Abstract:
Provided is a molding die including a die body for molding a molded article, which has an undercut, and a demolding core movable to approach and separate from the die body. When the demolding core is separated from the die body, the demolding core elastically deforms the molded article such that the undercut separates from the die body and makes it possible to remove the molded article from the die body. The die body includes a recess where the depth gradually reduces along the direction in which the demolding core separates from the die body. During molding of the molded article, a projection is formed on the undercut in accordance with the shape of the recess. During removal of the molded article, the engagement between the projection and the recess regulates the movement of the molded article in a direction intersecting the movement direction of the demolding core.

Description:
PRIORITY CLAIM 
     The present application is a National Phase entry of PCT Application No. PCT/JP2012/067458, filed Jul. 9, 2012, which application claims priority to Japanese Patent Application No. 2011-154136, filed Jul. 12, 2011, both of said applications being hereby incorporated by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     This invention relates to a molding die for a molded article made of synthetic plastic and more specifically, to a molding die with a demolding core. 
     As an example, Japanese Laid-Open Patent Publication No. H10-71634 discloses a conventional molding die relating to a similar technique. As shown in  FIGS. 21( a ) and 21( b ) , this molding die includes a movable die  52  by which a bumper  51  with an undercut  50  is formed, and a demolding core  53  that moves relative to the movable die  52  to demold the bumper  51  from the movable die  52 . As shown in  FIG. 21( b ) , in response to movement of the demolding core  53 , the bumper  51  deforms elastically such that the undercut  50  moves away from the movable die  52 , thereby allowing detachment of the bumper  51  from the movable die  52 . 
     If the bumper  51  is one that is to be attached to a vehicle, the bumper  51  generally has rigidity higher at its lower edge part than at its upper edge part, as shown in a partial view of  FIG. 22 . This is because the bumper merely has an abutment section  51   a  at its upper edge part that is to abut against right and left fenders or a front grille to be coupled thereto, whereas it has a flange section  51   b  at its lower edge part that is to extend to face lower part of a vehicle body. 
     If the bumper  51  is formed by a molding die such as that described in Japanese Laid-Open Patent Publication No. H10-71634 and the molded bumper  51  is to be detached from a movable die, the demolding core  53  is moved in the direction of arrow P to spread the undercut  50  outward (to the right of  FIG. 23 ), as shown in  FIG. 23 . Lower part of the bumper  51  has high rigidity. Thus, the lower part of the bumper  51  deforms by an amount smaller than the deformation amount of upper part of the bumper  51 . This enables the bumper  51  to move in a manner shown by arrow Q of  FIG. 23 . Specifically, the bumper  51  moves outward in the movement direction of the demolding core  53  and at the same time, it may also move in a direction perpendicular to the movement direction of the demolding core  53 . In this case, friction may be caused between a surface of the undercut  50  (surface of a hatched area of  FIG. 23 ) and a corner of the demolding core  53 , thereby making a scratch on this surface. Hence, if the bumper  51  is attached to a vehicle and the undercut  50  forms part of the interior of a wheel arch of the vehicle, a problem may be caused that the scratch is visible from outside the vehicle. 
     Japanese Laid-Open Patent Publication No. H07-52733 discloses a structure where multiple ribs are formed on the rear surface of an undercut of a bumper with the intention of enhancing the rigidity of the undercut of the bumper. However, these ribs increase the weight of the bumper. 
     SUMMARY OF THE INVENTION 
     The present invention was made for solving the above problems in the prior art. It is an objective of the present invention to provide a molding die that enhances the molding quality of a molded article at an undercut of the molded article. 
     To achieve the foregoing objective, and in accordance with one aspect of the present invention, a molding die is proposed that includes a die body for forming a molded article with an undercut, and a demolding core arranged to move toward and away from the die body. While moving away from the die body, the demolding core elastically deforms the molded article such that the undercut moves away from the die body, thereby making the molded article detachable from the die body. The die body includes a molding surface for forming the undercut. The molding die includes a recessed portion formed in the molding surface of the die body. Recessed portion is reduced in depth gradually in a direction in which the demolding core moves away from the die body. A projecting portion is formed on the undercut in conformity with the shape of the recessed portion during formation of the molded article. During detachment of the molded article, engagement between the projecting portion of the undercut and the recessed portion of the die body restricts movement of the molded article in a direction crossing the movement direction of the demolding core. 
     In this case, even if a factor such as difference in rigidity of a bumper observed in a direction crossing the movement direction of the demolding core generates force in the bumper that moves the bumper in this crossing direction, engagement between the recessed portion and the projecting portion restricts movement of the bumper in the crossing direction while the demolding core moves. This prevents sliding motion of the undercut along the demolding core, thereby avoiding a scratch on the undercut due to the sliding motion. 
