Abstract:
A method and apparatus for forming the base of a can are described. The base forming apparatus is typically a dome station for forming a dome on the base of a beverage can. The dome station is mountable onto a dome door of a bodymaker press, thereby avoiding the common failure of mounting bolts. A complete polyurethane ring ( 20 ) is used to create overtravel force and maintain consistent and symmetrical loading on the tooling. A compact biasing arrangement is used to simplify maintenance of the tooling and reduce the need for accurate alignment.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to base forming. In particular, it relates to an apparatus for forming the base of a can as it is carried on a punch of a can bodymaker. 
     The can bodymaker takes a formed cup and then draws and irons the cup to produce a can body. The cup is carried on a punch which passes through a series of draw and ironing rings, finally forcing the can body against a base forming apparatus. Such base forming apparatus is used for forming domed profiles on the base of a can where the base needs to be able to withstand high internal pressures, particularly for carbonated beverages. In this case, the apparatus is commonly referred to as a “doming station” or “domer”. 
     Doming stations are traditionally high wear items which need repair or replacement much more frequently than do other bodymaker tool parts. Since the domer must be able to withstand the force of the punch carrying the can as the punch reaches the end of its stroke, failure has been commonly found in the moving parts and components which provide a reactive force to counteract the punch force. Further failure has been common in bolts which carry the domer since these bolts take any excess force during the doming cycle. 
     It is increasingly desirable to produce cans from lightweight materials in order to reduce material costs. However, as the materials used become thinner, so the performance of the base profile becomes more critical. The dome produced must be able to withstand not only high internal pressures but also show good drop resistance. It is thus ever more important that the tolerances of the base profiles are tight and that consistent results are obtainable from the press and, in particular from the domer. 
     In order to ensure that these tight tolerances are obtained consistently, it is important that the doming station is accurately aligned and that forces within the structure of the doming station are evenly distributed. Thus, the doming stations of the prior art tend to have a complicated structure to handle the loads experienced by the punch striking the dome die and require fine adjustment whenever the apparatus is stripped down for repair or maintenance. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided an apparatus for forming a base profile on a container mounted on a punch, the apparatus comprising a die having a face with a profile complimentary to that of the desired base profile, an adjustable hold down for clamping the container against the punch during forming of the base profile and means for biasing the die against the base of the container, arranged so that the die is moveable against the action of the biasing means as the punch reaches the end of its stroke, characterised in that the biasing means is a continuous annulus of resilient material and creates a uniform overtravel force as the punch reaches the end of its stroke. 
     As the hold down clamps the container against the punch, the base profile or dome is formed over the dome die. To ensure that the dome is fully formed and tolerance repeatability can be achieved, the punch must “bottom out” on the dome die. The die may typically be set forward of the end stroke of the punch to produce an overtravel to ensure that the punch bottoms out. The annular biasing means of the present invention provides a reaction or overtravel force as this overtravel occurs. The use of a continuous annulus of resilient material gives better force distribution than an array of independent elements which may vary in size, rating and amount of wear between individual elements. Such variations between the individual biasing elements causes uneven force distribution around the circumference of the dome die. Preferably, the biasing means comprises a polyurethane ring or a single steel spring. 
     Additionally, the present invention provides an apparatus for forming a base profile on a container mounted on a punch, the apparatus comprising a die having a face with a profile complimentary to that of the desired base profile and associated biasing means for biasing the die against the base of the container, and an adjustable hold down and associated biasing means for clamping the container against the punch during forming of the base profile; characterised in that the hold down biasing means is arranged surrounding the dome die. 
     By adapting the hold down biasing means so that it can be arranged around the dome die, in front of the support for the dome die, the apparatus of the invention is more compact than the prior art devices. Furthermore, as there is no requirement for transmission of forces from the hold down ring through or around the dome die support, transmission rods or pins are not required. This simplifies the construction of the apparatus and reduces the number of co-operating, moving parts which require bushes, bearings or seals. This in turn, reduces the number of consumable parts which are subject to wear and therefore require routine replacement. 
     Furthermore, as the hold down ring biasing mechanism is positioned around the dome die and there is no complex structure behind the dome die support, this arrangement facilitates easy removal of coolant from the dome die by means of a central bore which runs along the centre of the dome die and through the dome die support. 
     In a preferred embodiment of the invention, the hold down is enlarged to act as a piston which is biased by fluid pressure. The dome die support may then be adapted to provide the sealed piston chamber within which the hold down piston operates. 
     Preferably, the hold down is biased to eject the container after the base profile has been formed. The fluid pressure, which is adjustable, is typically air pressure which pushes the hold down forward after the forming operation, thereby ejecting the can from the dome tooling. 
     The apparatus may further comprise a sensor for detecting overtravel of the die. This sensor detects more than one thickness of material in the dome station such as when double feeds occur. 
