EP0932461A1

EP0932461A1 - Reshaping of containers

Info

Publication number: EP0932461A1
Application number: EP97945004A
Authority: EP
Inventors: Christopher Francis Cheers
Original assignee: CarnaudMetalbox SA; CarnaudMetalbox PLC; Metal Box PLC
Current assignee: Crown European Holdings SA; Crown Packaging UK Ltd
Priority date: 1996-10-19
Filing date: 1997-10-13
Publication date: 1999-08-04
Also published as: GB9621831D0; WO1998017416A1; AU4632497A

Abstract

The reshaping of containers such a metal cans by expanding the container against a mould (1) using pressurised air is described. The mould is of the longitudinally split type and typically comprises three mould parts (5, 6, 7) separated by transverse gaps. During the forming operation the or each gap is reduced in size as two of the mould parts (5, 6) move towards the third (7). Air bearings are used to allow these moving mould parts to move freely and simultaneously to clamp longitudinally split parts together.

Description

RESHAPING OF CONTAINERS

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 "forming". Pneumatic reshaping of containers such as three piece steel aerosol cans is known from GB-A-2257073 (O73) 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 permit the container to shorten and 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. Furthermore, complex bearings are required for this type of mould. Only simple shapes with limited expansion are therefore possible with this apparatus.

A liquid forming method is proposed in US-A-3335590 (Early) for the reshaping of tube blanks. That patent describes a die which encloses the tube blank and comprises several annular 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 "bulges" 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 annular segments and gaps between the segments. As the tube expands, it also shortens in height until the gaps are completely 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 '073. 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 die to the degree which is necessary in the reshaping of thin walled can bodies at commercially acceptable speeds. Since the die segments comprise complete rings, no complex bearing system is required such as would be necessary if it were a longitudinally split mould, as in '073. It is not possible to form a necked container without the use of a split mould since vertical mould joints are required to get the container in and out of the mould.

The basic principle of 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 ('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 endure 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 '0737

The existence of a necked region necessitates the use of a split mould with vertical mould joints to fit the can into and unload it from the mould. However, all mould joints tend to leave visible marks known as "witness" marks or lines on the wall of the formed can after it has been pressed outwards by pressurised fluid against the mould wall. Another problem with split moulds in particular is the risk of tearing and bursting if the can is pressed against a sharp edge of the mould. This limits the maximum pressure which can be applied by the pressurised fluid and thus the definition available, since sharp definition requires high pressure.

According to the present invention, there is provided a method of reshaping a hollow container comprising: placing the container blank into a chamber defined by a split mould having perpendicular joints which comprise one or more longitudinal joints and at least one transverse gap; closing the longitudinal joint or joints of the split mould around the container blank; 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 chamber; and moving at least one of the mould parts towards another from a first position in which the parts are spaced from each other by said gap or gaps, which open into the mould chamber, to a second position in which either or both of the gap or gaps between the mould parts are closed or at least reduced in size, said moving step comprising allowing the moving mould part or parts to move freely on one or more air bearings.

The use of air bearings eliminates any lubrication requirements and air bearings^'" are much more resistant to contamination than are conventional high capacity roller bearings. Roller bearings were hitherto believed to be essential to withstand forces generated by the internal fluid pressure whilst allowing the mould parts to move longitudinally and the load bearing capacity of roller bearings is much higher than that of air bearings. Air bearings, however, require minimal maintenance and have a long life due to the absence of metal to metal contact. In order to overcome the limited load bearing capacity of air bearings, a high pressure is preferable. This is advantageously of the same order as the pressure which is used to form the can.

Preferably, the closing step further comprises pneumatically clamping the mould parts together across the or each longitudinal joint. The air bearings may also provide this pneumatic clamping. Active clamping of the mould parts provides reliable holding together of the mould parts which in turn eliminates failure from splitting due to contact with a sharp mould edge. This enables the process window to be widened, offering a more reliable process and the option to use higher pressures. Use of higher pressures permits the mould to be filled faster, leading to machines with higher output per mould. Another benefit of active clamping is that visible witness lines are minimised. The use of higher pressures will also improve the quality of the finished can by permitting shapes with better detail and definition to be made.

The supply of pressurised fluid and the moving of the mould parts on air bearings occurs concurrently, so that the mould parts move at ^"the same time as being subject to forces generated by this forming pressure. The pressurised fluid may be air and this air may be supplied both to the interior cavity and to the air bearings.

For certain desired shapes, gaps between the mould parts may be reduced in size but not fully closed up, still opening into the mould chamber, at the end of forming process.

