Patent Publication Number: US-11019886-B2

Title: Button

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Stage entry of PCT Application No: PCT/JP2017/021669 filed Jun. 12, 2017, the contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a button, especially a button such as an injection molded resin female snap button. 
     BACKGROUND ART 
     A synthetic resin female snap button (hereinafter simply referred to as “female snap” in this specification) which is engaged with and disengaged from a male snap button is widely used for clothes, bags, and the like. Such a female snap is described in, for example, JP2003-310310A.  FIGS. 10 and 11  are a perspective view and a bottom view of a conventional female snap  110 , and  FIG. 12  is a sectional view taken along the line D-D in  FIG. 11 . Hereinafter, an upper or upward side and a lower or downward side are based on  FIG. 12 , and a vertical (up-and-down) direction corresponds to the thickness direction of the female snap  110  in  FIG. 12 . The female snap  110  includes a substantially disk-shaped bottom portion  111  and a peripheral side portion  112  that protrudes upward from the radially outer end of the bottom portion  111 . In the center of the bottom portion  111 , a through-hole  113  is provided for passing a pin  122  of a button fastener  120  (see  FIG. 4 ). A space  114  above the bottom portion  111  and radially inward of the peripheral side portion  112  is a receiving portion  114  which detachably receives an engagement projection of a male snap as not shown. At the upper end on the radially inner surface of the peripheral side portion  112 A, a bulge  115  bulging radially inward is provided. The bulge  115  is temporarily bent radially outward when the engagement projection of the male snap is received in and removed from the receiving portion  114 . 
     When the above-mentioned female snap  110  is attached to a fabric such as clothes, the female snap  110  is held on an upper die and the button fastener  120  is set on a lower die of the button attachment machine with the fabric  1  (see  FIG. 4 ) being placed between the female snap  110  and the button fastener  120 . Then, the upper die is lowered toward the lower die. Thereby, the pin  122  of the button fastener  120  penetrates the fabric  1  upward to pass through the through-hole  113  of the female snap  110 . After that, the pin  122  is swaged on the bottom portion  111  of the female snap  110  by a punch of the upper die (See  FIG. 4 ). Thereby, the female snap  110  is fixed to the fabric. 
     CITATION LIST 
     Patent Literatures 
     JP2003-310310A 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the conventional injection-molded female snap  110  made of synthetic resin, there is a case where the bottom portion  111  of the female snap  110  cracks when the pin  122  of the button fastener  120  passes through the through-hole  113  of the female snap  110  at attaching the female snap  110  to the fabric  1 . The inventors have carefully observed the phenomenon that the female snap  110  cracks, and found out the following. The female snap  110  is molded by injecting a molten resin material into a cavity of a mold from an injection port called a gate at a side of the cavity. In the center of the cavity, there is a core (pin) for forming the through-hole  113 . The resin material injected through the gate is once divided into left and right flows by the core, and the flows meet and join again behind the core. The spot where the resin flows joined will appear as a weld line  116  on the outer and inner surfaces and the peripheral surface defining the through-hole  113  of the bottom portion  111  of the female snap  110  as a molded product.  FIGS. 11, 12  shows respectively the weld line  116  appearing on the outer surface of the bottom portion  111  and on the peripheral surface defining the through-hole  113 . Further, at a location corresponding to the gate in the female snap  110 , a trace of the gate position  117  (see  FIG. 11 ) remains slightly. The weld line  116  in a conventional product appears as a straight line substantially along a virtual straight line passing through the gate position  11  and the center of the female snap  110 . The weld line  116  is also linear in the thickness direction (vertical direction) of the bottom portion  111  and appears as a straight weld line  116  (see  FIG. 12 ) along the vertical direction on the peripheral surface defining the through-hole  113 . Although not shown, the weld line  116  appears as a straight line along the vertical direction on a cross section parallel to the cross section of  FIG. 12 . The weld line  116 , which is a material merging spot, is a low strength portion in the bottom portion  111  of the female snap  110 . Therefore, when the pin  122  of the button fastener  120  comes into the through-hole  113  along with a portion of the fabric  1  being pulled by the pin  122  at attaching the female snap  110  to the fabric  1 , the bottom portion  111  receives a force to radially expand the through-hole  113  by the portion of the fabric  1  pulled by the pin  122 . Such a force is considered to be the cause of cracking along the linear weld line  116 . 
