Patent Publication Number: US-7210323-B2

Title: Binder apparatus for sheet forming

Description:
TECHNICAL FIELD 
     The present invention generally relates to a binder apparatus for securing the edges of a sheet metal blank in a sheet forming process, especially a hot blow forming or stretch forming process. More particularly, the present invention pertains to a binder apparatus having sequentially movable sheet gripping segments on one side of the sheet metal blank for stepwise stretching of the sheet metal into a product of complex curvature without uneven thinning, tearing, or wrinkling of the sheet material. 
     BACKGROUND OF THE INVENTION 
     In sheet metal stretch forming processes, a hydraulic press machine is often used to support and move opposing forming tools required to form a flat sheet metal blank into a three-dimensional contoured article or product. The press moves the tools from an open position, in which a finished part is removed and a new blank inserted, to a closed position for stretching the sheet metal blank against the tools to form the product. Large presses for shaping large parts typically open and close along a vertical axis. A vertical press, thus, has a lower platen for supporting one of the tools, often a punch or male form tool, and an upper platen for carrying a complementary, opposing tool with a concave cavity, typically a female tool or die. Often the lower platen is raised by a hydraulically actuated ram to close the press. In hot stretch forming, the tools may be individually heated to maintain a suitable forming temperature for the sheet metal blank and the female tool may simply form a closed chamber against an upper surface or side of the sheet metal blank for introduction of a pressurized working gas to stretch the sheet metal blank against the male tool. 
     In order to stretch the sheet metal blank between the tools, the edges of the sheet metal blank must be gripped so that the interior part of the sheet metal blank is suitably stretched against a forming tool surface. This gripping function is accomplished by opposing binder surfaces. Depending upon the complexity of the shape of the product to be formed, the binding surfaces may be provided on the margins of the opposing tools, or a separate tool sometimes called a binder ring may provided at the margin of a tool to assist the binder function. Such a binder ring may be movable separately from the tool that it surrounds or with which it cooperates. 
       FIG. 1  illustrates tooling  10  typically used for hot stretch forming of a sheet blank of an aluminum alloy, e.g., AA 5083 formable at elevated temperatures, e.g., about 450° C. Some hot blow forming processes do not require an upper female forming die, but nonetheless include an upper tool  12  for clamping the sheet metal blank (not shown) about its periphery between the upper tool  12  and a lower tool  14 . An upper ram of a press (not shown) may carry the upper tool  12 , and a lower platen of the press (not shown) may carry a stationary male form die  16  wherein the lower tool  14  encircles the form die  16  and is either separate therefrom or is integral therewith. The lower tool  14  includes laterally opposed ends  18   a ,  18   b , and laterally opposed sides  20   a ,  20   b , each having corresponding upper surfaces  22   a ,  22   b ,  24   a ,  24   b . Likewise, the upper tool  12  includes laterally opposed ends  26   a ,  26   b , and laterally opposed sides  28   a ,  28   b , having corresponding lower surfaces  30   a ,  30   b ,  32   a ,  32   b  that correspond in kind to the upper surfaces  22   a ,  22   b ,  24   a ,  24   b  of the lower tool  14 . 
     In operation, the sheet metal blank is placed on top of the contoured surfaces  22   a ,  22   b  of the opposed ends  18   a ,  18   b  of the lower tool  14 . Then, the upper ram of the press drives the upper tool  12  toward the lower tool  14 , wherein the sheet metal blank is initially held just between the flat lower surfaces  32   a ,  32   b  of the upper tool  12  and the contoured surfaces  22   a ,  22   b  of the lower tool  14 . As the upper ram of the press continues to drive the upper tool  12  down, the sheet metal blank is first bent into engagement with the flat surfaces  24   a ,  24   b  of the lower binder  12  and is eventually bent into complete engagement between the contoured surfaces  22   a ,  22   b  of the lower tool  14  and the contoured surfaces  30   a ,  30   b  of the upper tool  12 . Thereafter, and in accordance with typical Quick-Plastic-Forming (QPF) processes, heating elements (not shown) in the upper tool  12 , lower tool  14 , and form die  16  heat the sheet metal blank, and pressurized gas is introduced through a port  34  in the side  28   a  of the upper tool  12 . The gas remains pressurized by virtue of a seal created between the upper press platen and an upper surface  36  of the upper tool  12  and by virtue of the seal created by the sheet metal blank which is squeezed between the upper tool  12  and the lower tool  14 . As is well-known, the pressurized gas forms the heated sheet metal blank over the form die  16  to create the finished product. 
