EP4688299A1

EP4688299A1 - Method and apparatus for calibrating a metal profile blank

Info

Publication number: EP4688299A1
Application number: EP24719095.2A
Authority: EP
Inventors: Frode Paulsen; Ulf Håkon TUNDAL
Original assignee: Norsk Hydro ASA
Current assignee: Norsk Hydro ASA
Priority date: 2023-04-03
Filing date: 2024-04-02
Publication date: 2026-02-11
Also published as: NO20230374A1; NO348322B1; WO2024208779A1; CN120957820A

Abstract

It is disclosed a calibration system (300) for calibrating an extruded hollow profile (200) by plastic deformation from an initial shape to a predetermined final shape, wherein the extruded hollow profile (200) comprises at least one hollow chamber (220) extending through a longitudinal direction (L) of the extruded hollow profile (200), the at least one hollow chamber (220) having openings (221) at opposite ends of the extruded hollow profile (200). The calibration system (300) comprises: - a liquid (310) for filling the at least one hollow chamber (220), - a first closure device (320) and a second closure device (320) configured to close the openings (221) to confine the liquid (310) in the at least one hollow chamber (220), - a die tool (340) configured to define an external boundary of the predetermined final shape of the extruded hollow profile (200), and - a pressing tool (330) configured to move the die tool (340) to compress the extruded hollow profile (200) such that an internal pressure (Pi) in the at least one hollow chamber (220) is increased and the extruded hollow profile (200) is plastically deformed. It is also disclosed a method for calibrating an extruded hollow profile (200) and a method for processing an extrusion billet (100) in which the calibration system may be used.

Description

Method and apparatus for calibrating a metal profile blank

TECHNICAL FIELD

The present invention relates to a method for processing an extrusion billet, and in particular calibrating an extruded hollow profile. Furthermore, the invention relates to a calibration tool for calibrating the extruded hollow profile according to the method.

BACKGROUND ART

Aluminium extrusions are increasingly used in automotive applications like body parts, chassis parts and in products that are essential to support the green transition, for example battery housings. Such applications typically have medium to high strength and good ductility to meet demanding requirements related to crash behaviour. To fully utilize the potential of the extrusion process, the typical hollow extruded profile can have many chambers and be relatively thin-walled to minimize material usage and contribute to weight saving. Due to the nature of the extrusion process, geometrical variations along extruded profile lengths are characteristically larger than in other forming processes like sheet rolling, hence making it difficult to meet the dimensional tolerance requirements of many automotive products without further refinements, as for example by machining. Such processing is expensive both from processing and investment point of views, in addition to the extra material that must be added to secure available material for the machining process.

One way to overcome the need for additional machining is calibration by introducing plasticity to the extruded profile, preferably before ageing to strength in case of extruding heat-treatable alloys, to keep springback low. Over the years, several methods have been proposed. One of the first methods used in the industry was so-called stretch calibration. In this method, the ends are fixed, and the profile is stretched into plasticity to remove twist and bow from the profile. A disadvantage with such methods is firstly that the ends are fixed and prevented from contraction as will happen for metals under plastic deformation. This creates a difference in the cross-section along the length of the profile. Secondly, the method is not able to calibrate the location of the profile walls relative to each other, as no tooling contact is present over the length of the profile.

Later calibration methods involve expanding the cross-section by the application of mandrels inside the profile. The mandrels are placed inside the profile and are thereafter moved apart, often by a wedge mechanism. As the mandrels are moved apart, the exterior profile walls are moved away from the centre line and the length of the periphery will increase and be deformed plastically. This will also have the effect of potentially removing bow and twist, in addition to calibrating the location of the surfaces to each other. A drawback with this method is the application of long and mechanically complex mandrels that are expensive to make and to function properly without breaking as a result of high forces. For multi-chamber extrusions, especially non-symmetrical ones, the design and operations of the mandrels will be complicated.

In US2022/0062965A1 it is disclosed a method for calibrating a metal profile blank configured as a hollow-chamber profile having at least one solid wall. Here, the profile cross-section is compressed into plasticity at least in one end region of the profile blank to be calibrated towards floating mandrels inside the profile. A drawback with this method is that mandrels must be machined and adapted to each profile geometry, which creates cost and complexity to the calibration operation. The present invention is directed to a method and a system that may solve or at least reduce at least one of the aforementioned problems or challenges.

SUMMARY OF INVENTION

The present invention is set forth and characterized in the independent claims, while the dependent claims describe other characteristics of the invention.

The present disclosure relates to a calibration system for calibrating an extruded hollow profile by plastic deformation from an initial shape to a predetermined final shape, wherein the extruded hollow profile comprises at least one hollow chamber extending through a longitudinal direction L of the extruded hollow profile, the at least one hollow chamber having openings at opposite ends of the extruded hollow profile, wherein the calibration system comprises:

- a liquid for filling the at least one hollow chamber,

- a first closure device and a second closure device configured to close the openings to confine the liquid in the at least one hollow chamber,

- a die tool configured to define an external boundary of the predetermined final shape of the extruded hollow profile, and

- a pressing tool configured to move the die tool to compress the extruded hollow profile such that an internal pressure Pi in the at least one hollow chamber is increased and the extruded hollow profile is plastically deformed into the predetermined shape.

It is thus achieved a calibration system wherein the outer wall will not collapse or bend unintentionally during calibration as the outer wall is held in contact with the die tool setting the predetermined shape.

The extruded profile may be formed of an aluminium or aluminium alloy.

The liquid may e.g. be water or oil. The available amount of the liquid should preferably at least equal the volume of the at least one hollow chamber in the initial shape of the extruded hollow profile.

Each opening may e.g. be closed by means of a cap placed on the end of the extruded hollow profile. Alternatively, each opening may be closed by means of a plug inserted into the opening. The two openings of each hollow chamber do not have to be closed in the same way, as an example, one opening may be closed with a cap while the other opposite opening may be closed with a plug. Other suitable ways of closing the openings known in the art will be available to the skilled person. The term closure device is understood as a generalization encompassing at least the above-mentioned alternatives suitable for closing the openings of the hollow chamber(s).

Each hollow chamber having two openings, these openings may be referred to as a first opening and a second opening.

