Patent Publication Number: US-2023150009-A1

Title: Method and apparatus for producing two-piece can bodies from a laminated metal sheet and a two-piece can body produced thereby

Description:
FIELD OF THE INVENTION 
     This invention relates to a method and apparatus for producing two-piece can bodies by drawing and ironing a laminated metal sheet, and more particularly to a processing method which prevents abrasion damage or scuffing of the laminate layer on the can body during its ironing, and a drawn and ironed two-piece can body produced thereby. 
     BACKGROUND OF THE INVENTION 
     Laminated metal sheet for packaging comprises a metal sheet and a laminate layer that covers one or both sides of the metal sheet where the laminate layer is produced by laminating the laminate layer onto the metal sheet by heat bonding or by direct extrusion onto the metal sheet. The laminate layer comprises one or more thermoplastic polymer layers. 
     Laminated metal sheet is used in the production of two-piece cans. Such a can consists of a can body comprising a base and a tubular body from sheet metal which is coated on at least one side with a laminate layer and a lid which is joined to the can body. 
     For the production of the can body a disc (usually circular) is produced from the laminated metal sheet, which disc is then deep-drawn into a cup which has a laminate layer at least on the outside, after which this cup is formed into a can body by wall ironing, the wall ironing taking place in a single stroke by punching the cup successively through a redraw ring and one or more wall-ironing rings (see  FIG.  1   ) using a punch in a drawing and ironing machine. The external shape of a punch is typically cylindrical, and thus rotation-symmetric, and may have the same diameter over the operative part, or the punch may have different diameters over the operative part, such as in JP2000042644, EP0402006, WO2019154743, GB2547016. 
     A separate punch typically is removably secured on the leading end of a reciprocating ram in a drawing and ironing machine. The punch provides an inner mandrel on which the can is shaped, drawn, and ironed as it passes through the one or more wall-ironing rings. The temperature of the punch increases due to the heat generated by the repeated frictional contact between the punch, the inside of the can body, and the one or more wall-ironing rings past which the punch moves. During wall ironing the shear forces can become excessively high in the laminate layer itself. This excessive shear results in an increased risk of damaging the laminate layer. One type of damage is so-called scuffing or abrasion, which damages the laminate layer and may result in direct contact between the metal substrate and the wall ironing tooling and/or a visually unacceptable laminate layer finish or, in very severe cases, rupture of the can body wall. Adequate lubrication between the wall ironing tooling and the laminate layer is important to prevent scuffing or abrasion damage, and this lubrication can be provided by the polymer layer itself (dry process). However, because of the deformation process the temperature of the metal sheet, the laminate layer, the redraw- and ironing rings and the punch rises. The risk of damaging the laminate layer increases if the temperature of the laminate layer increases. Consequently the temperature of the wall-ironing tooling must be kept below a critical value where the risk of damaging the laminate layer starts to occur, and this means that the production rate of the draw- and wall-ironing is limited thereby. This critical value is dependent on the composition of the laminate layer. In conventional can formation, externally applied cooling fluids maintain operational temperature conditions. However, in the dry DWI process no externally applied cooling fluids are used because the externally applied cooling fluids may contaminate the container surfaces which would then require post-forming cleaning processes that are costly and may be environmentally undesirable. 
     US2030084699 discloses a punch assembly comprising means to provide coolant to a circumferential channel that enables cooling the interior surface of the punch that is disposed at the end of a reciprocating ram in a drawing and ironing machine. However, application of this punch assembly is still found to cause abrasion damage and scuffing of the can bodies, especially at high reduction and high-speed production of can bodies. 
     OBJECTIVES OF THE INVENTION 
     It is an object of the invention to provide a method for producing can bodies for two-piece cans produced from laminated metal sheet without abrasion damage or scuffing of the laminate layer. 
     It is an also an object of the invention to provide a method for producing can bodies for two-piece cans produced from laminated metal sheet at higher speeds/reductions without abrasion damage or scuffing of the laminate layer. 
     It is also an object of the invention to provide a method for producing can bodies for two-piece cans without scuffing of the laminate layer at increased production speed. 
     It is also an object of the invention to provide an apparatus for producing can bodies according to the invention. 
     DESCRIPTION OF THE INVENTION 
     One or more of the objects is reached with a method according to claim  1 : A method for producing can bodies comprising a base and a tubular body for two-piece cans, from a laminated metal sheet by deep drawing and wall-ironing, wherein a disc is produced from the laminated metal sheet, which is deep-drawn into a cup, followed by redrawing the cup and subsequently forming the redrawn cup into a can body by wall ironing, wherein the wall ironing taking place in a single stroke by punching the redrawn cup through one or more wall-ironing rings by means of an internally cooled punch assembly, wherein the punch assembly comprises
         a ram ( 14 ),   a punch ( 1 ) which is, preferably removably, attached to the ram, the punch assembly comprising an internal annular cavity ( 15 ) below the surface of the punch between a position near the distal end of the punch ( 15   a ) and a position near the proximal end of the punch ( 15   b ),   a plurality of cooling fluid inlets ( 16 ) for supplying a cooling fluid into the internal annular cavity and a plurality of cooling fluid outlets ( 17 ) for removing the cooling fluid from the internal annular cavity, wherein the internal annular cavity is provided with means for improving the efficiency of the internal cooling of the punch,   wherein the means for improving the efficiency of the internal cooling of the punch consist of obstacles ( 18 ) in the internal annular cavity to increase the turbulence in the cooling fluid during its travel from the cooling fluid inlets to the cooling fluid outlets and to provide a larger cooling surface for extracting heat from the punch, wherein the obstacles consist of
           discontinuous obstacles ( 18 ) such as chevrons, cylinders, discontinuous walls or discontinuous zigzag walls, or of   continuous obstacles ( 18 ) in the form of a plurality of adjacent helical walls delimiting a plurality of helical cooling channels in the internal annular cavity to conduct the cooling fluid from the cooling fluid inlets to the cooling fluid outlets,   
           the ram comprising means for supplying cooling fluid to the cooling fluid inlets and removing cooling fluid from the cooling fluid outlets,
 
