FORMATION OF THE BASE OF CAN BODIES Field of the Invention This invention relates to a method of stuffing hollow articles from a blank piece. In particular, it relates to a method of embossing a blank in the form of a cup in a one-piece, drawn, and press-stretched (DWI) can body. BACKGROUND OF THE INVENTION In known methods of stuffing cans, the blank is held on a die and transported through a succession of dice to embed the surface cup and finally print the bottom edge to produce the desired base profile. For beverage cans, this base profile is typically a dome, while for food cans the base profile typically has a plurality of concentric annular panels surrounding a central panel. Alternatively, the base profile can be formed in a separate process that combines the pressing of the inner annular edges and then the formation by stamped profiles of a deeper "anti-bend" edge. The material used for the manufacture of cans is expensive and therefore efforts have been made in recent years to reduce the thickness of the material required in order to reduce the material costs accordingly. However, the limitations in the reduction of the thickness are imposed by the formation process and by the particular base profile, which is required in order to solve the thermal processing and pasteurization and with the conditions imposed by the product itself. , such as soft drinks. Food cans are often formed of ferrous material, for example reduced steel only once
(SR) or double reduced steel (DR). Steel is typically found in the tinplate form such as T57 tinplate. This tinplate has an elastic limit from
200 to 300 Nmpf1 and an ultimate tensile load
(UTS) from 330 to 410 Nmm "2. The minimum elongation for fracture is 23% and for the test / UTS is 80 to 90% Usually the finished tin used for food cans is matte although fusion tinplate is used momentary for some applications such as partially varnished cans.The tin coating is usually selected according to the product for which the tin is used, for example the tin cans T57 used for human food has a tin coating of 2.8 / 2.8 gm The profile used for the base of the one-piece can bodies formed in a single process exhibits dilution around the adjusted edge radius due to the tensile forces that arise during the shaping of the base. base are particularly high when the can wall is stretched in. Dilution is a particular problem at the innermost edge and if the material is too thin to drive the splitting of the base at this point. Consequently, the minimum thickness, which can be used for the formation of a DWI one-piece can body of 73mm diameter in a single process from tinplate T57, is 0.275mm SR, or 0.270mm SR for a food can DI of 65mm. Conventional bases can be formed from the 0.270mm SR material without the slit, but these are not strong enough to withstand some processing pressures. SUMMARY OF THE INVENTION According to the present invention there is provided a can body formed by the steps of: passing a cup over a die through a series of dice to increase the height of the side wall of the cup; introducing fluid between the die and the embossed cup after it leaves the dice; and pressing the pressed cup against a shaping tool of the base to form the desired base profile;
in which the can body has a side side integral with the end wall, the end wall includes at least one annular edge surrounding a central panel, the edge (s) have an internal radius between 0.8mm and 1.4mm. Typically the internal radius can be 1.4mm for a can body of 73mm in diameter, but can be reduced as low as 0.8mm for the same can body by the introduction of fluid according to the invention. These radii are much tighter than those found possible using conventional base shaping methods and the resulting base profile is much stronger. This radius can be what is commonly known as the "countersunk hole radius". The spokes do not refer to specific can diameters, but the typical can diameters, for which these profiles could be used, are 65 and 73 mm. The body of the can can be formed from a tinplate having a UTS value of up to 650 Nmm "2, preferably 500 Nmm" 2 or less. Tinplate can be double reduced steel and can have a thickness of at least 0.15mm. This can body is preferably crimped and pressed into the wall as it passes through the series of dice.