     In general, sheet metal that is subjected to a hot gas blow-forming process will undergo thickness reduction, or thinning, depending on factors such as the specific tool surface shape and relative shape and position of the blank. Extreme thinning must be avoided in order for the product to serve its structural purposes. It is also occasionally possible for a complex panel to wrinkle if the blank undergoes compressive stresses sometime during the forming operation. In other words, a finished panel will typically have wrinkles if the surface area of the sheet blank is greater than the final part shape. 
     In order to avoid the above-mentioned thinning and wrinkling problems, it has been proposed to use more than one forming stage, involving at least one hot blow forming tool. Such an alternative, however, can be cost prohibitive. Also it has been proposed to enlarge an addendum area of the blank, located between the blank holding margin of the blank and the finished component portion of the blank, in order to alleviate the non-uniform stretch condition between the flat clamping surfaces of the lower binder and the contours of the form die. Unfortunately, larger addendum areas increase the size of the blank, thereby leading to increased material costs. 
     Thus, there is a need to minimize or eliminate wrinkling and thinning conditions in metal forming processes, particularly hot blow forming processes, while avoiding the expense of current solutions to those problems. 
     SUMMARY OF THE INVENTION 
     The present invention meets this need by providing an improved binder apparatus for bending a sheet metal blank over a form die. The sheet metal blank has first and second opposed surfaces and is generally rectangular in outline and, thus, has opposite side edges and opposite end edges. Likewise, the form die is generally rectangular in outline with opposite sides and opposite ends and has a forming surface thereon. 
     The binder apparatus includes a first binder tool that is spaced apart from and that faces a second binder tool that generally circumscribes the form die. The first binder tool is generally rectangular in outline and includes a pair of laterally opposed end portions and a pair of laterally opposed side portions. The end and side portions have binder surfaces thereon that face complementary binder surfaces on the second binder tool. 
     The second binder tool is generally rectangular in outline and includes a pair of laterally opposed end segments that are positioned alongside the opposite ends of the form die and further includes a pair of laterally opposed side segments positioned alongside the opposite sides of the form die. The end and side segments have the complementary binder surfaces thereon that face the binder surfaces on the first binder tool. The binder surfaces on the end segments are elevated with respect to the binder surfaces on the side segments. In other words, the binder surfaces on the end segments are closer to their complementary binder surfaces on the first binder tool than the binder surfaces on the side segments are to their complementary binder surfaces on the first binder tool. Also, the end segments are separately movable with respect to the side segments in a direction that is substantially perpendicular to the opposed surfaces of the sheet metal blank. Preferably, the binder surfaces on the end segments have substantially similar contours, while the binder surfaces on the side segments have different contours from one another that tend to follow the contours on the respective sides of the form die. 
     In operation, the sheet metal blank is first preheated to a suitable hot blow forming temperature and is then placed against the elevated binder surfaces on the end segments of the second binder tool. Next, the first binder tool is moved toward the second binder tool such that, initially, only the binder surfaces on the end portions of the first binder tool contact the sheet metal blank. The first binder tool continues its movement toward the second binder tool, thereby bending the sheet metal blank into conformity between the complementary binder surfaces on the end binder portions of the first binder tool and the end segments of the second binder tool. Because of the difference in elevation between the end and side segments of the second binder tool, the first binder tool initially bends the sheet metal blank about the binder surfaces on the end segments before ever driving the sheet metal blank into contact with the side segments. Nonetheless, the first binder tool continues to travel toward its closed position against the second binder tool, thereby displacing the movable end segments and thereby driving the sheet metal blank into contact with the binder surfaces on the side segments of the second binder tool. The first binder tool travels even further toward the second binder tool thereby bending the sheet metal blank about the binder surfaces on the side segments of the second binder tool and simultaneously bending a central portion of the sheet metal blank over the forming surface of the form die until, finally, the sheet metal blank is fully clamped between the binder surfaces on the first and second binder tools. Thereafter, the sheet metal blank may be hot blow formed over the forming surface of the forming die in accordance with one aspect of the present invention. 