     It is preferable that the recessed portion has a groove shape and extends in the movement direction of the demolding core, and the projecting portion has a ridge shape. 
     In this case, only the projecting portion of a ridge shape is formed on a surface of the undercut, thereby preventing the undercut and eventually, the bumper from increasing in weight more than necessary. 
     In accordance with another aspect of the present invention, a molding die is proposed that includes a die body for forming a molded article with an undercut, and a demolding core arranged to move toward and away from the die body. While moving away from the die body, the demolding core elastically deforms the molded article such that the undercut moves away from the die body, thereby making the molded article detachable from the die body. The die body includes a molding surface for forming the undercut. The molding die being characterized by a projecting portion formed on the molding surface of the die body while being increased in height gradually in a direction in which the demolding core moves away from the die body. A recessed portion is formed in the undercut in conformity with the shape of the projecting portion during formation of the molded article. During detachment of the molded article, engagement between the recessed portion of the undercut and the projecting portion of the die body restricts movement of the molded article in a direction crossing the movement direction of the demolding core. 
     This aspect achieves the same operation and advantages as those achieved by the aforementioned aspect. 
     It is preferable that the projecting portion has a ridge shape and extends in the movement direction of the demolding core, and the recessed portion has a groove shape. 
     In this case, only the recessed portion of a groove shape is formed in a surface of the undercut, thereby preventing reduction in rigidity of the undercut. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing part of a molding die for a bumper according to one embodiment; 
         FIG. 2  is a cross-sectional view showing part of the molding die for a bumper according to the embodiment of  FIG. 1 ; 
         FIG. 3  is a perspective view of a bumper; 
         FIG. 4  shows an area S of  FIG. 3  in an enlarged manner; 
         FIG. 5  is a cross-sectional view of the molding die showing an undercut during demolding of the bumper; 
         FIG. 6  is a cross-sectional view taken along line  6 - 6  of  FIG. 5 ; 
         FIG. 7  is a cross-sectional view of the undercut showing a projecting portion; 
         FIG. 8  is a cross-sectional view taken along line  8 - 8  of  FIG. 7 ; 
         FIG. 9  is a perspective view of the undercut showing the projecting portion; 
         FIG. 10  is a cross-sectional view of the molding die showing how the projecting portion is formed; 
         FIG. 11  is a cross-sectional view taken along line  11 - 11  of  FIG. 10 ; 
         FIG. 12  is a cross-sectional view of the molding die showing the projecting portion during demolding of the bumper; 
         FIG. 13  is a cross-sectional view taken along line  13 - 13  of  FIG. 12 ; 
         FIG. 14  is a perspective view of an undercut showing a projecting portion according to a modification; 
         FIG. 15  is a perspective view of an undercut showing a projecting portion according to another modification; 
         FIG. 16  is a cross-sectional view of a molding die showing how a projecting portion is formed in a still further modification; 
         FIG. 17  is a cross-sectional view taken along line  17 - 17  of  FIG. 16 ; 
         FIG. 18  is a cross-sectional view of an undercut showing a recessed portion according to the modification of  FIG. 16 ; 
         FIG. 19  is a cross-sectional view taken along line  19 - 19  of  FIG. 18 ; 
         FIG. 20  is a perspective view of the undercut showing the recessed portion according to the modification of  FIG. 16 ; 
         FIGS. 21( a ) and 21( b )  are partial cross-sectional views of a conventional molding die showing how an undercut is formed; 
         FIG. 22  is a perspective view of an undercut of a bumper; and 
         FIG. 23  is a schematic view of an area R of  FIG. 22  taken from the rear surface of a product and schematically showing motion of the undercut during demolding. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment that embodies this invention is described next by referring to  FIGS. 1 to 13 . 
     A molding die  10 , a part of which is shown in  FIG. 1 , is configured to form a vehicle bumper out of synthetic plastic as a molded article. The molding die  10  includes a die body and a demolding core  12 . The die body is composed of a fixed die not shown in the drawings, and a movable die  11  that acts cooperatively with the fixed die to form a bumper  21  with an undercut  20 . The demolding core  12  functions to demold the molded bumper  21  from the movable die  11  toward a parting direction (Z direction). The demolding core  12  is moved toward a direction (Y direction) perpendicular to the parting direction (Z direction) to be away from the movable die  11 . The demolding core  12  engages the undercut  20 . As shown in  FIG. 2 , when the demolding core  12  is moved in the Y direction relative to the movable die  11 , the demolding core  12  elastically deforms the bumper  21  such that the undercut  20  moves away from the movable die  11  in the Y direction. Then, the bumper  21  becomes detachable from the movable die  11 .  FIGS. 1 and 2  show part of the molding die  10  corresponding to part of the bumper  21  where the undercut  20  is formed. The remaining part of the molding die  10  not shown in the drawings has the same structure as the aforementioned structure of the conventional molding die. 