According to a second aspect of the present invention, there is provided a can body formed by the steps of: passing a cup over a die through a series of dice to increase the height of the side wall of the cup; introducing fluid between the die and the embossed cup after it leaves the dice; and pressing the pressed cup against the base forming tool to form the desired base profile; in which the can body has a side side integral with the end wall, the end wall includes a peripheral channel portion having a depth of between 4% and 8% of the diameter of the body of the can. Typically the depth of the peripheral channel portion can be 4.7mm for a can body of 73mm diameter. This channel portion is much deeper than what has been found possible using conventional base shaping methods and the resulting base profile is much stronger. Preferably, an inner wall of the channel portion supports a central panel and at least one annular edge joins the portion of the channel to the central panel, the edge (s) having a radius of between 0.5mm and 2mm. Typically, the edge radius can be 0.76mm. The can body can be formed from a tinplate having a UTS value of up to 650 Nmitf2, preferably 500 Nmm "2 or less.The tinplate can be double reduced steel and can have a thickness of at least 0.15mm However, the thicker gauge steel is preferably reduced only once.The body of the can according to this aspect of the invention is preferably embossed and pressed into the wall, but having a base profile, which It has only been formally developed for embossed and re-embossed cans (DRD) .This base profile is considerably stronger than that of the first mode and it is better to withstand internal pressures, which arise during thermal processing without inversion of the base According to still another aspect of the present invention, a can body formed by the steps of: passing a cup on a die through a series of dice to increase the height is provided. of the side wall of the cup; introducing fluid between the die and the embossed cup after it leaves the dice; and pressing the pressed cup against the shaping tool of the base to form the desired base profile; wherein the can body has a side side integral with the end wall and is formed of a tinplate having a UTS value above 650 Nmm-2. In a preferred embodiment, the tinplate has a UTS of 500 Nmm "2 or less.The tinplate can be double reduced steel and can have a thickness of at least 0.15mm The can body of this embodiment of the present invention is it can form with a base profile according to any of the other two embodiments In each of the embodiments of the invention, the fluid is preferably introduced at least 20 ° before the bottom dead center, otherwise, they are not reduced The formation loads For steel food cans, it can be seen therefore that the basic advantage of light weight is achieved by means of the present invention, either by using higher strength material such as DR, or by using profiles of Stronger base, similar to those used in the present for DRD cans, or by a combination of the strongest material and the base profile.As a direct result of this invention, it has been found possible to produce a can from a thin hard material such as DR steel and / or to form a base having a stronger profile than usually possible in a single operation while the can body is still transported through the die. This is therefore a further advantage of the present invention for steel food cans, specifically the production of a stronger base profile in a single base forming operation. It would be appreciated that the present invention is not limited to steel can bodies in the tinplate form, or having basic profiles, which are only suitable for food products. For example, can bodies with dome-shaped base profiles are typically used for beverage products. In a further embodiment, it is believed that hard steels up to 500 Nmm "2 of production, 520 Nmm" 2 of UTS can be used for dome-shaped base profiles for beverage cans, in which the can body it is produced from a 0.18mm DR tinplate according to the present invention. This has not previously been possible without the splitting of the base support edge. The increased resistance for such steel cans for beverages is only obtained from the strength of the material. It is not possible to produce base profiles for stronger beverages due to problems that result during varnish spraying. Other embodiments of the can bodies for beverages made of aluminum are also within the scope of this invention. Typically, the shaping process of this invention allows 0.25mm gauge aluminum to be used, while the thinnest gauge for beverage cans has previously been 0.28mm. The significant advantage of light weight is obtained by a combination of the use of stronger aluminum alloys, having approximately 360 UTS and using stronger base profiles. These stronger base profiles are obtainable by producing smaller radii in the body manufacturer than in the present, typically between lmm and 1.5mm, and subsequently by reshaping to produce stronger base profiles. It has surprisingly been found that the method by which the can bodies of the present invention are manufactured, in which the fluid is forced between the die and the can wall during the forming operation of the base, considerably reduces the forces of traction at the base of the can during shaping. It is believed that this is as a result of the friction between the can and the die being reduced as the can is "in decline" during the formation - lu ¬
