CA2142531A1

CA2142531A1 - Resin film laminated aluminum sheet for can by dry forming

Info

Publication number: CA2142531A1
Application number: CA002142531A
Authority: CA
Inventors: Keiichi Shimizu; Fumio Kunishige; Atsuo Tanaka
Original assignee: Toyo Kohan Co Ltd
Current assignee: Toyo Kohan Co Ltd
Priority date: 1994-02-14
Filing date: 1995-02-14
Publication date: 1995-08-15
Also published as: GB2286364B; IT1278364B1; DE19504678C2; GB9502752D0; ITTO950091A0; DE19504678A1; ITTO950091A1; FR2716139B1; GB2286364A; FR2716139A1

Abstract

The invention relates to an aluminum sheet laminated on both sides with a resin film suitable for the production of a two-piece can having a can height about twice the length of the can diameter and a thin wall thickness from 30 to 70% of the original sheet thickness formed by a wall thickness reduction process without the use of a water-based coolant or lubricant.

Description

The present invention relates to a metal sheet laminated on both sides with a thermoplastic resin film suitable for the production of a two-piece can having a thin wall thickness formed by a wall thickness reduction process such as ironing, without the use of a water based coolant or a lubricant.
Two-piece cans are generally DRD (drawn and redrawn cans), (DRD cans) and drawn and wall ironed cans (DWI cans) produced from tin plated steel sheet, aluminum sheet, aluminum alloy sheet or electrolytically chromium coated steel (ECCS), and drawn thin-redrawn cans (DTR cans) have recently been put to practical use. DRD cans have a rather thick wall thickness which becomes thicker in proportion to the can height due to drawing and redrawing. It is used as a can having a rather low height from a view point of economy. The materials used for DRD are ECCS, tin plated steel sheet or aluminum alloy sheet. On the other hand, DWI can is used as a can having a high height because the wall thickness can be reduced to one third of that of the original sheet. The material presently used for DWI are tin plated steel sheet or aluminum alloy sheet. But there is a significant difference between DRD and DWI in that the former which is formed by drawing is made of a metal sheet precoated with an organic coating while the latter which is formed by ironing is coated after forming. This is due to the reduction ratio and the state of stress during processing which vary greatly between DRD and DWI. The application of a metal sheet covered with an organic coating for DWI where the reduction ratio and the surface pressure applied to the can wall are extremely high has not yet been put to practical use because of the seizing of the organic coating to the dies or the damage of the organic coatings on the outer and inner surfaces of the can.
On the other hand, drawn and thin-redrawn cans (DTR cans) are formed by thinning the can wall, which includes bending and bending-back at the corner of drawing dies having a small corner radius and applying a heavy tensile stress. DTR cans formed by the forming technique resembling drawing have a slightly thinner can wall thickness than that of the original sheet because the can wall is stretched. In addition, a high surface pressure is not applied to the can wall lying between the dies and the punch in the DTR process unlike in the ironing process, so that the surface pressure applied is not so high and the organic coating is hardly damaged, and thus a metal sheet covered with an organic coating can be applied to DTR. ECCS covered with a thermoplastic resin film is industrially used at present. But in the DTR process the can wall is apt to break during the forming because it is mainly formed with a tensile stress, and the wall thickness can only be about 80% of that of the original sheet and is thicker than that of DWI.
In this respect, aluminum sheets have not yet been used as substrates for DTR cans because they are less suited to the thi nni ng process by bending and bending back than ECCS.
As described above, there are advantages and disadvantages in DRD, DWI or DTR cans, and in the processing of same.
30It is therefore an object of the present invention to produce a steel sheet laminated on both sides with a thermoplastic resin film suitable for the production of a can having a can height about twice the can diameter and a thin wall thickness of 30 to 70% of the original sheet thickness like DWI.

