Patent Publication Number: US-6339949-B1

Title: Deep drawing method

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a method of deep drawing hard metal material such as magnesium. 
     Heretofore, there have been distributed extensively various kinds of plastic products and, as a result, a large quantity of plastic refuse and waste have been generated or dumped. Plastic waste has been incinerated together with combustible waste, which results in a serious problem of generation of atmospheric pollution and other environmental problems. Moreover, plastic products, even if they are withdrawn for the purpose of recycling, have disadvantages in that the cost associated with their recycling is high, whereby recycling thereof into new products is not profitable. Therefore, a rate of withdrawal of plastic products has been extremely low. 
     There has been advocated the use of magnesium material as substitutes or alternates for the plastic material, and casting of magnesium material has been proposed. However, cast products of magnesium are limited in their shape. Further, if deep drawing is carried out by press working, it is likely that products are cracked and, therefore, it has been difficult to provide successful press working. 
     An attempt has been made to overcome the difficulties as described above by using a press working die which is heated by a suitable electric heater or the like to about 250° C. However, this has a disadvantage in that dies must be heated every time a press working operation is to be carried out, which results in a serious drawback with regard to working efficiency. Further, a magnesium plate is increasingly hardened as it is repeatedly subjected to press working; and thus it becomes difficult to deep draw a magnesium plate and moreover, there is a serious problem in that cracks and seams are generated on surfaces of the formed products. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a new deep drawing method for magnesium plates. 
     Another object of the invention is to provide a new deep drawing method for magnesium plates, which are for structural purposes and have good recycling characteristics, environmental compatibility and energy saving properties. 
     According to an aspect of the present invention, there is provided a deep drawing method comprising using a die having a male mold with a convex forming surface and a female mold with a concave forming surface, comprising the steps of: 
     placing a vertically movably mounting table, which supports thereon a magnesium plate, between the male mold and the female mold, 
     placing a plurality of thin resin films on the magnesium plate, 
     keeping a central portion of the magnesium plate contacted with the convex forming surface of the male mold and moving the concave forming surface of the female mold relative to the convex forming surface of the male mold, thereby achieving a deep drawing of the magnesium plate. 
     In the deep drawing method according to the present invention, a layer of the thin resin films consists of at least two vinyl films each having a thickness of 0.02 mm. Also, it is preferred that the magnesium plate has a thickness of about 1.5 mm. 
     In the deep drawing method of the present invention, a pressure is applied to the magnesium plate by a base plate which is removably fitted on an upper surface of the concave forming surface of the female mold, while a vertical movement of the mounting table is adjusted by an extension device such as a hydraulic cylinder. 
     Further, in the deep drawing method of the present invention, the female mold has four female elements having an L-shaped cross section and fitted on a lower surface of the base plate. Each of the four female elements has a concave forming surface at its upper side of each inner corner surface for forming (i.e., for secondary molding) purposes, and an outwardly dilated guide surface, for molding purposes, continuously extending from the lower side of the concave forming surface. 
     Further, in the present invention, the female mold has four female elements having an L-shape cross section, and each inner corner surface has a concave forming surface. Each of the female elements is radially movably fitted to a lower surface of the base plate with a resilient element being mounted therein so that each of the female elements can be moved radially within a predetermined range. Further, supporting members each being positioned between adjacent female elements are fixed to the lower surface of the base plate to restrict a recovery position of the female elements, and the female elements are recovered to their original positions by a spring force of the resilient element that biases the female elements which have been radially pushed outwardly by the male element. 
     A shrinkage restriction plate can be fitted to either the male mold or female mold. 
     In the present invention described above, pre-pressing or molding (i.e., primary forming) is carried out by the outwardly dilated guide concave surface at the lower side of the female mold before further fabrication (i.e., secondary molding is performed) by the female mold, and then continuous finishing is performed by the concave forming surface, so that an excessive force is prevented from being applied concentrically to the curved surface of the mold. Further, two or three films of vinyl each having a thickness of 0.02 mm are disposed between the magnesium plate and the female guide surface (that is, the concave guide surface and the concave forming surface) and therefore, no shrinkage or contraction is produced between the die (or molds) and the magnesium plate and a smooth folding fabrication can be achieved without any trouble. 