     Preferably, the apparatus comprises an outer alignment ring in which the dome die, dome die support and hold down are mounted. The outer alignment ring is adjustable to ensure concentric alignment of the dome die with the punch. Once the outer alignment ring has been correctly aligned with the punch, the dome die, dome die support and hold down may be removed from the outer ring for inspection or maintenance without upsetting the alignment of the outer ring. When the dome die, dome die support and hold down are reinserted into the outer alignment ring, they self align within the outer ring. This arrangement simplifies maintenance procedures as the dome die, dome die support and hold down may be removed from the outer alignment ring and then reinserted without the need to realign the system. The outer alignment ring may be extended behind the dome die support and associated biasing means to form a housing for the dome die, dome die support and hold down. 
     Eccentric alignment means may be provided to adjust the orientation of the outer ring relative to the punch. These may comprise eccentric adjusters or pins which can be rotated from the rear of the dome door to align the outer ring with the punch, whilst the outer ring is located on the dome door but before the location bolts are fully tightened. 
     The apparatus may be adapted to be mounted in a dome door of a press by having a flanged housing, for example, which surrounds the domer and enables the domer to be locked onto the domer door. By mounting the domer in this way, cyclic loads are taken directly on the domer door rather than through bolts, thus avoiding component fatigue and risk of misalignment. None of the bolts used in the apparatus according to the invention are subjected to tensile loading during the doming operation. 
     The invention provides a simple and robust unit which can operate at high speed with reduced wear and simpler maintenance and alignment requirements than has been the case with prior art base forming apparatus. 
     Preferred embodiments of the invention will now be described, by way of example only, with reference to the drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side section of a first embodiment of base forming apparatus; 
     FIG. 2 is a side section of a second embodiment of base forming apparatus; 
     FIG. 3 is the side section of FIG. 1, with a can on a punch having a fully formed base; 
     FIG. 4 is the side section of FIG. 2, with a can on a punch having a fully formed base; and 
     FIG. 5 is a side section of a third embodiment of base forming apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiment of FIG. 1 comprises a dome shaped die  5  surrounded by and set back from a hold down ring  10 . A dome die support  15  comprises a “top hat” shaped component having a flange  18  behind which is a polyurethane ring  20 . This ring biases the dome die forwards towards the centre of the base of a can carried by a punch  85  (i.e. from right to left in the drawings, see in particular FIG.  3 ). The use of a complete ring ensures that there is symmetrical loading on the dome die without risk of misalignment during the forming operation. 
     The hold down ring  10  has an enlarged portion which acts as the piston in air piston  35  and the dome die support  15  defines the piston cylinder. The air piston  35  biases the hold down ring  10  towards an outer part of the base of the can. The hold down ring  10  is separated from a front retaining plate  45  by a bearing  30 . 
     An outer ring  25  can be aligned prior to locking the domer onto a bodymaker by using, for example, eccentric adjusters which are located at an angle to the central axis of the ring so that both horizontal and vertical adjustments can be achieved. The front retaining plate  45  may be attached to the outer ring  25  using bolts, interlocking lugs, an annular clamp or other suitable fixing techniques. 
     As can be seen from FIG. 4, a dome station (unnumbered) is readily mountable onto a dome door  50  of a press (not shown). The loads from the punch, carrying the can, striking the hold down ring  10  and dome die  5  are transmitted directly into the dome door  50  via the flange  18 . No loads are transmitted via the bolts holding the dome station in the dome door. 
     The dome station shown in FIG. 1 is easily dismantled for inspection and maintenance and then reassembled, without requiring re-alignment of the dome die  5  and hold down ring  10  in relation to the punch. Firstly, the front retaining plate  45  is removed from the outer ring  25  by removing the location bolts, twisting to release the co-operating lugs or releasing the clamp as appropriate. Bearing  30  remains located in the front retaining plate  45  when it is removed. The dome die support  15 , with the dome die  5  and hold down ring  10  still retained therein, may then be removed from the assembly, providing access to the strip bearings  31  and polyurethane ring  20 . Preferably, the strip bearings  31  are self lubricating and are either located in grooves in the outer ring  25  and back plate  26  or in grooves in the dome die support  15 . The hold down ring  10  may be removed from the dome die support  15 , providing access to the seals for the air piston  35 . On reassembly, the dome die support  15  is aligned in the outer ring by the strip bearings  31  and the hold down ring  10  is aligned by the bearing  30  in the front retaining plate  45  and by the dome die  5 . The front retaining plate  45  has an outer locating annulus which co-operates with a groove in the outer ring  25  to align the front retaining plate  45  with the outer ring  25 . As the outer ring  25  has not been moved from the dome door  50 , it remains aligned with the punch. The dome die  5  and hold down ring  10  self align with the punch due to the alignment of the dome die support  15  and front retaining plate  45  within the outer ring  25 . 
     FIG. 2 shows a second embodiment of the invention with like components given the same references. However, in this embodiment, the dome die support  60  is positioned directly behind the die and annular polyurethane spring  65  is positioned behind the dome die support  60 . The whole dome station is contained by a housing  70  which fits onto the dome door  50 . The hold down ring  10  is enlarged in diameter, to provide the piston of air piston  35 . The housing  70  defines the piston cylinder. 