It is another object of the invention to produce a steel sheet laminated with a thermoplastic resin film which can be formed into a can without the use of an emulsion or a water soluble lubricant presently used for cooling and lubrication in DWI
process.
The use of a metal sheet previously laminated with a thermoplastic resin film enables one to eliminate the coating and baking steps in the can producing process, thereby preventing the diffusion of solvent, and also to eliminate the subsequent rinsing, drying and waste water disposal steps. To the best of Applicant's knowledge, there has been no disclosure relating to a metal sheet laminated with a thermoplastic resin film, from which a can having a high can height and a thin wall thickness can be produced without a water-based coolant or lubricant, nor of such a can or of the process of producing same.
However, the following disclosures relating to the present invention have a different objective from that of the present invention.
Laid-Open Japanese Patent Publication N Sho. 62-275172 describes metal sheet covered with an organic coating for a two-piece can and its objective is to increase the retention of coolant (water-based cooling and lubricating agent) at the outer surface of a can in DWI process. It depends on the use of a water-based cooling and lubricating agent, and it is thus different from the present invention.
Published International Patent Application N W089/03303 describes a metal sheet for DWI cans of which one or both sides is covered with a polyester resin film. The production process of DWI cans made of this laminated metal sheet includes the use of suitably lubricated laminate in the drawing process and it is believed that the rinsing process cannot be totally eliminated. Therefore, a small rinser instead of a large can washer is still needed. It is quite different from the elimination of the rinsing process in the present application.
Laid-Open Japanese Patent Publication N Hei 4-91825 describes a metal sheet covered with a thermoplastic resin wherefrom a can having a thin wall is formed by bending and bending-back with a lubricant which volatilizes at a high temperature but without a water-based cooling and lubricating agent. It relates to DTR and, as can be seen from the Examples, the reduction ratio of the can wall thickness is about 20%
and thus smaller than the reduction rate achieved by the present invention.
In the present invention, the can height becomes higher in proportion to the increase of the reduction ratio of the can wall thickness from 30 up to 70%. This serves the aim of the present invention, but the greater the reduction ratio of the can wall thickness is, the more likely the seizure of the outer can wall to the dies and the damage to the resin film or the break of the wall area. Due to the absence of a water-based cooling and lubricating agent, the prevention of damage to the resin film on the outside can wall and the break of the can wall due to it are the most important objectives. In addition, it is a further object of the present invention to provide sufficient adhesive strength between the metal sheet and the resin film because it decreases in proportion to the reduction ratio of the can wall.
In accordance with the present invention, there is provided a resin laminated aluminum sheet for producing a drawn and ironed can produced by dry forming, the sheet comprising an aluminum sheet made of aluminum or of an alloy thereof and having a yield strength of 15 to 50 kg/mm2, a tensile strength of 15 to 55 kg/mm2, a center line average surface roughness height of 0.05 to 0.7 ~m, and a thickness of 0.15 to 0.50 mm, the aluminum sheet being laminated on both sides with a thermoplastic resin film having a thickness of 5 to 50 ~m and coated with a dry lubricant having a volatilizing temperature below the melting temperature of said thermoplastic resin.
The term "aluminum" as used herein refers to either aluminum or an aluminum alloy comprising more than 90 weight % of aluminum and less than 10 weight %
of other metals such as magnesium, manganese, etc.
Applicant has found quite unexpectedly that a two-piece can having a thin wall thickness can easily be formed by dry forming of the above resin laminated aluminum sheet, and that the resulting can has a sufficient pressure proof strength, corrosion resistance and adhesive strength between the aluminum sheet and the laminated resin film after forming. The aluminum sheet is electrolytically treated in a chromate solution in order to provide the desired adhesive strength between the aluminum sheet and the resin film.
The thermoplastic resin is preferably a crystalline polyester resin such as polyethylene terephthalate or a co-polyester resin composed mainly of ethylene terephthalate units, having a melting temperature of about 180 to about 260C. Preferably, the resin film has a thickness of about 10 to about 30 ,um.
In addition, the coating of a lubricant which volatilizes at a high temperature on the surface of the laminated resin film can improve the formability in the dry forming. The lubricant can be removed by heating 21~Z~31 the can after forming, thereby enabling the degreasing, rinsing and/or drying steps to be eliminated. The resin laminated aluminum sheet according to the invention enables one to practice the dry forming at high reduction ratios without any problem. The laminated aluminum sheet of the invention can also be subjected to a composite process consisting of a drawing and an ironing, without damaging the resin film laminate on both sides of the can nor breaking the can wall.
Further features and advantages of the present invention will become more readily apparent from the following description of preferred embodiments with reference to the accompanying drawings, in which:
Fig. 1 is a cross-sectional view of a resin laminated aluminum sheet according to the present invention;
Figs. 2A-2E are schematic views illustrating a process for producing a can having a thin wall thickness by dry forming, from a resin laminated aluminum sheet according to the invention;
Fig. 3 is a fragmentary sectional view of a tool arrangement for producing a can having a thin wall thickness and a high can height by dry forming, from a resin laminated aluminum sheet according to the invention;
Fig. 4 is a cross-sectional view of a can produced from a resin laminated aluminum sheet according to the invention; and Fig. 5 which is on the same sheet of drawings as Figs. 1 and 2 is a diagram showing the profile of the wall thickness of a can produced from a resin laminated aluminum sheet according to the invention.
Referring first to Fig. 1, there is illustrated an aluminum sheet 1 which is covered with a coating 2 of hydrated chromium oxide. Both sides of the 21~2531 coated aluminum sheet are laminated with a thermoplastic resin film 3 coated with a lubricant 4 which volatilizes at a high temperature. Such a resin laminated aluminum sheet can be reduced to a thin gauge by dry forming with a high reduction ratio.
