EP3501683A1

EP3501683A1 - Method of forming a metal can

Info

Publication number: EP3501683A1
Application number: EP17210346.7A
Authority: EP
Inventors: Henri Kwakkel
Original assignee: Tata Steel Ijmuiden BV
Current assignee: Tata Steel Ijmuiden BV
Priority date: 2017-12-22
Filing date: 2017-12-22
Publication date: 2019-06-26

Abstract

The invention relates to a method of forming a metal can comprising the steps of:
- providing a metal sheet (5) with a polymer coating on at least one side,
- cutting a blank (4) from the metal sheet (5),
- drawing the blank (4) into a cup (9),
- wall ironing the cup (9) into a can (18), and
wherein the blank (4) is drawn in a single drawing step into a cup (9), wherein the inner diameter of the cup (9) after the single drawing step matches the required inner diameter to be subjected to the wall ironing step.

Description

Field of the invention

The invention relates to a method of forming a metal can, more in particular a method of forming a steel can from a coated steel sheet.

Background of the invention

In can making sheet metal is formed into finished cans in which the important steps for two-part cans are cutting a blank from the steel sheet, deep drawing of the blank into a cup and wall ironing of the cup into a can with a predefined wall thickness. After the wall ironing step further steps may include the trimming, bottom forming, flanging, beading of the can and finally putting a closure on the can.
There are many important factors in the can making process, which include the type of material that is used, the thickness of the material, the work hardening exponent, the blank size, the draw ratio, the draw radius, the draw speed, the reduction of the wall thickness, the lubricant, the temperature, to name the more important factors.
Most of the process of forming a can is typically done with two separate devices, respectively the "cupper" and the "bodymaker". With the cupper a blank is cut from the metal sheet the shape of which is dependent on what the final shape of the can should be. Most cans are of a circular cross-section for which a round disc is cut from the metal sheet. Subsequently, the blank is subjected in the cupper to a deep drawing step wherein a cup is formed with a diameter of the bottom part larger than that of the final can to be formed as well as with a height of the wall far less than that of the final can.
In the bodymaker further forming steps are carried out starting with a redraw step wherein the cup is subjected to a second deep drawing step, the re-draw step, wherein the diameter of the bottom part is reduced and the height of the wall increased. Immediately after the re-draw step the cup is subjected to a wall ironing step in which the thickness of the wall is reduced therewith increasing the height of the wall. Typically a number of successive wall ironing steps are carried out to realise a wall height slightly larger than the height of the final can.
In the process of drawing, redrawing and wall ironing a lot of heat is generated. With steel sheet provided with a polymer coating this may result in a damaged polymer coating or in a polymer coating that partially peels off from the steel substrate. To avoid such damage to the polymer coating it should be prevented that the temperature of the cup or can increases above a certain maximum temperature.
Such problems do not arise with the use of uncoated sheet metal for can making such as tinplate, tin free steel (TFS) or electrolytically chrome coated steel (ECCS), wherein only after the can has been formed a coating is applied. In this process usually a lacquer is applied as a coating which however does not have the favourable properties of a polymer coating such as for instance good release properties.

Objectives of the invention

It is an objective of the present invention to provide a method of forming a metal can wherein it is prevented that a previously applied polymer coating is damaged in the can forming process.
It is another objective of the present invention to provide a method of forming a metal can wherein the temperature of the can is controlled in order not to exceed a predefined temperature.
It is another objective of the present invention to provide a method of forming a metal can which can easily be applied using existing can forming devices.
It is another objective of the present invention to provide a method of forming a metal can which allows to operate a can making line with the same speed and capacity as with non-polymer coated cans.

Description of the invention

The invention relates to a method of forming a metal can as defined in claims 1 - 15.
One or more of the objectives of the invention are realized by providing a method of forming a metal can comprising the steps of:

providing a metal sheet with a polymer coating on at least one side,
cutting a blank from the metal sheet,
drawing the blank into a cup,
wall ironing the cup into a can, and wherein the blank is drawn in a single drawing step into a cup, wherein the inner diameter of the cup after the single drawing step matches the required inner diameter to be subjected to the wall ironing step.