If the extruded hollow profile is vertically arranged (or at least arranged with the two openings at different vertical elevations), the first opening may be referred to as an upper opening, an uppermost opening, or a top opening. The second opening may then be referred to as a lower opening, a lowermost opening, or a bottom opening. If the extruded hollow profile is vertically arranged (or at least arranged with the two openings at different vertical elevations) during the calibration, the first closure device may be referred to as an upper closure device, an uppermost closure device, or a top closure device. The second closure device may then be referred to as a lower closure device, a lowermost closure device, or a bottom closure device.

One or both closure devices may comprise a port through which the pressure inside the hollow chamber(s) can be adjusted, i.e. increased or reduced.

One or both closure devices may comprise ports through which the pressure inside the hollow chambers can be equalized.

One or both closure devices may comprise a port through which the amount of liquid inside the hollow chamber(s) can be adjusted, i.e. increased or reduced.

One or both closure devices may comprise a port through which the hollow chamber(s) can be filled with liquid.

One or both closure devices may be provided with a pressure gauge and/or a temperature gauge.

One or both closure devices may be provided with a check valve configured to vent if the pressure from the liquid inside the hollow chamber exceeds a predetermined pressure value, e.g. in excess of 100 bar.

One or both closure devices may be provided with a valve for venting air from the hollow chamber, typically the upper closure device.

One or both closure devices may be provided with a plurality of ports corresponding to the number of hollow chambers of the extruded hollow profile. The ports will be arranged accord to the arrangement of the hollow chambers. As such, there may be specific closure devices for different extruded hollow profiles.

One or both closure devices may be provided with a fluid passage configured to provide fluid communication between two or more hollow chambers of the extruded hollow profile. The fluid passage may be externally arranged and connected to a plurality of ports in the closure device, wherein each port is in fluid communication with respective hollow chambers. Alternatively, the fluid passage may be integrated in the closure device, e.g. as one or more bores.

One or both closure devices may be attached to the extruded hollow profile. Alternatively, one or both closure devices may be forced against the end(s) of the extruded hollow profile and thus be held in place during the compressing operation. One or both closure devices may be forced against the extruded hollow profile by means of a biasing device, e.g. a spring or a piston. The applied force may be set to withstand a predetermined pressure inside the hollow profile. If the internal pressure exceeds the predetermined value, the biasing device may be configured to be pushed back such that the closure device is separated from the extruded hollow profile. The excessive pressure will then be vented from the hollow chamber and the biasing device will force the closure device against the extruded hollow profile once the pressure has dropped below the predetermined value.

The end of the extruded hollow profile will typically be cut perpendicular to the longitudinal direction of the extruded hollow profile. However, the ends may be cut with an angle that is less than 90° relative the longitudinal direction of the extruded hollow profile. The one or both closure devices that are forced against the end(s) of the extruded hollow profile may be configured to adapt to the angle of the ends of the extruded hollow profile. As an example, the closure device may be arranged on a ball joint or similar allowing the closure device to be adjusted according to the angle of the end while being forced against the end to close its opening(s). This may also be achieved with a spring-loaded closure device as the spring may allow some angular adjustment of the closure device.

One or both closure devices may comprise an elastomeric seal.

The pressing tool may be used to force one or both closure devices against the ends of the extruded hollow profile.

If the extruded hollow profile is arranged vertically and the pressing tool applies a force from above, i.e. on the upper closure device, the port/valve may preferably be arranged in the lower closure device to avid interference between the pressing tool and the port/valve.

When the extruded hollow profile is plastically deformed, the extruded hollow profile may be elongated, i.e. the length of the extruded hollow profile may increase. One or both closure devices may therefore be configured to move in response to such an elongation, e.g. by means of a spring load forcing the closure device(s) against the extruded hollow profile or by means of attaching the closure device(s) to the extruded hollow profile. The closure device(s) may preferably be configured to move in response to an elongation of the extruded hollow profile while maintaining the openings of the hollow chamber closed.

Due to water being incompressible, the water (or other type of liquid) confined inside the hollow chamber may prevent the compressing operation plastically deforming the extruded hollow to a predetermined final cross-section. In such cases it is advantageous to be able to drain the excessive liquid. That can be achieved with one or more of the above-mentioned closures.

The pressing tool may comprise one or more actuators configured to move the die tool.

The pressing tool may comprise a ball screw and a servomotor.

The pressing tool may comprise a hydraulic or mechanical press.

The die tool may be configured to enclose the extruded hollow profile.

The die tool may comprise:

- a plurality of die parts configured to be brought into contact with each other and stop dead at a point wherein the external boundary of the predetermined final shape is defined.

By coming to a dead stop during the compressing process, the die parts cannot move past the external boundary of the predetermined final shape. In that way there is no risk of over-compressing the extruded hollow profile, regardless of the applied force.

The die tool may comprise four dies. Wherein a first die part and a second die part are configured to be moved along a first horizontal direction X and a third die part and a fourth die part are configured to be moved along a second horizontal direction Y perpendicular to the first horizontal direction X.

As a first step during the compressing operation, the first die part and/or the second die part may be moved along the first horizontal direction X to reduce the distance between them. As a result, the extruded hollow profile can be compressed in the first horizontal direction X.

As a second step during the compressing operation, the third die part and/or the fourth die part may be moved along the second horizontal direction Y to reduce the distance between them. As a result, the extruded hollow profile can be compressed in the second horizontal direction Y. Alternatively, the first die part and/or the second die part may be moved along the first horizontal direction X to reduce the distance between them simultaneously as the third die part and/or the fourth die part is/are be moved along the second horizontal direction Y to reduce the distance between them.

If the pressing tool comprises a ball screw and a servomotor, the ball screw and the servomotor may be configured to move the die parts directly.

The pressing tool may comprise:

- a housing comprising a plurality of walls configured to enclose the die tool, and

- a plurality of wedge-shaped cams configured to move between the walls and the die parts to force the die parts away from the walls and towards the extruded hollow profile.

The pressing tool may comprise four wedge-shaped cams (a first wedge-shaped cam, a second wedge- shaped cam, a third wedge-shaped cam, and a fourth wedge-shaped cam). The first wedge-shaped cam may be configured to interact with the first die part, the second wedge-shaped cam may be configured to interact with the second die part, the third wedge-shaped cam may be configured to interact with the third die part, and the fourth wedge-shaped cam may be configured to interact with the fourth die part. The wedge-shaped cams may be configured to interact with inner surfaces of the walls of the housing.

The wedge-shaped cams may be configured for a sliding interaction with the walls and the dies. The material of the wedge-shaped cams and their surface roughness should preferably be selected accordingly.