     The present invention has been made in view of the above problem, and an object of the present invention is to provide an injection-molded resin button, which does not crack or is unlikely to crack at attaching the button to a fabric. 
     Solution to Problem 
     In order to solve the above-mentioned problems, according to the present invention, there is an injection-molded resin button comprising a button body having a through-hole at the center thereof: wherein the button body comprises a left half part and a right half part, the left and right half parts being segmented left and right by a virtual straight line passing through a gate position, on the button body, corresponding to a gate of a mold for molding the button and the center of the button body, wherein the button body includes one or more concave portions or one or more convex portions on an outer surface of the button body, the concave portions or the convex portions being provided asymmetrically in the left and right half parts. The above “outer surface” means a surface of the button body which will come into contact with a fabric when the button is attached to the fabric. The outer surface corresponds to a lower surface of a bottom portion  11  represented by reference numeral  11   a  in  FIG. 3 . Further, left and right arc concave portions or left and right arc convex portions and first and second arc concave portions or arc convex portions as described below are examples of the concave portions or the convex portions. 
     In one embodiment of the present invention, the button body includes a weld line, and wherein the weld line appears as a nonlinear line at least on the outer surface of the button body. In another embodiment of the present invention, the weld line appears as a nonlinear line on an inner surface of the button body. In still another embodiment, the weld line appears as a nonlinear line on a peripheral surface defining the through-hole of the button body. Further, in one embodiment of the present invention, the weld line appearing on the outer surface of the button body is at least partially zigzag. 
     In the present invention, one or more concave or convex portions are provided on the outer surface of the button body asymmetrically in the left half part and the right half part of the button body which are segmented left and right by the virtual straight line connecting the gate position (a gate trace, which slightly remains on a peripheral side surface of the button body and which corresponds to a gate of a mold for molding the button) of the button body and the center. Such concave or convex portions are molded by convex or concave portions on a cavity surface of the mold. A melted resin material injected through the gate is once divided into left and right resin flows by a core that is at the center of the cavity for molding the through-hole. In other words, the material is divided into a left half space and a right half space of the cavity which are segmented by a virtual straight line passing through the gate and the center of the cavity (or the core). These left and right flows join again behind the core. Resistances to the left and right divided flows or velocities thereof are different in the left and right half spaces of the cavity, because the convex or concave portions on the cavity are provided asymmetrically in the right and left spaces of the cavity. Thereby, a weld line as a spot where the right and left flows meet and join will not be linear along the virtual straight line unlike before. Therefore, a nonlinear weld line (see reference numeral  16  in  FIG. 2 , etc.) appears on exposed surfaces including the outer surface of the button body. The weld line is also nonlinear in the thickness direction (the vertical direction based on  FIG. 3 ) of the button body (see reference numeral  16  in  FIG. 3 , etc.). Thereby, the length of the weld line on outer or inner surfaces, etc. of the button body will be longer than that of the weld line  116  in the conventional product. Therefore, the area of a spot on which the left and right resin flows join increases, enhancing joining strength of the flows. Thereby, it will be possible to eliminate or reduce cracking of the button body along the weld line when the button is attached to the fabric. Concave or convex portions provided in the left and right half parts of the button body are not limited in number, shape, arrangement, etc. as long as they are bilaterally asymmetrical. However, it is preferable to form concave or convex portions so as to make the weld line zigzag. Thereby, even if the button body receives a force to radially expand the through-hole by a portion of a fabric pulled by a pin of a button fastener when the button is attached to the fabric, it is possible to reduce an effect of cracking along the weld line. 