to efficiently internally cool the punch during the production of the can bodies to prevent abrasion damage or scuffing of the laminate layer on the tubular body of the can body, wherein
   a. the cooling fluid inlets are arranged nearer the distal end of the punch and wherein the cooling fluid outlets are arranged nearer the proximal end of the punch, preferably wherein the inlets and outlets are arranged in a regular pattern around the circumference of the punch, or wherein   b. the cooling fluid inlets are arranged nearer the proximal end of the punch and wherein the cooling fluid outlets are arranged nearer the distal end of the punch, preferably wherein the inlets and outlets are arranged in a regular pattern around the circumference of the punch, or wherein   c. the cooling fluid inlets of part of the helical cooling channels are arranged nearer the distal end of the punch and wherein the corresponding cooling fluid outlets are arranged nearer the proximal end of the punch, and wherein the cooling fluid inlets of the other helical cooling channels are arranged nearer the proximal end of the punch and wherein the corresponding cooling fluid outlets are arranged nearer the distal end of the punch, so that some of the helical cooling channels conduct cooling fluid from the distal end to the proximal end of the punch and the other cooling channels conduct cooling fluid from the proximal end to the distal end of the punch, preferably wherein the direction of the cooling fluid alternates from one helical cooling channel to its adjacent helical cooling channel, preferably wherein the inlets and outlets are arranged in a regular pattern around the circumference of the punch.       

