Abstract:
An alternative stamp die is designed with tapered areas and non-tapered areas to provide for a longer lived stamp die while avoiding the problems associated with slug pulling.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    In the art of stamp press dies, power press tools are used for forming a predefined shaped part, called a leadframe, by punching the leadframe from a sheet or strip of material. FIG. 1 shows an elevation view of a typical stamp tool  101 . Here a punch  102 , which can be made of carbide, punches through a strip of material  103 . The punch  102  can be held within a punch holder  104  which in turn can be held by a punch shoe  105  and driven by a ram  106 . A stripper  107  can be provided to hold the material  103  and to guide the punch  102  during punching, also known as blanking. A die shoe  108  is provided for supporting the strip material  103  and for holding at least one stamp press die  109 . The stamp press die  109  has an aperture  111  with an edge against which the strip material  103  is pressed by the punch  102 . As the punch  102  presses the material  103 , a leadframe or slug  110  is formed by being cut by the stamp press die  109 , and pressed through the aperture  111  of the stamp press die  109 .  
           [0002]    [0002]FIG. 2 shows a press cycle for a typical stamp tool  101 . In FIG. 2 a,  the punch  102  is loaded above the strip material  103  as the strip material  103  is fed into position between the stripper  107  and the stamp press die  109 . In FIG. 2 a,  the punch  102  forces itself through the strip material  103  thereby removing a section of the material  103 . This section is referred to as a slug  110 . As shown in FIG. 3 a,  upon achieving the ultimate tensile strength of the strip material  103 , the slug  110  is then forced into the die  109 , which is the mating component of the punch  102 . The edges of the die cavity effectively cut the sheet material  103  into the slug  110  and the shape of the slug  110  conforms closely to the shape of the die cavity. The punch  102  is then retracted from the die  109  and material  103  during the upwards cycle of the press stroke, leaving the stamped out slug  110  contained within the die  109 . After the punch stroke, the punch  102  returns to the loaded position, as shown in FIG. 2 a,  and the process can begin again.  
           [0003]    In a typical process, a stamping press can power the tool  101  in vertical strokes at rates of 500 strokes per minute, while metal strip material  103  is fed through the tool  101 . At each stroke of the press, hundreds of carbide punches pierce the metal strip. Slugs  110  are extracted similar in geometry to the punch  102  and are pushed into the die  109 . After each cycle, one slug  110  after another is pushed into the die  109 . Because the shape of each slug  110  conforms closely with the shape of the die cavity, after a number of slugs  110  have been pushed into the die  109  these slugs  110  can become jammed in the die  109 . This condition, called packing, prevents further slugs  110  from being pushed into the die  109 .  
           [0004]    In order to overcome the problems of packing, conventional dies are tapered as shown in FIG. 3. Tapers facilitate the passage of slugs  110  through the die  109  during the blanking process. Tapering provides a subtle widening of the die cavity  301  from the cutting aperture  302  of the die  109  to the exit aperture  303  of the die  109 . Conventional dies  109  use uniform tapers of approximately 0.1° from the cutting aperture  302  to the exit aperture  303  and are provided around the perimeter of the die cavity. Thus, as additional slugs  110  are pushed into the die, the lateral forces against the side of each slug  110  decrease in the direction of the widening taper. As a result, slugs  110  pushed through the die  109  are free to fall out the exit aperture  303 .  
           [0005]    Standard dies are provided with a uniform taper around the die cavity  301 . During normal maintenance, the cutting aperture  302  of the die  109  is periodically sharpened through surface grinding in order to maintain a sharp edge along the cutting side of the die  109 . Removal of die material, while required to maintain die sharpness, increases the size of the cutting aperture  302  of the die  109  due to the taper. As a result, the tool is more inclined to slug pulling due to reduction in the lateral forces that retain the slug  110  within the die  109 . After a number of maintenance treatments, the die  109  may fail to retain the slug  110  with the undesirable consequence of the slug  110  following the punch  102  on extraction.  
           [0006]    When slug pulling occurs, the loose slug  110  is free to damage the leadframe during subsequent cycling of the press and tool  101  thereby creating excessive downtime, yield loss and adding costs to the operation. The slug  110  can be pulled from the die  109  and deposited onto or under the strip material  103 . Subsequent stroking of the tool with the dislodged slug produces severe damage to the stamp stripper  107 . Slug pulling is an industry wide problem and efforts to overcome slug pulling have limited success. One such approach is honing, which includes sharpening of the die to provide a controlled chamfer of the edge of the die, and is a very time consuming process. However, in some cases the only remedy is the premature replacement of the die  109  component of the press tool  101 .  
         SUMMARY OF THE INVENTION  
         [0007]    Accordingly, a need has arisen in the art for an alternative stamp die which reduces slug pulling and has a longer operational life than those stamp dies known before can provide.  
           [0008]    It is an object of the invention to provide a cost effective stamp die which has prolonged tool life. It is a further object to provide a stamp die which provides greater process consistency by providing the same slug retention characteristics throughout the tool life of the stamp die. It is a further object of the invention to provide a stamp die with the capability to be installed into existing press tools.  
           [0009]    In accordance with the present invention, a stamp die is provided having a cutting edge side with a cutting edge which defines a cutting aperture on the cutting edge side of the die, and an exit side having an exit aperture. A tapered area is provided extending from a tapered portion of the cutting edge to a corresponding tapered portion of the exit side such that the size of the corresponding portion of the cutting aperture is less than that of the exit aperture. A non-tapered area is also provided extending from a non-tapered portion of the cutting edge to a non-tapered portion of the exit side such that the size of a non-tapered portion of the cutting aperture remains the same as the size of a corresponding non-tapered portion of exit aperture.  