from the base. Preferably, the fluid, which is introduced, comprises cooling fluid or other liquid and is advantageously introduced through ducts, which pass along the longitudinal axis of the die and exit the die around the perimeter of the die, at the top of the side wall of the cup. The main advantage of having ducts in the top wall is, that in this way it is easier to select a tool that is the same to avoid ironing material from the wall of the can inside the holes. In addition, this avoids fatigue failure, which can arise if the fluid is introduced into the angle between the upper wall of the transition between the thick and thin material on a side wall stretched in wall press. The use of a cooling fluid has been proposed, which is introduced into the transient point described above in EP-A-0045116 to assist in the splitting of the can body from the die after forming. However, that application does not suggest that the introduction of a cooling fluid between the die and the body of the can allows the formation of a can body of a thinner material and / or having a stronger base profile. Although it is possible to use gas or air as the fluid, this is not a preferred alternative since the gas should be maintained at a constant pressure, which is difficult to achieve in a controlled manner due to the compressibility of the gas. Furthermore, it is preferable for convenience that the fluid be introduced at the same time that the air passes through the die to the base, in order to assist in the splitting of the can from the die. Generally, this can be 60 ° before the bottom dead point (BDC). However, it would be appreciated that this timing is only for convenience and that the fluid can be introduced at any time, or even permanently, after the cup has left the sausage / ironing dice. It is important that the fluid does not enter during ironing as the reduction of friction between the die and the cup at this stage allows an unevenness in forces on the side wall of the cup, resulting in a tear. It is also important to keep the cup feeding area free of refrigerant fluid. The fluid can usually be introduced at a pressure of 200 psi, although pressures between 150 and 2000 psi are also acceptable. BRIEF DESCRIPTION OF THE DRAWINGS The preferred embodiments of the present invention will now be described with reference to the drawings, in which: Figure 1 is a side section in part of an apparatus for forming a pressed-in-the-wall and pressed-out can body;; Figure 2 is a side section of the upper wall profile of a high pressure split die of the apparatus of Figure 1; Figure 3 is a partial side section of a first base profile of the body of the can; and Figure 4 is a partial side section of a second base profile of the body of the can. DETAILED DESCRIPTION OF THE INVENTION A mechanical press, part of which is shown in Figure 1, typically comprises a frame, which supports a tool package comprising a re-dumped die, two ironing rings or dice and an extractor, a through which a die 10 can pass. A lower training cushion 28 is axially aligned with the tool pack. The die 10 has a longitudinal fluid conduit 20, which is connected to the perimeter of the die in the wide part of the die through a series of radially extending channels 22. A second longitudinal duct 25 passes through the length of the die and exits on the front face of the die. In use, the cups are fed from a feed duct to the die and each embossed cup is pressed against the surface of the die re-imbedded in the tool holder. Subsequently, the reembutted cup is pushed through the ironing rings to make the can body 30 have a side wall that is thinner than its bottom wall. After the exit of the dice / rings, the fluid is introduced through radial channels 22 at a point of approximately 60 ° before the BDC, as shown in Figure 2, simultaneously with the provision of pressing air to the face of the die through a second conduit 25. The cup is then glued to the lower forming cushion and the desired layer profile is formed in a single operation. On the return stroke of the die the body of the can 30 is extracted from the die by means of the extractor. Comparative Example 1 A DWI can body of 73mm diameter 0.275mm tinplate SR T57 (see the above specification) having a conventional DWI base profile as shown in Figure 3 and formed in the conventional manner, that is, without the introduction of fluid between the die and the cup, it was cut open in order to measure the thickness of the flanged base at different points along the radius of the base. Table 1 shows the thicknesses at different points along the radius. A spreading test was carried out on an equivalent DWI can body and produced a thickening pressure of 3,103 bar (50 psi). Table 1 All dimensions are in mm: A 0.270 E 0.261 I 0.258 B 0.264 F 0.270 J 0.267 C 0.270 G 0.258 K 0.240 D 0.270 H 0.270 L 0.264
Example 1 A DWI »0.22mm tin plate body of DR having a tensile breaking load was made
(UTS) of 460 Nmm "2, according to the method of the present invention, introducing refrigerant fluid between the die and the cup at 60 ° before top dead center (TDC), and the same tests were carried out as in the comparative example 1. The base profile was that of Figure 3, the profile conventionally used for DWI cans, The results of these tests are shown in table 2. The equivalent thickening data was 2,689 bars (39 psi). ).
