Patent Publication Number: US-2004040970-A1

Title: Metal can being a pressure tight metal packaging

Description:
[0001] The invention relates to a metal can, being a pressure tight metal packaging such as a two piece steel light gauge aerosol can, comprising an ironed side wall stretching essentially along an axial direction and a circumferential direction, and a first end closure adjoining the side wall in a first wall-to-closure transition portion, and a second end closure adjoining the side wall in a second wall-to-closure transition portion, which side wall comprises a main side wall portion stretching from the first wall-to-closure transition portion to the second wall-to-closure transition portion, wherein the main side wall portion is provided with means of increased side wall thickness supporting the side wall essentially along the circumference the can, having superior vacuum resistance.  
       [0002] It is common practice in a metal can that the thickness of the side wall in its wall-to-closure transition portions is greater than the thickness in the main side wall portion.  
       [0003] There is a constant strive to down gauge the metal sheet from which metal cans are manufactured in general and the steel sheet use therefor in particular, in order to save material and to reduce “dead” weight in the distribution chain. However, customers of can makers, such as the fillers of the cans, are reluctant to reduce the wall thickness of aerosol cans because of the suspected lower vacuum performance of such a down gauged can. A lower vacuum performance increases the risk of collapsed cans during filling in case of dented cans.  
       [0004] A metal can with improved vacuum performance is known. American patent U.S. Pat. No. 3,951,296 discloses a wall-ironed container having a plurality of reinforcing ribs being spaced from the ends of the side walls and from each other, in order to achieve increased resistance against buckling of the side wall and having improved resistance against vacuum. In axial cross section, the ribs are trapezium shaped, with a central surface parallel to the inside surface of the side walls having an axial length of 2,36 mm, and both wedging surfaces inclined at an angle of 24° from the inside surface of the side walls to the central surface of the reinforcing ribs. The ribs protrude inwards over a distance of 167 μm. The known ribs are formed in the side wall by means of ironing the side wall in an ironing ring against a punch in which grooves have been formed spaced from one another along the longitudinal axis of the punch. In an ironing operation, the side wall is passed through the ironing ring by which the side wall is reduced in thickness and elongated.  
       [0005] The known can has satisfactory vacuum performance, but it is difficult to fabricate in a plural-ring ironing process. As the side wall elongates under the action of a second ironing ring, the reinforcing ribs move out of the respective grooves in which they were formed. Their destruction has to be prevented.  
       [0006] According to the present invention, it is an objective to provide a novel can concept wherein considerable material and weight savings, in the order of 5% and even more, can be achieved without unacceptably affecting the performance, notably the vacuum performance.  
       [0007] According to the invention a metal can, being a pressure tight metal packaging, is provided, comprising an ironed side wall stretching essentially along an axial direction and a circumferential direction, and a first end closure adjoining the side wall in a first wall-to-closure transition portion, and a second end closure adjoining the side wall in a second wall-to-closure transition portion, which side wall comprises a main side wall portion stretching from the first wall-to-closure transition portion to the second wall-to-closure transition portion, wherein the main side wall portion is provided with means of increased side wall thickness supporting the side wall essentially along the circumference of the can, wherein the means supporting the side wall consist of one annular portion in which the thickness of the side wall is greater than the side wall thickness in the main side wall portion outside the annular portion.  
       [0008] For the purpose of the application, the annular portion is considered to comprise also an annular portion in which substantial regions along the circumference have a side wall thickness that is greater than the side wall thickness in the main side wall portion outside the annular portion.  
       [0009] Surprisingly the vacuum performance can be greatly enhanced by supporting the can wall only locally along its circumference by providing only one supporting annular portion, for instance a reinforcement rib, which forms part of the wall and is thus integrated therein. A side wall with only one reinforcement rib can be advantageously fabricated using multiple ironing steps in a plural-ring ironing process, since a formed rib will not be destructed by following ironing steps.  