The pressing tool may comprise a flat cam.

The flat cam may be configured to receive one die part and holding the die part in place during the compressing operation. The pressing tool may then comprise one wedge-shaped cam less than the number of dies. The compressing operation may be performed without moving the cam being held by the flat cam. As an example, the pressing tool may comprise one flat cam and three wedge-shaped cams.

As an alternative to the flat cam, the housing may be configured to receive and hold one of the dies. The pressing tool may then comprise three wedge-shaped cams.

The pressing tool may comprise one or more spacer cams configured to guide the movement of the die parts and/or the movement of the wedge-shaped cams.

A side surface of one or more die parts may be inclined such that a side surface of the wedge-shaped cam can interact with a side surface of the die part in a parallel manner when the wedge-shaped cam is inserted between the wall and the die. In that way, the force exerted on the extruded hollow profile can be more evenly distributed. The extruded hollow profile may thus be more precisely compressed.

Alternatively, one or more walls of the housing may be inclined such that a side surface of the wedge- shaped cam can interact with a side surface of the die part in a parallel manner when the wedge- shaped cam is inserted between the wall and the die. In that way, the force exerted on the extruded hollow profile can be more evenly distributed. The extruded hollow profile may thus be more precisely compressed.

If the pressing tool comprises a ball screw and a servomotor, the ball screw and the servomotor may be configured to move the wedge-shaped cams. The wedge-shaped cams may be moved in succession or in parallel.

The wedge-shaped cams typically taper from a first end towards an opposite second end being thinner than the first end. To effectuate the compressing of the extruded hollow profile, the wedge-shaped cams can be moved in the direction of the thinner end between the walls and the dies. As the wedgeshape cams are moved in between the walls and the dies, the more the wedge-shaped cam will separate the die parts from the walls, i.e. moving the die parts away from the walls and thus towards the extruded hollow profile.

The compressing of the extruded hollow profile may be released, by means of moving the wedge- shaped cams in the direction of the first end (i.e. the wider end of the wedge-shaped cam) to allow the die parts to move away from the extruded hollow profile.

The wedge-shaped cams may be moved in the vertical direction Z to cause a movement of the die parts in the horizontal directions X,Z, when the extruded hollow profile is vertically arranged (i.e. the wedge-shaped cams may be moved parallel to the longitudinal direction of the extruded hollow profile to move the die parts to compress the cross-section of the extruded hollow profile).

The housing may be box-shaped and have an open top. The extruded hollow profile, the die tool and the cams may then be introduced in the housing through the open top. The box may then comprise four walls arranged perpendicular to each other forming a rectangle.

The housing may have a closed bottom and then serve as a liquid tank.

The pressing tool and the die tool are preferably configured to compress an extruded hollow profile that is vertically arranged.

The die tool may have the same length as the extruded hollow profile.

If a spring back effect is expected in the extruded hollow profile after the compressing operation, such spring back effect can be taken into account when setting the external boundary of the predetermined final shape defined by the die tool.

One or both closure device may be held in place by the housing. The housing may comprise a hole in one of the walls through which a closure device may enter and a recess or hole in the opposite wall through/into which the closure device may enter. The hole and recess/hole may be arranged such that the closure device entered therein can close one opening of the hollow chamber. The closure device may then slide in and out of the housing to close and unclose the opening. The housing may then prevent movement of the closure device in the longitudinal direction of the extruded hollow profile arranged in the housing.

A closure device configured to enter the housing may be configured to allow three wedge-shaped cams to enter the housing while the closure device is closing the opening. Two wedge-shaped cams may be entering the housing on opposite sides of the closure device and a third wedge-shaped cam may enter the housing through a split in the closure device.

The calibration system may comprise:

- a liquid tank configured to hold the liquid and to receive the extruded hollow profile.

The liquid tank is preferably configured such that the entire extruded hollow profile can be submerged in the liquid. The hollow chamber can then be filled by means of submerging the extruded hollow profile in the liquid. The liquid tank is preferably configured to receive a vertical arranged extruded hollow profile. The liquid tank should then have a depth equal or greater than the length of the extruded hollow profile.

The liquid tank may comprise a circulation pump for exchanging the liquid, a cooler for cooling the liquid, a filter for cleaning the liquid and/or a temperature sensor.

The calibration system may comprise:

- a robotic arm configured to handle the extruded hollow profile, and

- a control system configured to control movement of the robotic arm.

The calibration system may thus be automated.

Handling by the robotic arm may involve positioning the extruded hollow profile for calibration and removing the extruded hollow profile after calibration. Handling may also involve filling the hollow chamber with the liquid. Handling may also involve engagement and disengagement of the closure devices.

The control system may be configured to control operation of the pressing tool.

The control system may comprise a computer.

The present disclosure relates to a method for calibrating an extruded hollow profile having an initial shape, wherein the method comprises the steps of:

- providing an extruded hollow profile, wherein the extruded hollow profile comprises at least one hollow chamber extending through a longitudinal direction L of the extruded hollow profile, the at least one hollow chamber having openings at opposite ends of the extruded hollow profile,

- filling the at least one hollow chamber with a liquid,

- confining the liquid in the at least one hollow chamber by means of closing the openings,

- compressing the extruded hollow profile causing an internal pressure P, in the at least one hollow chamber to increase and plastically deforming the extruded hollow profile to a predetermined final shape, and

- releasing the compression on the extruded hollow profile and unclosing the openings of the at least one hollow chamber.

It is thus achieved a calibration system and a calibration method for calibrating an extruded hollow profile, i.e. an extruded profile with a hollow chamber. By compressing the extruded hollow profile, a pressure is built up inside the hollow chamber. The pressure inside the hollow chamber will support plastic relocation of the outer profile walls. Thus, unwanted geometrical distortions along the profile will be removed.

The calibration system and the calibration method will calibrate the extruded hollow profile such that the cross-section is within given tolerances, e.g. according to industry standards, throughout the entire extruded hollow profile.

One or more parts of the extruded hollow profile may be plastically deformed, in some cases the entire extruded hollow profile may be plastically deformed.

The extruded hollow profile is calibrated once the die parts are in position such that the predetermined final shape has been achieved. The extruded hollow profile may comprise one or more hollow chambers. If the extruded hollow profile comprises a plurality of hollow chambers, all the hollow chambers will extend through the longitudinal direction of the extruded hollow profile, i.e. through the entire length of the extruded hollow profile. In cross-section, i.e. perpendicular to the longitudinal direction, the hollow chambers may be different in size and/or geometry.

