Patent Publication Number: US-5829290-A

Title: Reshaping of containers

Description:
This is a continuation of application Ser. No. 08/621,795, filed Mar. 22, 1996, now abandoned. 
    
    
     This invention relates to a method and apparatus for reshaping of containers. In particular, it relates to the reshaping of containers such as metal cans. Reshaping of cans will also be referred to hereinafter as &#34;forming&#34;. 
     Pneumatic reshaping of containers such as three piece steel aerosol cans is known from GB-A-2257073 (&#39;073) which uses a split mould comprising sleeves around a liner to define the desired final shape. A mandrel acts as a space saver within the container to be reshaped and supplies air to the interior of the container within the mould to cause it to expand outwardly against the liner. Both ends of the container are held in position by slidable clamping members. As the container expands outwardly, either or both of the upper and/or lower clamping members is/are free to move inwardly to reduce thinning of the container body. This movement requires careful design to avoid any gaps into which the side wall of the container body might expand during reshaping. Only simple shapes with limited expansion are therefore possible with this apparatus. 
     A liquid forming method is proposed in U.S. Pat. No. 3,335,590 (Early) for the reshaping of tube blanks. That patent describes a split die which encloses the tube blank and comprises several segments positioned between a stationary top section and a moveable bottom section. The enclosed die position is within ±0.01 inch. A single piston creates an axial load on the tube blank to cause the die segments to close up whilst a liquid volume control system &#34;bulges&#34; the tube within the die. A balance system balances the axial load pressure and bulge pressure to maintain the enclosed die position. The patent describes a three part die which effectively has floating segments and splits between the segments. As the tube expands, it also shortens in height until the splits are closed up to maintain the enclosed die position when the final shape of the tube is reached. Early uses a hydraulic process which is much easier to control than forming using air, such as is described in GB-A-2257073. 
     Although the system of Early purports to be capable of forming complex parts in a controlled manner, in practice, the system is not capable of controlling the movement of the tube within the mould to the degree which is necessary in the reshaping of thin walled can bodies at commercially acceptable speeds. 
     Another liquid forming method is described in CH-A-388887 which uses a press assembly to provide an axial load to a hollow body in a two part mould. In a first forming steep, the press closes the mould and compresses the hollow member. A second step uses a piston which forces liquid into the hollow body to cause it to expand outwardly against the mould. 
     As with the Early patent, the use of a liquid results in contamination of the inner surface of the item to be reshaped. Furthermore, this system is not capable of forming complex shapes. 
     The basic principle of all the above methods is to force the wall of the tube/body to expand to take on the shape of the closed up cavity. As is noted in the first example (&#39;073), it is desirable to minimise thinning during the forming process. This is particularly the case with modern can bodies which are already made of material which is extremely thin in order to reduce raw material costs. Additionally, if the can is a two piece drawn and wall-ironed can, then the material of the wall-ironed can side wall is thinner than that of the neck region and has been subjected to work hardening. The more the material has been worked, the less strain it can take before fracture. Consequently, wall-ironed cans are even more susceptible to splitting during the forming process than are cans with a seamed side wall of constant thickness such as are described in &#39;073. 
     According to the present invention, there is provided a method of reshaping a hollow container comprising: placing a container blank into a chamber defined by a mould having three parts; supplying a pressurised fluid to the interior cavity of the hollow container to expand the container radially outwards onto the inner surface of the mould; and moving the mould parts towards each other from a first position in which the parts are spaced from each other by gaps which open into the could chamber, to a second position in which the gaps between the mould parts are reduced in size whilst still opening into the mould chamber. 
     As the container expands outwardly, a loss of height occurs. Since the gaps between the mould parts are not completely closed up at the end of the forming operation, it is ensured that height of the container is &#34;lost&#34; from the gap positions throughout most of the forming process. If the mould were to close up completely before the end of the forming process, then the side wall of the container may split due to excessive longitudinal tension. The initial gaps between the mould parts are typically set to a height which is greater than the expected height loss in the container after reshaping. Clearly this height will vary according to the degree of expansion required. 