     The invention therefore embodies three different variants: a, b and c. Variant a and b are different in their location of the inlets and outlets for the cooling liquid in the internal annular cavity with the discontinuous obstacles or the continuous obstacles (helical cooling channels). 
     Variant c relates only to the embodiment wherein the internal annular cavity is provided with continuous obstacles in the form of helical cooling channels. 
     Preferable embodiments are provided in the dependent claims. 
     The punch is attached to the end of the ram. Preferably the punch is removably attached to the ram. This means that the punch, which outside surface contacts the can body directly, can be replaced without the need to replace the ram, e.g. if the punch is worn or damaged. However, the invention is also embodied by the ram and the punch forming one integral part, i.e. wherein the punch is not removably attached to the ram, and wherein the internal annular cavity forms a cavity in the integral ram&amp;punch combination. If the punch portion is welded to the ram, then the punch is considered to form an integral part of the ram&amp;punch combination, because the punch cannot be easily removed from the ram any longer. In such a construction the ram&amp;punch combination must be replaced if the punch portion is worn or damaged. 
     In case the punch is removably attached then obstacles in the internal annular cavity preferably are part of the punch, and thus the obstacles are removed with the punch when the punch is removed. Less preferably the obstacles are formed directly on the ram, and thus the obstacles stay behind on the ram if the punch is removed from the ram. These two embodiments are morphologically identical when the punch is mounted on the ram. 
     The method according to the invention is based on the improvement of the internal cooling of the punch assembly by increasing the cooling efficiency. This is achieved by increasing the degree of turbulence in the cooling fluid being passed through the punch and by increasing the contact surface between the cooling fluid and the punch. The way the invention achieves the increase in turbulence in the cooling fluid and increasing the contact surface between the cooling fluid and the punch is by placing discrete or continuous obstacles in the internal annular cavity. The punch comprises an internal annular cavity that extends over a length of the punch just below the surface of the punch that comes into contact with the side walls of the can bodies during the wall ironing step. The distance between the surface of the punch and the internal annular cavity, i.e. the wall thickness, has to be thick enough to withstand the mechanical stresses of the deep drawing and wall-ironing process and maintain its dimensions, but also thin enough to maximise the heat transfer from the surface of the internal annular to the cooling fluid running through the cavity. Through this internal annular cavity the cooling fluid can be led from cooling fluid inlets at one end of the internal annular cavity to the cooling fluid outlets at the other end of the internal annular cavity. This way heat can be led away from the punch by the cooling fluid, and the surface temperature of the punch can be kept below the critical value to prevent scuffing or abrasion damage. In the absence of any obstacles in the internal annular cavity, which is the prior art situation, the cooling fluid flows between the cooling fluid inlets and outlets directly and laminarly. This means that the cooling fluid has little time to absorb heat, and also as a result of the laminar flow, the cooling capacity of the cooling fluid is not efficiently used. It is noted that usually the redraw ring and ironing rings also have internal cooling channels, which cool the outside laminate layer of the can body. However, the majority of the heat will be dissipated by the cooled punch, because this has a much longer time of contact and a much larger area of contact with the laminate layer. 
     The cooling fluid is not particularly limiting. Water, preferably demineralised water, has proven to be very suitable. Anti-corrosion agents may be added to the cooling fluid. 
     The obstacles break-up the flow of the cooling fluid and also increase the cooling surface of the punch, so the ability to pass on the heat from the punch into the cooling fluid increases both as a result of the increase in cooling surface and the increase in turbulence, because a turbulent flow is able to absorb more heat than a laminar flow can. 
     In an embodiment of the invention the discontinuous obstacles in the internal annular cavity may for instance comprise of pillars (cylindrical or otherwise), chevrons, discontinuous short walls which are perpendicular or angled to the flow of the cooling fluid through the cavity, or discontinuous zig zag walls. The external shape of the punch is preferably rotation-symmetrical with respect to the centreline of the ram. 
     The continuous obstacles prolong the contact time between the cooling fluid and the punch because the obstacles force the cooling fluid to take a longer path between the cooling fluid inlets and the cooling fluid outlets, and the contact surface of the internal annular cavity becomes larger as a result of the presence of the discontinuous obstacles in the internal annular cavity. Also, as a consequence of the path being longer due to the presence of the continuous obstacles, the area through which the cooling fluid has to flow becomes smaller, and this in turn increases the turbulence in the fluid. 
     The method according to the invention therefore results in a more efficient cooling of the punch and therefore of a lower surface temperature of the punch in comparison to the prior art punches. The cooler punch temperature results in lower laminate layer temperatures during the wall-ironing process, and thus prevents scuffing or abrasion damage in laminate layers prone to such damage, and enables increasing the production speed of the can bodies because the punch temperature at which the risk for scuffing or abrasion damage in laminate layers becomes prominent is reached at higher production speeds compared to the prior art situation. 
     In one embodiment of the invention (variant a) the cooling fluid inlets are arranged nearer the distal end of the punch and the cooling fluid outlets are arranged nearer the proximal end of the punch. Preferably the inlets and outlets are arranged in a regular pattern around the circumference of the punch. In another embodiment of the invention (variant b) the cooling fluid inlets are arranged nearer the proximal end of the punch and wherein the cooling fluid outlets are arranged nearer the distal end of the punch, preferably wherein the inlets and outlets are arranged in a regular pattern around the circumference of the punch. In variant a and b the means for improving the efficiency of the internal cooling of the punch consist of discontinuous obstacles or consist of continuous obstacles in the form of a plurality of adjacent helical walls delimiting a plurality of helical cooling channels in the internal annular cavity. 
     In another embodiment of the invention (variant c) the continuous obstacles are helical walls in the internal annular cavity thus forming helical cooling channels in the internal annular cavity. In this embodiment part of the cooling fluid inlets are arranged nearer the distal end of the punch and wherein the other cooling fluid inlets are arranged nearer the proximal end of the punch, so that some of the helical cooling channels conduct cooling fluid from the distal end to the proximal end of the punch and the other cooling channels conduct cooling fluid from the proximal end to the distal end of the punch, preferably wherein the direction of the cooling fluid alternates from one helical cooling channel to its adjacent helical cooling channel. 
     It is noted that helix is a shape like a spiral staircase. It is a type of smooth space curve with tangent lines at a constant angle to a fixed axis. A circular helix of radius a and slope b/a (or pitch  2   nb ) is described by the following parametrisation: 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 x(t) = a · cos(t) 
                 y(t) = a · sin(t) 
                 z(t) = b · t 
               
