EP3488944A1

EP3488944A1 - Process for the production of a can body by wall ironing

Info

Publication number: EP3488944A1
Application number: EP18208554.8A
Authority: EP
Inventors: Emile JASPERSE; Frederik Jonker
Original assignee: Tata Steel Ijmuiden BV
Current assignee: Tata Steel Ijmuiden BV
Priority date: 2017-11-27
Filing date: 2018-11-27
Publication date: 2019-05-29

Abstract

The invention relates to a process for the production of a can body comprising a base and a tubular body from sheet metal which is coated on at least one side with a polymer layer, in which process, firstly, a round disc is produced from the sheet metal, which disc is then deep-drawn into a cup which has a polymer layer at least on the outside, after which this cup is formed into a can body by wall ironing, the wall ironing taking place in a single stroke by moving the cup successively through a redraw die and one or more wall-ironing rings.

Description

The invention relates to a process for the production of a can body comprising a base and a tubular body from sheet metal which is coated on at least one side with a polymer layer, in which process, firstly, a round disc is produced from the sheet metal, which disc is then deep-drawn into a cup which has a polymer layer at least on the outside, after which this cup is formed into a can body by wall ironing, the wall ironing taking place in a single stroke by moving the cup successively through one or more wall-ironing rings.
A process of this nature is described in EP0402006-A1 , which is based on a laminate comprising an aluminium sheet. This patent proposes that the problems with processing of this laminate be solved by employing a combination of a proposed exit angle from a wall-ironing ring and an entry angle thereof which is selected between 1 and 4° and external cooling after each wall-ironing step. This patent also proposes a specific selection of material for the wall-ironing ring.
US Patent A-3,765,206 proposes the wall ironing of cans from coated steel utilizing a single wall-ironing ring with an entry angle of between 4 and 6°. In this case, the entry angle is understood to mean the angle which the plane of entry to a wall-ironing ring forms with the axis of the said ring. However, this document relates only to steel sheet with a metallic coating.
EP0298560-A2 describes a method of wall ironing in two die regions in which in one die region hydrodynamic lubrication under pressure is applied, whereas in the other die region there is no lubrication.
It has been found that various problems may arise with wall ironing for the production of a can body from a laminate based on a steel sheet and a polymer layer. Some of these problems relate to the polymer layer. During wall ironing the shear forces can become excessively high in the coating itself. This excessive shear results in an increase of roughness of the polymer layer. This increase in roughness is also known as "scuffing". This increase in roughness results in damage to the coating and possibly to contact between the metal substrate with the wall ironing tooling and/or a visually unacceptable can wall finish. Or in very severe cases rupture of the can body wall.
It is the object of the invention to provide a method of wall ironing which reduces or eliminates scuffing of the polymer layer on the can body wall.
It is also an object of the invention to provide a method of wall ironing which prevents or reduces the roughening of the polymer surface.
It is also an object of the invention to provide a method of wall ironing which prevents rupture of the polymer film.
One or more of these objects is reached with a process for the production of a can body comprising a base and a tubular body from sheet metal which is coated on one side with a polymer layer, in which process first a round disc is produced from the sheet metal, which disc is then deep-drawn into a cup, wherein the outside of the cup is provided with said polymer layer, after which this cup is redrawn and subsequently formed into a can body by wall ironing, the wall ironing taking place in a single stroke by moving the redrawn cup successively through one or more wall-ironing rings by means of a punch, characterized in that the used sheet metal is selected from the group of sheet metals consisting of (uncoated steel sheet (blackplate), tin coated steel sheet (tinplate), chromium-chromium oxide coated steel sheet (ECCS), tinplate which was diffusion annealed to form an iron-tin alloy consisting of at least 80% of FeSn (50 at.% iron and 50 at.% tin) thereupon, chromium-chromium oxide coated steel sheet produced by electroplating from a trivalent chromium electrolyte (TCCT)) and wherein the entry angle (α) for the first wall-ironing ring is between 3.5 and 4.5° and the exit angle (β) for the first wall-ironing ring is between 2.5 and 3.5°.
Preferable embodiments are provided in the dependent claims.