     Further, the forming method of the present invention comprises the steps of: placing a shrinkage restriction plate, which has an opening for receiving the male mold inserted therein, between the male mold and the female mold; placing at a lower portion of the shrinkage restriction plate a cantilever plate that is vertically movable in the female mold so that an upper surface of the cantilever plate is placed on the same plane as the female mold; providing a coupling rod so that the cantilever plate is disposed on an upper end of the coupling rod; providing a supporting base for fixing a lower end of the coupling rod; providing a resilient support member for supporting the supporting base; and inserting the male mold into the female mold, with a circumferential end portion of the magnesium plate being held by the shrinkage restriction plate. The shrinkage restriction plate can be fitted to either the male mold or the female mold. 
     Also, in the forming method of the present invention, the shrinkage restriction plate can be vertically movably mounted on the base plate for the male mold, and a spring element can be provided to downwardly bias the shrinkage restriction plate. If necessary, the female mold can be fitted at its upper surface by a fixing member. 
     Further, the forming method of the present invention utilizes a male mold or die that includes a generally rectangular surface having four corner portions, and a female mold or die having a correspondingly shaped opening. Each corner portion has a first side and a second side interconnected by an obliquely extending end surface. A first arc located within an angle formed by the first side and the end surface is tangent to the first side and the end surface at first and second locations, respectively. And, a second arc located within an angle formed by the second side and the end surface is tangent to the second side and the end surface at third and fourth locations, respectively. The first second, third and fourth locations along with a portion of the end surface located between the second and fourth locations are interconnected by a continuous curve. 
     In the method described above, several contact portions, that is five concentrations of force are provided at the corners or the curved surface portions to diffuse or disperse the concentration of the force to five cross points, so that the concentrated force can be dispersed. Therefore, the concentration of the force by the press working is dispersed to at least five locations so that generation of shrinkage or cracks on the curved portions can be prevented effectively. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an explanatory diagram showing a male mold and a mounting table used for a deep drawing method according to a first embodiment of the present invention. 
     FIG. 2 is a front view of the male mold that is a first die of the first embodiment. 
     FIG. 3 is a front view of the female mold that is a second die of the first embodiment. 
     FIG. 4 is a bottom view of the female mold of the first embodiment. 
     FIG. 5 is an explanatory diagram of a female mold of a second embodiment, viewed from the bottom thereof. 
     FIG. 6 is a front view of a male mold and a shrinkage restriction plate used for the deep drawing method according to a third embodiment of the invention. 
     FIG. 7 is a sectional view of the female mold of the third embodiment on which a magnesium plate is placed. 
     FIG. 8 is a sectional view of the male mold and the female mold for carrying out the deep drawing method of the third embodiment of the invention. 
     FIG. 9 is a front view of a second male mold according to the third embodiment of the invention. 
     FIG. 10 is a sectional view of a second female mold according to the third embodiment of the invention. 
     FIG. 11 is an explanatory diagram of the second male mold and the second female mold of the third embodiment, showing the condition of performing the deep drawing according to the invention. 
     FIG. 12 is a perspective view of a product which is produced by form working by providing three points for concentration of force on a curved surface of a magnesium plate. 
     FIG. 13 is an explanatory, exaggerated diagram of a corner portion shown in FIG.  12 . 
     FIG. 14 is a perspective view of a product which is produced by press working by providing five points for concentration of force at a corner portion of a magnesium plate. 
     FIG. 15 is an explanatory diagram showing the five points at the corner of the product shown in FIG.  14 . 
     FIG. 16 is a bottom view of a male mold which has five cross points at its corner, seen from a lower portion thereof. 
     FIG. 17 is a plan view of a female mold resulting in five cross points at a corner of the product. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     A first embodiment of the invention will be described with reference to the accompanying drawings wherein FIG. 1 shows a relationship between a male mold  1 , a vertically movable magnesium plate  5  to be shaped, and a mounting table  3  which has, at its central portion, a substantially rectangular opening portion  4  for receiving therein a convex portion  12  of the male mold  1 , which will be described presently. 
     On the mounting table  3 , a magnesium plate  5  is placed so that it covers the opening portion  4  of the mounting table  3  and, on top of the magnesium plate  5 , a plurality of, preferably two or three, resin thin films  6  such as styrene, vinyl or the like, each having a thickness of about 0.02 mm, are superposed or disposed in an overlapping relation. By providing the two or three thin resin films as described above, each of the resin films can be freely moved or slid relative to each other, and accordingly, generation of shrinkage or contraction of the magnesium plate  5 , which is positioned between a female mold or die  2  and the convex portion  12  of the male mold or die  1 , can be prevented to thereby provide a desired finished surface. 