               
                   
                   
               
            
           
         
       
     
     According to the invention the number of helical continuous obstacles must be such that a plurality, and preferably at least three helical cooling channels are formed. It is preferable that each helical cooling channel is provided with its own cooling fluid inlet and its own cooling fluid outlet. The inventors found that three or more helical cooling channels leads to a very efficient cooling because the length of the channels is such that the cooling fluid is able to effectively and efficiently cool the work surface of the punch. With one or two cooling channels the efficiency of the cooling is significantly reduced and the danger of scuffing or abrasion damage of the laminate layers increases. Preferably the punch comprises at least four adjacent helical cooling channel, more preferably at least five adjacent helical cooling channel and even more preferably at least six adjacent helical cooling channels. The inventors found that six channels resulted in an optimal combination of cooling capacity without excessively complicating the design of the punch. Preferably the inlets and outlets for the cooling fluid are arranged in a regular pattern around the circumference of the punch, i.e. 60° between each inlet or outlet around the circumference for the six-channel embodiment, or 72° for a five-channel embodiment. Although it is possible to feed more than one helical channel with one inlet or bleed more than one helical channel with one outlet, it is preferable that each channel is fed with its own cooling fluid inlet and bled with its own cooling fluid outlet. Individual inlets and outlets for cooling fluid also allows to alternate the flow direction between adjacent helical cooling channels, thereby potentially achieving an even more homogeneous cooling of the punch. 
     The advantage of inlets being arranged at the distal end or the proximal end of the punch, and the corresponding outlets being arranged at the proximal end or the distal end of the punch means that the cooling liquid only travels directly from the inlet at one end of the punch to the other end so that the maximum cooling effect can be achieved. In prior art such as JP2006055860, JP2006-055860 and JP2005-288483 the cooling liquid has to travel up and down the punch because the inlets and outlets for the cooling liquid are all located at the proximal end of the punch. JP2006055860 discloses a punch with continuous zig-zag channels, whereas 3P2006-055860 and 3P2005-288483 show an embodiment with a single helical channel and embodiments with or continuous zig-zag channels through which the cooling liquid is led. 
     In prior art such as JP2006055860, JP2006-055860 and JP2005-288483 the returning warmed-up cooling liquid meets cooler cooling liquid thereby heating the incoming cooling liquid. This diminishes the cooling capacity of the incoming cooling liquid and thereby reduces the cooling efficiency of the cooled punch as a whole. So these prior art configurations have a very considerably reduced cooling capacity compared to the configurations according to the invention. 
     The external temperature of the punch can be monitored continuously, e.g. by direct contact or contactless measurement of the punch temperature or by monitoring the temperature of the cooling fluid entering and leaving the punch. In an embodiment of the invention the temperature of the punch assembly is controlled by means of a temperature control unit wherein the temperature control unit is able to control the temperature of the punch by adapting the speed of production of can bodies and/or by adapting the flow rate of the cooling fluid entering the internal annular cavity and/or by adapting the temperature of the cooling fluid entering the internal annular cavity. 
     In an embodiment the redraw ring and the one or more ironing rings also have internal cooling channels, which enable cooling the outside laminate layer of the can body during the deep drawing and wall-ironing. 
     The invention is also embodied in the punch assembly according to claim  6 . Preferable embodiments are provided in the dependent claims. 
     The punch according to the invention may be provided with the obstacles in two ways. Since the internal annular cavity provided with the discontinuous or continuous obstacles is quite complicated structurally, it is preferable to produce the punch by means of additive manufacturing, such as  3 D-printing. By means of additive manufacturing the punch with the external surface contacting (in use) the laminate layer of the can bodies, including the complicated internal structure in the internal annular cavity can be produced as one integral part, in one production step. For instance, the punch as depicted in  FIG.  4    may be produced in one production step by additive manufacturing so that the punch sleeve  19  and the insert  20  can be made as one part where the sleeve and insert are combined, and therefore inseparable, in one part, i.e. one integral part. The channels or obstacles in the internal annular cavity are produced simultaneously with the rest of the punch as the punch is being produced by additive manufacturing. The production of such a punch with the intricate shapes of the discontinuous obstacles in the internal annular cavity cannot be produced by classical machining in one part, i.e. one integral part. 