     Accordingly, the sheet metal blank is sequentially locked between the first and second binder tools—first between complementary binder surfaces at opposed ends of the first and second binder tools, and then between complementary binder surfaces at opposed sides of the first and second binder tools. This progressive process results in more gradual bending and closer conformity of the shape of the sheet metal blank with respect to the shape of the forming surface of the forming die. Thus, by using a binder apparatus having elevated and movable end segments and having stationary side segments, with contoured surfaces on all of the segments, metal is more easily stretched over a form die so as to minimize thinning and wrinkling. 
    
    
     
       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 a hot stretch-forming apparatus in accordance with the prior art; 
         FIG. 2  illustrates a hot stretch-forming apparatus in accordance with the present invention; 
         FIG. 3A  illustrates the hot stretch-forming apparatus of  FIG. 2  being used to form a flat sheet of material in accordance with a method of the present invention; 
         FIG. 3B  illustrates the hot stretch-forming apparatus of  FIG. 3A  wherein the flat sheet of material is being partially formed over a portion of the apparatus in accordance with the method of the present invention; 
         FIG. 3C  illustrates the hot stretch-forming apparatus of  FIG. 3B  wherein the flat sheet of material is being further formed over another portion of the apparatus and is also being super-plastically formed in accordance with the method of the present invention; and 
         FIG. 3D  illustrates the hot stretch-forming apparatus of  FIG. 3C  wherein the flat sheet of material has been completely formed by the apparatus in accordance with the method of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now in detail to the drawing figures,  FIG. 2  illustrates a hot stretch-forming apparatus  110  in accordance with the present invention, including a rectangular upper tool or binder  112 , a four-sided lower tool or binder apparatus  114  mounted generally vertically opposed to and below the upper binder  112  for clamping a two-dimensional sheet metal blank (not shown) therebetween, and a form die  116  that is substantially circumscribed by portions of the lower binder apparatus  114  and that is provided for forming an impression of itself on the sheet metal blank to produce a three-dimensional component (not shown). The hot stretch-forming apparatus  110  is adapted for use within a press (not shown), which has an upper ram for driving the upper binder  112  in a downward direction toward the lower binder apparatus  114  and form die  116 . The press also has a lower platen  118  for supporting the lower binder apparatus  114  and form die  116  on an upper surface  119  thereof. 
     The upper binder  112  is essentially an upper die or pressure flask that has an upper surface  120  adapted for mounting to a flat upper platen of a press (not shown) and that has electrical heating elements (not shown) therein for maintaining a desired forming temperature of the sheet metal blank, which is usually pre-heated. The upper binder  112  is preferably mounted on a load bearing insulation layer (not shown) and a sub plate (not shown) that is attached to the upper platen (not shown). The upper binder  112  includes laterally opposed ends  122   a ,  122   b  with contoured lower binder surfaces  124   a ,  124   b  and further includes laterally opposed sides  126   a ,  126   b  with contoured lower binder surfaces  128   a ,  128   b . Corners  130  of the upper binder  112  are adapted for initial contact with a sheet metal blank (not shown) to be formed. A port  132  is provided through one of the sides  126   a  to communicate pressurized gas into a cavity defined by the upper platen of the press, the upper binder  112 , and an upper surface of the sheet metal blank when the tools are in their closed position. The upper binder  112  is driven by the upper platen of the press in a direction toward the lower binder apparatus  114  and the form die  116 . 
     The form die  116  is preferably fixedly mounted to the upper surface of the lower platen  118  with a layer of insulation (not shown) positioned therebetween. The form die  116  includes a generally convex upper surface  134  having various structural design features  136  therein for embossing or otherwise forming the sheet metal blank. Alternatively, the form die  116  could be movably mounted to the lower platen  118  to provide double-action motion for forming the sheet metal blank. In any event, the form die  116  is generally circumscribed by the lower binder apparatus  114 . 