If the extruded hollow profile comprises a plurality of hollow chambers, the internal pressure may be equalized between the hollow chambers during the compressing operation. The increase in the internal pressure caused by the compressing operation will then be the same in all the hollow chambers. As such there will be no differential pressure between the hollow chambers. In that way the inner walls, i.e. walls separating two or more hollow chambers, may not be affected by the internal pressure.

By not equalizing the internal pressure between two or more hollow chambers, deformation of the inner wall may be prevented or reduced. As an example, if the inner wall is starting to bulge into one of the hollow chambers during the compression operation, the volume of that hollow chamber will be reduced, and the internal pressure Pi increased. At the same time the volume of an adjacent hollow chamber will be increased, and its internal pressure Pi reduced. As such, the inner wall will experience a differential pressure working against the bulging, preventing or at least reducing the extent of the bulging.

An advantage of the calibration system and the calibration method is that the extruded hollow profile can be calibrated without the need of complex profile specific mandrels. The cost associated with fabrication of the mandrels can then be saved. Furthermore, not having to insert and remove the mandrels makes the method less time consuming and less complex, as the mandrels are typically difficult to remove after calibration.

An advantage of the calibration system and the calibration method is that the calibration can correct geometrical deviations in an initial cross-section that are both larger and smaller as compared to a given final cross-section.

The initial cross-section is understood as the cross-section of the extruded hollow profile after extruding and prior to calibrating. The final cross-section is understood as the cross-section after calibrating.

The pressure inside the extruded hollow profile may typically increase 0.5-100 bar during the compressing operation. When the openings of the hollow chamber are closed, some leakage can be acceptable as long as the required internal pressure is allowed to build up inside the hollow chamber(s).

The system and method can be used to calibrate extruded hollow profiles of any length, however, the present system and method is especially suitable for extruded hollow profiles that have been cut to a predefined length. A typical length of an extruded hollow profile may be 1.5-2.5 meters for sills and frame components. A typical length of an extruded hollow profile may be 0.1-0.2 meters for chassis components.

The longitudinal direction of the extruded hollow profile is the direction in which it was extruded. The length of the extruded hollow profile is measured along its longitudinal direction.

The extruded hollow profile may typically have a wall thickness in the range 1.5-5.0 mm. The temperature of the liquid may typically be at ambient temperature, such as 5-30°C, and may be heated or cooled as required.

One of the openings of the hollow chamber may be closed prior to filling of the hollow chamber with the liquid, typically if the extruded hollow profile is not submerged in the liquid.

After unclosing the openings, the water can be drained from the hollow chamber.

The compression will typically be released prior to unclosing the openings of the hollow chamber. However, depending on the type of compressing, the unclosing may be performed prior to releasing the compression.

The extruded hollow profile may be processed prior to the calibration. The processing may involve mechanical or chemical operations to change or preserve the extruded hollow profile.

The initial shape of the extruded hollow profile may have an initial cross-section and the final shape of the extruded hollow profile may have a final cross-section, wherein the initial cross-section is larger than the final cross-section.

The filling of the at least one hollow chamber with liquid may be performed by means of submerging the extruded hollow profile in the liquid.

An advantage of submerging the extruded hollow profile in the liquid is that all surfaces of the extruded hollow profile will be in contact with the liquid. If the liquid has a different temperature than the extruded hollow profile, a temperature change in the material of the extruded hollow profile can be more uniform, as compared to only filling the hollow chamber(s). The liquid will typically be colder than the extruded hollow profile.

Submerging the extruded hollow profile is a time efficient way of filling the hollow chamber(s) with liquid.

The extruded hollow profile may typically be submerged in a tank containing the liquid, e.g. a liquid tank according to the present disclosure. The extruded hollow profile may typically be oriented in a vertical direction, i.e. with its longitudinal direction oriented vertically, while being submerged.

The extruded hollow profile may typically be oriented in a vertical direction during the compressing operation.

The openings may be closed by means of a first closure device and a second closure device.

The closure devices used to close the openings may be a closure device according to the present disclosure.

The calibration method may comprise the steps of:

- placing the first closure device (lower closing device) on a supporting surface, the supporting surface may be submerged in the liquid, the supporting surface may e.g. be a bottom surface of a liquid tank, a floor, or a part of the pressing tool,

- arranging the extruded hollow profile vertically on top of the first closure device, thus closing the first opening (lower opening) of the hollow chamber,

- placing the second closure device (upper closing device) on top of the extruded hollow profile, thus closing the second opening (upper opening) of the hollow chamber, and optionally (depending on the type of closure devices) - applying a force on the second closure device, the applied force being sufficient to withstand the internal pressure (in the hollow chamber) required to plastically deform the extruded hollow profile.

The compressing of the extruded hollow profile may be performed by means of a pressing tool 330.

The pressing tool used to compress the extruded hollow profile may be a pressing tool according to the present disclosure.

If the pressing tool comprises a plurality of wedge-shaped cams, the compressing operation may comprise the steps of:

- moving the wedge-shaped cams parallel to a longitudinal direction of the extruded hollow profile (typically in the vertical direction Z) forcing the die tool towards the extruded hollow profile (typically in the first horizontal direction X and/or the second horizontal direction Y) causing a compression of the extruded hollow profile.

A die tool may be used to define an external boundary of the predetermined final shape of the extruded hollow profile.

The die tool used to define the external boundary of the predetermined final shape of the extruded hollow profile may be a die tool according to the present disclosure.

If the die tool comprises four die parts (a first die part, a second die part, a third die part and a fourth die part), the compressing operation may comprise the steps of:

- moving the first die part and/or the second die part along the first horizontal direction X to reduce the distance between them and to compress the extruded hollow profile in the first horizontal direction X, and

- moving the third die part and/or the fourth die part along the second horizontal direction Y to reduce the distance between them and to compress the extruded hollow profile in the second horizontal direction Y.

The calibration method may be performed using a calibration system as described herein.

The extruded hollow profile may be made of an aluminium alloy, preferably a 6XXX series aluminium alloy or a 7XXX series aluminium alloy.

The extruded hollow profile is made of an aluminium alloy having a composition (in wt%) 0.40-0.8 Si; up to 0.7 Fe; 0.10-0.40 Cu; up to 0.15 Mn; 0.8-1.2 Mg; 0.04-0.35 Cr; up to 0.25 Zn; up to 0.15 Ti, others up to 0.05 each, in total up to 0.15, and balance Al and incidental impurities.