           [0010]    The stamp die can provide either a partial aperture, which when joined with a similar stamp die forms a complete aperture, or the stamp die can be provided with a complete aperture such that whereby the cutting aperture and the exit aperture cooperate to form a cavity.  
           [0011]    The stamp die can be further provided with a non-tapered area and a tapered area disposed on one side of one cutting edge such that one cutting edge of the die has a tapered portion and a non-tapered portion. It is further provided that the size of the tapered portion of the cutting edge can be greater than the size of the non-tapered portion. The stamp die further provides that the cutting aperture can forms substantially a circle or a quadrilateral. 
       
    
    
       [0012]    Other technical advantages of the present invention will be readily apparent to one skilled in the art from the following figures, descriptions, and claims.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    For a more complete understanding of the present invention and its advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawing, wherein like reference numerals represent like parts, in which:  
         [0014]    [0014]FIG. 1 is an elevation view of a cross-section of a typical stamp die tool;  
         [0015]    [0015]FIG. 2 is a simplified elevation diagram showing a cycle process for the typical stamp die tool of FIG. 1;  
         [0016]    [0016]FIG. 3 is an elevation view of a cross-section a typical stamp die;  
         [0017]    [0017]FIG. 4 is an elevation view of an alternative stamp die according to the invention;  
         [0018]    [0018]FIG. 5 is an elevation view of an alternative stamp die according to the invention; and  
         [0019]    [0019]FIG. 6 is an elevation view of an alternative stamp die according to the invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    The proposed invention involves an alternative stamp die. An embodiment of the stamp die according to the invention is illustrated in FIG. 4.  
         [0021]    The alternative die according to the invention provides certain areas of the die that do not have taper. Not only does this solution provide for reduced slug pulling during the blanking process, it can also extend die life.  
         [0022]    The solution prevents slug pulling by providing sufficient lateral pressure for slug retention throughout the die life of the tool. The solution also provides sufficient taper in order to avoid packing by allowing free passage of the slug through the die. By providing taper in certain areas and no taper in others, the alternative press die according to the invention allows the slug to continuously contact portions of the die wall throughout the life of the die, resulting in improved slug retention characteristics.  
         [0023]    [0023]FIGS. 4, 5 and  6  show several exemplary embodiments of the alternative stamp die  109  according to the invention. The stamp die  109  is provided with an aperture  111  in the shape of the desired configuration of the slug  110  to be produced. The stamp die  109  can be formed of a single piece having an aperture  111  or of multiple pieces the form the configuration of the desired aperture  111  when joined. Similarly, the stamp die  109  can be formed having a plurality of apertures  111  along a cutting edge side  304  of the die  109 .  
         [0024]    The aperture  111 , as shown in FIG. 4, is defined by the cutting edge  302  on a cutting edge side  304  of the stamp die  109 . The cutting edge  302  of the stamp die  109  is in turn comprised of a side of at least one tapered area  401  and a side of at least one non-tapered area  402 . The tapered area  401  and non-tapered area  402  extend from the cutting edge side  304  of the die  109  to the exit side  305  of the die  109 . As shown in FIG. 4, the non-tapered areas  402  can be provided in pairs and can be disposed substantially opposite each other along the perimeter of the aperture  111  in order to provide effectively cooperating lateral forces that counteract the forces that contribute to slug pulling. However, it can be appreciated that different configurations are possible depending on the configuration of the piece to be produced. For example, in FIG. 5, a circular slug  110  can be produced by a circular aperture  111  having a cutting edge  302  comprised of tapered portions  401  and non-tapered portions  402 . The non-tapered portions need not be provided substantially opposite one another in order to provide cooperating forces which counteract the slug pulling forces.  
         [0025]    The size of the non-tapered areas  402  relative to the tapered areas  401  depend on a number of factors including the thickness of the material  103  being stamped. The non-tapered areas  402  are provided with as much area as avoids packing while still maintaining sufficient lateral force to counteract the slug pulling forces. Generally, the thicker the strip material  103 , the more prone are the slugs  111  to packing and therefore the die  109  can accommodate relatively less non-tapered area  402 . In contrast, the thinner the strip material  103 , more non-tapered area  402  is provided to provide greater contact with the slug to counteract the slug pulling forces.  
         [0026]    For example, a die  109  with a rectangular aperture  111 , such as shown in FIGS. 4 and 6, can have dimensions of 0.080 inches in length by 0.035 inches in width and can have a depth of 0.750 inches. Strip materials  103  that can be accommodated by such a die  109  typically have a thickness of 0.006 inches to 0.010 inches. Accordingly, for a stamp die  109  to process relatively thicker strip material  103 , the stamp die  109  can have one pair of non-tapered areas  402 , as shown by FIG. 4 a.  The non-tapered areas can each be 0.020 inches in length and can be disposed at a mid-point along each of the longer sides of the aperture  111 .  
         [0027]    For a stamp die  109  provided for processing relatively thinner strip material  103 , the stamp die  109  can be provided with relatively more non-tapered area  402 . Such a stamp die  109  is shown in FIG. 6 which can have two pairs of non-tapered areas  402 , each pair disposed on a side of the aperture  111 . Each non-tapered area  402  of one pair of non-tapered areas  402  can be disposed on each of the longer sides of the aperture ill, each area having a length of 0.020 inches along the cutting edge  302 . The second pair of non-tapered areas can be disposed on the shorter sides of the aperture and each area can have a length of 0.035 inches, which is equal to the width of the aperture.  
         [0028]    Although the invention has been described here by reference to specific embodiments thereof, such embodiments are susceptible of variation and modification without departing from the spirit and scope of the provided claims.