     As shown in  FIG. 3 , the bumper  21  forms lower part of a front grille and respective front parts of right and left fenders of a vehicle. Upper edge part of the bumper  21  is formed as an abutment section  21   a  that is to abut against lower edge part of the front grille and to be connected to the front grille. Lower edge part of the bumper  21  is formed as a flange section  21   b  that is to extend to face lower part of a vehicle body. The undercut  20  to form a front side of a wheel arch is formed on each of the right and left sides of the bumper  21 . As shown by arrows T of  FIG. 4 , part of the molding die  10  corresponds to part of the bumper  21  where the undercut  20  is formed. 
     As shown in  FIGS. 5 and 6 , a molding surface  11   a  of the movable die  11  to form the undercut  20  is gradually inclined in the movement direction (Y direction) of the demolding core  12  to be shifted in the parting direction (Z direction). A recessed portion  13  of a groove shape is formed in the molding surface  11   a  for the undercut  20  to extend in the movement direction of the demolding core  12 . The recessed portion  13  is formed to be reduced in depth linearly toward the movement direction of the demolding core  12 . A bottom surface  13   a  of the recessed portion  13  extends parallel to the movement direction of the demolding core  12 . Further, as shown in  FIG. 6 , opposite side surfaces  13   b  of the recessed portion  13  of a groove shape extend parallel to the movement direction (Y direction) of the demolding core  12 . 
     As shown in  FIGS. 7, 8 and 9 , the recessed portion  13  forms a projecting portion  14  of a ridge shape on a surface of the undercut  20  that projects toward the front of the bumper  21 . The projecting portion  14  is gradually reduced in height as it approaches a lateral inner surface  21   c  of the bumper  21 . The recessed portion  13  engages the projecting portion  14  when the demolding core  12  is moved in the Y direction. Thus, the recessed portion  13  restricts movement of the undercut  20  in a direction (X direction) perpendicular to the movement direction (Y direction) of the demolding core  12 . The bottom surface  13   a  of the recessed portion  13  is not always required to extend parallel to the movement direction of the demolding core  12  but it may tilt in the tilting direction of the molding surface  11   a . Specifically, the recessed portion  13  can have any shape that allows movement of the demolding core  12  in the Y direction while making the recessed portion  13  engage the projecting portion  14  in the X direction during this movement of the demolding core  12 . 
     Described next is how the molding die  10  of the aforementioned structure operates while the bumper  21  formed by the molding die  10  is demolded from the molding die  10 . 
     As shown in  FIG. 1 , the bumper  21  is formed by the molding die  10 . A surface of the undercut  20  of the bumper  21  is provided with the projecting portion  14  of a ridge shape formed by the recessed portion  13  in the molding surface  11   a  of the movable die  11 , as shown in  FIGS. 9, 10 and 11 . The projecting portion  14  is formed to be gradually reduced in height as it approaches the lateral inner surface  21   c  of the bumper  21 . In this embodiment, two recessed portions  13  are provided in the movable die  11 , and two projecting portions  14  are formed on the surface of the undercut  20  by these recessed portions  13 . However, the number of the projecting portions  14  is not limited to two. 
     In this condition, when the demolding core  12  is moved in a direction (Y direction) perpendicular to the parting direction (Z direction), the undercut  20  is pressed in the Y direction by the demolding core  12 . This elastically deforms the bumper  21  to move the undercut  20  in the Y direction, as shown in  FIG. 12 . 
     As shown in  FIGS. 12 and 13 , while the demolding core  12  moves, the opposite side surfaces of the projecting portion  14  of the bumper  21  slide on the opposite side surfaces of the recessed portion  13  of the movable die  11 . This restricts displacement of the bumper  21  relative to the movable die  11  in a direction (X direction) perpendicular to the movement direction (Y direction) of the demolding core  12 . As a result, even if the rigidity of the bumper  21  differs between an area near the abutment section  21   a  and an area near the flange section  21   b , the bumper  21  will not move in a direction perpendicular to the movement direction of the demolding core  12  during demolding. 
     Further, as shown in  FIGS. 2 and 5 , when the undercut  20  is moved in the Y direction by the demolding core  12  to be away from the movable die  11 , the bumper  21  becomes capable of being demolded from the movable die  11  in the parting direction (Z direction). At this time, the bumper  21  can be detached from the movable die  11  by moving the movable die  11  in the parting direction (Z direction). 
     The embodiment described in detail above has the following features. 