     In one embodiment of the present invention, the left half part and the right half part are different in volume. The left half part and the right half part can have the same volume as long as the number, shape, arrangement, etc. of the concave or convex portions are asymmetrical right and left. However, it is possible to help increase the length of the weld line by making the concave or convex portions different in volume between the left and right half parts. The difference in volume in the left and right half parts can be determine by measuring the weight each of the left and right half parts of a button, which are split along the above-mentioned virtual straight line connecting the gate position and the center. That is, if the left and right half parts are equal in weight, their volumes are equal, and if the left and right half parts are different in weight, heavier part is large and lighter part is small in volume. 
     In one embodiment of the present invention, the left half part includes one or more left arc concave portions or left arc convex portions extending or arranged in an arc shape in the circumferential direction around the center, wherein the right half part includes one or more right arc concave portions or right arc convex portions extending or arranged in an arc shape in the circumferential direction, and wherein the left arc concave portions or left arc convex portions and the right arc concave portions or right arc convex portions are displaced from each other in the radial direction. Since the left arc concave portions (or convex portions) and the right arc concave portions (or convex portions) are radially displaced from each other, a joining spot of the left and right resin flows divided at molding varies leftward and rightward in the radial direction. Thereby, a weld line becomes zigzag, making the weld line longer. 
     In one embodiment of the present invention, the left half part includes one or more left arc concave portions or left arc convex portions extending or arranged in an arc shape in the circumferential direction around the center, wherein the right half part includes one or more right arc concave portions or right arc convex portions extending or arranged in an arc shape in the circumferential direction, and wherein the left arc concave portions or left arc convex portions and the right arc concave portions or right arc convex portions are at the same location in the radial direction and different in length in the circumferential direction. In a case where the left and right arc concave (or convex) portions are different in circumferential length even though they are at the same radial location, a joining spot of the left and right resin flows divided at molding will be easy to vary leftward and rightward in the radial direction. Thereby, the joining spot varies left and right relative to the virtual straight line as the center line at the same radial location where the left and right arc concave (or convex) portions lie, making the weld line longer. 
     In one embodiment of the present invention, the left half part or the right half part includes one or more first arc concave portions or first arc convex portions extending or arranged in an arc shape in the circumferential direction around the center, and one or more second arc concave portions or second arc convex portions extending or arranged in an arc shape in the circumferential direction, the second arc concave portions or second arc convex portions being provided with an interval in the radial direction relative to the first arc concave portions or first arc convex portions. In this way, by providing the first arc concave portions (or convex portions) and the second arc concave portions (or convex portions) with an interval between them in the radial direction in the left or right half parts, it is possible to change a resistance or a velocity, in the radial direction, of one flow (one the side where the first and second arc concave (or convex) portions and the interval to be formed) of the left and right resin flows divided at molding relative to the other flow. Thereby, it is possible to vary, left and right in the radial direction, a spot where the one and other flows join. Thereby, the weld line is easy to be zigzag, making the weld line longer. 
     In one embodiment of the present invention, only one of the left half part and the right half part includes one or more concave portions or convex portions on the out surface. Thereby, it is possible to displace, relative to the virtual straight line, a joining spot of the left and right resin flows divided at molding, making the weld line longer. 
     In one embodiment of the present invention, the button is a female snap button, wherein the button body is a bottom portion of the female snap button, and wherein the female snap button includes a peripheral side portion protruding from a radially outer end of the bottom portion. 
     Examples of a resin of which the button in the present invention is made include thermosetting resins such as phenol resin (PF), epoxy resin (EP), melamine resin (MF), urea resin (UF), unsaturated polyester resin (UP), alkyd resin, polyurethane (PUR), thermosetting polyimide (PI), polyethylene (PE); thermoplastic resins such as polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), polyvinyl acetate (PVAc), polyurethane (PUR), polytetrafluoroethylene (PTFE), ABS resin (acrylonitrile butadiene styrene resin), AS resin, acrylic resin (PMMA); and the like, but not limited thereto. 