       [0010] It is remarked that a semi-product is known from European patent EP B 0 122 651, which semi-product comprises a can side wall which has a greater wall thickness in one annular centre portion than in the other portions of the side wall. To obtain two end products, this semi-product is first cut transversely across the annular centre region, before it is provided with a closure to form a metal can having a side wall with a greater wall thickness adjacent the closures than at other regions. In the end-products, the greater wall thickness is localised in the wall-to-closure transition portions of the can walls, for the formation of a flanged connection with the closure. The thus obtained cans do not have a circumferential rib in the main side wall portion of their walls.  
       [0011] In an embodiment of the invention, the annular portion is intersected along essentially the entire circumference by a cross sectional plane through the metal can, located halfway between the first wall-to-closure transition portion and the second wall-to-closure transition portion. Such a located rib provides the best vacuum performance of the metal can.  
       [0012] In a suitable embodiment, the annular portion is a circumferential rib protruding inwardly from the inside surface of the side wall into the packaging. This has the advantage that the outside surface of the packaging is undisturbed.  
       [0013] In an embodiment of the invention, the side wall thickness in the annular portion is not more than 40 μm thicker than the side wall thickness in the remainder of the main side wall portion. When the annular portion extends more than 40 μm from the main side wall of the packaging, it becomes problematic to strip the side wall from a wall ironing tool. For instance, stripping from a wall ironing punch becomes problematic in the case the annular portion protrudes into the packaging by more than 40 μm.  
       [0014] By preference, the side wall thickness is not more than 30 μm thicker than the side wall thickness in the remainder of the main side wall portion. Herewith the stripping problems are limited to a more acceptable level.  
       [0015] More by preference, the side wall thickness is not more than 20 μm thicker than the side wall thickness in the remainder of the main side wall portion. The stripping behaviour of a side wall with an inward protrusion of 20 μm has been found to be approximately equal to that of a straight wall can.  
       [0016] In an embodiment of the invention, the annular portion comprises, when seen in a longitudinal section of the metal can, a portion wherein the side wall thickness is constant over an axial distance. This provides a further advantage in fabrication, in that such a metal can side wall is better stripped from a wall ironing tool, for instance a wall ironing punch.  
       [0017] Additionally, in the case that the first end closure is integral to the side wall, the thickness of the side wall in the annular portion of the main side wall portion, when seen in a longitudinal section of the metal can and measured at increasing distances from the first end closure, first gradually increases from the thickness of the side wall outside the annular portion to a maximum thickness of the side wall inside the annular portion over a section with an axial length D 1 , and then decreases from the maximum thickness to the thickness in the remainder of the main side wall portion outside the annular portion over a section with an axial length D 2 , which length is shorter than D 1 . This is done with regard to the stripping direction of the metal can side wall, and further improves the stripping behaviour from the wall ironing punch.  
       [0018] In an embodiment wherein the first end closure is integral to the side wall, it is advantageous that, when seen in a longitudinal section of the metal can, the thickness of the side wall in a section of the annular portion of the main side wall portion, measured at increasing distances from the first end closure, gradually increases from the thickness of the side wall outside the annular portion to a maximum thickness of the side wall inside the annular portion, in which section the surface of the side wall inside the packaging is wedged with respect to the corresponding surface of the side wall on the outside of the packaging at an angle between 0.01 and 5°. The lower limit of the range of angles is related to the available space between the annular portion and a wall-to-closure transition portion. The upper limit marks an angle above which it becomes increasingly problematic to strip the side wall from a wall ironing tool.  
       [0019] By preference, this surface is wedged at an angle between 0.01 and 1°. Herewith, stripping problems are even better avoided.  
       [0020] More by preference, this surface is wedged at an angle between 0.01 and 0.25°. Herewith stripping behaviour can be approximately equal to that of a straight wall can.  
       [0021] Further, a can wall according to the invention may be manufactured by conventional drawing and wall ironing processes.  
       [0022] The invention is also embodied in a method of forming a metal can wall according to the invention, wherein on the side of the wall facing the inside of the packaging the wall during forming is supported by a punch and on its other side it is during forming brought into contact with a forming die, characterised in that a profiled punch is used having a cavity in its working surface that corresponds with the protrusion.  
       [0023] Finally the invention is also embodied in a punch having a cavity accommodating and causing when in use the protrusion in the wall portion according to the invention in the method according to the invention. 
     