     The convex portion (that is, a convex forming surface)  12  of a desired shape is provided on an upper surface of a male base  11 , which is a lower portion of the male mold  1 , so that the convex portion  12  is placed in a confronting relation with the female mold  2  which will be described presently. Extension devices  7 ,  7  which are formed of hydraulic cylinders or the like are mounted to both of each of the male base  11  and the mounting table  3  so that vertical movement of the mounting table  3  relative to the male base  11  can be realized. 
     As illustrated in FIG. 2, the convex portion  12  of the male mold  1  has a flat top surface  15 , a vertical side surface  16  and an arc surface  17  which is located between, and connects, the two surfaces  15  and  16 . The arc surface  17  is defined by an incomplete arc-shaped configuration that results from cutting one end portion of an arc adjacent to the vertical side surface  16 . 
     With reference to FIGS. 3 and 4 showing the female mold  2 , there are provided four female mold elements  21   a  at corners of a lower surface of a base plate  21  which has a substantially rectangular shape similar to the male mold  1 , and the female mold elements  21   a  each have an L-shaped transversal cross section. At an upper side of an internal angle surface of the female mold element  21   a,  a concave surface portion (concave forming surface)  22  is provided for finishing purposes or secondary fabrication. The concave surface portion  22  extends vertically from the lower surface of the base plate  21  for a predetermined distance “n”. Preferably, the concave surface portion  22  extends to a mid-portion of the female mold element  21   a.    
     On a lower side (that is, at the lowest portion) of the internal angle surface of the female mold elements  21   a,  an arc shaped surface extending in an outwardly dilated fashion is provided. This arc shaped surface is a concave guide portion  23  that extends downwardly in a fan shape for the purpose of performing prefabrication or a primary fabrication. In other words, the female mold element  21  a has a concave surface portion  22  for the finish or secondary fabrication at an upper portion thereof and, at a lower portion thereof, a concave guide portion  23  which is outwardly dilated for primary fabrication. 
     In FIG. 5 which shows a female mold or die  30  in a second embodiment of the invention, the female mold  30  is fitted on a lower surface of a base plate  31  and is radially movable from a center thereof In otherwords, the female mold  30  includes four female mold elements  30   a  in such a manner that each female mold element is provided on a lower surface of the base plate  31  and has, at its internal angle surface, a concave forming surface  22   a  so that each female mold element  30   a  is in a confronting relation with each other and is movable in a radial direction within a predetermined range. 
     In the female mold element  30   a,  a resilient element  33  such as a spring, rubber element, etc. is mounted, and supporting members  37 ,  37  are fixed to a lower surface of the base plate  31  and placed between the female mold elements  30   a  for limiting the position of each of the female mold elements, so that each of the female mold elements can be returned to its original position. 
     One end of the resilient element  33  is engaged with each of the female mold elements  30   a,  and the other end is engaged with an interior of a supporting member  35  which has an L-shape and is positioned outside of the female mold element and fixed to the lower surface of the base plate  31 . The female mold element  30   a  is pushed toward the convex portion  12  of the male mold  1  and retracted in the radial direction as shown by arrows in FIG. 5 so that the resilient element  33  is compressed to generate a spring-biasing force for the recovery. 
     Thus, when the pushing force is released or, in other words, when the concave forming surface  22   a  of the female mold  30  is released from the convex portion  12  of the male mold  1 , the female mold elements  30   a  are returned to their original positions. In this case, the supporting members  37 ,  37  serve to restrict the female mold elements  30   a  from moving radially inwardly or toward a center, and cause the female mold elements  30   a  to stop at the same position. 
     In the embodiment of the present invention described above, a female mold  2  or  30  is positioned at an upper portion of the male mold  1 , and the magnesium plate  5  is disposed between the convex portion  12  of the male mold  1  and the concave surface portion  22  of the female mold. On the upper surface of the magnesium plate  5 , a plurality of (preferably two or three) thin resin films  6  such as a vinyl film or the like having a thickness of about 0.02 mm are superposed such that each of the superposed thin films  6  can be moved independently and separately from the adjacent films  6  and therefore, generation of a shrinkage or contraction between the die and the magnesium plate can be prevented so that a desired finish can be achieved. 