     As an alternative the punch can be produced from at least two parts: a punch sleeve and an insert which, when joined together form the internal annular cavity with the discrete or continuous obstacles, e.g. the adjacent helical channels. The insert with the obstacles can be produced by additive manufacturing (AM) wherein the insert also may comprise the cooling fluid inlets and the cooling fluid outlets from a material suitable for AM such as a tool steel, a cemented carbide, such as WC, or copper or a copper alloy. Alternatively the insert may be produced by machining the insert, for instance from a tool steel or another suitable material such as stainless steel, copper or a copper alloy. 
     The material of the punch with the integral internal structure in the internal annular cavity or the insert with the discrete or continuous obstacles in the internal annular cavity (after assembly with the punch sleeve) preferably is a material suitable for AM such as a cemented carbide, such as WC, or copper or a copper alloy. 
     The invention is also embodied in a can body produced in accordance with the method or apparatus according to the invention. 
     Laminated metal sheet for packaging comprises a metal sheet and a laminate layer that covers at least one side of the metal sheet. Such a laminated metal sheet is produced by laminating the laminate layer onto the metal sheet. The laminate layer may be applied to the metal sheet by heat bonding the laminate layer to the metal sheet or by using an adhesion promoter between the laminate layer and the metal sheet or by using a laminate layer comprising an adhesion layer. The laminate layer may be produced in-line and laminated onto the metal sheet in an integrated lamination step, or a pre-produced laminate layer may be laminated onto the metal sheet in a separate lamination process step. An alternative lamination method is to extrude a laminate layer by means of a flat die and laminate the laminate layer directly onto the metal sheet. 
     The ironing method of the present invention is particularly effective for ironing a metal sheet, selected from the group of metal sheets such as cold rolled steel, blackplate, tinplate, ECCS, TCCT®, galvanised steel or aluminium or aluminium alloy. The metal sheet is preferably supplied in coiled form. 
     The metal sheet is preferably coated on one or both sides with an organic resin selected from polyester, polyolefin, polyamide and other thermoplastic resins. The resin film to which the present invention is applicable may be a film formed by a single layer, or two or more layers, and is preferably a film of a thermoplastic resin, especially a polyester resin. 
     The polyester resin preferably has an ester unit such as ethylene terephthalate, ethylene isophthalate, butylene terephthalate or butylene isophthalate, and is preferably a polyester consisting mainly of at least one kind of ester unit selected therefrom. Each ester unit may be a copolymer, or the polyester may be a blend of homopolymers or copolymers of two or more kinds of ester units. It is also possible to use other ester units containing e.g. naphthalenedicarboxylic acid, adipic acid, sebacic acid or trimellitic acid as their acid component, or e.g. propylene glycol, diethylene glycol, neopentyl glycol, cyclohexanedimethanol or pentaerythritol as their alcohol component. 
     The polyester may be a laminate of two or more polyester layers composed of homopolyesters or copolyesters, or a blend of two or more thereof. For example, the polyester film may have a copolymerized polyester layer of high thermal adhesion as a lower layer, and a polyester or modified polyester layer of high strength, heat resistance and barrier property against corrosive substances as an upper layer. 
     The resin film preferably has a thickness of 5 to 100 μm and more preferably 10 to 40 μm when it is a single-layer film. Any film having a thickness below 5 μm is very difficult to laminate on a surface-treated steel sheet, is likely to give a defective resin layer upon drawing, or drawing and ironing and is unsatisfactory in impermeability to corrosive substances when a can is formed and filled with its contents. An increase in thickness gives satisfactory impermeability, but any thickness over 100 μm is economically a disadvantage. The proportions in thickness of the layers of a multi-layer film depend on formability, impermeability, etc., and the thicknesses of the layers are so controlled as to give a total thickness of 5 to 60 μm. 
     The resin film may be formed from a resin to which a colouring pigment, a stabilizer, an oxidation inhibitor, a lubricant, etc. have been added to the extent not impairing the necessary properties thereof. It is possible to use a metal sheet having a pigment-free polyester resin film laminated on its side supposed to define the inner surface of a can, while a polyester resin film containing a pigment, such as titanium oxide, is laminated on its side supposed to define the outer surface of the can. 
    
