METHOD FOR EXPANDING A CYLINDRICAL METALLIC PRECURSOR IN THE MANUFACTURE OF A THREE-PIECE CAN AND SHAPING MANDREL FOR EXPANDING A CYLINDRICAL METALLIC PRECURSOR

Description

FIELD OF THE INVENTION

The present invention relates to methods for shaping cylindrical metallic precursors used for manufacturing cans and to cans so produced, in particular for packaging powdered products such as infant nutrition and milk formula. The present invention further relates to an apparatus suitable for performing such methods.

BACKGROUND ART

Metal cans have been used for the packaging of powdered materials for many years as they may be sealed for long term storage and are relatively easy and cheap to produce. The aesthetics of such metal cans play a huge role in the customer experience. The shape of a can may be both attractive to the eye, as well as providing an actual function, such as a hand grip. Various can constructions are known, including what are commonly referred to as two-piece cans and three-piece cans. In the case of two-piece cans, the base and sidewall are produced in one step from a single piece of metal, usually be a combination of deep drawing and necking. Three-piece cans generally comprise a cylindrical sidewall with a longitudinal seam and two ends that are connected to the sidewall by a single or double folded seam.

Although two-piece and three-piece cans bear many similarities, the fundamental differences in their production lead to significant differences. Two-piece cans generally have no longitudinal seam and may be subjected to significantly higher distortion forces and pressures during manufacture. In a three-piece can, the longitudinal seam will always be a point of asymmetry, which will distort differently to other areas of the circumference. Expansion of the can may be limited by the strength of this seam. Due to the greater deformations common in two-piece can production, this is usually only applicable to aluminium or softer alloys. Three-piece cans are more generally of steel. Another important difference is the need to trim the upper end of two-piece cans prior to applying the closure. Due to the longitudinal extension during the deep-drawing process, the upper edge of the can body may no longer be level. To achieve the required tolerance for correct seaming, the upper ends of such can bodies are usually trimmed, requiring an additional step. Such cutting processes introduce metal particulates into the production environment and may be less preferred for certain sensitive goods. Three-piece cans need not be drawn in the longitudinal direction and thus can remain well within the tolerances acceptable for end-seaming.

To create a specific body shape for a three-piece can, an expansion shaping process may be used, in which a cylindrical mandrel is expanded, stretching portions of a cylindrical precursor from an initial diameter to a larger diameter. Such mandrels typically comprise a plurality of longitudinally extending segments disposed radially about a longitudinal axis of the mandrel. The segments have arcuate surfaces which engage an inner surface of the precursor as the mandrel expands by action of wedges or cam surfaces. It will be understood that the depth of any profile, relief or contour in the final can will be dependent on the amount by which the can is stretched.

In existing procedures attempting to achieve a high degree of stretching, the final body shape has been found to show vertical split lines at the gaps between the segments of the mandrel. These split lines may be present at different locations around the full circumference of the body. The split lines may be visible with the naked eye and are detrimental to the aesthetics of the final can, especially on otherwise smooth or unicolour surfaces. The split lines form due to local stretching of the metal sidewall over the edges of the segments. This is exacerbated as the segments move further apart and the unsupported sidewall between segments forms flat strips that no longer follow the same curvature as the arcuate surfaces. Often, a soft, more ductile steel grade needs to be used to avoid problems of cracking at the location of these split lines. This has the effect of making the final package less strong and more vulnerable for dents.

Patent publication DE102011100506A1 describes a method for expanding a cylindrical tube, in particular to produce tin cans that are circular in cross section. In a first expansion step, shaped lamellae are expanded outwards by an actuator arm provided with wedge surfaces, with the tube being expanded to the desired shape. The shaped lamellae are then moved back again, so that they are no longer in contact with the inner wall of the tube. The shaped lamellae are then rotated around their longitudinal axis with respect to the tube. In a second expansion step, the shaped lamellae are again moved radially outward to the same extent as in the first expansion step. In this second expansion step, the shaped lamellae shape the areas of the cylinder which had no direct contact with the shaped lamellae at the end of the first expansion.

Such existing two-stage methods of expansion are cumbersome as they require a rotation step, involving additional time and tooling. Even then, such methods may be unable to achieve high degrees of expansion without the previously mentioned cracking problems occurring at the end of the second stage. Additionally, rotation of the mandrel is unsuitable for producing certain profiled surfaces, since the rotation of the mandrel may upset the intended position of the profile. In general, only rotational symmetric shapes can be made with this process.