In a preferred embodiment, the method further comprises applying a load to either or both ends of the container. Longitudinal tension is avoided or at least 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 piston or a pair of pistons which act on either or both ends of the mould parts, to cause the mould parts to move towards each other and, simultaneously, provide a compressive force which reduces or overcomes the longitudinal tension in the container side wall.

The piston or pistons may typically be actuated by fluid pressure, usually air pressure. This pressure may be applied independently or to any combination of pistons and container cavity. Preferably, a single air pressure supply is used for one piston 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.

Each piston preferably acts 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 pistons or other loading means. The piston or pistons preferably will not reach the limit of its/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 a pair of pistons which may need to be either different or matched according to the complexity of the shape required.

Advantageously, the mould comprises three parts. In one embodiment, both the top and bottom are movable and two separate pistons are used. In another embodiment, the top and middle parts are moveable in which case the top part may be driven by a single piston.

As the container expands outwardly, a loss of height occurs. Preferably, the width of the gaps between the mould parts are selected so that the height of the container is "lost" from the gap positions throughout most of the forming process.

The gaps between the mould parts are advantageously positioned at the points of maximum expansion of the container. This limits the leάgth 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, ie 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.

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 split mould having three parts spaced from each other by transverse 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 pressurised fluid to the interior cavity of the hollow container to expand the container radially outwards onto the inner surface of the mould; and allowing two of the mould parts to move towards the third, as the can is being expanded, at least one of the moving parts moving freely on one or more air bearings.

According to a still further aspect of the present invention, there is provided an apparatus for reshaping a hollow container comprising: a split 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; means for moving two of the mould parts towards the third 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 closed or at least reduced in size whilst still opening into the mould chamber; and one or more air bearings for allowing the moving mould parts to move freely.

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: Figure 1 is a schematic sectioned side view of an apparatus for reshaping a can body;

Figure 2 is a schematic sectioned plan view of the apparatus of figure 1; Figure 3 is a circuit diagram for a circuit to supply pressurised air to a piston, air bearings and a can cavity;

Figure 4 is a schematic sectioned side view of a second embodiment of the present invention; and Figure 5 is a circuit diagram for the embodiment of figure 4.

In figures 1 and 2 there is shown a mould 1 for reshaping ("blow forming") a can body. The can body is a drawn and wall ironed (DWI) 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 10 and 11. These gaps permit the can to shorten during the forming operation, thereby reducing the risk of splitting the can wall.

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.

The mould is of the split mould type in order that a necked DWI can body can be fitted into the mould and removed from it after forming. Such a mould has a single longitudinal (vertical) joint so that the mould is separable into two halves. In the example shown, only the two upper mould parts 5, 6 have this vertical joint. In the three part mould shown, there are therefore only two perpendicular joints. A seal and support ring 15 and a rubber sealing ring 16 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 30 and a lower piston 32, which together apply a load to both ends of the can in the mould chamber 20 and cause the neck ring 5 and base support 7 respectively to move towards the stationary middle part 6. Lower piston 32 is moveable upwards by means of a pressurised air supply which is fed to the piston 32 via passages 35. Similarly, the upper piston 30 is moveable downwards by means of a pressurised air supply which is fed to the piston via passages 36 and 37. The middle part 6 is stationary in the example shown, parts 5 and 7 moving towards it when acted on by the pistons during forming.

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.

Upper mould part 5 is provided with a pair of air bearings 40,42 for each side of the mould part. The bottom mould part 7 is not split vertically and must be pushed up around the can base. Since the split middle is stationary and the base moves but is not split vertically, there is no need for air bearings for these parts. Air bearings 40,42 are supplied with pressurised air via longitudinal air passage 45 and radial holes 46,47 respectively. The source of this pressurised air may be the same as that for supplying the piston 30 and cavity 20 (see figure 3) . The air bearings 40,42 serve a dual purpose.

Firstly, they act as a simple bearing to permit free axial movement of the upper mould part 5 during forming, allowing the can to shorten. Secondly, the air bearings clamp corresponding halves of the split mould together across the vertical joint during axial movement, thus eliminating mould split failure and reducing any witness lines to a minimum.

Figure 2 shows a schematic plan view of the neck ring 5 and air bearings 40, 42 of figure 1. From this figure, it can be seen that each air bearing comprises a cushion of air and at each end of the air cushions there are small gaps 43, 44. Optionally, these gaps 43, 44 may be fitted with seals.

The projected area A_a of the air bearings is greater than the projected area A-, of the mould shape.

Consequently, when the same pressure is applied to both the mould cavity and the air bearings as described above, there is a net force pushing inwards and therefore clamping the mould parts together.