     The lower binder apparatus  114  basically includes laterally opposed stationary binder segments or sides  138   a ,  138   b  on either side of the form die  116 , and laterally opposed movable binder segments or ends  140 ,  140  on either end of the form die  116 . The binder segments  138   a ,  138   b ,  140 ,  140  closely circumscribe the form die  116 , and are spaced from the form die  116  according to dimensions that are consistent with current one-piece binders known in the art. Uniquely, however, the binder segments  138   a ,  138   b ,  140 ,  140  are contoured, positioned, and mounted in a manner which is heretofore unknown in the art. 
     The stationary binder sides  138   a ,  138   b  are mounted to the upper surface  119  of the lower platen  118  with a layer of insulation  142  therebetween. The stationary binder sides  138   a ,  138   b  are generally rectangular in shape, but have contoured upper binder surfaces  144   a ,  144   b  that preferably, but not necessarily, conform closely with sides of the form die  116  that are relatively proximate the stationary binder sides  138   a ,  138   b . The contoured upper surfaces  144   a ,  144   b  have convex crest portions  146   a ,  146   b ,  146   c  that represent the peak in height of the stationary binder sides  138   a ,  138   b . One of the stationary binder sides  138   b  has a depression  148  formed in the contoured upper surface  144   b  that follows a particular contour of the form die  116 . To complement the contoured upper surfaces  144   a ,  144   b  of the stationary binder sides  138   a ,  138   b , the upper binder  112  is similarly contoured. The sides  126   a ,  126   b  of the upper binder  112  include the contoured lower surfaces  128   a ,  128   b  that have concave crest portions  150   a ,  150   b ,  150   c  that substantially match the respective convex crest portions  146   a ,  146   b ,  146   c  of the stationary binder sides  138   a ,  138   b . Likewise, one of the sides  126   b  includes a projection  152  that closely complements the depression  148  of one of the stationary binder sides  138   b . Thus, due to the complementary contours, when the upper binder  112  eventually closes down on the stationary binder sides  138   a ,  138   b , the sheet metal blank gets clamped therebetween in a substantially uniform sealing manner. Moreover, seal beads (not shown) may be provided on the lower surfaces  124   a ,  124   b ,  128   a ,  128   b  of the upper binder  112  to further enable sealing in this regard. Finally, the contoured upper surfaces  144   a ,  144   b  of the stationary binder sides  138   a ,  138   b  include laterally opposed end portions, or shoulders  154 , against which the movable binder ends  140 ,  140  abut. 
     The movable binder ends  140 ,  140  are mounted to the upper surface  119  of the lower platen  118  via cradles  156 . The cradles  156  are supported and biased in an upward direction by cushion devices  158  positioned under flange portions  160  of the cradles  156 . The cushion devices  158  include pistons  162  that are mounted within cylinders  164 , which may be gas, hydraulic, spring, or the like. In any event, the cushion devices  158  provide the means by which the cradles  156  are elevated with respect to the upper surface  119  of the lower platen  118 . The cushion devices  158 , however, do not support the cradles  156  in a lateral direction. Accordingly, alignment devices  166  are mounted between the upper surface  119  of the lower platen  118  and the flange portions  160  of the cradle  156  to support the cradle  156  in a lateral direction and maintain the cradles  156  in precise relation to the stationary binder sides  138   a ,  138   b  and the form die  116  such that a predetermined gap is controlled therebetween. The alignment devices  166  include guide posts  168  that are mounted to the upper surface  119  of the lower platen  118  and are fitted within bearing sleeves  170  that are press fit into the flange portions  160  of the cradles  156 . The alignment devices  166  may be any type of bearing device such as a linear bearing assembly and the like. 