It would be desirable to provide alternative methods for expanding cylindrical metallic precursors, in particular suitable for manufacture of three-piece cans for both symmetrical and asymmetrical designs.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a method for expanding a cylindrical metallic precursor in the manufacture of a three-piece can, the method comprising the steps of: arranging the precursor, having an initial diameter, around a shaping mandrel, the shaping mandrel having a plurality of longitudinally extending segments disposed radially about a longitudinal axis of the shaping mandrel, the segments having arcuate surfaces for engaging an inner surface of the precursor wall; actuating, in a first step, a first subgroup of the plurality of segments to expand outwards to engage with the inner surface of the precursor wall and expand at least a portion of it to a first diameter that is larger than the initial diameter of the precursor; and actuating, in a second step, a second subgroup of the plurality of segments to expand outwards and also engage with the inner surface of the precursor wall whereby the second subgroup expands at least a portion of the precursor to a second diameter that is larger than the first diameter. The method allows a greater degree of expansion of the wall of a cylindrical metallic container, whereby greater variations in diameter can be achieved between recessed portions and the remainder of the wall.

In the following, reference to recessed portions will refer to areas of the container wall that have deliberately been expanded less than other areas of the wall for the purpose of producing a profiled outer surface. These recessed portions may still be at a position corresponding to the initial diameter or may have been expanded but by a lesser amount than the remainder of the wall. The method is particularly suited to welded steel containers made of high grade steel. In the past, high degrees of expansion could only be achieved by the choice of more ductile steels in order to avoid problems of cracking. The possibility of using such stronger steels allows better performance in top load, side load and dent resistance of the final packaging. This enables a lower wall thickness for the can. Because individual segments expand outwards in different steps, a better distribution of the stretch forces in the metal is created. As a result of this, the final can may be up to 25% thinner than existing cans: where regular cans are made of 0.24 to 0.25 mm wall thickness, a thickness of 0.18 to 0.20 mm can be used Furthermore, the precursor can be expanded by up to 30% of its initial diameter without cracking.

The segments may be distributed into any suitable subgroups that allow for the desired expansion procedure in one stroke of the tool. There may be more than two subgroups and not all subgroups need to be equal in number. Nevertheless, in one preferred embodiment, the first subgroup consists of alternate segments about the longitudinal axis of the shaping mandrel and the second subgroup consists of all remaining segments. In this case, the number of segments in each subgroup will be identical. Expanding the body first with only half of the segments at once and then subsequently with all of segments, allows a better distribution of the stretch forces during the critical final stages of expansion.

The segment groups may be actuated to expand at different velocities, depending on their initial positions and the stage of overall expansion reached. The skilled person will be familiar with the actuators and wedges required for such expansion. The absolute rate of expansion of the precursor wall may be greater during the initial stages of expansion than at the final stages of expansion. The relative velocities of the different subgroups will also vary. In an embodiment, during the first step, the segments of the second subgroup may be actuated to expand outwards at a velocity that is at least slightly greater than a velocity at which the segments of the first subgroup expand outwards. This is because they will generally be initially retracted further than the segments of the first subgroup and thus need to catch up with them before participating in the expansion of the wall in the second step. In this context, reference to the first step is intended to refer to the portion of the expansion, where only the segments of the first subgroup engage the inner surface. Reference to the second step is intended to refer to the portion of the expansion in which both subgroups are in contact with the inner surface. Thus, in the first step, contact with the wall is limited to one subgroup of segments while the second subgroup remains void of contact with the wall. Nevertheless, it should be understood that despite the reference to two steps, these are merely temporal stages in the operation and the process can be otherwise continuous without requiring removal or re-adjustment of the mandrel between the steps.

In the second step, the first subgroup also expands from the first diameter to the second diameter. In this manner, all longitudinal portions of the precursor contacting the segments are enlarged to a cylinder with all of its circumference at the second larger diameter. The method therefore allows to expand most or all of a precursor to a cylinder of a larger diameter, that being the second diameter.

Once the segments of the second subgroup have caught up with the segments of the first subgroup, all the segments may be actuated to expand outwards at the same velocity during the second step. The two subgroups of segments may alternatively be expanded at different velocities. The skilled person will however understand that the precise speed of expansion will depend on the design of the cam or wedge surfaces used and the interrelation between the groups of segments.

For the sake of the present description, the second step may be considered to start when the second subgroup contacts the precursor wall. It will nevertheless be understood that the actuation and movement of the subgroups may be continuous. In an embodiment the second step may comprise an initial stage where the arcuate surfaces of the second subgroup are engaged with the inner surface of the precursor yet still radially inwards of the arcuate surfaces of the first subgroup. At this stage, only a part of the arcuate surfaces of the second subgroup contacts the precursor wall.

