacturing costs. Moreover, the peripheral seal enables use of a gap between the tooling, and thereby facilitates “hot” tooling to be swapped out for “cold” tooling without having to wait for the hot die to cool and contract away from the hot flask. This yields reduced time required for tool changes, which leads to reduced manufacturing cost. Similarly, the peripheral seal enables use of tooling materials having different coefficients of thermal expansion, thereby providing more tooling options for a tooling designer, which thereby leads to increased part quality and reduced part cost. Thus, the present invention provides parts of uncompromised quality at a reduced manufacturing cost, while providing the flexibility to rapidly change tooling on a low volume production basis. 
     It should be understood that the invention is not limited to the embodiments that have been illustrated and described herein, but that various changes may be made without departing from the spirit and scope of the invention. For example, the materials, dimensions, and the like disclosed herein are for exemplary purposes only. Accordingly, it is intended that the invention not be limited to the disclosed embodiments, but that it have the full scope permitted by the language of the following claims.