Figure 3 is a circuit diagram of the air supply to the mould cavity, pistons and air bearings of figures 1 and 2. In brief, the circuit comprises a pressure regulator 50 and reservoir bottle 51 via which pressurised air is supplied through "blow" valve 52 to pistons, cavity and air bearings. Regulators 54 and 55 control the flow of air to the upper piston and cavity, and to the lower piston respectively. Pressurised air is supplied to the air bearings along lines 60 and 62. An exhaust valve 53 expels the air after a can has been formed.

In an alternative embodiment, shown in figure 4, one of the air bearings for neck ring 5 has been replaced by a rolling element bearing 48 and the remaining air bearing 42 loads both of the neck ring split mould parts against the rolling element bearing.

In this embodiment, only an upper piston 30 is used and the middle mould part 6 is "floating" on a second air bearing 41 and roller bearing 48. Both neck and middle parts 5 and 6 are moveable on their air bearings during the forming operation. The base in this embodiment is stationary. Since only one piston is used, the force at the stationary base of the can is simply a reaction to the force applied at the top (ie equal and opposite) so that perfect balancing is achieved automatically.

When using a single piston, the can is pressed out by the pressurised air during the blowing operation until it contacts middle mould part 6. Since the middle part is free to move, the frictional forces between the can and the mould part 6 cause this part to follow the movement of the can.

In order to ensure that this middle part is correctly positioned at the start of the forming operation, small pneumatic pistons are used to fix the floating middle part in its start position, with gaps between it and the neck ring 5 and base support 7. This operation is the same as is used to position the top neck split mould parts in the embodiment of figure 1 and in standard moulds.

A schematic circuit diagram which shows how air is supplied to the piston, bearings and can cavity is shown in figure 5.

In both embodiment, the pressure of the air supplied to the piston 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 sufficient piston pressure. 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 piston. If the stroke were limited, the can might not be fully expanded against the mould wall before the piston 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 whole of the mould inner wall prevents further movement of the piston. The balance between the can cavity pressure and the piston pressure should be maintained at all times throughout the forming cycle so the rate of pressure rise in the cavity and behind the pistons must be balanced throughout the cycle.

Usually, the gaps 10 and 11 will not close up completely by the end of the shaping operation. Any final gap, however, 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.

Although not specifically described, it will be understood that other embodiments are also possible, derived from combinations of the embodiments described above .

For example, in the tooling shown in figure 1, one of the air bearings could be replaced by a roller bearing. This would give an assembly comprising two pistons 30 and 32, two fixed split mould parts 6 and two moving split mould parts 5, one on an air bearing, the other on a roller bearing.

Conversely, in the tooling shown in figure 4, either or both of the roller bearings shown could be replaced by air bearings.

Claims

CLAIMS :

1. A method of reshaping a hollow container comprising: placing the container blank into a chamber defined by a split mould having perpendicular joints which comprise one or more longitudinal joints and at least one transverse gap; closing the longitudinal joint or joints of the split mould around the container blank; 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 chamber; and moving at least one of the mould parts towards another from a first position in which the parts are spaced from each other by said gap or gaps, which open into the mould chamber, to a second position in which either or both of the gap or gaps between the mould parts are closed or at least reduced in size, said moving step comprising allowing the moving mould part or parts to move freely on one or more air bearings.

2. A method according to claim 1, in which said closing step further comprises pneumatically clamping the mould parts together across the or each longitudinal joint.

3. A method according to claim 1 or claim 2, in which the air bearings provide the pneumatic clamping

4. A method according to any one of claims 1 to 3, in which the supply of pressurised fluid and the moving of the mould parts on air bearings occurs concurrently.

5. A method according to any one of claims 1 to 4, in which the pressurised fluid is air.

6. A method according to claim 5, in which the pressurised air is supplied both to the interior cavity and to the air bearings .

7. A method according to any one of claims 1 to 6, in which the gaps still open into the mould chamber at the end of the forming process.

8. A method according to any one of claims 1 to 7, in which the mould comprises three parts, the bottom part being fixed axially.

9. 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 split mould having three parts spaced from each other by transverse 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 pressurised fluid to the interior cavity of the hollow container to expand the container radially outwards onto the inner surface of the mould; and allowing two of the mould parts to move towards the third, at least one of the moving parts moving freely on one or more air bearings, as the can is being expanded.

10. An apparatus for reshaping a hollow container comprising: a split 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; means for moving two of the mould parts towards the third 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 closed or at least reduced in size whilst still opening into the mould chamber; and one or more air bearings for allowing the moving mould parts to move freely.