     The movable binder ends  140 ,  140  are mounted to the cradles  156  with a layer of insulation  172  therebetween. The movable binder ends  140 ,  140  are generally rectangular in shape, but have contoured upper binder surfaces  174  that preferably, but not necessarily, conform closely with ends of the form die  116  that are relatively proximate the movable binder segments. The contoured upper surfaces  174  have convex crest portions  176  that represent the peak in height of the movable binder ends  140 . To complement the contoured upper surfaces  174  of the movable binder ends  140 , the upper binder  112  is similarly contoured. The ends  122   a ,  122   b  of the upper binder  112  include the contoured lower surfaces  124   a ,  124   b  that have concave crest portions  178  that substantially match the respective convex crest portions  176  of the movable binder ends  140 . Thus, when the upper binder  112  closes down on the movable binder ends  140 , the sheet metal blank gets clamped therebetween in a substantially uniform sealing manner. Again, seal beads (not shown) may be provided on the lower surfaces  124   a ,  124   b ,  128   a ,  128   b  of the upper binder  112  to further enable sealing in this regard. Finally, the contoured upper surfaces  174  of the movable binder ends  140  include laterally opposed end portions, or shoulders  180 , against which the ends  154  of the stationary binder segments abut  138   a ,  138   b . Accordingly, the crest portions  176  and shoulders  180  of the movable binder ends  140  are relatively elevated with respect to the crest portions  146   a ,  146   b ,  146   c  and ends  154  of the stationary binder sides  138   a ,  138   b , to enable sequential clamping or locking of the sheet metal blank between the upper binder  112  and the lower binder apparatus  114 , as will be described in more detail below with regard to the method of the present invention. 
     The method of the present invention is illustrated in reference to  FIGS. 3A through 3D . Referring now to  FIG. 3A , a blank sheet  182  of material may be preheated to its desired forming temperature and then be placed between the upper and lower binders  112 ,  114 . In the discussion below, many of the elements of the die apparatus  110  may be obscured from view by the blank sheet  182 . Therefore, in the discussion below  FIG. 2  may be referenced in addition to  FIGS. 3A–3D . The blank sheet  182  may be loaded atop the crest portions  176  of the movable binder ends  140  or may be initially elevated with respect thereto. The blank sheet  182  may be loaded manually or automatically, and may be held in place with the aid of a gravity-operated blank loading device (not shown) such as that described in U.S. Pat. No. 6,085,571, which is assigned to the assignee hereof and is incorporated by reference herein. The blank sheet  182  may be steel, titanium, or polymeric material, but is preferably an aluminum alloy such as AA5083 for hot stretch-forming. In any case, the blank sheet  182  is substantially two-dimensional in that it is generally planar or flat with no substantial three-dimensional projections provided therein. The blank sheet  182  is sized such that a central portion  184  thereof is centered over the form die and a marginal portion or area  186  thereof extends over the contoured upper surfaces  144   a ,  144   b ,  174  of the lower binder apparatus  114 . The blank sheet  182  has an upper surface  188 , a lower surface  190 , ends  192 , sides  194 , a longitudinal axis  196  along its length, and a transverse axis  198  perpendicular to the longitudinal axis  196 . The blank sheet  182  may be in contact with the contoured upper surfaces  174  of the movable binder ends  140  or may be initially elevated with respect thereto. 
     When the blank sheet  182  is in place, the binders  112 ,  138   a ,  138   b ,  140 , the form die  116 , and the blank sheet  182  itself may be heated such as by electrical resistance elements (not shown), to maintain a desired QPF temperature such as about 500 degrees C. in the forming environment. An upper ram of the press (not shown) then slowly drives or lowers the upper binder  112  toward the lower die platen  118  such that the lower corners  130  of the upper binder  112  engage respective corners in the marginal area  186  of the blank sheet  182 . The upper binder  112  continues its downward travel so as to drive the blank sheet  182  downward so that the lower surface  190  of the blank sheet  182  initially engages the crest portions  176  of the contoured upper surfaces  174  of the movable binder ends  140 . At this point in the process, the movable binder ends  140  remain in their upwardly biased position, elevated with respect to the stationary binder sides  138   a ,  138   b.    
     Referring now to  FIG. 3B , the upper binder  112  continues to be driven downwardly by the upper ram of the press (not shown) so as to bend the blank sheet  182  about its longitudinal axis  196  (shown in  FIG. 3A ) until the lower corners  130  of the upper binder  112  drive the respective corners of the blank sheet  182  into initial engagement with the shoulders  180  of the movable binder ends  140 . Accordingly, the ends  192  of the blank sheet  182  are bent into conformity between the complementary contoured surfaces  124   a ,  124   b ,  174 . The sides  194  of the blank sheet  182 , however, remain straight because the sides  194  have not yet been formed over the stationary binder sides  138   a ,  138   b . At this point in the process, the sides  194  of the blank sheet  182  may be in initial engagement with the crest portions  146   a ,  146   b ,  146   c  of the contoured upper surfaces  144   a ,  144   b  of the stationary binder sides  138   a ,  138   b  or may be elevated with respect thereto. Note, however, that the shoulders  180  of the movable binder ends  140  remain elevated with respect to the ends  154  of the stationary binder sides  138   a ,  138   b . Thus,  FIG. 3B  represents the first stage of a sequence of forming the blank sheet  182  over the lower binder apparatus  114 . 