It is noted that the phrase 'moving the redrawn cup successively through one or more wall-ironing rings by means of a punch' means that the external diameter of the can body is the same everywhere along the can body, and that there is no remaining flange on the can body after the drawing. In the process according to the invention the direction of movement of the cup and can is the same (see figure 1). E.g. GB2547016 discloses a process where there is always a flange remaining on the can body. This means that the can body is not pushed through the rings but is halted to retain the flange and the can must be retracted to remove it from the process. The direction of movement of the cup and can is reversed near the end. Moreover, the flange in GB2547016 has to be removed afterwards leading to significant waste compared to the optional trimming of the can body according to the invention.
The invention therefore consists in the fact that, when a sheet metal is used which has been selected from the group consisting of i). uncoated steel sheet (blackplate), ii). tin coated steel sheet (tinplate), iii). chromium-chromium oxide coated steel sheet (ECCS), iv). tinplate which was diffusion annealed to form an iron-tin alloy consisting of at least 80% of FeSn (50 at.% iron and 50 at.% tin) thereupon, or v). a chromium-chromium oxide coated steel sheet produced by electroplating from a trivalent chromium electrolyte (TCCT), the advantage of the wall-ironing ring or rings according to the invention is that scuffing due to shear in the polymer coating during wall ironing is prevented or minimised. If more than one ring is used, the entry angle for each successive wall-ironing ring has to be smaller than that of the preceding ring. In the second and any further wall-ironing rings following the first wall-ironing ring the entry angle should become smaller in order to prevent scuffing. It was found that the entry angle for the first wall-ironing ring should be between 3.5 and 4.5° in order to prevent the expansion force in this first ring becoming excessive.
It has been found that if the entry angles α for the wall- ironing rings 6 and 7 are made to conform with the conditions described above, good results for the surface of the can body 9 formed are obtained without producing impermissibly high expansion forces in the wall-ironing rings and, most importantly, without hairs. Such good results are obtained, for example, if the entry angles α for the wall- ironing rings 6 and 7 are selected, for example, to be 4° and 2°, respectively. Selecting the material of the polymer coating as described above results in cans with an intact coating, and the risk of the coating becoming detached from the metal base is negligible. The process according to the invention can be used without external coolant. With an external coolant a coolant is meant that is applied directly to the can during the wall-ironing operation as in EP0402006-A1 . The coolant usually also contains lubricant or provides lubrication by itself to facilitate the wall-ironing operation. In the process according to the invention the polymer layers provide the lubrication. It is possible to use internal cooling in the form of internal cooling of the punch and/or the wall-ironing rings or the spacers between the rings. In this case no external coolant is necessary. This process, referred to as a dry process, is not hampered by large amounts of coolant that need to be processed and the can bodies need not be rinsed to remove the excess coolant and dried afterwards.
The polymer layer preferably comprises two or more layers, each with their specific properties. It is preferable to use a three-layer polymer coating system on each side of the substrate. The three layers of coating on each side of the substrate comprise an adhesion layer, a main layer and a surface layer with optimised interface properties, such as release properties, an optimised adhesion to steel is provided by the adhesion layer, and the main layer has a more general functionality such as providing barrier properties. The table below gives an overview.
The inventors found that it is beneficial if a cylindrical land zone having a length L is present between the entry and the exit of each wall ironing ring wherein L is at most 0.6 mm, preferably at most 0.5 mm, more preferably at most 0.3 mm, preferably wherein L of the first wall ironing ring is different from L of the second wall ironing ring.
The wall-ironed can body is sometimes very tightly adhered to the punch as a result of the smoothness of the punch and inner surface of the can body and the retained tension in the can. In an embodiment the reduction in the second wall-ironing ring (RED2) or, in case of more than two wall-ironing rings being used, the reduction in the last wall-ironing ring (RED_Last), is chosen such so as to remove tension in the can body thereby facilitating the stripping of the can body from the punch. For this purpose the reduction RED2 (or RED_Last) is preferably chosen low, preferably between 0.1 and 10%.
In a preferred embodiment the wall thickness of the cup is reduced by a value RED1 between 10 and 60 % in the first wall-ironing ring and wherein, if present, the wall thickness is further reduced by a value RED2 of between 0.1 and 30% in the second wall-ironing ring. More preferably the wall thickness of the cup is reduced by a value RED1 between 20 and 55 % in the first wall-ironing ring and/or the wall thickness of the cup is further reduced in the second wall-ironing ring by a value RED2 of at least 2%, preferably more than 5 %.

