Patent Publication Number: US-2010107719-A1

Title: Necking die with shortened land and method of die necking

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority right of co-pending U.S. provisional patent application Ser. No. 61/197,976 filed on Oct. 31, 2008 by applicants named herein. The disclosure of the aforesaid provisional patent application is specifically incorporated herein in its entirety by this reference. 
    
    
     BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     This invention relates to the shaping of metal containers by means of a succession of necking steps using dies that gradually modify the container walls into a desired finished shape. More particularly, the invention relates to the design of dies to improve die necking operations and to methods of die necking. 
     II. BACKGROUND ART 
     Thin walled metal foodstuff containers, beverage cans, aerosol canisters, and other such containers for consumer or industrial products are often provided with inwardly- or outwardly-flared walls for aesthetic reasons or for reasons of practicality or economy. For example, beverage can bodies are often provided with an inward flare adjacent to their upper ends primarily to reduce the size of the required metal end closure walls. Such end closure walls are necessarily made of a metal of a much thicker gauge than that required for the walls of the container bodies, so any reduction in their size results in a considerable saving of metal. Containers of this kind are often made from rolled metal sheet that is cut into blanks, cupped, drawn and ironed to elongate the side walls, and then finally trimmed to produce a straight-walled open-ended container body pre-form. Such container body pre-forms are then provided with flared ends or other shapes of the above-mentioned kind by a process known as die necking whereby the open end of a tubular pre-form is forced into a succession of shaped annular dies of ever-decreasing diameter until the desired size reduction of the tubular wall at the open end is achieved. A large number of small changes of diameter are carried out in order to avoid metal buckling, ripping or tearing that generally occurs if abrupt size changes are attempted in a single step. More details of a typical necking operation may be obtained from U.S. Pat. No. 5,497,900 issued to Caleffi et al. on Mar. 12, 1996 and from PCT publication WO 2007/136608 A2, published on Nov. 29, 2007 (the disclosures of which documents are specifically incorporated herein by reference). 
     The necking dies work in conjunction with correspondingly-sized knockout punches that fit within the central openings of the dies and help to support and shape the container pre-forms to be necked. The purpose of the die is to reduce the diameter of the opening and impart a shape that is aesthetically pleasing at the top portion of the container pre-form. The purpose of the knockout punches is to control the metal by diverting it back towards the die so that the size reduction of a particular die necking stage is not larger than intended, to prevent failures of buckling in the neck, and to knock the container pre-form out of the die after shaping has been accomplished in that stage. 
     As the container pre-form is forced into the die, considerable friction is generated even though a lubricant is usually present on the die and metal surfaces. The friction thus generated hinders the smooth shaping operation and increases the risk of metal buckling. It also limits the degree of necking (the extent by which the open end may be necked-in at each necking step) because the container pre-form can sustain only a certain maximum axial load without deformation, and a greater degree of necking-in requires a greater degree of axial load regardless of the generated friction. Therefore, increased friction detracts from the axial load that can be applied to necking-in. 
     U.S. Pat. No. 5,711,178 which issued to Hogendoorn et al. on Jan. 27, 1998 discloses a die for use in a die-necking process of a metal body. The die is designed to reduce axial force needed in the die necking steps. 
     U.S. Pat. No. 4,881,394 which issued to Jansen on Nov. 21, 1989 and U.S. Pat. No. 5,168,742 which issued to Heyes et al. on Dec. 8, 1992 may also relate to the minimization of axial force, but these patents relate to ironing dies which are quite different from necking dies. 
     It would therefore be advantageous to provide alternative ways of reducing the amount of friction generated between the metal pre-form and a necking die without compromising a desired shaping operation. 
     SUMMARY OF THE EXEMPLARY EMBODIMENTS 
     One exemplary embodiment of the invention provides a necking die set for necking-in a metal container preform. The die set comprises a knockout punch having a generally cylindrical surface and a die having, in an axial direction from front to back of the die, an inwardly tapering in-feed surface, a forming radius, a generally cylindrical land defining a die bore diameter, a discharge surface following the land, and a relief surface having a diameter larger than the die bore diameter. The land has an axial length of less than 0.1 inch. This dimension limits the number of metal contacts with the land to one or two as a metal container preform is necked in the die in co-operation with the knockout punch. 
     The axial length of the land is preferably from 0.010 to 0.0950 inches, and still more preferably from 0.0127 to 0.0827 inches. 