The 6XXX series and 7XXX serial alloys as referred to herein refer to alloys as listed in the "International Alloy Designations and Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys" published by The Aluminum Association.

The method for calibrating an extruded hollow profile may be computer implemented. The method for calibrating an extruded hollow profile may be automated.

The present disclosure relates to a method for processing an extrusion billet produced from an aluminium alloy, wherein the method comprises the steps of:

(a) homogenizing the extrusion billet,

(b) preheating the extrusion billet prior to the extruding operation, (c) extruding the extrusion billet to form an extruded hollow profile,

(d) cooling the extruded hollow profile down to room temperature after the extruding operation, and

(e) artificially aging the extruded profile, the method further comprises the step of: calibrating the extruded hollow profile according to the calibration method described herein.

The homogenizing step temperature may be in the range between 470 °C and 590 °C. A 7XXX series aluminium alloy may be homogenised e.g. at a temperature range of between 470 to 520 °C. A 6XXX series aluminium alloy may be homogenised e.g. in a temperature range of 520 to 590 °C.

The calibrating step is preferably performed before the artificially aging step (e).

The calibration of the extruded hollow profile may be performed by means of a calibration system according to the present disclosure.

The processing method may comprise the step of: soft annealing the extrusion billet at a temperature between 350 and 450 °C, after the homogenizing step (a) and before the preheating step (b).

The cooling rate from the homogenizing operation to the soft annealing operation is at least 100 °C per hour, or preferably at least 200 °C per hour. By soft annealing the extrusion billet at a temperature between 350 and 450 °C a high amount of MgzSi particles is precipitated and thereby the material is made soft allowing a high extrusion speed.

The processing method may comprise the steps of:

- exposing the extruded hollow profile of step (d) to a solution heat treatment and quenching operation.

Before the solution heat treatment, the extruded hollow profile may be cut into predetermined lengths. The solution heat treatment may be performed at temperatures of above about 470 °C. In general, a 6XXX series aluminium may be solution heat treated at a temperature above 520 °C, such as between 540 and 580 °C to obtain fast dissolution of MgzSi particles. A non-recrystallized 6XXX extrusion and extrusions of 7XXX series alloys may be solution heat treated in the lower part of the temperature range.

The quenching may be performed by submerging the solutionized hollow profile into a quenching bath. In a preferred method, a hollow profile having 2 or more voids is submerged in a quenching bath in a vertical direction to ensure filling of the hollow voids with the quenching liquid which enables fast cooling of across the entire profile cross-section. During the quenching operation, the extruded hollow profile may be submerged for less than 30 seconds, typically in the range of 25-15 seconds or less.

The calibration operation may be performed after the quenching operation.

The calibration operation may be performed within a short time after the quenching operation. Preferably the calibration operation is performed immediately after the quenching. The quenching operation and calibration operation may be performed in an in-line process. Furthermore, the in-line process may additionally comprise a partial or final artificial aging process. By performing the artificial aging process in an in-line process with the quenching and calibration it may be possible to obtain extra strength of 20-30 MPa in the final profile. The steps of quenching, calibration and artificial aging may be performed in an in-line process.

When the calibration operation and the quenching operation may be performed simultaneously, the hollow chamber is preferably filled with liquid by means of submerging the extruded hollow profile in the liquid. Preferably the quenching and calibration operations are performed in separate operation to keep the temperature of the quenching bath at a low temperature, such as below about 30 °C. A more efficient cooling of the extruded hollow profile can then be achieved as the heat transfer from the extruded hollow profile

The method will typically comprise artificially ageing of the extruded hollow profile subsequent the quenching operation. The artificial aging may comprise one or more heating steps.

The method for processing an extrusion billet may be computer implemented. The method for processing an extrusion billet may thus be automated.

The calibrated extruded hollow profile may be used as components in the automobile industry, some examples are battery boxes, side sills, door sills and front rails.

The present disclosure relates to a computer program comprising instructions which, when the program is executed by a control system, cause the control system to carry out any one of the steps of the methods described herein.

The control system may comprise a computer. The computer program may be installed on the computer.

BRIEF DESCRIPTION OF DRAWINGS

Following drawings are appended to facilitate the understanding of the invention. The drawings show embodiments of the invention, which will now be described by way of example only, where:

Fig. 1 is a perspective view of an extrusion process wherein an extrusion billet is forced through an extrusion tool to make an extruded hollow profile.

Fig. 2a is a perspective view of an extrusion billet.

Figs. 2b-d are cross-sectional views of three different extruded hollow profiles that could all be made from the extrusion billet of Fig. 2a.

Figs. 3a-f show a perspective view, side views and cross-sectional views of an early stage of a calibration process, wherein the extruded hollow profile is placed inside a die tool and filled with a liquid.

Figs. 4a-f show a perspective view, side views and cross-sectional views of a stage in the calibration process subsequent the stage shown in Figs. 3a-f, wherein openings at both ends of the extruded hollow profile have been closed by means of closure devices.

Figs. 5a-f show a perspective view, side views and cross-sectional views of a stage in the calibration process subsequent the stage shown in Figs. 4a-f, wherein the extruded hollow profile has been compressed in the first horizontal direction X.

Figs. 6a-f show a perspective view, side views and cross-sectional views of a stage in the calibration process subsequent the stage shown in Figs. 5a-f, wherein the extruded hollow profile has been compressed in the first horizontal direction X and the second horizontal direction Y to be calibrated to a predetermined shape.

Fig. 6g shows a detail view of Fig. 6d, wherein internal pressure Pi in the hollow chambers are increased because of the compression such that an outer wall of the extruded hollow profile is forced against the die tool during the calibration process.

Fig. 7 is a schematic overview of a calibration system.

Fig. 8 shows the same detail view as Fig. 6g for a situation wherein the extruded hollow profile is compressed without the use of closure devices, thus without a pressure increase in the hollow chambers.

DETAILED DESCRIPTION

In the following, embodiments of the invention will be discussed in more detail with reference to the appended drawings. It should be understood, however, that the drawings are not intended to limit the invention to the subject-matter depicted in the drawings.