Examples

A three layer polymer coating system with a total thickness of 30 µm is applied to one side of a steel strip (the side becoming the outside of the can) with a thickness of between 0.10 and 0.50 mm by means of film lamination. In this example the coated strip obtained is used to produce, in two steps, a cup with a diameter of 73 mm, the polymer-coated side forming the outside of the cup. In the first step, a cup with a diameter of 100 mm is deep drawn from a round disc with a diameter of 150 mm. In the second step, this cup is formed into a cup having the final diameter of 73 mm by a further deep-drawing operation. See Fig. 1 for the schematic representation. This cup is fed to a wall ironing machine in which the wall thickness of the cup is reduced by wall ironing at a speed of between 180 and 600 strokes per minute and using a redraw ring followed by a first wall-ironing ring with an entry angle α₁ and an exit angle β₁, which reduces the wall thickness of the cup by a value (RED1) between 10 and 60 % and a second wall-ironing ring an entry angle α₂ and an exit angle β₂, which reduces the wall thickness of the cup by a value RED2 between 2 and 25%.

Application	Adhesion layer	Main layer	Surface layer
Food	Optimised for sterilisation	Optimised for non-blushing performance or coating colour	Optimised for forming performance or content release
Aerosol	Optimised for heat resistance	Optimised for barrier performance or coating colour	Optimised for print
General Line (Paint)	Optimised for chemical resistance	Optimised for barrier properties or coating colour
Beer and beverage	Optimised for adhesion	Optimised for barrier performance, deformation stresses and coating colour	Optimised for forming performance and print

These experiments showed that an entry angle of 4° and an exit angle of 3° without external coolant provided excellent results with the majority of polymer coated strips. No scuffing was observed. Comparative experiments showed that the angle is critical in obtaining a good result. The method according to the invention is particularly suitable for polymer coatings which contain no or only insignificant amounts of titanium dioxide. However, the inventors found that white layers are somewhat more prone to scuffing because of the loading of the film with hard particles, such as Ti0₂ which have an abrasive effect even though these films could also be processed with the settings as claimed. The inventors found that when using a white coating on the intended outside of the can which is pigmented with titanium dioxide that the entry angle α for the first wall-ironing rings is preferably between about 1.5 and 2.5 °. This is believed to be caused by the hard titanium dioxide particles have a scouring effect that increases the risk of damage to the film when being processed with an entry angle for the first wall-ironing ring of between 3.5 and 4.5°. A white titanium dioxide pigmented coating can be further processed with the exit angle (β) of the first ring between 2.5 and 3.5° and an entry angle (α) for the second wall-ironing ring, if present, of between 1.5 and 2.5° and the exit angle (β) is between 2.75 and 3.25°, similar to the method according to the invention.

Examples of successful combinations (OK=no scuffing, Not OK=scuffing or damaged, Just OK=acceptable), exit angle = 3°.

# Rings	Entry angle (°)	Reduction RED1 (%)	Entry angle (°)	Reduction RED2 (%)	Result
1	4	43	-	-	OK
1	4	32			OK
1	4	26			OK
1	4	45			OK
1	4	41			OK
1	4	31			OK
1	4	46			OK
1	4	45			OK
1	4	48			OK
1	4	50			OK
1	4	48			OK
1	4	48			OK
1	4	25			OK
1 (Comparison)	5	48			Not OK
2	4	48	2	6.9	OK
2			2	14.6	OK
2			2	21.5	OK
2			2	27	OK
2			2	13	OK
2			2	19.2	OK
2			2	10	OK
2 (Comparison)	4	48	3	13	Not OK
2 (Comparison)	4	48	4	13	Not OK
1*	2	46	-	-	OK
1*	2	50			OK
1*	2	48			OK
1*	2	25			OK
2*	2	48	2	31	OK
2*			2	50	OK
2*			2	48	OK
2*			2	25	OK
2*(Comparison)	2	48	4	32	Not OK
2*	4	45	-	-	Just OK

*White film, TiO₂ pigment.

The polymer coated steel substrates that can be processed by the process according to the invention are preferably based on polycondensates, such as polyesters, copolyesters (including PET, PBT, polyethylene furanoate (PEF), poly(lactic acid) (PLA)) or polyamides, polyolefins, elastomers, crystallisable polyaddition polymers, or any other polymer that can be formed in a film by extrusion. The polymer coating may consist of one or more layers. Preferably the polymer coating layer comprises or consists of polyethylene terephthalate, IPA-modified polyethylene terephthalate, CHDM-modified polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polyethylene furanoate, poly (lactic acid) or copolymers or blends thereof.
The procedure of the novel process is illustrated in more detail in the appended figures, in which Fig. 1 shows various processing systems in various processing phases, Fig. 2 shows a detail of a wall ironing operation and Fig. 3 shows a schematic detail of the workface of an ironing ring with a land zone between the (frusto-conical) entry and exit plane.
Fig. 1 illustrates how a preformed deep-drawn cup 3 is formed into a finished wall-ironed can body 9. The cup 3 is placed between a redraw sleeve 2 and a redraw die 4. When punch 1 moves to the right, the cup 3 is brought to an internal diameter of the final finished can 9 by the redrawing step.
Then, the punch 1 successively forces the product through (in this example) two wall-ironing rings 6 and 7. Ring 8 is an optional stripper ring. Wall ironing provides the can body 9 to be formed with its ultimate wall thickness and wall length. Finally, the base of can body 9 is formed by moving punch 1 towards an optional base tool 10.
Retracting punch 1 allows to detach can 9 from the punch 1 so that it can be discharged in the transverse direction. The optional stripper ring may assist in this. The can 9 is then subsequently trimmed, optionally necked and provided with a lid after filling.
Fig. 2 provides a detailed illustration of the passage of a part of the can wall to be formed through, for example, wall-ironing ring 5. Punch 1 is indicated diagrammatically.
The entry plane for wall-ironing ring 5 runs at an entry angle α to the direction of the axis of the wall-ironing ring. The thickness of the material of the wall to be formed is reduced between punch 1 and wall-ironing ring 5. This material comprises the actual metal can body wall 11 with layers of polymer 12 and 13 on either side. The layer of polymer 12 becomes the outside of the can body, and the layer of polymer 13 becomes the inside of the can body, eventually coming into contact with the contents of the can. The figure illustrates how the thickness of all three layers 11, 12 and 13 is reduced.
Fig. 3 shows a schematic detail of the workface of an ironing ring with a land zone between the (frusto-conical) entry and exit plane. The radii of the transfer between the land zone and the entry plane and the radius of the transfer between the land zone and the exit plane are between 0.1 and 10 mm, preferably between 0.2 and 5 mm.
The wall ironing rings are preferably provided with a land zone of length L which is located between connects the frusto-conical entry surface and the frusto-conical exit surface of the ring. The land zone is a cylindrical ring and has a length of at most 0.6 mm, preferably of at most 0.5 mm, and even more preferably of at most 0.3 mm.