In an embodiment the second step may comprise an intermediate stage where the arcuate surfaces of the second subgroup are radially aligned with the arcuate the first subgroup. This intermediate stage may commence from a position where the edges of the arcuate surfaces of adjacent segments are initially engaging i.e. the arcuate surfaces form an almost complete circumference without gaps. It will be understood that although the circumference may be complete, it may not be a perfect circle, since the individual segments may have curvatures of a slightly larger diameter than the precursor at this point of expansion. After an initial stage of the second step where a central part of the second subgroup of segments has engaged the precursor, a point will be reached where the complete surface of all segments will engage the precursor wall. Thereafter, all segments may be expanded to a final and almost uniform curvature that corresponds with the arcuate surfaces of the second subgroup. During this intermediate stage (which may also be the final stage), the segments will move slightly apart, leaving gaps between the adjacent segments.

In an embodiment, the second step may comprise a final stage, following the intermediate stage, where the arcuate surfaces of the second subgroup move radially outwards beyond the arcuate surfaces of the first subgroup. This final step can provide for a slight overstretching of the precursor to remove any remaining marks caused by the edges of the segments of the first subgroup during the first step.

Although other diameters may be considered, the precursor may be expanded in the first step to a first diameter that is preferably between 10% and 20% greater than the initial diameter of the precursor. A first step with an expansion to a first diameter in this range can generally be achieved without unacceptable levels of stress on the precursor walls at the edges of the segments, in particular when working with high-grade steel.

After the second step, a circumferential gap can be present between edges of the arcuate surfaces of adjacent segments. The manner in which expansion is achieved and the number of segments, allows the width of this gap to be limited. In embodiments, the gap may not exceed 5 mm, preferably it does not exceed 2 mm and more preferably it does not exceed 1.5 mm. As a result, the visibility of vertical split lines in the final form may be prevented or avoided.

In an embodiment, the second diameter is preferably between 20% and 30% greater than the initial diameter of the precursor. The method and its associated apparatus provides for a larger expansion of a precursor than may normally be possible for high grade steels. This gives more scope for upgrading the aesthetics of a can made of a durable material. Expanding only part of the surface of the container in a first step while carefully providing an ideal force distribution of the material in the second step, ensures optimum expansion without detrimental split lines or other distortion. Thereby an optimally expanded container body with a smooth outer surface with no flaws may be obtained.

In an embodiment the first and second steps may be performed without rotation of the precursor with respect to the mandrel. The method of the invention prevents the necessity to rotate the precursor, at any time during the process. This is more time efficient and reduces potential errors in realigning the precursor for further expansion. Furthermore the expansion can be obtained with a relatively simple and single apparatus. It is however not excluded that a slight rotation could be provided in order to further avoid the presence of split lines or to enhance a particular intended contour or profile.

The method may make use of arcuate segment surfaces that all have the same curvature at a given longitudinal position along the longitudinal axis and/or which have a constant curvature at all positions along their longitudinal extent. Nevertheless, designs with differently shaped surfaces may also be considered. Arcuate surfaces of one or more of the plurality of segments may be profiled in the longitudinal direction such that after the second step, the expanded precursor body has a profiled outer surface. The method can thus provide for an expanded cylinder with a gripping portion and/or any pattern that is satisfactory to the customer. In order to avoid variations in the upper and lower edges of the precursor body, each segment may have the same longitudinal profile length despite variations in the actual profile. In this way, trimming of the ends of the expanded precursor can be avoided. In this context, longitudinal profile length is the path length when following the surface of a segment (or the precursor body) from a first reference position to a second reference position corresponding to the top and bottom of the can. In general, it may be desirable that the upper and lower ends of the expanded precursor do not deviate by more than 0.5 mm from a flat plane perpendicular to a longitudinal axis of the body, preferably less that 0.25 mm or less than 0.1 mm.

In an embodiment, at least a first segment may have a different longitudinal profile from a second segment or all segments may have different longitudinal profiles, while all segments have the same longitudinal profile length. The skilled person will recognise that various appealing can designs can be created while respecting the geometric requirement that every longitudinal section of the wall should have equal length. In particular, designs that are non-rotationally symmetric about the longitudinal axis of the can may be created.

The longitudinal profile may comprise, at an end portion of all of the segments, a curve towards the longitudinal axis of the shaping mandrel. This profile of the segments allows a bottom or top portion of the can to remain at or near the initial diameter, or at least to maintain a diameter that is significantly smaller than the second diameter. This can be useful in ensuring a smaller base for stacking purposes or for reducing the presence of a sharp corner at the base. This curvature at the base is advantageous for access with a scoop e.g. having a corresponding curved shape. At the upper end of the can, a smaller diameter can facilitate the connection with a lid assembly, allowing the outer circumference of the lid assembly to remain within the outer circumference of the can.