     The embodiment of FIG. 2 is more robust than that of FIG.  1  and so is best suited for forming the base on a steel can. However, since there is less friction resistance in the first embodiment, and consequently less heat generated, that embodiment is ideally suited for forming the base on an aluminum can but could also be used for steel cans. 
     The dome station of FIG. 2 is easy to dismantle for inspection and maintenance, without affecting the alignment of the dome die.  5  and hold down ring  10  in relation to the punch. The housing  70  is accurately aligned with the punch and access to the dome die  5 , dome die support  60  and hold down ring  10  may be obtained without affecting the alignment of the housing  70 . The front retaining plate  45  is removed, as previously discussed in relation to FIG. 1, and the hold down ring  10 , dome die  5  and dome die support  60  may be removed from the housing  70 , providing access to the polyurethane ring  65  and the seals for the air piston  35 . 
     FIGS. 3 and 4 show the two types of dome station with a can  80  carried by a punch  85  in the position at which the dome has been fully formed. 
     In order to form a dome on the base of a can  80  carried by the punch  85 , the hold down ring  10  first clamps the metal of the can body between itself and the punch. In this state the hold down ring, driven by the punch, moves back from the position shown in FIG. 1, against fluid pressure, to that of FIG. 3 (and similarly for FIGS.  2  and  4 ). In order to achieve the final definition of the dome, the hold down ring  10  bottoms out against the dome die  5 , which is initially set forward of the end stroke of the punch, and will move by a small overtravel, thereby compressing the polyurethane ring  20 . Compression of the polyurethane ring  20  thus provides a symmetrical reaction or overtravel force. 
     Once the dome has been fully formed and the punch is driven back by the bodymaker, the air cylinder  35  pushes the hold down ring  10  forward and assists in ejection of the can. The large diameter of the hold down ring  10  provides a large surface area in cylinder  35  for ease of pressure control and increased hold down force. The cylinder  35  is operated by means of a pressurised fluid supply fed through a small accumulator close to the unit (not shown). This ensures that a consistent force is applied as the hold down ring  10  reduces the cylinder volume during the formation of the dome. 
     If there is more than one thickness of material in the dome station, for example if there has been a double feed of cans, then the sensor  40  detects this and stops the bodymaker, ejecting the can from the tooling. 
     As the dome is formed, a large force acts on the dome station. In conventional dome stations, this force is taken by bolts which are used to mount the domer. However, in the present invention, the domer is mounted directly onto the dome door so that the load from the whole unit is taken directly on the dome door rather than through bolts. This minimises any risk of failure of the domer due to cyclic loads taken solely on the bolts. 
     A further embodiment which improves load distribution is shown in FIG.  5 . The dome station of this embodiment is similar to that of FIGS. 1 and 3 and uses the same reference numerals where appropriate. The bearing support at the rear of the dome station of FIG. 1 is removed in the variant of FIG.  5 . The dome die support  15  is then able to rotate slightly about plain bearing  22 . It is believed that this feature may have the advantage of distributing the load more evenly throughout the dome die support  15 , should the punch and domer be misaligned, thereby reducing the risk of failure through over-stressing or fatigue. 
     The harmonics of the apparatus of FIG. 1 are improved by carrying out the modification of FIG. 5 since the dome die of FIG. 5 is allowed to follow the centre line of the punch as a dome is produced. This reduces oscillations of the ram after the dome has been formed, i.e. on the return stroke, and consequently reduces machine vibrations and increases tool life. 
     The embodiment shown in FIG. 5 may be modified further by removing the plain bearing  22  and providing an arcuate surface on the extremity of the flange on the dome die support  15  where it contacts the outer ring  25 . The arcuate contact surface allows controlled rocking of the dome die support  15  within the outer ring  25 , to correct misalignment between the dome die  5  and hold down ring  10  and the punch. 
     It can be seen that the dome stations described are much simpler than known dome stations, having fewer moving parts and fewer and simpler bearings. This means that the domers are less subject to dome maintenance requirements. Furthermore, there are, overall, fewer components which are lightweight where possible and yet which are still robust enough to operate at can making speeds of typically 500 cans per minute. For example, the dome die support  15  may be made from aluminum, single components are used to limit wear and components have been wasted where possible. 
     The dome stations of FIGS. 1 and 5 are particularly easy to maintain, in contrast with known domers. Specifically, the front retaining plate  45  is removable and then the domer can be stripped down in situ on the dome door. Only the outer ring  25  need remain in place since this is set up concentric by means of eccentric bushes as described above. Provided that this outer ring remains set up, the other domer components simply fit into this and self align without disturbing the initial machine set-up. 
     It will be appreciated that the invention has been described above by way of example only and that changes may be made within the scope of the invention as defined by the claims.