     (1) The recessed portion  13  is formed in the molding surface  11   a  of the movable die  11  to be reduced in depth gradually toward the movement direction of the demolding core  12 . The projecting portion  14  is formed on the undercut  20  in conformity with this shape of the recessed portion  13 . While the demolding core  12  moves in the movement direction (Y direction), engagement of the projecting portion  14  with the recessed portion  13  restricts movement of the bumper  21  in a direction (X direction) perpendicular to the movement direction (Y direction). Hence, even if a factor such as difference in rigidity of the bumper  21  observed in the aforementioned perpendicular direction (X direction) generates force in the bumper  21  that moves the bumper  21  in this perpendicular direction, movement of the bumper  21  in this perpendicular direction (X direction) is restricted while the demolding core  12  moves. This prevents sliding motion of the undercut  20  in the aforementioned perpendicular direction along the demolding core  12 . Thus, scratches on the undercut  20  due to the sliding motion are avoided, thereby enhancing the quality of the bumper  21 . 
     A terminal portion of the bumper disclosed in Patent Document 2 includes multiple ribs formed on the rear surface of the terminal portion to extend as far as to reach a lateral inner surface with the intention of increasing the rigidity of the terminal portion corresponding to an undercut. Unlike the ribs of Patent Document 2, the projecting portion  14  of the present invention does not extend as far as to reach the lateral inner surface  21   c  of the bumper  21 , as shown in  FIG. 7 . Specifically, the projecting portion  14  is not intended to enhance the rigidity of the undercut  20 , so that the rigidity of the undercut  20  with the projecting portion  14  is the same as that of the undercut  20  without the projecting portion  14 . 
     (2) The recessed portion  13  of a groove shape is formed to extend in the movement direction of the demolding core  12 . During formation by the molding die  10 , the projecting portion  14  of a ridge shape is formed on a surface of the undercut  20  in conformity with the shape of the recessed portion  13 . Hence, only the projecting portion  14  of a ridge shape is formed on the surface of the undercut  20 , thereby preventing the undercut  20  and eventually, the bumper  21  from increasing in weight more than necessary. 
     (3) The molding die is used to form the bumper  21  made of synthetic plastic to be used in a vehicle. Forming the bumper  21  by this molding die can prevent scratches on the undercut  20  of the bumper  21  that might be made during demolding. Thus, if the undercut  20  forms part of a wheel arch viewable from outside, this reduces the probability of formation of a scratch on the wheel arch. 
     Embodiments of the present invention are not limited to the aforementioned embodiment but it may be modified as follows. 
     In the aforementioned embodiment, the projecting portion  14  of a ridge shape is formed on a surface of the undercut  20  by the recessed portion  13  of a groove shape formed in the molding surface of the movable die  11 . The recessed portion  13  may also be rectangular. In this case, the recessed portion  13  may be configured to form the projecting portion  14  on the surface of the undercut  20  having parallel outer side surfaces  14   a  while becoming lower gradually toward the movement direction of the demolding core  12 , as shown in  FIG. 14 . 
     In the aforementioned embodiment, the projecting portion  14  of a ridge shape is formed on a surface of the undercut  20  by the recessed portion  13  of a groove shape formed in the molding surface of the movable die  11 . As shown in  FIG. 15 , the recessed portion  13  may also be configured to form the projecting portion  14  on the surface of the undercut  20  composed of two projecting sections triangular in cross section as viewed in the movement direction of the demolding core  12  while having two outer side surfaces  14   a  on the opposite sides thereof extending parallel to the movement direction of the demolding core  12 . In this case, the recessed portion  13  is also configured to form the projecting portion  14  that is reduced in height gradually toward the movement direction of the demolding core  12 . 
     As shown in  FIGS. 16 and 17 , a projecting portion  30  of a ridge shape may be formed on the molding surface  11   a  of the movable die  11  to extend in the movement direction (Y direction) of the demolding core  12  while being increased in height gradually toward this movement direction. As shown in  FIGS. 18, 19 and 20 , in conformity with this shape of the projecting portion  30 , a recessed portion  31  of a groove shape is formed in a surface of the undercut  20  to be increased in depth gradually toward the movement direction of the demolding core  12 . When the demolding core  12  moves in the Y direction for demolding of the bumper  21 , the projecting portion  30  engages with the recessed portion  31  to restrict movement of the undercut  20  in the X direction. This structure also prevents the undercut  20  from sliding on the demolding core  12  in a direction perpendicular to the movement direction of the demolding core  12  during demolding, thereby avoiding scratches on the undercut  20 . Additionally, this structure merely involves formation of the recessed portion  31  of a groove shape in the surface of the undercut  20 , thereby preventing reduction in rigidity of the undercut  20  due to the recessed portion  31 .