Fig. 1 illustrates an extrusion process for making an extruded hollow profile 200. An extrusion billet 100 made of aluminium or an aluminium alloy is forced through an extrusion tool 400 to form the extruded hollow profile 200. The extruded hollow profile 200 has a longitudinal direction L corresponding to the direction in which it was extruded. The extruded hollow profile 200 has a crosssection which is perpendicular to the longitudinal direction L. The profile cross-section is given by the extrusion tool 400. The finished extruded hollow profile 200 may have one or more hollow chambers 220. The one or more hollow chambers 220 extends through the longitudinal direction L of the extruded hollow profile 200, i.e. through the entire extruded hollow profile 200. After the extruding process, the one or more hollow chambers 220 have openings 221 at opposite ends of the extruded hollow profile 200.

Fig. 2a illustrates a typical extrusion billet 100. Different types of extruded hollow profiles 200 can be made from the extrusion billet 100, some examples are given in Figs. 2b-d. Fig. 2b illustrates a crosssection of a single chamber extruded hollow profile 200'. The single chamber extruded hollow profile 200' has one hollow chamber 220 enclosed by outer walls 210. Fig. 2c illustrates a cross-section of a double chamber extruded hollow profile 200". The double chamber extruded hollow profile 200" has two hollow chambers 220 enclosed by outer walls 210 and divided by one inner wall 230. Fig. 2d illustrates a cross-section of a triple chamber extruded hollow profile 200'". The triple chamber extruded hollow profile 200'" has three hollow chambers 220 enclosed by outer walls 210 and divided by two inner walls 230.

The present disclosure provides a method and an associated calibration system for calibrating an extruded hollow profile 200 after it has been extruded and cut to a predefined length. The present calibration method and calibration system 300 use a liquid 310 to support the extruded hollow profile 200 during calibration. Figs. 3a-6g illustrate four stages of such a calibration method using the calibration system 300 to calibrate an extruded hollow profile 200.

Figs. 3a-f illustrate an early stage of the calibration method, wherein Fig. 3a is a perspective view, Fig. 3b is a first side view, Fig. 3c is a second side view, Fig. 3d is a horizontal cross-section indicated with B-B in Fig. 3b, Fig. 3e is a first horizontal cross-section indicated with C-C in Fig. 3c, and Fig. 3f is a second horizontal cross-section indicated with F-F in Fig. 3b. Figs. 3a-f are all illustrating the same stage of the calibration operation.

In Figs. 3a-f, the extruded hollow profile 200 is positioned inside a die tool 340, the die tool being arranged inside a pressing tool 330. The extruded hollow profile 200 may be placed inside the die tool 340 and pressing tool 330 by means of a robotic arm 350 or a human operator.

The pressing tool 330 may comprise a housing 331. The illustrated housing 331 is box-shaped with a closed bottom and an open top, allowing it to hold liquid 310 and thus function as a liquid tank. By filling the housing with liquid 310, the extruded hollow profile 200 will be submerged when placed inside the pressing tool 330 and thus the hollow chambers 220 also being filled with the liquid 310, as seen in Fig. 3d.

As an alternative, the pressing tool 330 may be configured to be submerged in liquid, and then placing the pressing tool 330 in a liquid tank, which in turn would cause the extruded hollow profile 200 when placed inside the pressing tool 330 to be submerged.

As a further alternative, only the hollow chamber(s) 220 may be filled with a liquid 310 after the extruded hollow profile 200 is placed in the pressing tool 330.

The pressing tool 330 and the die tool 340 are configured to receive the extruded hollow profile 200 and hold the extruded hollow profile 200 in a vertical orientation, i.e. with the longitudinal direction L of the extruded hollow profile 200 arranged in the vertical direction Z, as illustrated in Figs. 3e and 3f.

The die tool 340 may comprise a plurality of die parts 341, 342, 343, 344, as illustrated inter alia in Figs. 3d-f. The die parts 341, 342, 343, 344 are preferably configured to compress the extruded hollow profile 200 horizontally from all sides and over the entire length of the extruded hollow profile 200.

The pressing tool 330 may comprise wedge-shaped cam(s) 332, a flat cam 333 and spacer cam(s) 334. The wedge-shaped cams 332 are configured to be moved in and out of the housing 331 during the calibration operation to exert a force on the die tool 340. In the example illustrated in the figures, three wedge-shaped cams 332 are used. The flat cam 333 may be configured to stay inside the housing 331 during the calibration operation. In the example illustrated in the figures, one flat cam 333 is used. The spacer cams 334 may be configured to stay inside the housing 331 during the calibration operation to guide the movement of the die parts 341, 342, 343, 344 and/or to guide the movement of the wedge-shaped cams 332.

In Fig. 3d, a first die part 341 is held in place inside the pressing tool 330 by a flat cam 333. A second die part 342 is held in place inside the pressing tool 330 by two spacer cams 334, such that the second die part 342 can move in the first horizontal direction X but not in the second horizontal direction Y. A third die part 343 is held in place inside the pressing tool 330 by the flat cam 333 and one spacer cam 334, such that the third die part 343 can move in the second horizontal direction Y but not in the first horizontal direction X. A fourth die part 344 is held in place inside the pressing tool 330 by the flat cam 333 and another spacer cam 334, such that the fourth die part 344 can move in the second horizontal direction Y but not in the first horizontal direction X.

At the stage illustrated in Figs. 3a-f, the three wedge-shaped cams 332 are lifted to a position in which they do not force the die parts 341, 342, 343, 344 towards the extruded hollow profile 200. Figs. 3e-f illustrate how the opening 221 at a lower end of the extruded hollow profile 200 may be closed by a closure device 320. The closure device 320 is arranged in the bottom of the housing 331 and the extruded hollow profile 200 is placed standing on top of the closure device 320.

In Figs. 3a-f, the extruded hollow profile 200 has not yet been calibrated. The shape of the extruded hollow profile 200 at this stage may be referred to as an initial shape, i.e. an uncalibrated shape.

Figs. 4a-f illustrates a stage in the calibration process subsequent the stage shown in Figs. 3a-f. Figs. 4a-f are all illustrating the same stage of the calibration operation. At this stage the opening 221 at an upper end of the extruded hollow profile 200 is closed by a closure device 320. The liquid 310 inside the hollow chambers 220 is thus confined.

The closure device 320 used to close the opening 221 in the upper end of the extruded hollow profile 200 may be held in place by the housing 331, such that movement of the closure device 320 in the vertical direction Z is prevented. The closure device 320 may be configured to slide in and out of the housing 331 through holes in two opposing walls.