     The gaps or &#34;split lines&#34; in the mould are advantageously positioned at the points of maximum expansion of the container. This limits the length of can side wall which will slide over the mould cavity wall during the process. Initially, as the pressurised fluid is introduced to the container cavity, the side wall moves outwards until it contacts the narrowest parts of the mould. In a simple shape, if the gaps are at the points of maximum expansion, the container material will not move on the points of contact with the mould during further expansion, since movement of material will occur where there is least resistance to such movement, i.e. where there is no contact with the mould. 
     In a more complex shape, once the container contacts the mould, the metal of the container tends to slide on the contact points with the mould, giving rise to local frictional forces. As these frictional forces increase, so does the longitudinal tension in the container side wall. Only minimal elongation in the side wall is then possible before splitting ensues. It is therefore beneficial to minimise longitudinal tension and frictional forces. By positioning two gaps at the points of maximum expansion in the mould of the present invention height can be lost throughout most or all of the forming process, so that longitudinal tension is limited. 
     In a preferred embodiment, the method further comprises applying a load to both ends of the container. Longitudinal tension is also kept to a minimum by this loading of the container during reshaping, since the load advantageously balances the cavity pressure to avoid any splitting of the wall. The load may be either a constant or a variable load as required by the shape desired. 
     Usually the load is applied by a pair of pistons which act both on the mould parts, to cause them to move towards each other and, simultaneously, provide a compressive force which reduces or overcomes the longitudinal tension in the container side wall. 
     The pistons may typically be actuated by fluid pressure, usually air pressure. This pressure may be applied independently or to any combination of the pistons and the container cavity. Preferably, a single air pressure supply is used for one of the pistons and the cavity. That supply is advantageously split for the piston and cavity as close as possible to the piston so as to minimise losses and to maintain the same pressure supplied to the cavity and piston. The cavity pressure and piston pressure are thus automatically balanced throughout the process and any variability in the supply pressure will not affect the process as much. By using two pressure supplies, it is possible to vary the pressure and the timings of pressurisation between the cavity and the pistons. As a result, the process becomes more versatile. 
     The pistons preferably act on an area which is the cross sectional area of the unformed container or slightly larger. If the pressure in the piston and the container is the same, the force from the piston cancels out the longitudinal force resulting from the internal pressure. 
     In a preferred embodiment, only contact of the expanded container with the mould wall prevents further movement of the pistons or other loading means. The pistons preferably will not reach the limit of their stroke before the container is fully reshaped. 
     The method may also comprise means for regulating the air flow to control the rate of pressure rise in the two pistons and the cavity. Flow regulation provides fine control of the pressure balance between the pistons which may need to be either different or matched according to the complexity of the shape required. 
     According to a further aspect of the present invention, there is provided a method of reshaping a two piece can into a shape having two or more enlarged regions, the method comprising: placing the container blank into a chamber defined by a mould having three parts spaced from each other by gaps which open into the mould chamber and each of which is at, or substantially at, the position of maximum expansion of one of the enlarged regions; supplying a pressurized fluid to the interior cavity of the hollow container to expand the container radially outwards onto the inner surface of the mould; and moving the mould parts towards each other as the can is being expanded. 
     According to a still further aspect of the present invention, there is provided an apparatus for reshaping a hollow container comprising: a mould having three parts defining a chamber to accommodate a container blank; means for supplying a pressurised fluid to the interior cavity of the hollow container to expand the container radially outwards onto the inner surface of the mould; and means for moving the mould parts towards each other from a first position in which the parts are spaced from each other by gaps which open into the mould chamber to a second position in which the gaps between the mould parts are reduced in size whilst still opening into the mould chamber. 
     This apparatus may advantageously be used to carry out either of the methods described above. 
    