The overall design of the expanded precursor at termination of the second step will generally depend on the combined outer surfaces of all of the segments and the degree to which they have expanded. The expanded precursor body may have a profiled outer surface with portions of the outer surface having the second diameter and other portions of the outer surface being recessed with respect to the second diameter by at least 10%, 12% or 15% and preferably at least 20%. The recessed portions may form a design or pattern in relief with respect to an otherwise uniform outer surface. In this context, uniform is intended to denote a constant diameter cylindrical surface i.e. 2D curved. It is however not excluded that an external tool may also be provided for engaging an outer surface of the precursor, in particular for exerting a force to recess portions of the outer surface inwards e.g. into contours of the arcuate surfaces of some or all of the segments.

A can may be manufactured by expanding a metallic cylindrical precursor by the method described above followed by: attaching a base to one of two open end portions of the expanded precursor body. In the present context, reference to manufacture of a three-piece can is not intended to require that three pieces are necessarily combined together but merely to the technique for forming a side wall without ends. In embodiments, only the base may be attached in a manufacturing step and an upper open end may be folded over or otherwise provided with a rim and closed with a separate closure. Nevertheless, in preferred embodiments the method may further comprise attaching a separate rim to the other of the two open end portions to form a three-piece can. Thereby a closed container with sufficient volume and attractive shape may be formed, for preservation of products, in particular infant nutritional products.

Attachment of the base and/or rim preferably takes place without first trimming the end portions of the expanded precursor body. As discussed above, the expanded precursor preferably has a constant and accurate length around its circumference within the tolerances for seaming, whereby trimming is not required. It will be understood that avoiding trimming is desirable, since any such procedure could create metal debris. In the past, length variation and the need for trimming has also been avoided by retaining or clamping the ends of the can during expansion. This however can lead to other drawbacks such as wrinkling and/or cracking and imposes further limitations both on equipment and design. The present solution also avoids the need for clamping of the can ends during expansion.

The invention also includes the three-piece can as described above and hereinafter. The can body may have an outer diameter and a profiled outer surface, wherein a first portion of the outer surface is recessed by at least 10% or 12% or 15% and preferably by at least 20% with respect to a second portion of the outer surface having the outer diameter. The method allows for a large choice of designs, with a high expansion threshold, preferably up to 30% of the initial diameter even when using thin, high strength steel as discussed above. In preferred embodiments, highly anisotropic steel may be used that has little directionality, such as tin plated steel (TPS) TS275 according to EU Packaging Steel standard EN10202, preferably having a Yield/0.2% Proof strength (Rp) of between 225 and 325 N/mm2.

In a particular embodiment parts of the first portion and the second portion can be located at a same longitudinal position of the outer surface i.e. at the same height on the can wall. The can outer design may be rotationally asymmetric about the longitudinal axis. Instead, reflectional symmetry with respect to a chosen face of the can may be achieved, which is highly desirable for branding purposes. This is in particular achievable if each longitudinal section of the can body has the same longitudinal profile length.

In one desirable embodiment, the profile comprises a circumferential recessed groove that surrounds the can, wherein a longitudinal position of the groove varies around the circumference. The groove may have a constant shape i.e. cross-sectional shape, around the circumference or may vary in shape e.g. depth and width around the circumference. Nevertheless, this variation should remain within the overall requirement that each longitudinal section of the can body has the same longitudinal profile length.

In certain embodiments, the first recessed portion may be a minor part of the can outer surface and the second portion having the outer diameter may be a major part of the outer surface. The major part may be more than 50% or more than 60% or more than 70% or more than 80% of the can outer surface. This major part may be generally flat i.e. with constant curvature corresponding to the maximum outer diameter

According to another aspect of the invention, there is provided a shaping mandrel for expanding a cylindrical metallic precursor in the manufacture of a three-piece can. The mandrel may comprise an actuator arm and a plurality of longitudinally extending segments, arranged about the actuator arm, each of the segments having an inner cam surface and an arcuate outer surface for engaging an inner surface of a precursor positioned over the mandrel. The segments each have a longitudinal profile and in an embodiment, at least a first segment may have a different longitudinal profile from a second segment, while all segments have the same longitudinal profile length.

In certain embodiments all segments may have different longitudinal profiles but the same overall longitudinal profile length. For example, each segment may have a generally flat profile with a recessed channel and the longitudinal position, cross-sectional shape and/or orientation of the recessed channel may vary from segment to segment around the circumference. In this manner, a groove or contour may be created all around the body of a can formed on the mandrel, without causing variations in the overall length of the can around its circumference.