     Advantageous Effect of Invention 
     In the present invention, concave portions or convex portions on the outer surface of the button body are provided asymmetrically in the left part and the right part. Thereby, a weld line becomes nonlinear. The weld line is generated as follows. At molding, a melted resin material injected into a cavity through a gate of a mold at molding is once divided into left and right resin flows by a core at the center of the cavity, and the left and right divided resin meet and join behind the core, resulting in the nonlinear weld line. Thereby, the length of the weld line will be longer than that in a conventional product. Further, since the area of the joining spot of the left and right resin flows increases, enhancing joining strength of the flows. Thereby, it will be possible to eliminate or reduce cracking of the button body along the weld line when the button is attached to the fabric. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view of a female snap according to a first embodiment of the present invention. 
         FIG. 2  is a bottom view of the female snap of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view taken along the line A-A in FIG. 
         FIG. 4  is a cross-sectional view showing a state in which a female snap is attached to a fabric by a button fastener. 
         FIG. 5  is a bottom view of a female snap according to a second embodiment of the present invention. 
         FIG. 6  is a bottom view of a female snap according to a third embodiment of the present invention. 
         FIG. 7  is a cross-sectional view taken along the line B-B in  FIG. 6 . 
         FIG. 8  is a bottom view of a female snap according to a fourth embodiment of the present invention. 
         FIG. 9  is a cross-sectional view taken along the line C-C in  FIG. 8 . 
         FIG. 10  is a perspective view of a conventional female snap. 
         FIG. 11  is a bottom view of the female snap in  FIG. 10 ; 
         FIG. 12  is a cross-sectional view taken along the line D-D of  FIG. 11 . 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Hereinafter, some embodiments of the present invention will be described with reference to the drawings. However, the invention is not limited to the embodiments, and those skilled in the art can suitably modify or alter them within the scope of the claims or the range of equivalents.  FIGS. 1 and 2  are a perspective view and a bottom view of a female snap  10  as a button according to an embodiment of the present invention.  FIG. 3  is a cross-sectional view taken along the line A-A in  FIG. 2 . Hereinafter, an upper or upward side and a lower or downward side are based on  FIG. 3 , and a vertical (up-and-down) direction is the thickness direction of the female snap button of  FIG. 3 . The female snap  10  is an injection-molded product made of synthetic resin, and includes a substantially disc-shaped bottom portion  11  as a button body and a peripheral side portion  12  that protrudes upward from a radially outer end of the bottom portion  11 . In the center of the bottom portion  11 , a through-hole  13  is provided for allowing the pin  122  of the button fastener  120  to pass when the female snap  10  is attached to a fabric  1  (see  FIG. 4 ). A space  14  above the bottom portion  11  and radially inward of the peripheral side portion  12  of the female snap  10  is a receiving portion  14  that detachably receives an engagement projection of a male snap as not shown. At an upper end on the radially inner surface of the peripheral side portion  12 , a bulge  15  bulging radially inward is provided. The bulge  15  is temporarily bent radially outward when the engagement projection of the male snap is received in and removed from the receiving portion  14 . The reference numeral  18  in  FIGS. 1 to 3  represents an annular edge  18 , which slightly protrudes radially outward and is provided at the boundary with the bottom portion  11  on the radially outer side surface of the peripheral side portion  12 . The lower surface (the outer surface in the claims)  11   a  of the bottom portion  11  gradually lowers radially inward from the annular edge  18  and then becomes horizontal. Thus, it can be said that the lower surface  11   a  of the bottom portion  11  is slightly convex downward. 