Since drawing, redrawing and wall ironing of the drawn cup generates a lot of heat the temperature of the cup or can as the case may be increases considerably. This is not an issue with cans that are provided with a coating after completion of the can forming process, but for cans made from pre-coated sheet material, more in particular for polymer pre-coated sheet material, this is a real issue. If the temperature increases to above a certain critical temperature the coating will definitely be damaged, leading to loss of production and possibly the need to stop the process and clean the forming devices. In this respect polymer coated cans comprise one sided and two sided coated cans with a single or multi-layered polymer coating. The polymer coating on opposite sides may be different in the number of coating layers and the layers of the coating may be of different chemical composition.
In the conventional can forming process a cupper and a bodymaker are used wherein in the cupper a blank is cut and deep drawn into a cup and wherein the further forming steps are carried out in the bodymaker comprising a re-draw step and immediately thereafter one or more wall ironing steps. Since re-drawing generates heat the temperature of the re-drawn cup increases already considerably immediately before the wall ironing step the end temperature of the re-drawn can is very likely to be above the critical temperature of a polymer coating. Although it would be possible to integrate an extra cooling step between the re-draw step and the first wall ironing step in addition to the normal cooling of the bodymaker this would mean large and costly modifications to the bodymaker.
By providing a single drawing step before wall ironing wherein the step of drawing the blank into a cup is carried out in a first device and the step of wall ironing the cup is carried out in a second device the re-drawn step is removed from the can forming process. With that the increase of the temperature of the cup immediately before the wall ironing step is avoided.
An alternative would be to maintain the known procedure with a re-draw step and implementing the re-draw step in the first device. This again would mean a costly modification in the can making line, more in particular that the cupper device should be adapted to be able to also apply a re-draw step. A further disadvantage is that with a draw and an immediate successive re-draw step the temperature of the cup would also increase considerably and could increase up to or over the critical temperature limit that the coating can be exposed to without being damaged. With a single draw step the temperature will increase but will stay far from the critical temperature limit.
By providing that the inner diameter of the cup after the single drawing step matches the required inner diameter to be subjected to the wall ironing step in the second device, the drawn cup is ready to be processed in existing second devices. In practice this means that the inner diameter of the cup corresponds to the outer diameter of the punch sleeve with which the cup is drawn through the successive ironing die rings.
The terms "drawing" and "deep-drawing" are used interchangeable in the description, unless indicated otherwise.
Since the single drawing step will also result in an increase in temperature it is further provided that the drawn cup is cooled before subjecting the drawn cup to the wall ironing step. By cooling the drawn cup before applying the wall ironing step allows for a larger ironing reduction without that the temperature increases up to or over the critical temperature limit. According to a further aspect the drawn cup is conveyed from the first device to the second device and wherein the drawn cup is cooled from a first temperature to a second temperature during conveyance of the drawn cup to the second device.
It is further provided that the conveying speed and distance over which the drawn cup is conveyed from the first device to the second device is such that there is sufficient time to cool the cup to the second temperature. Cooling down the drawn cup to the second temperature when conveying the drawn cup to the second device can be carried out in a number of different ways for instance cooling by using a cold liquid, like for instance water, or by using cold air. Although very well possible this again requires further equipment which needs to be built in the can making line.
According to a further aspect of the invention cooling is carried out by exposing the drawn cup to ambient air during conveyance of the cup from the first to the second device. The temperature of the steel sheet increases considerably during the drawing step, but because the drawn cups are formed out of relatively thin sheet metal the amount of heat energy stored in a drawn cup is not very large. Moreover, because of the ratio between relatively limited thickness of the sheet material and large surface area of the drawn cup the heat stored in the drawn cup is rapidly lost. In typical can making lines the conveying distance between first and second device is large enough and because of that enough conveying time between successive forming devices to sufficiently cool the drawn cans.
The second temperature is ambient temperature or ambient temperature with a tolerance range of +10°C or a tolerance range of +20°C. Ambient temperature is to be understood as a temperature in a range of 5 - 30°C, depending largely on the temperature in the production facility. The second temperature should preferably be at most 40°C and more preferably at most 30°C.
By eliminating the re-draw step in the forming process and applying only a single drawing step before the wall ironing step different sized blanks are used. The maximal possible draw ratio is in the range of 1.9 - 2.1, wherein the draw ratio is the blank diameter divided by the cup diameter, and for that reason a smaller blank is used for a cup with an inner diameter corresponding to the inner diameter of the final can.
With a smaller diameter blank also the wall height of the cup after the single drawing step according to the method will be lower than with the conventional method with a re-draw step. According to the method the lower height of the cup wall is compensated by providing a blank with a larger thickness than would be used for the same can in the conventional forming process, and subject the wall with the larger thickness to a larger ironing reduction to arrive at a can with the same thickness as a can made according to the conventional process.
The larger reduction of the wall thickness in wall ironing the cup is carried out in multiple successive steps. Because of the low temperature of the drawn cup when arriving at the second device, which is about ambient temperature, it is possible to subject the drawn cup to the necessary number of successive ironing steps to realise the required larger ironing reduction.
As far as necessary the wall ironing of the cup is controlled such that the temperature of the cup does not exceed a temperature of 225°C, preferably does not exceed a temperature of 200°C. Above the 225°C limit the coating will almost certainly be damaged and to be on the safe side and prevent any damage it is provided that the temperature does not exceed the 200°C limit. Control of the temperature as far as necessary can be done by increasing the cooling realised by the standard cooling system present in the second device.
For the same reason it is provided that the wall ironing of the cup is carried out in at most 5 successive ironing steps, typically in at most 4 successive ironing steps.
By restricting the drawing step to a single drawing step and providing sufficient cooling between the first and second device, the temperature of the drawn cup at the start of wall ironing of the cup will be sufficiently decreased to be able to carry out the necessary wall ironing steps to realise the required wall height of the can without exceeding the predefined temperature limit.
According to a further aspect it is provided that the blank diameter for a given can diameter is in a range of 80 - 95%, typically in a range of 85 - 92% of a blank diameter used in a method comprising a re-draw step in the second device. With a reduced diameter more blanks can be cut from a metal sheet though the metal sheet has to be thicker in order to make a can of the same standard dimensions.
The draw ratio in the single drawing step is in a range of 1.9 - 2.1, typically in a range of 1.9 - 2.0. This is about the maximum or close to the maximum possible draw ratio without the chance that any wrinkling or other negative drawing effects will occur.
In order to realise a can of the same standard dimensions from a blank of smaller diameter it is provided that the blank has an initial thickness which is a factor 1.2 - 2.0, typically 1.3 -1.8, larger compared to the thickness of a blank used in a method comprising a redraw step in the second device.
To arrive at a can with the same standard wall thickness the total reduction in the wall ironing step is in a range of 50 -70%, typically in a range of 55 - 65%.