Additionally or alternatively, the segments comprising a first subgroup of segments and a second subgroup of segments, wherein the actuator arm is longitudinally movable relative to the segments and has a plurality of wedge surfaces, arranged to contact the cam surfaces of the respective segments and move the first subgroup and the second subgroup radially outwards at respective different speeds in a series of steps to expand the mandrel from a first state to a final state. The shaping mandrel of the invention allows expansion of a cylinder of high-grade steel in a single process, which provides better control of stretching forces on the material.

The first subgroup may consist of an even number of segments equal to or greater than 6, preferably equal to or greater than 8 or even 12 or more. The second subgroup may consist of an identical number of segments. By increasing the number of segments, better distribution of forces may be achieved and, for a given expansion, the overall gap between adjacent segments at completion of the operation can be reduced. A total of up to 24 segments may thus be present.

The plurality of wedge surfaces may comprise a first set of wedge surfaces radially aligned with segments of the first subgroup and a second set of wedge surfaces radially aligned with segments of the second subgroup. With each segment having an associated wedge surface, control of the expansion of each segment can be achieved. As a result each step of the method of expansion of the invention can be accurately performed. The wedge surfaces may be separate surfaces on a single wedge body or may be formed by multiple wedge bodies assembled together. It will also be understood that wedge surfaces may be arranged in tandem in the longitudinal direction in order to provide a balanced expansion force at two points along the longitudinal axis for each segment.

Each of the plurality of wedge surfaces may be defined by a wedge angle relative to the longitudinal axis of the shaping mandrel. A wedge angle of the first set of wedges may be constant over the longitudinal movement of the actuator arm and the wedge angle of the second set of wedges may also be constant but higher than the wedge angle of the first set of wedges. In an embodiment, this wedge angle may be initially higher than and subsequently equal to the wedge angle of the first set of wedges. By controlling the wedge angle of the second set of wedges to be different and larger than the angle of the first set, the second subgroup of segments may catch up with the first subgroup of segments.

In an embodiment the first subgroup of segments may be undercut, whereby the second subgroup of segments can be recessed behind the arcuate outer surfaces of the first subgroup in the first step. Thus, the overall initial diameter of the mandrel can be reduced for insertion into the precursor. Also, on termination of the expansion process, the mandrel can be collapsed again for removal, even if some regions of the precursor are still at or close to the initial diameter.

The arcuate surfaces of the segments of the first and second subgroups may have different sizes. In particular, the segments of the first subgroup may be larger than those of the second subgroup. In a preferred embodiment, the arcuate surfaces of the first subgroup and the second subgroup may be equal in size. This ensures that in the final critical expansion step, the inner surface of the precursor is subject to balanced expansion and any gaps between segments are equally spaced.

It will be understood that the ability to provide significant expansion gives considerable scope for forming cans with different aesthetic, functional and ergonomic forms, while still being possible to extract the mandrel after completion of the expansion step. In certain designs, it may be desirable to only fully expand minor regions of the can outer surface to a maximum extent, leaving major regions with a lesser expansion. In general, in the final state, a major part of the arcuate surfaces of all the segments may have a diameter corresponding to a maximum outer diameter of the can and a minor part may be recessed by at least 10%, preferably 15% and more preferably 20%, with respect to the outer diameter. For the sake of definition, where reference is given to the diameter of the can, this will thus refer to the maximum outer diameter. The major part may be more than 50% or more than 60% or more than 70% or more than 80% of the can outer surface. This major part may be generally flat i.e. with constant curvature corresponding to the maximum outer diameter.

An additional advantage of such high expansion is that for the same volume of can, less material is needed. The material reduction may be at least 2.5%, or 5% or even 7.5% of the total weight of the tin.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be described by way of example, with reference to the attached drawings, in which:

FIG. 1A shows a flowchart representing a sequence of steps for manufacture of a three-piece can from a metallic blank and comprising an expansion step,

FIG. 1B shows an illustration of a top and a side wall of a cylindrical metallic precursor expanded according to methods known in the art,

FIG. 2A shows a cross-sectional view of an expansion apparatus, for expanding a cylindrical metallic precursor, the apparatus shown in a contracted state before expansion, with the precursor positioned around the apparatus,

FIG. 2B shows a cross-sectional view of the expansion apparatus of FIG. 2A, after completion of expansion,

FIG. 3 shows a cross-section through the mandrel of FIG. 2A at position III-III,

FIGS. 4A to 4C show a longitudinal-section taken at position IV-IV in FIG. 3 taken at different stages during expansion of the mandrel;