     On the lower surface  11   a , the upper surface  11   b  and the peripheral surface defining the through-hole  13  of the bottom portion  11  of the female snap  10 , a nonlinear weld line  16  (see  FIGS. 2 and 3 ) emerges. Further, on the peripheral side portion  12 , a trace of the location corresponding to a gate (injection port) of a mold used for molding the female snap  10  slightly remains, and this is a gate position  17 . The gate position  17  is located at the top in  FIG. 2 . The bottom portion  11  of the female snap  10  is virtually segmented into a left half part  11 L and a right half part  11 R by a virtual straight line L connecting the gate position  17  and the center O of the female snap  10 . In the present embodiment, the weld line  16  on the lower surface  11   a  of the bottom portion  11  extends radially outward from the through-hole  13  to the radially outer end of the bottom portion  11  as follows. Firstly, in the radially inward end on the through-hole  13  side, the weld line  16  begins in the right half part  11 R and goes into the left half part  11 L across the virtual straight line L and then goes back from the left half part  11 L to the right half part  11 R across the virtual straight line L. After the weld line  16  extends in the above-mentioned zigzag shape, the weld line  16  further extends radially outward almost along the virtual straight line L. After this, the weld line  16  goes into the left half part  11 L and then terminates at the radially outer end. The weld line  16  is also nonlinear in the vertical direction of the bottom portion  11 , and the vertically zigzag weld line  16  appearing on the peripheral surface defining the through-hole  13  is shown in  FIG. 3 . Although not shown, the weld line  16  appearing on the upper surface  11   b  of the bottom portion  11  is different in shape from the weld line  16  appearing on the lower surface  11   a.    
     In the left half part  11 L of the bottom portion  11 , a first left arc concave portion  21  and a second left arc concave portion  22  are formed. The first and second left arc concave portions  21 ,  22  extend in an arc shape in the circumferential direction around the center O, and the second left arc concave portion  22  are located radially outward from the first left arc concave portion  21  with an interval  41 . In this embodiment, the width along the radial direction of each of the first and second left arc concave portions  21 ,  22  is the same, and the degree of depression from the lower surface  11   a  of each of the first and second left arc concave portions  21 ,  22  is also almost the same. In the right half part  11 R of the bottom portion  11 , a first right arc convex portion  31  and a second right arc convex portion  32  are formed. The first and second right arc convex portions  31 ,  32  extend in an arc shape in the circumferential direction, and the second right arc convex portion  32  are located radially outward from the first right arc convex portion  31  with an interval  42 . The first and second left arc concave portions  21 ,  22  and the first and second right arc convex portions  31 ,  32  extend about 170 degrees in the circumferential direction. In this embodiment, the width along the radial direction of the second right arc convex portion  32  is about twice the width along the radial direction of the first right arc convex portion  31 . Further, the degree of protrusion from the lower surface  11   a  of each of the first and second right arc convex portions  31 ,  32  is almost the same. Further, the first right arc convex portion  31  is located, in the radial direction, between the first and second left arc concave portions  21 ,  22 . In other words, the first right circular arc convex portion  31  is displaced in the radial direction from the first and second left arc concave portions  21 ,  22 . The second right arc convex portion  32  is formed radially outside the second left arc concave portion  21 . As described above, in the left and right half parts  11  L,  11 R of the bottom portion  11  of the female snap  10 , the first and second left arc concave portions  21 ,  22  as concave portions and the first and second right arc convex portions  31 ,  32  as convex portions are provided bilaterally asymmetrically. In this embodiment, the volume of the left half part  11 L including the first and second left arc concave portions  21 ,  22  is smaller than the volume of the right half part  11 R including the first and second right arc convex portions  31 ,  32 . 