     The mounting table  3  for placing thereon the magnesium plate  5  is vertically movably disposed above the convex portion  12  of the male mold  1 , and extension devices  7 ,  7  having hydraulic cylinders or the like are provided at both ends of the mounting table  3 . In this state, the concave surface portion  22  of the female mold which is shown in FIG. 3 is placed on the mounting table  3  in a confronting relation with the convex portion  12 , so that the base plate  21  of the female mold is pressed downwardly for starting a press working operation. 
     The convex portion  12  is projected through the opening  4  at a center of the mounting table  3  until it contacts a lower surface of the magnesium plate  5 . A further projection or lowering of the female mold  2  presses a circumferential portion of the magnesium plate  5  by virtue of the female mold  2  having the aforementioned concave surface portion  22  to thereby achieve the predetermined deep draw forming. 
     The convex portion  12  should not initially cooperate with the concave surface portion  22  but rather a primary, pre-deep drawing should be conducted by the concave guide portion  23  which is disposed on the lower side of the concave surface portion  22  on the internal angle surface of the female mold element  21   a.  In this case, there is a small gap between the convex portion  12  and the concave guide portion  23 . In the next step, the secondary or finishing fabrication should be conducted by the concave surface portion  22  which is disposed continuously at an upper portion of the concave guide portion  23 , which gradually decreases in diameter. 
     In case of the step described above, if the concave surface portion  22  and the magnesium plate  5  contact directly with each other, it is likely that scratches and cracks are produced on a surface of the magnesium plate  5  and an excessive force is applied to the surface of the plate and therefore, it is quite difficult to provide a desired finish on the curved surface of the product. In the embodiment of the invention described above, however, provision of the resin thin films  6  serves to provide a desired sliding effect among the guide concave portion  23 , the concave surface portion  22  and the magnesium plate  5 . Thus, unnecessary and unreasonable force is not applied locally to the curved surface of the magnesium plate  5 . Therefore, no shrinkage is produced on the surface of the magnesium plate  5  and generation of cracks or seams can be effectively prevented. Thus, a desired finish of a surface of the press-worked products can be obtained with no cracks or scratches. 
     An operation of the second female mold  30  will be described. When the female mold  30  shown in FIG. 5 is placed on the mounting table  3  above the male mold shown in FIG. 1; that is, when the male mold  1  is inserted into an opening C of the female mold which is positioned at a center of the four female mold elements  30   a,  a deep drawing is conducted at the circumferential portion of the magnesium plate to be formed, by virtue of the four female mold elements  30   a  which constitute the female mold  30 . 
     The retraction of the female mold elements  30   a  in the direction shown by the arrows serves to lessen a concentrated burden or load at the corners of the deep drawn portion and enables a smooth fabrication of the deep drawn portion to be achieved. In this case, since two or three thin resin films  6  such as vinyl films are disposed between the magnesium plate  5  and the concave forming surface, a desirable sliding effect can be obtained between the magnesium plate  5  and the female mold  30 . 
     A third embodiment of the invention will be described with reference to FIG. 6, in which a male mold or die  40  having a rectangular shape is fitted, facing in a downward direction, to a central portion of a lower surface of a rectangular plate  41  which is larger than the male mold  40 . The plate  41  has at its four corners insertion holes  42  extending in a vertical direction, and supporting members  43  are vertically movably inserted into the insertion holes  42 . A shrinkage restriction plate  44 , which is larger than the male mold  40 , is fixed at its four corners to lower ends of the supporting members  43 , and springs  45  are mounted on the supporting members  43  between the shrinkage restriction plate  44  and the plate  41 . The shrinkage restriction plate  44  has at its central portion an opening  46  for receiving therein the mold  40 . 
     With reference to FIG.  7  and FIG. 8, at the lower end of the each supporting member  43 , a female mold or die  50 , in a paired relation with the male mold  40 , has a female mold portion  51  at its central portion and is placed on a upper central portion of a weighing plate  53 . The weighing plate  53  is fitted on an upper portion of a base  55  (of substantially the same size) and provides a space  54  in the base  55 . A plurality of resilient support members  56 ,  56  disposed in an upright posture within the space  54  are fitted at their upper ends to a supporting base  57 , and a plurality of connecting rods  58 ,  58  are fitted to an upper surface of the supporting base  57  and extend through guide holes  59 ,  59  to be coupled with a lower surface of a cantilever plate  60 , which is vertically movably positioned within the female mold portion  51 . Ordinarily, the supporting base  57  is lifted upwardly by a resilient force of the resilient supporting members  56 , and the cantilever plate  60  which is fitted to the upper end of the connecting rods  58  is set so that an upper surface of the cantilever plate  60  is positioned on the same plane as an upper surface of the female mold  50 . 