    
     A preferred embodiment of the present invention will now be described, by way of example only, with reference to the drawings, in which: 
     FIG. 1 is a sectioned side view of an apparatus for reshaping a can body; and 
     FIG. 2 is a circuit diagram for a circuit to supply pressurised air to two pistons and a can cavity. 
    
    
     In FIG. 1 there is shown a mould 1 for reshaping (&#34;blow forming&#34;) a can body. The can body is a drawn and wall ironed can body having an integral base and side wall and necked at its upper open end. 
     The mould has three die parts 5, 6 and 7 which comprise neck ring, side wall and base support respectively. The die parts are separated from each other by gaps or &#34;split lines&#34; 10 and 11. For ease of machining, the base support die 7 is made in two parts, with a central part 8 supporting the base dome of the can body. The neck ring 5 provides simple support to the necked portion of the can body. These components together define a chamber 20 to receive the can body and are machined to the desired final shape of the can body after blow forming. 
     A pair of seal and support rings 15, 16 and a rubber sealing wing 17 are provided to seal the top edge of the container body. A space saving mandrel 22 passes through the centre of the seal and support rings to a position just above the base support dome 8. The mandrel 22 supplies air to the cavity of a can body within the chamber 20 via a central bore 24 and radial passages 26. The apparatus further includes an upper piston and a lower piston 30, 32 which together apply a load to both ends of the can in the mould chamber 20. Lower piston 32 is moveable upwards by means of a pressurised air supply which is fed to the piston via passage 35. Similarly, the upper piston is moveable downwards by means of a pressurised air supply which is fed to the piston via passages 36 and 37. In the preferred embodiment shown, the passage 36 is connected to the central bore 24 of the mandrel 22 so that the upper piston and can cavity share a common air supply. The common air supply is split for the piston 30 and cavity at the junction of the air passage 37 and the central mandrel bore 24, within the piston 30 so as to minimise losses and to maintain the same pressure supplied to the cavity and piston. The cavity pressure and piston pressure are thus automatically balanced throughout the process. 
     A schematic circuit diagram which shows how air is supplied to the pistons and can cavity is shown in FIG. 2. In the figure, the upper piston 30 and seal and support rings 15,16 are shown schematically as a single unit 30&#39;. Likewise, the base support 7,8 and lower piston 32 are shown as a single unit 32&#39;. Units 30&#39; and 32&#39; and neck ring 5 are movable, whereas the side wall die 6 of the mould is fixed. 
     The circuit comprises two pressure supplies. Pressure supply 40 supplies pressurised air to the top piston 30 and cavity of the can within the mould chamber 20. Pressure supply 50 supplies pressurized air to the lower piston 32 only. 
     The two supplies each comprise pressure regulators 42,52, reservoirs 44,54, blow valves 46,56 and exhaust valves 48,58. In addition, the lower pressure supply 50 includes a flow regulator 59. Optionally, the upper pressure supply 40 may also include a flow regulator, although it is not considered essential to be able to adjust the flow in both supplies. Reservoirs 44, 54 prevent a high drop in supply pressure during the process. 
     Typically, high pressure air of around 30 bar is introduced to the can cavity and to drive the top of the can. The air pressure to drive the bottom piston 32 is typically around 50 bar, depending on the piston area. The air pressure within the can cavity provides the force which is required to expand the can outwards but also applies an unwanted force to the neck and base of the can which leads to longitudinal tension in the can side wall The two pistons are thus used to drive the top and the bottom of the can, providing a force which counteracts (i.e. balances) this tension in the can side wall. 
     The pressure of the air supplied to the pistons is critical in avoiding failure of the can during forming due to either splitting or wrinkling. Splitting will occur if the tension in the can side wall is not counteracted by the piston pressure since the pressure is too low. Conversely, the pressure of the air supplied should not be so high that this will lead to the formation of ripples in the side wall. 
     For this reason, no stops are required to limit the stroke of the pistons. If the stroke were limited, the can might not be fully expanded against the mould wall before the pistons reached the stops. If this occurs, the tension in the can side wall would cease to be balanced by the piston pressure with a consequent risk of splitting. In effect, the contact of the expanded can with the side wall of the mould prevents further movement of the pistons. 
     It should be noted therefore that the balance between the can cavity pressure and the piston pressure must be maintained at all times throughout the forming cycle so that the rate of pressure rise in the cavity and behind the pistons must be balanced throughout the cycle. The rate of pressure rise can be controlled by the flow regulator 59 or by adjusting the supply pressure via the pressure regulators 44,54. 
     In order to form the can, the blow valves 46,56 are first opened. It is possible to have a short delay between the opening times of the blow valves if required to obtain a better match between the piston and cavity pressures but there will then need to be a higher rate of pressure rise for one circuit in order to maintain this balance. A delay can also be used to compensate for different pipe lengths, maintaining a pressure balance at the time of forming. The upper supply 40 is split for the piston 30 and cavity as close as possible to the piston 30 as described above for FIG. 1. 
     The apparatus is designed so that, at the latest, when each piston reaches its maximum travel the can is fully reshaped and the gaps 10,11 are not closed up at the end. Closing of the gaps leads to splitting of the can due to excessive tension in the side wall in the same way as does limiting movement of the pistons before full expansion has occurred. However, the final gap should not be excessive since any witness mark on the side wall becomes too apparent, although removal of sharp edges at the split lines alleviates this problem. 
     Once the reshaping operation is completed, the air is exhausted via valves 48 and 58. Clearly the exhaust valves are closed throughout the actual forming process. It is important that both supplies are vented simultaneously since the compressive force applied by the pistons to balance the cavity pressure (longitudinal tension) may be greater than the axial strength of the can so that uneven exhausting leads to collapse of the can. 
     EXAMPLE 
     Two piece can bodies were &#34;blow formed&#34; using the apparatus of FIG. 1 to give a maximum expansion of 8%. The relevant dimensions before and after forming are given in table 1 below. 
     
                       TABLE 1                                                     
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Dimension   Original (mm) After forming (mm)                              
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Can height  168           165.3                                           
Neck diameter                                                             
            62.16         unchanged                                       
Outside diameter                                                          
            65.953        8% max increase                                 
Upper split line                                                          
            2.3           0.375                                           
Lower split line                                                          
            1.15          0.375                                           
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