     As in the above example, the weld line  16 , which is generated by left and right resin flows joined at molding the female snap  10 , can be extended in a zigzag shape such as by providing the left half part  11 L with the concave portions ( 21 ,  22 ) and providing the right half part  11 R with the convex portions ( 31 ,  32 ); further displacing the concave and convex portions ( 21 ,  22 ;  31 ,  32 ) in the radial direction; changing the radial width of each of the convex portions ( 31 ,  32 ). Furthermore, the first and second left arc concave portions  21 ,  22  are arranged in the radial direction of the bottom portion  11  with the interval  41  between them, and similarly the first and second right arc convex portions  31 ,  32  are also radially arranged with the interval  42  between them. This way, providing the intervals  41 ,  42  also helps to extend the weld line  16  so as to increase leftward and rightward swing in a zigzag shape. In this embodiment, the radial width of the interval  41  of the left half part  11 L is larger than radial width of the first right arc convex portion  31 , which is arranged at the same radial position as with the interval  41 . This is also considered to promote a zigzag extension of the weld line  16 . The interval  42  of the right half part  11 R has almost the same radial width as that of the second left arc concave portion  22 , which is arranged at the same radial position as with the interval  42 . In this way, it is preferable that the width in the radial direction each of the intervals  41 ,  42  is larger than or equal to the radial width of convex or concave portion arranged, in other half part, at the radial position corresponding to the intervals  41 ,  42 . 
     The first and second left arc concave portions  21 ,  22  as described above are molded by convex portions provided on a cavity surface of a mold (not shown) for molding the female snap  10 , and the first and second right arc convex portions  31 ,  32  are molded by concave portions on the cavity surface. In this way, since the mold is provided with convex and concave portions bilaterally asymmetrically, the nonlinear weld line  16  shown in  FIG. 2 , etc. is generated. That is, when the female snap  10  is molded, a melted resin material injected into the cavity through the gate of the mold is once divided into left and right flows by a center core corresponding to the through-hole (the material is divided into a left half part and a right half part of the cavity by a virtual straight line passing through the gate and the center of the cavity (core)). The left and right divided flows meet and join again behind the core. At this time, resistances to the left and right divided flows or velocities thereof are different in the left and right half spaces of the cavity, and therefore the spot where the resign flows joined will not become a linear line, resulting in the weld line  16  that is at least partially zigzag. Thereby, when the female snap  10  is attached to the fabric  1 , the bottom portion  11  does not crack or is unlikely to crack along the weld line  16 . 
       FIG. 4  is a cross-sectional view showing a state in which the female snap  10  is attached to the fabric  1  with the button fastener  120 . The button fastener  120  comprises a substantially disk-shaped base  121  and a pin  122  protruding from a center area of the base  121 . When the female snap  10  is attached to the fabric  1  using the button attachment machine, the pin  122  of the button fastener  120  penetrates the fabric  1  upward and passes through the through-hole  13  of the female snap  10 . After this, the pin  122  is swaged as being crushed by a punch as not shown. Thereby, the female snap  10  is fixed to the fabric  1 . Since the pin  122  of the button fastener  120  comes into the through-hole  13  of the female snap  10  along with a portion of the fabric  1  being pulled by the pin  122 , the bottom portion  11  receives a force to radially expand the through-hole  13 . Such a force is considered to be the cause of cracking along the weld line  16  having a relatively low strength. In this embodiment, with the bilaterally-asymmetric convex and concave portions as stated above, it is possible to intentionally extend the weld line  16  in a zigzag shape, the weld line  16  being a spot of joining left and right resin flows. Thereby, strength of the resin-joining spot increases, decreasing or eliminating the above-mentioned cracking. As shown in  FIG. 4 , the female snap  10  is attached to the fabric  1  so that the lower surface  11   a  of the bottom portion  11  faces the fabric  1 . Therefore, concave portions (or convex portions) provided on the lower surface  11   a  of the bottom portion  11  are not seen by a user after the female snap  10  is attached to the fabric  1 . On the other hand, the upper surface  11   b  of the bottom portion  11  is visible to a user even after being attached to the fabric  1 . Therefore, it is possible to form concave portions (or convex portions) on the lower surface  11   a  of the bottom portion  11  without considering aesthetic appearances when the arbitrary, nonlinear weld line  16  is formed. 