The closure devices 320 may comprise flat surfaces configured to be pressed against the openings 221.

The closure device 320 may be configured to equalize the internal pressure Pi in the hollow chambers 220 of the extruded hollow profile 200. This may be achieved by draining water from the hollow chamber 220 wherein the pressure is too high. It may also be achieved by providing a passage between the hollow chambers 220 through the closure device(s) 320.

Figs. 5a-f illustrates a stage in the calibration process subsequent the stage shown in Figs. 4a-f. Figs. 5a-f are all illustrating the same stage of the calibration operation. At this stage the pressing tool 330 has been operated to compress the extruded hollow profile 200 in the first horizontal direction X. The first die part 341 and the second die part 342 have been moved closer to each other by means of a wedge-shaped cam 332 being lowered in between the housing 331, the second die part 342, and the two spacer cams 334, as illustrated in Figs. 5d-e.

The wedge-shaped cam 332 is preferably configured to not compress the extruded hollow profile 200 beyond a predetermined point. This may e.g. be achieved by adapting the size of the edge-shaped cam 332 accordingly.

During compression, the volume of the hollow chambers 220 may change and an internal pressure Pi in the hollow chambers 220 will increase. The die tool 340 will performing a compressive action against the outer walls 210 of the extruded hollow profile 200, causing the internal pressure Pi in the hollow chambers 220 to increase. The internal pressure Pi will prevent the outer walls 210 of the extruded hollow profile 200 from leaving the surfaces of the die tool 340 and hence avoid undesirable deformation of any outer walls 210 being supported by the die tool 340.

Figs. 6a-g illustrates a stage in the calibration process subsequent the stage shown in Figs. 5a-f. Figs. 6a-g are all illustrating the same stage of the calibration operation. At this stage the pressing tool 330 has been operated to compress the extruded hollow profile 200 also in the second horizontal direction Y. The third die part 343 and the fourth die part 344 have been moved closer to each other by means of two wedge-shaped cams 332. One wedge-shaped cam 332 has been lowered in between the housing 331, the flat cam 333, the spacer cam 334, and the third die part 343. The other wedge-shaped cam 332 has been lowered in between the housing 331, the flat cam 333, the spacer cam 334, and the fourth die part 344. The die tool 340 is preferably configured to not compress the extruded hollow profile 200 beyond a predetermined point. This may e.g. be achieved by adapting the die parts 341, 342, 343, 344 to come to a dead stop at the predetermined point. As illustrated in Fig. 6d, the third die part 343 may come to a dead stop as it meets the first die part 341 and the second die part 342, and the fourth die part 344 may come to a dead stop as it meets the first die part 341 and the second die part 342.

As illustrated e.g. in Figs. 6e-f, the die parts 342, 343, 344 may have an inclined side surface for interaction with the wedge-shaped cams 332.

In Figs. 6a-g, the extruded hollow profile 200 has been compressed to a predetermined shape. The outer walls 210 of the extruded hollow profile 200 have been brought into a plastic, permanent deformation, removing unwanted distortions like bow and twist, and re-locating the outer walls 210 to the desired position. As a result of the compressing, the internal pressure Pi in the hollow chambers 220 is greater than the external pressure. This causes the internal pressure Pi to force the outer walls 210 against the die parts 341, 342, 343, 344, as illustrated in Fig. 6g. If the internal pressure Pi is different in the hollow chambers 220, the inner wall(s) 230 will experience a differential pressure. The closure device 320 may be configured to equalize such a differential pressure.

After the final calibration step, the closing devices 320 and the die parts 341, 342, 343, 344 must be moved away to release the extruded hollow profile 200 after calibration. As the die tool 340 is released, the elastic energy stored in the extruded hollow profile 200 from the calibration will be released.

Under plastic deformation, most metals will act like being incompressible, meaning that the metal volume is conserved during plastic flow. This may imply that the compressive action undertaken to calibrate the extruded hollow profile 200, which in the end should be plastic deformation to secure a permanent final shape, will affect the length of the extruded hollow profile 200 during calibration. To control the internal pressure Pi and to avoid too high die tool forces, one or more of the closing devices 320 may be spring loaded. This is a way to regulate the internal pressure Pi in the hollow chambers 220 and to avoid high forces on the means supporting the closing devices 320. This calibration method may be used for calibrating an extruded hollow profile 200 having at least one hollow chamber 220. There can, however, be as many hollow chambers 220 as is practical from an extrusion point of view.

Fig. 7 schematically illustrates a calibration system 300 for calibrating an extruded hollow profile 200. As illustrated, the calibration method may be automated and may then typically comprise a control system 360 and a robotic arm 350. The robotic arm 350 may be configured to handle the extruded hollow profile 200, and the control system 360 may be configured to control the robotic arm 350. The robotic arm 350 may typically place the extruded hollow profile 200 into the pressing tool 330 and/or the die tool 340, and subsequently remove it after calibration. The robotic arm 350 may also handle the extruded hollow profile 200 as it undergoes heat treatment operations, e.g. solution heat treatment (solutionizing) and quenching.

In the preceding description, the calibration system has been described with a vertical arrangement. The calibration system may alternatively have a horizontal arrangement. The calibration method can also be performed with a horizontally arranged calibration system. The horizontally arranged calibration system may comprise all the same features as the vertically arranged calibration system.

The calibration system may comprise a hydraulic or mechanical press configured to drive the compressing operation. Fig. 8 illustrates a situation wherein the extruded hollow profile 200 is compressed with the die tool 400 without closure devices 320 closing the openings 221 of the hollow chambers 220. The volume inside the hollow chambers 220 may thus change freely and no pressure will build up inside the hollow chambers 220. The internal pressure in the hollow chambers 220 will not force the outer walls 210 against the die tool 340 and the predetermined final shape may thus not be achieved. As exemplified in Fig. 8, two outer walls 210 may bulge inwards. Furthermore, the inner wall 230 may bulge into one of the hollow chambers 220 without an internal pressure building up to force the inner wall 230 back.

In the preceding description, various aspects of the disclosure have been described with reference to the illustrative embodiment. For purposes of explanation, specific numbers, systems and configurations were set forth in order to provide a thorough understanding of the invention and its workings. However, this description is not intended to be construed in a limiting sense. Various modifications and variations of the illustrative embodiment, as well as other embodiments, which are apparent to persons skilled in the art to which the disclosed subject matter pertains, are deemed to lie within the scope of the present invention.