Claims

Process for the production of a can body (9) comprising a base and a tubular body from sheet metal which is coated on one side with a polymer layer (12), in which process first a round disc is produced from the sheet metal, which disc is then deep-drawn into a cup (3), wherein the outside of the cup is formed by said polymer layer (12), after which this cup is redrawn and subsequently formed into a can body (9) by wall ironing, the wall ironing taking place in a single stroke by moving the redrawn cup (5) successively through one or more wall-ironing rings (6, 7) by means of a punch (1), characterized in that the used sheet metal is selected from the group of sheet metals consisting of (uncoated steel sheet (blackplate), tin coated steel sheet (tinplate), chromium-chromium oxide coated steel sheet (ECCS), tinplate which was diffusion annealed to form an iron-tin alloy consisting of at least 80% of FeSn (50 at.% iron and 50 at.% tin) thereupon, chromium-chromium oxide coated steel sheet produced by electroplating from a trivalent chromium electrolyte (TCCT)) and wherein the entry angle (α) for the first wall-ironing ring is between 3.5 and 4.5° and the exit angle (β) for the first wall-ironing ring is between 2.5 and 3.5°.
Process according to claim 1 wherein the sheet metal is coated on both sides with a polymer layer (12, 13).
Process according to claim 1 or 2 wherein additional wall-ironing rings (7), positioned behind the first wall-ironing ring (6), are used wherein the entry angle of each successive wall-ironing ring is smaller than that of the preceding ring.
Process according to any one of claims 1 to 3 wherein no external coolant is directly applied to the can body during the wall-ironing operation
Process according to any one of claims 1 to 4 wherein the entry angle (α) for the second wall-ironing ring (7), if present, is between 1.5 and 2.5° and the exit angle (β) is between 2.75 and 3.25°.
Process according to any one of the preceding claims wherein the entry angle (α) for the first wall-ironing ring (6) is at least 3.75° and/or at most 4.25.
Process according to any one of claims 1 to 6 wherein the entry angle (α) for the second wall-ironing ring (7), if present, is at least 1.75 and/or at most 2.25°.
Process according to any one of claims 1 to 7, wherein a cylindrical land zone having a length L is present between the entry and the exit of each wall ironing ring and wherein L is at most 0.6 mm, preferably at most 0.5 mm, more preferably at most 0.3 mm.
Method according to any one of the preceding claims wherein the reduction in the second wall-ironing ring (7) is chosen to remove tension in the can body thereby facilitating the stripping of the can body (9) from the punch (1).
Method according to any one of the preceding claims wherein the wall thickness of the cup is reduced by a value RED1 between 10 and 60 % in the first wall-ironing ring and wherein, if present, the wall thickness is further reduced by a value RED2 of between 0.1 and 30 % in the second wall-ironing ring.
Method according to claim 10 wherein the wall thickness of the cup is reduced by a value RED1 between 20 and 55 % in the first wall-ironing ring.
Method according to claim 10 or 11 wherein the wall thickness of the cup is further reduced in the second wall-ironing ring by a value RED2 of at least 2 %, preferably of at least 5 %.
Method according to any one of the preceding claims wherein the polymer layer is selected from polycondensates such as polyester, co-polyester, polyethylene furanoate (PEF), poly(lactic acid) (PLA)), polyamides, polyolefins, elastomers, crystallisable polyaddition polymers.
Method according to claim 13 wherein the polymer layer comprises or consists of polyethylene terephthalate, IPA-modified polyethylene terephthalate, CHDM-modified polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polyethylene furanoate, poly (lactic acid) or copolymers or blends thereof.
Can produced according to the process of any one of claims 1 to 14.