     The knockout punch preferably has an outer diameter and the die preferably has a bore diameter effective to leave a gap therebetween, the gap being greater than the top-wall thickness of a container preform necked in the die set. Alternatively, the gap may be the same as the top-wall thickness or less than the top-wall thickness, to effect re-drawing of the container preform during the necking step. When the gap is less than the top-wall thickness, the gap is preferably up to 10% smaller than the thickness of the top-wall, and more preferably up to 5% smaller. 
     The container preform may preferably have a top-wall thickness of 0.0058 to 0.010 inch, and the die may preferably have a forming radius of 0.2 and 0.5 inches. 
     Another exemplary embodiment provides a necking die set for necking-in a metal container preform. The die set comprises a knockout punch having a generally cylindrical surface and a die having, in an axial direction from front to back of the die, an inwardly tapering in-feed surface, a forming radius, a generally cylindrical land defining a die bore diameter, a discharge surface and a relief surface having a diameter larger than the die bore diameter. The land has an axial length that limits the number of metal contacts to one. 
     The exemplary embodiments also extend to dies designed for use in the aforesaid die sets. 
     Another exemplary embodiment provides a method of necking-in a metal container preform having a top-wall thickness. The method includes the steps of directing an open end of a metal container preform surrounding a cylindrical knockout punch into an annular necking die to effect necking-in of the container preform, and then using the knockout punch and optionally pressurized gas to knock out the container body preform from the die. The method also includes providing the die with, or selecting the die to have, a land having an axial length effective to limit a number of contacts between the container preform with the land during the necking step to one or two. The distance between the land and the knockout punch is also preferably reduced to cause a resizing or redistribution of metal of the necked-in part of the container wall. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-section of one form of necking die and knockout punch combination showing surfaces normally provided within such a die; 
         FIG. 2  is an enlargement of part of the cross-section of  FIG. 1  showing the parts within the dashed-line circle of  FIG. 1 ; 
         FIG. 3  is a view similar to  FIG. 2 , but also showing part of a wall of a container body pre-form inserted into the die; 
         FIG. 4  is an enlargement of the land region of the die of  FIG. 3  showing points of metal contact with the land; 
         FIG. 5  is a view similar to  FIG. 4  according to an exemplary embodiment, showing a modification of the region of the land to minimize metal contact; 
         FIGS. 6A and 6B  are exaggerated schematic views showing a prior art die ( FIG. 6A ) and a die according to an exemplary embodiment ( FIG. 6B ); and 
         FIG. 7  is a view similar to  FIG. 5  but showing a further alternative exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     In the following, for the sake of simplicity, reference is made to a “container” rather than a “container body pre-form”, although the latter is generally intended. 
       FIGS. 1 and 2  of the accompanying drawings show a necking-in die set  10  comprising a combination of a die  11  and a knockout punch  12 . This combination is shown in cross-section but is generally symmetrical about an axial centerline (central axis  14 ). The die has a front  16  at the left hand side of the drawing (where the container is inserted during necking) and a rear  17  at the right hand side. In a direction front to back of the die  11 , the internal surfaces of the die are made up of an inwardly-tapering in-feed surface  20 , a forming radius  21  (sometimes referred to as an inflection point), an annular land  22  having a surface that is parallel to the axial centerline  14  and a specific bore diameter D 1 , a discharge surface  23  at the rear end of the land, and a relief surface  24  (often referred to as a cut-back surface) of greater diameter D 2  than the land. Knockout punch  12  fits within the central opening or bore of the annular die and is generally cylindrical in shape with a diameter D 3 . This diameter is typically 0.001 to 0.002 inch smaller than the bore diameter D 1  minus twice the thickness of the metal  25  (see  FIG. 3 ) forming the top of the container wall undergoing the die necking stage under consideration (the so-called top-wall thickness). Because of this smaller diameter D 3 , there is a spacing or degree of free play between the wall of the container and the opposing land  22  and knockout punch  12  during a die necking step. For aluminum beverage containers, the top wall of the un-necked container will generally range between 0.0058 and 0.010 inch, depending on whether the container body is intended for an aluminum beverage can or an aluminum bottle. 