FIG. 5A shows a cross-sectional view of two adjacent segments in the mandrel of FIG. 3 prior to a first expansion step,

FIG. 5B shows a cross-sectional view of two adjacent segments in the mandrel of FIG. 3 at the start of a second expansion step,

FIG. 5C shows a cross-sectional view of two adjacent segments in the mandrel of FIG. 3 at a further stage of expansion,

FIG. 5D shows a cross-sectional view of two adjacent segments in the mandrel of FIG. 3 at a final stage of expansion,

FIG. 5E shows a cross-sectional view of two adjacent segments in the mandrel of FIG. 3 at an alternative final stage of expansion, and

FIG. 6-8 show three exemplary container bodies expanded according to the method of the invention.

DESCRIPTION OF EMBODIMENTS

FIG. 1A shows a sequence 100 of steps 110-160 for manufacture of a three-piece can from a metallic blank 1. This sequence comprises in particular an expansion step 140. Other steps will be known to the person skilled in the art and only briefly described. In step 110, a metallic sheet is cut into a flat blank 1. In step 120, the blank may be coated and cured. After this optional step, the flat blank 1 is welded into a generally cylindrical precursor 2 in step 130. The precursor 2 has a metallic body with two open ends and a vertical seam (not shown) present where the sidewall has been joined to itself to form a cylinder. The metallic body of the cylindrical precursor 2 is generally a steel body, prepared from a steel blank. In step 140 a portion of the wall of the cylindrical metallic precursor 2 is expanded outward. The resulting expanded container 3 has a shape that is contoured and deviates from the original cylindrical shape. This step may be performed according to the method of the invention, described later. In step 150 the expanded container 3 may be subject to any of flanging (F, as shown) and/or other processes known in the art comprising for instance any of printing or beading (not shown). In step 160 the expanded container 3 may be provided with closures 4, 5 attached at each of the two open ends with or without sealing and/or pre-filling. This terminates the manufacturing cycle of the three-piece can. It is noted that this sequence is serving merely as an example and the expansion step of the invention may also be part of other sequences known in the art for manufacturing a three-piece can. It will however be noted that in the illustrated embodiment, there is no requirement for trimming of the container ends prior to closure step 160.

FIG. 1B shows an illustration of a top and a side view of a cylindrical precursor 2 of diameter d before and after expansion into an expanded body 3 according to expansion methods known in the art. The illustration of the expanded body exaggerates the shape of the precursor in order to illustrate the problem of expansion methods known in the art. Instead of a round body of diameter D the surface of the resulting body 3 has a number of vertical split lines(S). These correspond with locations of high stretching of the metal wall during expansion. The vertical split lines have formed between adjacent segments of an expansion mandrel (not shown) and do not follow the curvature of the mandrel. In this illustration, it would correspond to the locally flat section between each two of six segments.

FIG. 2A shows a cross-sectional view of a conventional expansion apparatus 10, which is shown to explain the underlying principle for expanding a metallic cylindrical precursor 2. The apparatus is shown in a retracted state, i.e. before expansion. The expansion apparatus 10 comprises a housing 11 and a shaping mandrel 13 extending from the housing 11. The shaping mandrel 13 comprises a plurality of similar segments 14 spaced about an expander arm 12. Each segment 14 has an arcuate contacting surface A for contacting or engaging an inner surface I of a cylindrical precursor 2 placed around the segments 14. Each segment 14 further has a cam surface 18 at its inner side, facing towards the expander arm 12. In the retracted state the contacting surfaces A of the segments 14 are spaced radially inward from the inner surface I of the container body 2. A proximal end 12A of the expander arm 12 and ends 20 of the segments 14 are received within the housing 11. A distal end 13B of the mandrel 13, has a reduced diameter.

The expander arm 12 is movable axially within the housing (along axis Z) to cause expansion of the segments 14. The segments 14 extend longitudinally along the expander arm 12. A pair of wedges 16 are positioned about the expander arm 12 in tandem. The expander arm 12 and the wedges 16 form the actuator of the shaping mandrel 13. Each wedge 16 has a plurality of wedge surfaces 17, each of which contacts a mating cam surface 18 of the segments 14. Axial movement of the expander arm 12 together with the wedges 16, causes the wedge surfaces 17 to ride along the cam surfaces 18 to expand the segments 14 outwards to contact the precursor 2 inner surface I.