The difficulties such as heat generated due to processing, softening or melting of the resin film due to the generated heat, resultant direct contact of the base aluminum with the forming dies and the break of can wall have to be overcome. The heat generated due to processing is based on the deformation of the aluminum sheet and the friction. The heat generation based on the deformation decreases when the reduction ratio and the deformation resistance is low, and in the ironing of the composite process of the present invention, the heat generation due to friction, in proportion to (surface pressure) x (friction coefficient) decreases when the deformation resistance is low. In addition, when the resin is heated, the damage to the resin film diminishes when the surface pressure is low. Thus, the damage to the resin film diminishes when the deformation resistance is as low as possible. As the laminate is drawn and redrawn before it is ironed as shown in Fig. 3, it is desirable that the work hardening is as little as possible. For these reasons, an aluminum sheet is selected as the metal substrate for the resin laminated metal sheet of the present invention.
If the finished can is positively or negatively pressurized in use, then the can bottom and can wall should have sufficient strength in order to endure such a pressure. In particular, when a can is positively pressurized, the pressure proof strength of the can bottom is critical. As the pressure proof 214~531 strength is roughly in proportion to (sheet thickness)2 x (yield strength), it depends on the sheet thickness and the yield strength. The required pressure proof strength also depends on the content in the can. The lower limit of the yield strength, the tensile strength and the thickness of the aluminum sheet are defined on the basis of the pressure proof strength. On the other hand, the upper limit of the yield strength and the tensile strength are defined on the basis of the degree of damage to the resin film during ironing. Based on these conditions, the yield strength and the tensile strength of the steel sheet according to the present invention preferably range from 15 to 50 kg/mm2 and from 15 to 55 kg/mm2, respectively. When the yield strength and the tensile strength are higher than 50 kg/mm2 and 55 kg/mm2, respectively, the can wall is apt to break due to damage to the resin film. Also, the yield ratio which is represented by (yield strength/tensile strength) is preferably more than 0.7 or less than 1 because a higher yield strength which affects the can bottom strength and a lower deformation resistance in the processing which affect the damage to the resin layer during the ironing are preferable. The upper limit of the aluminum sheet thickness is defined as 0.5 mm based on the pressure proof strength of the formed can (that of more than 0.5 mm is scarcely required) and also on the decrease in cost. The lower limit of the steel sheet thickness is defined as 0.15 mm based on achieving a stably continuous and high-speed production of an aluminum sheet having a uniformthickness.
As mentioned hereinabove, the resin which is laminated to the aluminum sheet is a thermoplastic resin, preferably a crystalline polyester having a thickness in the range from 10 to 30 um and a melting 21~2S31 temperature of 180 to 260C. In a dry forming process, such a thermoplastic resin can make the lubrication during ironing more effective. When the resin is softened by the heat generated by the friction between the outer surface of a can and ironing dies during the ironing, the lubrication effect occurs. The higher the temperature of the dies, the more effective the lubrication effect of the resin is. However, the higher the temperature of the dies, the further the resin in the ironing dies is softened. The resin is thus damaged by the surface pressure in proportion to the deformation resistance of the aluminum sheet. When the aluminum sheet directly contacts the ironing dies, the can wall breaks. Therefore, excessive softening of the thermoplastic resin should be avoided and the temperature of the ironing dies is preferably kept within a suitable range, more preferably from 25C to the glass transition temperature of the thermoplastic resin. A thermoplastic resin which softens at a low temperature is thus not preferable and a thermoplastic resin having a melting temperature (it is used as an index representing the softening sensitivity) higher than 180C is preferably applied because it improves the formability by the dry forming. In industrial production, the drawing and ironing steps are successively practiced and the temperature of the can wall sometimes increases above 100C. If use is made of a thermoplastic resin having a low melting temperature, the resin then softens or melts so that the appearance of the formed can is damaged or the aluminum is exposed in the can and the corrosion resistance is deteriorated. The thermoplastic resin also seizes on the forming tools so that a continuous production can not be performed. For this reason, the thermoplastic resin should- have a melting temperature higher than 180C. On the other hand, if the melting temperature is higher than 260C, there is insufficient lubrication during forming due to the softening of the resin.
Accordingly, the melting temperature of the thermoplastic resin preferably ranges from 180 to 260C. On the other hand, the thickness of the thermoplastic resin film which is laminated to the aluminum sheet is defined as ranging from 5 to 50 ~m.
If the resin thickness is less than 5 ~m, there is a possibility that the ironing dies directly contact the aluminum sheet on the outer surface of the can during the ironing, thereby causing the can wall to break.
There is also a possibility that the corrosion resistance on the inner surface of the can is reduced.
In addition, it is hard to laminate a thermoplastic resin to an aluminum sheet continuously and uniformly.
The upper limit of the resin thickness is defined as 50 um based on the avoidance of wrinkles caused during the drawing and also on the decrease in cost.
Among the thermoplastic resins having a melting temperature ranging from 180 to 260C, use is preferably made of a polyester resin such as polyethylene terephthalate or polybutylene terephthalate, a co-polyester resin mainly composed of ethylene terephthalate units or a polyester resin composed of a mixture of these resins. For example, a co-polyester resin composed of 75 mole % of polyethylene terephthalate and 25 mole % of polyethylene isophthalate, polyethylene sebacate or polyethylene adipate, or a polyester resin composed of polyethylene terephthalate or of the above-mentioned polyester resin blended with polybutylene terephthalate may be used.