    
    
     [0024] The invention will be illustrated in the following in more detail, also using the drawings wherein:  
     [0025]FIG. 1 shows a longitudinal section partly in perspective of a can body showing an inwardly protruding thicker wall portion;  
     [0026]FIG. 2 shows a longitudinal section through the centre line of a can body showing an inwardly protruding thicker wall portion indicating dimension symbols;  
     [0027]FIG. 3 shows the actual dimensions of several cans tested;  
     [0028]FIG. 4 shows the relation between applied vacuum and collapsing force for different can types;  
     [0029]FIG. 5 shows the dimensions of a carbide punch used in the method according to the invention;  
     [0030]FIG. 6 shows the dimensions of a steel punch used in the method according to the invention. 
    
    
     [0031] For investigations three types of cans were produced on a commercially available bodymaker:  
     [0032] A first type of a known can concept with a straight wall with a wall thickness of 0.15 mm.  
     [0033] A second type of a can according to the invention with a wall thickness of 0.15 mm and a mid wall step of 0.02 mm×40 mm, i.e. with H1=44 mm, H2=52 mm, H3=92 mm, H4=96 mm and X1=0.170 mm, see FIG. 2;  
     [0034] A third type of a can according to the invention with a wall thickness of 0.15 mm and a mid wall step of 0.03 mm×20 mm, i.e. with H1=50 mm, H2=62 mm, H3=82 mm, H4=88 mm and X1=0.180 mm, see FIG. 2.  
     [0035] A fourth type of can commercially available in the market having a straight wall with a wall thickness of 0.167 mm was used as a reference.  
     [0036] For the test cans and the reference cans T57 packaging steel grade was used. This is a regular material commercially supplied to the market to manufacture such cans.  
     [0037] Five hundred trial cups were made in two stages: The first draw was done on a separate cupping press.  
     [0038] The second draw was carried out on a separate cupping press to the final diameter of 45 mm.  
     [0039] For a can of the first type a punch was used with diameter 44.917 mm. This punch was reground afterwards for a can of the second type with a step of 0.02 mm×40 mm while changing the nominal diameter to 44.913 mm.  
     [0040] A new tool steel punch was manufactured for a can of the third type with a step of 0.03 mm×20 mm.  
     [0041] The cans of the second type and the third type were provided with an inwardly protruding thicker wall portion. This has the advantage that the outside surface of the packaging is undisturbed.  
     [0042] The transition zone length in front and at the end of the punch step was chosen to be different. This is done with regard to the stripping direction of the cans.  
               TABLE 1                          shows the used ironing dies:                                        1 st  die      2 nd  die      3 rd  die      4 th  die           [mm]   [mm]   [mm]   [mm]                                                     0.15 cans   45.56   45.33   45.20   45.21           0.15 “step” cans   45.54   45.32   45.22   45.21                      
 
     [0043] All cans were wall ironed on one and the same body maker.  
     [0044] The stripping behaviour of the test cans of the second type with X1=0.170 mm was approximately equal to that of the straight wall cans. Some stripping problems occurred with cans of the third type with a X1=0.180 mm. The reason for this is the difference in step size and the use of a tool steel punch, which gives a higher friction.  
     [0045] A profile of the wall thickness of the different can types was made with a thickness scanner. The wall thickness profile of the cans with a step was also measured after the third die in order to check the effect of moving the thick wall by reducing the can in the fourth die, see FIG. 3 and table 2.  
               TABLE 2                          Measured thickness                             Thickness thick   Thickness thin           wall [μm]   wall [μm]                                             0.17 reference cans   167   167           0.15 reference cans   152   152           0.15, step 0.02 cans 3 rd  die   188   167           0.15, step 0.03 cans 3 rd  die   205   175           0.15, step 0.02 cans   172   150           0.15, step 0.03 cans   183   155                      
 
     [0046] From FIG. 3 and table 2 it can be concluded that the wall thickness between the different can types is approximately equal. The thickness of the step is 22 and 28 microns respectively. There is no measurable material displacement in the step, caused by the fourth die operation.  
     [0047] Each type of can was tested on vacuum performance using a force-gauge. A force perpendicular to the centre line of the can, hereafter called the T-bar force, was applied on the wall of the can in the middle of the can height, i.e. where the test can according to the invention had a local annular wall portion with a greater thickness. The can was de-pressurised to different vacuum levels. For each vacuum level, ten cans were tested by increasing the T-bar force. In FIG. 5 the results are summarised.  
     [0048] Each point in FIG. 5 represents an average of 10 measurements. Some cans from a can of the second and third type were tested by applying the force at a quarter of the can height, where the wall was thin.  
     [0049] From the results it is concluded that the vacuum performance of the second can type according to the invention is approximately equal to the performance of the reference can. That means that a local wall thickness increase according to the invention does increase the vacuum performance. Further, it is clear that the length of the thick wall also plays a role in increasing the vacuum performance.  
     [0050] According to the invention it is now feasible to down gauge packaging steel for aerosol type cans without losing vacuum performance by providing the can wall with supporting means such as a supporting thicker portion along its circumference as dislosed herein.