FIG. 2B shows a cross-sectional view of the same expansion apparatus 10 of FIG. 2A, shown in a finally expanded state. This view shows how the expander arm 12 has been moved proximally in the direction of the housing 11 (Z-direction). This axial movement has caused the wedge surfaces 17 to cooperate with corresponding cam surfaces 18 of the segments 14 to force the segments 14 radially outwards into engagement with the inner surface I of the precursor 2. The inner surface I of the precursor 2 is deformed accordingly to adopt the shape of the expanded mandrel 13. The distal end 13B of the mandrel defines the smallest internal diameter of the expanded precursor 2.

When the expansion is completed, the expander arm 12 is moved axially away from the housing 11 and the segments 14 may be brought back into a collapsed or retracted position. The precursor 2 may then be removed from the shaping mandrel 13. It will be understood that the mandrel 13 must collapse sufficiently such that the largest outer diameter can pass through the smallest diameter portion of the precursor 2. In conventional expansion devices such as the apparatus 10 of FIG. 2, around 6 or 8 segments 14 may be provided, all of which are substantially identical.

FIG. 3 shows a cross-sectional view in the plane (XY), of a shaping mandrel 12 of an expansion apparatus 10 according to the invention. Like numerals will be used for like components as in the case of the conventional apparatus of FIG. 2. The mandrel 13 is shown in its initial state, prior to any expansion, with an outer diameter of d1.

The shaping mandrel 13 comprises twelve segments 14, which surround the expander arm 12. In this case the segments 14 are divided into two subgroups, namely first subgroup segments 14A and second subgroup segments 14B. The first subgroup segments 14A are spaced outwardly of the second subgroup segments 14B at the initial outer diameter d1. It will also be noted that the segments 14A have undercut flanks 19. This allows the second subgroup segments 14B to be recessed behind the arcuate contacting surfaces A of the first subgroup segments 14A in this initial state of the mandrel 13.

The first subgroup segments 14A have first cam surfaces 18A at their inner side, while the second subgroup segments have second cam surfaces 14B at their inner side. Due to the presence of the undercut flanks 19, the second cam surfaces 18B are slightly narrower than the first cam surfaces 18A even though the respective arcuate contacting surfaces A of all the segments 14 are identical in size.

The expander arm 12 carries a wedge 16, with wedge surfaces 17, which are also divided into first wedge surfaces 17A and second wedge surfaces 17B. These engage respectively with the first and second cam surfaces 18A, 18B of the segments 14.

FIG. 4A shows schematically the wedges 16 illustrating the profiles of the wedge surfaces 17 and cam surfaces 18 of the shaping mandrel 13 of FIG. 3 in a longitudinal section at position III-III. First wedge surfaces 17A, actuating the first subgroup segments 14A are shown in the top half of the figure, while second wedge surfaces 17B actuating the second subgroup segments 14B are shown in the bottom half of the figure. Also shown in this view is a channel 40 formed in the arcuate surfaces A of the mandrel 13. The channel 40 has a depth that is almost equal to the overall expansion of the mandrel 13. It also corresponds to a diameter of a distal portion 13B of the mandrel 13. The channel 40 extends around the complete circumference of the mandrel 13 but varies in its longitudinal position from segment to segment. Nevertheless, the width and depth of the channel 40 is for each segment 14 arranged such that the profile length of each segment 14 following the arcuate surface A in the longitudinal is identical.

Dealing first with the first wedge surfaces 17A and the mating first cam surfaces 18A, these have a constant angle α1 that generally corresponds to that of the conventional apparatus 10 of FIG. 2. In the case of the second wedge surfaces 17B, these have a two-step surface. A first part of the surface denoted 17Bi has an angle α2, that is greater than the angle α1 of the first wedge surfaces 17A. A second part 17Bii of the surface, has an angle α1 that again corresponds to that of the first wedge surface 17A. In the illustrated embodiment, the first and second parts 17Bi and 17Bii are of approximately equal length. Similarly, the second cam surfaces 18B are also in two parts, with a first part 18Bi having an angle α2 and a second part having an angle α1.

FIG. 4B shows the shaping mandrel 13 of FIG. 4A, with the expander arm 12 and wedges 16 partially withdrawn in the proximal direction with respect to the segments 14. The first wedge surface 17A has progressed along the respective first cam surface 18A, causing the first subgroup segments 14A to move radially outward. The second wedge surface 17B has also progressed along the respective second cam surface 18B. As depicted, during this part of the movement, the first part 17Bi of the second wedge surface and the first part 18Bi of the second cam surface are in sliding engagement and dictate the expansion of the segment 14B at the angle α2. Since the angle α2 is greater than the angle α1, the second subgroup segment 14B has moved radially outward by a distance that is greater than that of the first subgroup segment 14A.