LIST OF REFERENCE NUMBERS

100 Extrusion billet

200 Extruded hollow profile

200' Extruded hollow profile with single chamber

200" Extruded hollow profile with double chamber

200"' Extruded hollow profile with triple chamber

210 Outer wall

220 Hollow chamber

221 Opening

230 Inner wall

300 Calibration system

310 Liquid

320 Closure device

330 Pressing tool

331 Housing

332 Wedge-shaped cam

333 Flat cam

334 Spacer cam

340 Die tool

341 First die part 342 Second die part

343 Third die part

344 Fourth die part

350 Robotic arm

360 Control system

400 Extrusion tool

Pi Internal pressure in the hollow chamber

X First horizontal direction

Y Second horizontal direction

Z Vertical direction

L Longitudinal direction of the extruded hollow profile

Claims

1. A calibration system (300) for calibrating an extruded hollow profile (200) by plastic deformation from an initial shape to a predetermined final shape, wherein the extruded hollow profile (200) comprises at least one hollow chamber (220) extending through a longitudinal direction (/.) of the extruded hollow profile (200), the at least one hollow chamber (220) having openings (221) at opposite ends of the extruded hollow profile (200), wherein the calibration system (300) comprises:

- a liquid (310) for filling the at least one hollow chamber (220),

- a first closure device (320) and a second closure device (320) configured to close the openings (221) to confine the liquid (310) in the at least one hollow chamber (220),

- a die tool (340) configured to define an external boundary of the predetermined final shape of the extruded hollow profile (200), and

- a pressing tool (330) configured to move the die tool (340) to compress the extruded hollow profile (200) such that an internal pressure (Pi) in the at least one hollow chamber (220) is increased and the extruded hollow profile (200) is plastically deformed into the predetermined shape.

2. The calibration system (300) according to claim 1, wherein the die tool (340) comprises:

- a plurality of die parts (341, 342, 343, 344) configured to be brought into contact with each other and stop dead at a point wherein the external boundary of the predetermined final shape is defined.

3. The calibration system (300) according to claim 2, wherein the pressing tool (330) comprises:

- a housing (331) comprising a plurality of walls configured to enclose the die tool (340), and

- a plurality of wedge-shaped cams (332) configured to move between the walls and the die parts (341, 342, 343 ,344) to force the die parts (341, 342, 343 ,344) away from the walls and towards the extruded hollow profile (200).

4. The calibration system (300) according to any one of the preceding claims, wherein the calibration system (300) comprises:

- a liquid tank configured to hold the liquid (310) and to receive the extruded hollow profile (200).

5. The calibration system (300) according to claim 4, wherein the calibration system (300) comprises:

- a robotic arm (350) configured to handle the extruded hollow profile (200), and

- a control system (360) configured to control movement of the robotic arm (350).

6. A method for calibrating an extruded hollow profile (200) having an initial shape, wherein the method comprises the steps of:

- providing an extruded hollow profile (200), wherein the extruded hollow profile (200) comprises at least one hollow chamber (220) extending through a longitudinal direction (/.) of the extruded hollow profile (200), the at least one hollow chamber (220) having openings (221) at opposite ends of the extruded hollow profile (200),

- filling the at least one hollow chamber (220) with a liquid (310),

- confining the liquid (310) in the at least one hollow chamber (220) by means of closing the openings (221),

- compressing the extruded hollow profile (200) causing an internal pressure (P, in the at least one hollow chamber (220) to increase and plastically deforming the extruded hollow profile (200) to a predetermined final shape, and

- releasing the compression on the extruded hollow profile (200) and unclosing the openings (221) of the at least one hollow chamber (220).

7. The method according to claim 6, wherein the initial shape of the extruded hollow profile (200) has an initial cross-section and the final shape of the extruded hollow profile (200) has a final crosssection, wherein the initial cross-section is larger than the final cross-section.

8. The method according to claim 6 or 7, wherein the filling of the at least one hollow chamber (220) with liquid (310) is performed by means of submerging the extruded hollow profile (200) in the liquid (310).

9. The method according to any one of claims 6-8, wherein the openings (221) are closed by means of a first closure device (320) and a second closure device (320).

10. The method according to any one of claims 6-9, wherein the compressing of the extruded hollow profile (200) is performed by means of a pressing tool (330).

11. The method according to any one of claims 6-10, wherein a die tool (340) is used to define an external boundary of the predetermined final shape of the extruded hollow profile (200).

12. The method according to any one of claims 6-11, wherein the method is performed using a calibration system (300) according to any one of claims 1-5.

13. The method according to any one of claims 6-11, wherein the extruded hollow profile (200) is made of an aluminium alloy, preferably a 6XXX series aluminium alloy or a 7XXX series aluminium alloy.

14. The method according to any one of claims 6-13, wherein the extruded hollow profile (200) is made of an aluminium alloy having a composition (in wt%)

0.40-0.8 Si; up to 0.7 Fe;

0.10-0.40 Cu; up to 0,15 Mn;

0.8-1.2 Mg;

0.04-0.35 Cr; up to 0.25 Zn; up to 0.15 Ti, others up to 0.05 each, in total up to 0.15, and balance Al and incidental impurities.

15. A method for processing an extrusion billet (100) produced from an aluminium alloy, wherein the method comprises the steps of: (a) homogenizing the extrusion billet (100),

(b) preheating the extrusion billet (100) prior to the extruding operation,

(c) extruding the extrusion billet (100) to form an extruded hollow profile (200),

(d) cooling the extruded hollow profile (200) down to room temperature after the extruding operation, and

(e) artificially aging the extruded profile, the method further comprises the step of: calibrating the extruded hollow profile (200) according to any one of claims 6-14.

16. The method according to claim 15, wherein the method further comprises the step of: soft annealing the extrusion billet (100) at a temperature between 350 and 450 °C, after the homogenizing step (a) and before the preheating step (b).

17. The method according to any one of claims 15-16, wherein the method comprises the steps of:

- exposing the extruded hollow profile (200) of step (d) to a solution heat treatment and quenching operation.

18. The method according to claim 17, wherein the calibration operation is performed after the quenching operation.

19. The method according to claim 18, wherein the steps of quenching, calibration and artificial aging are performed in an in-line process.

20. A computer program comprising instructions which, when the program is executed by a control system (360), cause the control system (360) to carry out the steps according to any one of claims 6- 19.