Brief description of the drawings

The invention will be further explained on basis of the example shown in the drawing, in which:

fig.1: shows a schematic cross section of a first device to cut a blank and draw the blank, and
fig.2: shows a schematic cross section of a second device for the wall ironing step of the cup drawn with the first device.

Detailed description of the drawings

In fig.1 part of a first device 1 is shown in a first position at the left half of the figure and in a second position at the right half of the figure. The first device 1 has a cutter 2 with a cutting tool 3 to cut a blank 4 from a metal sheet 5. At the left half of the figure blank 4 is cut from metal sheet 5 and clamped between blank holder 6 and die 7. A punch 8 is in a position directly above blank 4.
At the right half of the figure the punch 8 has been pushed in downward direction, as seen in the figure, therewith drawing blank 4 into a cup 9. The diameter of punch 8 corresponds to the diameter of the punch 11 in the second device 10 (see fig.2), the device for the wall ironing of cup 9.
In fig.2 a schematic cross section is shown of the second device 10 for the wall ironing of cup 9. Device 10 is provided with a punch 11 guided in sleeve 12 and a number of wall ironing rings 13,14,15,16, a stripper tool 17 and a domer tool 18.
With respect to a conventional bodymaker device the second device 10 does not comprise a redraw die for a re-draw step in which a cup is re-drawn to a smaller diameter and increased wall height. The cup 9 conveyed from the first device 1 to the second device 10 already has an inner diameter corresponding to the diameter of punch 11 and is directly subjected to the wall ironing step in second device 10.
After passing through all wall ironing rings 13,14,15,16, the formed can 18 arrives at the domer tool in which the bottom of can 18 is formed. The stripper tool 17 allows the withdrawal of punch 11 from can 18, after which formed can 18 is released from second device 10.

Claims

Method of forming a metal can comprising the steps of:
- providing a metal sheet with a polymer coating on at least one side,

- cutting a blank from the metal sheet,

- drawing the blank into a cup,

- wall ironing the cup into a can,
characterised in that the blank is drawn in a single drawing step into a cup, wherein the inner diameter of the cup after the single drawing step matches the required inner diameter to be subjected to the wall ironing step.
Method according to claim 1, wherein the cup is cooled from a first temperature to a second temperature before wall ironing the cup into a can.
Method according to claim 1, wherein the step of drawing the blank into a cup is carried out in a first device and the step of wall ironing the cup is carried out in a second device.
Method according to claim 3, wherein the drawn cup is conveyed from the first device to the second device and wherein the drawn cup is cooled from a first temperature to a second temperature during conveyance of the drawn cup to the second device.
Method according to claim 4, wherein the conveying speed and distance over which the drawn cup is conveyed from the first device to the second device is taken such that there is sufficient time to cool the cup down to the second temperature.
Method according to one or more of claims 2-5, wherein the cooling is carried out by exposing the drawn cup to ambient air during conveyance of the cup from the first to the second device.
Method according to one or more of claims 4-6, wherein the second temperature is ambient temperature.
Method according to one more of the previous claims, wherein the wall ironing of the cup is carried out in multiple successive steps.
Method according to claim 8, wherein the wall ironing of the cup is carried out in at most 5 successive ironing steps.
Method according to one more of the previous claims, wherein the wall ironing of the cup is controlled such that the temperature of the cup does not exceed a temperature of 225°C, preferably does not exceed a temperature of 200°C.
Method according to one more of the previous claims, wherein the blank diameter for a given can diameter is in a range of 80 - 95%, typically in a range of 85 - 92%, of a blank diameter used in a method comprising a re-draw step in the second device.
Method according to one more of the previous claims, wherein the draw ratio in the single drawing step is in a range of 1.9 - 2.1, typically in a range of 1.9 - 2.0.
Method according to one more of the previous claims, wherein the blank has an initial thickness which is a factor 1.2 - 2.0, typically 1.3 -1.8, larger compared to the thickness of a blank used in a method comprising a re-draw step in the second device.
Method according to one more of the previous claims, wherein the total reduction in the wall ironing step is in a range of 50 -70%, typically in a range of 55 - 65%.
Method according to one or more of the previous claims, wherein the coating is a single or multi-layered polymer coating.