FIG. 4C shows the shaping mandrel 13 of FIG. 4A, with the expander arm 12 and wedges 16 almost fully withdrawn in the proximal direction with respect to the segments 14. The first wedge surface 17A has continued along the respective first cam surface 18A, causing the first subgroup segments 14A to continue to move radially outward at the same rate, as dictated by the angle α1. The second wedge surface 17B has progressed further along the respective second cam surface 18B such that now, the second first part 17Bii of the second wedge surface 17B and the second part 18Bii of the second cam surface 18B first enter into sliding engagement. From this point of the trajectory, further proximal movement of the expander arm 12 causes expansion of the second subgroup segment 14B to be dictated by the slope of these parts. This will occur at the angle α1 at the same speed as that of the first subgroup segment 14A.

FIG. 5A shows an enlarged view of two adjacent segments 14A, 14B of FIG. 3 at the beginning of a first step of expansion. The arcuate surface A of the first subgroup segment 14A contacts the inner surface I of the precursor 2 and causes the curvature of the inner surface I to conform to its curvature. The arcuate surface A of the second subgroup segment 14B is spaced radially inwards from the inner surface I by a distance Rx. Also visible are the undercut flanks 19 of the first subgroup segment 14A and a pair of transition ears 30 behind which the second subgroup segment 14B is recessed. During this stage of expansion, all stretching of the precursor is caused by the first subgroup segments 14A, in contact with just about half of the inner surface I.

FIG. 5B shows an enlarged view of the segments 14A and 14B at the point at which the second subgroup segment 14B first enters into contact with the inner surface I of the precursor 2. At this point, the second subgroup segment 14B is still radially inwards of the first subgroup segment 14A. Although from this point, some of the stretching of the precursor 2 will take place by engagement of the second subgroup segment 14B, nevertheless, the lateral edges of the first subgroup segments 14A at the position of the transition ears 30 are a critical location P, where stretch marks may be created in the precursor 2. With reference to FIG. 4B, this figure still corresponds to the period of expansion depicted where the first part 17Bi of the second wedge surface and the first part 18Bi of the second cam surface are in sliding engagement and dictate the expansion of the segment 14B at the angle α2.

FIG. 5C shows an enlarged view of the segments 14A and 14B during a second stage of the expansion step at the point at which the arcuate surfaces A of all of the segments 14 first align. In the illustrated embodiment, this also corresponds to the point where the lateral edges of adjacent segments 14 just touch and there is thus no gap between adjacent segments 14. It will be understood that this is a chosen position and it is not necessary for the mandrel 13 to pass through this no-gap configuration. This corresponds to the position depicted in FIG. 4C, where first part 17Bi of the second wedge surface and the first part 18Bi of the second cam surface first enter into sliding engagement. From this point forwards, all segments will expand at the same rate and the arcuate surfaces A of all of the segments 14 will remain radially aligned.

FIG. 5D shows an enlarged view of the segments 14A and 14B during a final stage of the expansion step. The arcuate surfaces A of all of the segments 14 are still aligned but have now expanded to a second diameter d2 that is larger than the initial diameter d1. During the expansion from the position depicted in FIG. 5C, the segments 14A, 14B have moved apart, creating a circumferential gap 32 between adjacent segments 14A, 14B. It will be understood that the circumferential gap 32 also extends in a longitudinal direction of the mandrel 13.

FIG. 5E shows an enlarged view of the segments 14A and 14B at an alternative final stage of expansion. In this situation, the second subgroup segment 14B has expanded to a diameter that is slightly larger than the second diameter d2. This can be used to stretch the precursor 2, slightly away from the edges of the first subgroup segment 14A at the position of the transition ears 30. This may be used to provide a slight over-stretch at the critical location P, to remove previously created stretch marks. This alternative final stage could be achieved by a further part of the second cam surface 18B, following the second part 18Bii.

FIG. 6 shows the completed can 3 after finishing the expansion step and after providing top and bottom closures 4, 5. A profile 42 is provided around the outer surface, corresponding to the channel 40 in the mandrel 13. FIGS. 7 and 8 depict alternative cans 3, all of which have contoured outer surfaces. As a result of the improved expansion achievable with the disclosed mandrel, variations in diameter of up to 30% are achievable, while ensuring an otherwise smooth, flaw-free surface of the can 3.

The invention has been described by reference to certain embodiments discussed above. It will be recognized that these embodiments are susceptible to various modifications and alternative forms well known to those of skill in the art. In particular, different numbers of segments and different wedge angles and profiles can be used to create cans of different designs. Accordingly, although specific embodiments have been described, these are examples only and are not limiting upon the scope of the invention