    
     
       DRAWINGS AND FIGURES 
       The invention is further described by means of the following, non-limiting drawings and figures. 
         FIG.  1    illustrates how a preformed deep-drawn cup  3  is formed into a finished wall-ironed can body  9 . The cup  3  is placed between a redraw sleeve  2  and a redraw die  4 . When punch  1  moves to the right, the cup  3  is brought to an internal diameter of the final finished can  9  by the redrawing step. Then, the punch  1  successively forces the product through (in this example) two wall-ironing rings  6  and  7 . Ring  8  is an optional stripper ring. Wall ironing provides the can body  9  to be formed with its ultimate wall thickness and wall length. Finally, the base of can body  9  is formed by moving punch  1  towards an optional base tool  10 . Retracting punch  1  allows to detach can  9  from the punch  1  so that it can be discharged in the transverse direction. The optional stripper ring may assist in this. The can  9  is then subsequently trimmed, optionally necked, flanged and provided with a lid after filling. 
         FIG.  2    provides a detailed illustration of the passage of a part of the can wall to be formed through, for example, wall-ironing ring  6 . Punch  1  is indicated diagrammatically. The entry plane for wall-ironing ring  6  runs at an entry angle α to the direction of the axis of the wall-ironing ring. The thickness of the material of the wall to be formed is reduced between punch  1  and wall-ironing ring  6 . This material comprises the actual metal can body wall  11  with laminate layers  12  and  13  on either side. The laminate layer  12  becomes the outside of the can body, and the laminate layer  13  becomes the inside of the can body, eventually coming into contact with the contents of the can. The figure illustrates how the thickness of all three layers  11 ,  12  and  13  is reduced. 
         FIG.  3    shows the punch  1  which typically is removably secured on the leading end of a reciprocating ram  14  in a drawing and ironing machine.  FIG.  3    shows the punch on top of the ram in one of the embodiments of the invention in detail. The internal annular cavity runs between  15   a  and  15   b  and is more prominently outlined in  FIG.  4    with the dashed boxes in the cross section of the punch. 
         FIG.  4    shows a cross-section of the punch with the continuous obstacles forming the adjacent helical cooling channels. In this figure the punch consists of a punch sleeve  19  and an insert  20 . The internal annular cavity formed by the joined punch sleeve and insert is filled with the continuous obstacles. 
         FIG.  5   a    shows the outside surface of the insert  20  with the helical continuous obstacles  18 . It shows six adjacent helical cooling channels (channel a-f), each with their own individual inlet and outlet for cooling fluid.  FIG.  5   b    shows a cross section of the same insert as used in  FIG.  4   . 
         FIG.  6    shows six different examples of the internal annular cavity: without obstacles (A: prior art); with discontinuous obstacles (B: short walls perpendicular to the flow of the cooling fluid; C: circular pillars; D: chevrons; E: zig zag channels); and with continuous obstacles (F: six adjacent helical cooling channels formed by the continuous obstacles forming the walls of the channels). The distance between the cooling fluid and the work surface of the punch is the same for all embodiments. 
         FIG.  7    shows the surface temperature of the various examples. It is clear that all the examples according to the invention provide a very homogeneous temperature profile along the length of the punch (running from about 40 to about 110 mm). The prior art shows a significantly higher surface temperature of the punch, showing the improvement that the invention is able to provide with regard to the surface temperature of the punch. The lowest surface temperature is achieved with the continuous obstacles forming the helical channels, irrespective of the flow direction of the cooling fluid in the channels (distal to proximal, proximal to distal or mixed). All embodiments of the invention show a significant improvement over the prior art. 
         FIG.  8    shows the effect of this lower surface temperature. The closed circles show the temperature during the production of can bodies at  165  cans/minute using the prior art annular internal cavity without obstacles. The triangles show the punch temperature at the same production rate and identical boundary conditions for the embodiment with the six adjacent helical cooling channels, and a steady state temperature of about 65° C. is reached compared to 95° C. for the prior art punch. This means that the production rate can be increased. In the example the production rate is increased 70% to  280  cans/minute and this leads to a maximum temperature of the punch of just below 90° C., which still is below the prior art situation at a much lower production rate. The cans made at an increased production rate according to the method of the invention were free of scuffing and no abrasion damage was observed. 
     
    
    
     The results of the discontinuous obstacles are only marginally less favourable compared to the continuous obstacles and also allow a significant improvement in surface temperature control and associated production rate improvement of about 60-65%.