     If desired, the shrinkage restriction plate  44  shown in FIG. 6 can be separated from the aforementioned plate  41  and be fitted to an upper surface of the female mold  50  by virtue of a fixture device  62  such as a bolt or the like as shown in FIG.  10 . In operation, a circumferential portion of the magnesium plate  5  is caught and held by the shrinkage restriction plate  44  as shown in FIG. 8 and, as shown in FIG. 10, while the circumferential portion of the shrinkage restriction plate  44  is fitted to and held by the upper surface of the female mold  50  by the fixture device  62 , the male mold  40  is inserted into the female mold portion  51  in the female mold  50  to thereby perform the press working, as shown in FIGS. 8 and 11. 
     In this case, the lower surface of the magnesium plate  5  is contacted by the cantilever plate  60 , and therefore, as the male mold  40  is lifted upward after the press working is finished, a pressed product  5   a  is simultaneously lifted upward by a resilient force of the resilient supporting members  56  so that the product  5   a  will be discharged from the female mold  50 . Although the thin resin films  6  are not illustrated in FIGS. 8 and 11, it should be anticipated that a desired number of thin resin films, such as two or three films, can be provided. 
     In the conventional method of deep draw forming by performing known press working, as illustrated in FIGS. 12 and 13, a force is concentrated on a relatively small number of places, such as three positions as two crossing points G and H along an arc B, which is inscribed in a region defined by a right-angled corner portion A and flat side surfaces E and F extending from the right-angled corner portion A, and a circumferential surface portion D that exists between the crossing points G and H. This has resulted in generation of cracks particularly when the material being defined is hard or rigid. Therefore, it has been proved that the press working has had substantial difficulty with regard to mass production due to a substantial reduction of yield. Thus, it is extremely difficult to place the arc B closer to the corner A. 
     In view of the foregoing difficulties in the conventional technique, a chamfering line s is provided by the present invention such that the chamfering line s is oblique relative to a right-angled corner a or mutually perpendicular side lines e and f which form the right-angled corner a, and cross points c and d exist where the chamfering line s and the side lines e and f intersect. Further, the internal angles between the chamfering line and the side lines are connected by two arcs b 1  and b 2  as illustrated in FIG.  15 . Further, cross points or locations g and j, exist where the two side lines e and f are the tangent with the arcs b 1  and b 2 . And, cross points or locations h and i exist where chamfering line s is tangent with the arcs b 1  and b 2 , with a portion k of the chamfering lines existing between contact points h and i. 
     As described above, at corner portion a, five locations g, h, k, i and j, which is more than the three locations in the conventional technique, are formed for dispersing the forming force to the five locations. Thus, since there are five locations at which the force is dispersed, whereby the force applied to each of the locations is dispersed, a desired curved surface can be formed without generation of cracks and seams on the curved surface of a hard material such as the magnesium plate  5 . 
     The curvature of the curved surface R of the arcs b 1  and b 2 , which are inscribed within an internal angle formed by the combination of the side surface portions e and f, is determined so that the angles e-c-d and angle f-d-c form smooth curved surfaces. And, a radius of the arcs b 1  and b 2  is determined in accordance with products to be produced by the press working. 
     On the corner surfaces of the male mold  40  shown in FIG.  16  and on the corner portions of the female mold elements of the inner surface portions of the female mold  50 , a flat surface portion which coincides with the aforementioned portion k, so that the number of contact locations are increased from three to five. Thus, concentration of force is dispersed to five locations to prevent the force from being concentrated locally, so that generation of cracks or seams on the curved surface portions can be prevented. The thus formed male mold is illustrated in FIG. 16 in which corner portions providing as many as five contact locations are provided for dispersing the force at these five locations and, similarly, the female mold for the same purposes is illustrated in FIG.  17 . 
     Although the present invention has been described with reference to the examples which employ the magnesium plate as a molded material, it is to be understood that the present invention is also applicable to other hard metals which present some difficulty in forming. Further, the present invention can be applied to generally known soft metals into a desired shape. 
     The molding method according to the present invention provides advantages in that it can be applied not only to a general press working of hard metals such as magnesium, but also to deep draw forming of hard metals without using a large-scale instrument.