       FIG. 5  is a bottom view showing a female snap  10   a  as a button according to a second embodiment of the present invention. Since the female snap  10   a  is common to the female snap  10  except for concave portions  23 ,  24 ,  25 ,  26  as described below, same reference numerals as with the female snap  10  are used except for the concave portions  23 ,  24 ,  25 ,  26  and descriptions thereof are omitted. The female snap  10   a  is provided with concave portions on the lower surface  11   a  of the bottom portion  11  as follows. The left half part  11 L of the bottom portion  11  includes four first left concave portions (one left arc concave portion)  23  arranged in an arc shape in the circumferential direction around the center O, and four second left concave portions (one left arc concave portion)  24  arranged radially outward from the first left concave portions  23  with an interval and in a circumferentially arc shape. Each of the first left concave portions  23  has a square or trapezoidal shape with rounded corners. Each of the second left concave portions  24  has a square shape with rounded corners, and is approximately twice as large as each of the first left concave portions  23 . Further, the right half part  11 R of the bottom portion  11  includes four first right concave portions (one right arc concave portion)  25  arranged in an arc shape in the circumferential direction, and four second right concave portions (one right arc concave portion)  26  arranged radially outward from the first right concave portions  25  with an interval and in a circumferentially arc shape. Each of the first right concave portions  25  has a square shape with rounded corners. Each of the second right concave portion  26  has a square or trapezoidal shape with rounded corners, and is approximately twice as large as each of the first right concave portions  25 . Further, the first right concave portions  25  are provided, in the radial direction, between the first and second left concave portions  23 ,  24 , and the second right concave portions  26  are provided radially outward from the second left concave portions  24 . Furthermore, each of the first right concave portions  25  is larger than each of the first left concave portions  23 , and each of the second right concave portions  26  is larger than each of the second left concave portions  24 . In the female snap  10   a , the volume of the left half part  11 L is larger than that of the right half part  11 R. The female snap  10   a  is molded by a mold, which includes a cavity having convex portions corresponding to the concave portions  23 ,  24 ,  25 ,  26 . On the lower surface  11   a  of the bottom portion  11  of the female snap  10   a , a zigzag weld line  16   a  shown in  FIG. 5  is generated. The radially inner end of the weld line  16   a  is located on the virtual straight line L. The weld line  16   a  goes radially outward into the left half part  11 L from the radially inner end; then returns rightward going into the right half part  11 R across the virtual straight line L; then returns leftward going into the left half part  11 L across the virtual straight line L again; then returns rightward again going into the right half part  11 R across the virtual straight line L; and extends to and terminates at the radially outer end. In the female snap  10   a , the bottom portion  11  hardly cracks along the weld line  16   a  when the female snap  10   a  is attached to the fabric. 
     Further, in the female snap  10   a  of the second embodiment, there are three flat portions between two of the four first left concave portions  23  formed in the left half part  11 L of the bottom portion  11   a . These flat portions are not concave or convex and define the lower surface  11   a  of the bottom portion  11 . It can be said that each of the flat portions is a spot in which more resin material gathered than in each of the first left concave portions  23 . That is, in a circumferentially arc shaped region where the four first left concave portions  23  are provided, the portions (flat portions) having more resin are also arranged in an arc shape along the circumferential direction. Similarly, as to the second left concave portions  24  and the first and second right concave portions  25 ,  26 , the portions (flat portions) each having more resin material than each corresponding concave portion are formed and arranged in a circumferentially arc shape between two of the respective second left, first and second right concave portions  24 ,  25 ,  26 . In this way, in the bottom portion  11 , portions (flat portions) having more resin material than concave portions are arranged in an arc shape in the circumferential direction. Thereby, it is possible to make the weld line  16   a  nonlinear, strengthening the female snap  10 . 