     During neck forming, as represented in  FIG. 3 , the die is usually held stationary and the open end  26  of a container  27  is pushed into the die. The open end  26  is guided into the die by the in-feed surface  20  which causes a reduction of diameter of the container and provides the container body with a desired shoulder shape  30 . The wall of the container passes the forming radius  21  of the die (where bending commences), contacts the knockout punch  12  and is fed into the die bore formed by land  22  to form a neck-in portion  31  of reduced diameter. This operation is assisted by the knockout punch  12  which is moved in the same direction as the container and at approximately the same speed during this stage of the operation. At the end of the necking step, the knockout punch  12  is reversed in direction and, usually assisted by pressurized air introduced into the container through a central channel (not shown) in the knockout punch, is used to push the container out of the die. A step  28  of enlarged diameter formed on the knockout punch  12  contacts the open end of the container to effect this pushing step. The discharge surface  23  at the end of the land  22 , being ramped, aids the removal of the necked-in portion  31  of the container from the die without damaging the surface coating (if any) provided on the outer surface of the container. This may be necessary as the container may have a slightly larger diameter than the bore diameter at the land  22 , particularly at the open end where there is often an outward flare, as shown. 
     As previously mentioned, and as shown more clearly in the still further enlarged view of  FIG. 4 , the spacing between the outer surface of the knockout punch  12  and the adjacent surface of the land  22  is preferably slightly greater than the thickness of the metal of the container wall at this stage of the operation (usually between 0.0005 inch to 0.001 inch greater) so that the metal does not wedge itself in the gap between the die and the knockout punch (which could result in axial collapse of the container). As the metal forced into the die passes the forming radius  21 , it first contacts the outer surface of the knockout punch  12 , and is then diverted back into contact with the surface of the land  22  (shown as “Contact  1 ” in the drawing). The metal is then diverted back again into contact with the knockout punch  12 , and is once again diverted back into contact with the land  22  (“Contact  2 ”). This kind of rebound of the metal between the knockout punch and the land may occur several more times or, after the second contact, the metal may stay in continuous contact with the knockout punch before the metal passes the land and enters the gap  33  between the knockout punch  12  and the relief surface  24 . Alternatively, in some instances, the metal may remain in continuous contact with the land  22 , or there may be a mix of rebounds and continuous contact. As previously mentioned, at this stage of the necking operation, the knockout punch  12  moves in the same direction as the container wall  25  (from front to back of the die, as indicated by the arrow) and offers no real resistance to the movement of the metal since the knockout punch is moved at approximately the same speed as, or faster than, the metal and no resistive friction is developed. However, friction is generated at the positions where the metal contacts the surface of the land  22  and the friction increases with the number of contacts (or area of contact) between the metal and the land. As the friction increases, the force or axial loading required to push the container into the die increases, and there is a greater chance of the container jamming in the die or buckling under the pushing force. 
     An exemplary embodiment of the present invention minimizes the friction generated in this way by reducing the axial length of the land below the minimum length conventionally employed (about 0.1 inch). This decreases the number of times the metal contacts the surface of the land  22  and/or decreases the area of contact. Ideally, the land is made so short that there is only one contact of the metal with the land, but as many as two metal contacts may be accepted. This is illustrated schematically in  FIG. 5  of the accompanying drawings. This limited number of contacts reduces the friction, which will in turn reduce the axial load required to push the container into the die during necking. 
     It may be possible to determine the number of contacts made with the land by microscopic examination of the land surface of a well-used die since the contacts change the surface appearance or physical wear on the land surface, which appear as surface bands. Moreover, a test die made of a tough transparent material, e.g. polycarbonate or other strong polymer, may be used to allow visual observation of movements of the container wall during the necking-in process. 
     The axial length of the land  22  required to produce the desired reduction in friction is a function of the forming radius  21 , the metal properties of the top wall of the container, the top wall thickness of the un-necked container body (generally 0.0058 to 0.010 inch), and the clearance between the knockout punch and the land. For most applications using aluminum can body stock (e.g. container bodies made of alloys AA3004, AA3014, X319, X343, etc.) the forming radius  21  will fall between 0.2 and 0.5 inches, and the knockout punch and die clearance over metal (the metal being at the gauge to which it thickens in that stage of the necking operation) will fall between 0.0005 and 0.001 inch per side. In such circumstances, the preferred land lengths will be within the range of 0.027 and 0.060 inch in axial length. It should be noted that the length of the land is the length of the portion that is parallel to the central axis  14  and does not include any part of the discharge surface  23  or the forming radius  21 . The preferred working range for the length of the land is 0.010 to 0.0950 inch, and generally an amount less than 0.1 inch. These dimensions are normally suitable for all conventional necking speeds and stroke lengths. 
     Table 1 below shows the land lengths that are optimum for achieving a single contact with the die land. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 FORMING 
                 MINIMUM 
                 MAXIMUM 
               