Table 2 All dimensions are in mm: A 0.215 E 0.215 I 0.210
B 0.215 F 0.218 J 0.215
C 0.218 G 0.213 K 0.200 D 0.218 H 0.217 L 0.218
Comparative Example 2 A DRD can body of 73 mm diameter DR steel of 0.18mm in the tinplate shape having a tensile tensile load (UTS) of 650 Nmm "2 and having the base profile shown in figure 4 it was formed in a conventional manner by a single pressing operation and cut open in order to measure the thickness of the base at various points along the radius.An equivalent can body produced peak data of 2,793 baria (40.5 psi) These results are presented in Table 3. Table 3 All dimensions are in mm: A 0.171 E 0.176 B 0.171 F 0.171 C 0.171 G 0.170 D 0.163 H 0.171 I 0.176 Example 2 A body of 0.22mm ID can of DR tinplate with a tensile tensile load (UTS) of 460 Nmm-2 having a base profile similar to the DRD can of Comparative Example 2 and Figure 4 but having radii in E, F, G and H of lmm and a conical external wall, in a single pressing operation it uses ndo a lower edge that has the appropriate profile. This body was also cut open, giving the thickness data in table 4. Finally, an equivalent can body produced a peak data of 3.52 bar (51 psi). Table 4 All dimensions are in mm: A 0. 209 E 0. 215 B 0. 209 F 0. 209 C 0. 207 G 0 206 D 0. 199 H 0. 208 I 0. . 215 Example 3 DWI cans with a standard DWI base profile corresponding to that shown in Figure 5 were produced from a 0.12mm SR T57 tinplate. This caliber contrasts with the lower caliber for the SR material used to estimate the production that is 0.275mm (although it is believed possible to use tinplate of 0.27mm caliber with conventional processes). The tin coating was 2.8 / 2.8 gm "2 and matte finish The profile of figure 5 is that of the machining of the lower edge, forming the profile of a base using this machining that has a complementary profile. The profile of Figure 5 are given in Table 5. When the cans were formed in a conventional manner, that is, without fluid being introduced between the die and the embossed cup, there was a high incidence of splitting of the base. the remaining unscrewed cans were burst through the air outlet system, decreasing the pressure of the air outlet, preventing the bases from being blown resulting in the implosion of the cans during the overflow, when fluid was introduced to produce cans with the profile of figure 5 from the same tinplate, there was no incidence of splitting, bursting or implosion Table 5 All Dimensions are shown in mm Position Radius 1 1.21 2 to 6 1.4 Example 4 The DI cans with the standard DWI profile of Figure 5 were produced from 0.22mm DR tinplate having a tensile strength of 350 Nmm "2, in contrast to the conventionally used tinplate which has a resistance to the traction of 270 Nmm "2. The resistance produced was 423 Nmm "2 and the tensile breaking load (UTS) was 450 Nmm" 2. The elongation to the fracture was 15.8%, the test / UTS of 94.4% and the tin coating was 2.0 / 2.0. The bases of all the cans were formed without the introduction of fluid splitting. This was not surprising since it is well known that tinplate having a reduced gauge and increased tensile strength is more susceptible to splitting when formed. Despite this dissuasive aspect, the cans were formed from the above tinplate using hydraulic aid of the method of the present invention with the surprising result that none of the cans were split. Example 5 Cans with a conventionally high performance DRD base profile style, as shown in Figure 6, were produced from 0.285mm T57 tinplate. The curvature pressure for this profile was 76 psi, in contrast to a curvature pressure of 56 psi achieved for the same material that has the base profile of Figure 5. The radii for the profiles of Figure 6 are given in the Table 6. Table 6 All dimensions are shown in mm Position Radio 10 1.13 20 0.8 30 0.8 35 3.0 40 2.5 50 1.82 60 1.0