       FIG. 6  is a bottom view of the female snap  10   b  according to the third embodiment of the present invention.  FIG. 7  is a cross-sectional view taken along the line B-B in  FIG. 6 . Regarding part of the female snap  10   b  that is common to the female snap  10 , same reference numerals as with the female snap  10  are used and descriptions thereof are omitted. The female snap  10   b  includes one concave portion  27  that extends in an arc shape in the circumferential direction in only the right half part  11 R on the lower surface  11   a  of the bottom portion  11 . The concave portion  27  extends approximately 170 degrees in the circumferential direction. That is, there are no concave and convex portions in the left half part  11 L. In this way, by providing the concave portion  27  only in the right half part  11 R, the following weld line  16   b  is formed on the lower surface  11   a  of the bottom portion  11  of the female snap  10   b . That is, referring to  FIG. 6 , the weld line  16   b  extends radially outward from a left-right intermediate point on the through-hole  13  side almost on the virtual straight line L and gradually goes into the right half part  11 R. Then, the weld line  16   b  extends radially outward further, gradually turning leftward toward the virtual straight line L from an almost intermediate point in the radial direction, and terminates at the radially outer end of the bottom portion  11  almost on the virtual straight line L. That is, the weld line  16   b  is generated so as to enter the right half part  11 R where the concave portion  27  lies. The weld line  16   b  is a curved line not including a zigzag portion and is nonlinear. This weld line  16   b  can also prevent the female snap  10   b  from cracking when it is attached to the fabric  1 . 
       FIG. 8  is a bottom view of the female snap  10   c  according to the fourth embodiment of the present invention, and  FIG. 9  is a cross-sectional view taken along the line C-C in  FIG. 8 . Regarding part of the female snap  10   c  that is common to the female snap  10 , same reference numerals as with the female snap  10  are used and descriptions thereof are omitted. The female snap  10   c  includes a left arc concave portion  28  extending in the circumferential direction on the lower surface  11   a  of the left half part  11 L of the bottom portion  11  and a right arc concave portion  29  circumferentially extending on the lower surface  11   a  of the right half part  11 R. The left arc concave portion  28  circumferentially extends approximately 40 degrees in the upper half of the left half part  11 L based on  FIG. 8 . The right arc concave portion  29  circumferentially extends approximately 170 degrees in the right half part  11 R. The circumferential length of the left arc concave portion  28  is approximately half of that of the right arc concave portion  29 . The left arc concave portion  28  and the right arc concave portion  29  are at the same position in the radial direction. Further, the radial width of the left arc concave portion  28  is slightly larger than that of the right arc concave portion  29 . In this way, by providing the left and right arc concave portions  28 ,  29  located at the same radial position and having different circumferential lengths, the following zigzag weld line  16   c  is generated on the lower surface  11   a  of the bottom portion  11  of the female snap  10   c . That is, referring to  FIG. 8 , the weld line  16   c  extends from a left-right intermediate point on the through-hole  13  side almost on the virtual straight line L to go into the left half part  11 L; then turns rightward toward the virtual straight line L; goes into the right half part  11 R across the virtual straight line L; and then turns leftward to terminate at the radially outer end of the bottom portion  11  almost on the virtual straight line L. The weld line  16   c  enters the right half part  11 R at a radial position where the left and right arc concave portions  28 ,  29  lies. This weld line  16   c  can also eliminate or reduce cracking of the female snap  10   c  at attaching the female snap  10   c  to the fabric  1 . 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           10 ,  10   a ,  10   b ,  10   c  female snap button (button) 
           11  bottom portion (button body) 
           11   a  lower surface of bottom portion (outer surface) 
           11   b  upper surface of bottom portion (inner surface) 
           12  peripheral side portion 
           13  through-hole 
           14  receiving portion 
           15  bulge 
           16 ,  16   a ,  16   b ,  16   c  weld line 
           17  gate position 
           21 ,  23  first left arc concave portion (concave portion) 
           22 ,  24  second left arc concave portion (concave portion) 
           25  first right arc concave portion (concave portion) 
           26  second right arc concave portion (concave portion) 
           27  concave portion 
           28  left arc concave portion (concave portion) 
           29  right arc concave portion (concave portion) 
           31  first right arc convex portion (convex portion) 
           32  second right arc convex portion (convex portion) 
         L virtual straight line 
         O center