               
                 RADIUS 
                 LAND LENGTH 
                 LAND LENGTH 
               
               
                 (inch) 
                 (inch) 
                 (inch) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 0.01 
                 0.0127 
                 0.0129 
               
               
                 0.1 
                 0.0217 
                 0.0341 
               
               
                 0.2 
                 0.027 
                 0.0421 
               
               
                 0.3 
                 0.0314 
                 0.0484 
               
               
                 0.5 
                 0.0385 
                 0.0583 
               
               
                 0.8 
                 0.0468 
                 0.0701 
               
               
                 1.0 
                 0.0515 
                 0.0767 
               
               
                 1.2 
                 0.0558 
                 0.0827 
               
               
                   
               
            
           
         
       
     
     Thus, it can be seen from the above table that, for forming radii below 1.2 inch, the land preferably has a minimum length above about 0.1 inch and a maximum length below about 0.095 inch, and preferably below 0.09 inch. 
       FIGS. 6A and 6B  are somewhat exaggerated schematic views illustrating the difference between conventional necking dies and those of some of the exemplary embodiments. Thus, it will be seen that in the exemplary embodiments illustrated by  FIG. 6B , the land  22  is much shorter than that in the conventional die design illustrated in  FIG. 6A . 
     The inventor has also surprisingly found that, when the length of the land is shortened in the indicated manner, the free play or spacing conventionally provided between the container wall and the confronting surfaces of the land  22  and knockout punch  12  may be eliminated without increasing friction unacceptably and without increasing any tendency of the container to jam in the die. This was evident by the fact that containers necked in this way did not collapse during the necking step, even when provided with a weakened mid section (e.g. a narrow waist). It is also noted that, when the free play or spacing is eliminated, or the spacing between land and knockout punch is made slightly less than the thickness of the adjacent container wall (top-wall thickness), the metal in the container wall may be redistributed or “re-sized” to eliminate or reduce circumferential irregularities of thickness that may build up as the container is necked-in. Indeed, the wall thickness after the neck reduction may be reduced in this process by 10% or less, and preferably 5% or less, when compared to an equivalently necked container where conventional free play or spacing is provided. This wall thickness reduction produces an even wall-smoothing effect. This advantage can be achieved without further modifying the land, i.e. while maintaining the flat cross-sectional profile of the surface of the land  22 , as shown. It is theorized that, although the re-sizing of the container wall can accomplished in this way, this can be done without significant increases in friction because the re-sizing of the metal takes place to only over a short axial distance due to the reduced length of the land. 
     This embodiment is illustrated in  FIG. 7  of the accompanying drawings, from which it can be seen that the gap between the knockout punch  12  and the land  22  is the same as the adjacent topwall thickness of the container  27 . Again, there is just one contact, but the container remains in contact with the land  22  over substantially the entire length of the land. Essentially, metal rebound between the knockout punch and the land has been eliminated, but the area of metal contact (compared to a conventional die) has been reduced.