       FIG. 3C  represents the second stage of that sequence. The upper binder  112  continues its downward travel, so as to bend the blank sheet  182  about its transverse axis  198  (shown in  FIG. 3A ). Continued downward travel of the upper binder  112  will overcome the upward bias force provided by the cushion devices  158  and thus will displace the movable binder ends  140  and cradles  156  until the cradles  156  bottom out on the upper surface  119  of the lower platen  118  or at least until the shoulders  180  of the movable binder ends  140  are in substantial elevational alignment with the ends  154  of the stationary binder sides  138   a ,  138   b . Accordingly, the margins at the sides  194  of the blank sheet  182  are bent into conformity between the complementary contoured surfaces  128   a ,  128   b ,  144   a ,  144   b . At this point in the process, the blank sheet  182  is fully clamped about its marginal area  186  between the upper binder  112  and lower binder apparatus  114 . 
     With full closure of the binders  112 ,  114 , the blank sheet  182  is gripped in gas-tight sealing engagement via the lockbeads (not shown) on the upper binder  112 . Accordingly, high pressure gas may be admitted against the upper surface  188  of the blank sheet  182  through the port  132  in the upper binder  112 , or upper platen, or the like in accord with customary practice in the art. Concurrently, gas may be vented from the opposite side of the blank sheet  182  through similar suitable ports (not shown), as is also known in the art. Thus, the high temperatures and gas pressure combine to stretch the blank sheet  182  into compliance with the contoured convex surfaces  134  of the form die  116 . 
       FIG. 3D  illustrates the completion of the process. Here, the upper binder  112  has been retracted, by raising the upper platen of the press. As can be seen, the sheet metal blank of  FIG. 3A  has been formed into a formed three-dimensional component C with a scrap margin M therearound. Also, the sides  194  and ends  192  of the sheet  182  are bent into substantial conformity with the contoured surfaces  124   a ,  124   b ,  128   a ,  128   b  of the binders  112 ,  114 . 
     The sheet metal blank  182  was sequentially clamped, first between ends  124   a ,  124   b ,  140  of opposed binders  112 ,  114  about a first axis  196 , and then between sides  128   a ,  128   b ,  144   a ,  144   b  of the opposed binders  112 ,  114  about a second axis  198  transverse to the first axis  196 . Accordingly, the sheet metal blank  182  is preformed in a compound manner to avoid wrinkling thereof during the forming process, which minimizes wrinkling in the finished component C. 
     Thinning and wrinkling defects can be avoided by implementing a more complex forming process, wherein a pre-forming stage defines a suitable pre-formed panel shape with relatively even thinning behavior and further wherein the panel is situated against the final forming surface in such a way to guarantee a wrinkle-free final forming process. The pre-forming stage or operation can be achieved by a stamping method or hot gas blow forming. With respect to tool design, the punch can be the only moving element for stretching the blank, or the binder ring can be designed to move around a stationary punch, with an identical effect as the moving punch concept. The functionality of the ring can be substantially increased if the ring is provided in separate sections to enable a sequence of stretching operation to achieve an optimum pre-formed panel shape. 
     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 present invention could be adapted for use in traditional steel sheet metal stamping if the movable binder segments incorporated a lock bead to control blank draw in. Likewise, the present invention may also be adapted for use in plastic sheet forming. Moreover, the present invention has been described in reference to generally rectangular binders, but is equally applicable to binders of any shape including square, circular, oblong, and the like. Finally, words of orientation such as upper and lower have been used herein to set forth an example of the present invention, but should not be construed as limiting the present invention. In other words, the present invention can be carried out in any orientation. 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.