21~2531 The above-described polyester resin can be laminated to the aluminum sheet by the following methods:
(1) A melted polyester resin is directly extruded on both sides of an aluminum sheet.

(2) A non-oriented or oriented polyester resin film is thermally laminated to both sides of an aluminum sheet.
Both methods can be applied to the production of the resin laminated aluminum sheet of the present invention, but a biaxially oriented polyester resin film is preferably applied based on such required characteristics of the formed can as the impact resistance of the resin layer and the permeation resistance against the corrosive content. In such a case, it is preferable to laminate a biaxially oriented double layered polyester resin film to an aluminum sheet in order that the innermost layer (directly in contact with the aluminum surface) of the resin film has a planar orientation coefficient of 0.00 to 0.05 and the outermost layer (the furthest from the aluminum surface) of the resin film has a planar orientation coefficient of 0.01 to 0.10. When the planar orientation coefficient of the innermost layer is more than 0.05, the resin film is apt to peel off during the processing, so it is not practical. On the other hand, where the planar orientation coefficient of the outermost layer is less than 0.01, the biaxial orienta-tion in the whole resin film has almost disappeared.
When the aluminum sheet laminated with such a resin film is processed into a drawn and ironed can by dry forming, cracks are sometimes caused in the polyester resin film so that it cannot be used for a can to be packed with a corrosive content. Also, in the case where the planar orientation coefficient of the outermost layer is more than 0.10, the resin film has an insufficient extendability so that cracks are sometimes caused in the resin film under severe processing. Therefore, when using a biaxially oriented polyester resin film, it is preferable that such a resin film has an outermost layer with a planar orientation coefficient in the range of 0.01 to 0.10 and an innermost layer with a planar orientation coefficient in the range of 0.00 to 0.05. The lamination of the polyester resin film to an aluminum sheet by laying an adhesive between the resin film and the aluminum sheet is suitable for the inside surface of a can where a corrosive content is packed. In this case, the control of the planar coefficient of the resin film as describèd above is not necessary. A known adhesive can be applied, but a thermosetting resin containing an epoxy group in its molecular structure is more preferable. It can be applied on the one side to be laminated to the aluminum surface of the resin film or on both sides of the aluminum sheet.
The planar orientation coefficient which is defined as the orientation of the innermost and outermost layers of the biaxially oriented polyester resin film is determined by the following method.
Firstly, the polyester resin film is removed from the aluminum sheet by dipping the laminate into a diluted hydrochloric acid solution which only dissolves the aluminum sheet. After rinsing in water and drying the film, the refractive indexes in the lengthwise, widthwise and thickness directions of the innermost and outermost layers of the polyester resin film are measured with a refractometer. Then, the planar orientation coefficient is determined according to the following equation:

A = (B + C)/2 - D
where A represents the planar orientation coefficient of the polyester resin film, B represents the refractive index in the lengthwise direction of the polyester resin film, C represents the refractive index in the widthwise direction of the polyester resin film, and D represents the refractive index in the thickness direction of the polyester resin film.
The refractive indexes measured by the method described above show an average value within 5 ~m from the outermost layer (of either side of the resin film) so it is possible to determine the planar orientation coefficient of the innermost layer from that of the outermost layer.
In addition, in the present invention, it is also possible to apply a biaxially oriented double layered film having outer and inner layers with different melting temperatures in order that the planar coefficient of either side of the film be easily controlled within the desired range.
Furthermore, the intrinsic viscosity (IV
value) of the polyester resin film is also an important factor. The intrinsic viscosity which is in proportion to the molecular weight of the resin greatly affects the stiffness and the formability of the resin film. In the case where the resin film has an intrinsic viscosity less than 0.50, the resin layer on the drawn and ironed can has a poor impact resistance, even if the planar coefficient of the resin film laminated to the aluminum sheet is maintained within the desired range, thereby causing many micro cracks in the polyester resin layer on the inside of the impacted area and exposing the aluminum substrate. On the other hand, a resin film having an intrinsic viscosity more Z1~2531 than 0.70 encounters a high viscous resistance during the ironing process, which sometimes causes practical problems.
The lamination of a pigmented thermoplastic resin film to one side of the aluminum sheet constituting the outside of a can is also an important factor from an artistic point of view. It is also possible to add white pigment based on titanium dioxide to the resin during the production of the resin in order to improve the print contrast of the design applied on the outside of a can. Inorganic or organic pigment, which may have a color other than white can be used and be selected depending on the uses. The preferable print contrast can be obtained by the addition of 1 to 20 weight % of pigment.
Furthermore, it is possible to apply other thermoplastic resins such as bisphenol A polycarbonate, a polyamide resin selected from nylon 6, nylon 66, copolymer nylon 666, nylon 610; nylon 7 and nylon 12, and polyethylene naphthalate. These resins can be used alone or can be co-extruded with other resins to define the upper layer or an intermediate layer of a double or triple layered film. Also, a resin composed of the aforementioned polyester resin blended with these thermoplastic resins can be used. A double layered film composed of an upper layer of the aforementioned polyester resin and a lower layer of the above blended resin can also be used. In some cases, additives such as antioxidants, stabilizers, antistatic agents, lubricants and corrosion inhibitors can be added so long as they do not deteriorate other characteristics during the manufacturing process of the polyester resin used for the present invention.
The grain size and the centerline average height of the surface roughness of the aluminum sheet 21~2~31 affect the adhesion of the thermoplastic resin to it and the corrosion resistance.
The grain size is defined as the average value measured in the grain sizes of 3 larger grains selected from those observed in the 3 cm x 3 cm visual field of 200 magnification (real area : 150 ~m x 150 ~m) in a section parallel to the rolling direction of the aluminum sheet. The grain size of each grain is the average value of the longer width and the shorter one measured in the grain cross. The longer width is defined as the length of the longest segment line passing through the center of the grain, and the shorter width is defined as that of the segment line passing through the center of the grain and also perpendicular to the longest segment line. The grain size definition deduced from the sizes measured in larger grains as mentioned above depends on the following:
Supposing that all the aluminum grains are composed of spheres having the same diameter, the sections of grains are observed as circles having different diameters to each other. The longest diameter of these circles having different diameters is a diameter of the sphere, that is the real grain size.
For the reason mentioned above, the aluminum grain size is defined depending on those of larger grains.
When the grain size is more than 50 ~m, a rough surface is caused during the drawing and deteriorates the adhesion of the thermoplastic resin.
Film defects are also caused and the corrosion resistance is deteriorated. On the other hand, when the grain size is less than 10 ~m, the aluminum sheet becomes harder and has to be rapidly heated when it is produced.

21~2531 The center line average height is defined as follows:
In the length 1 of the measured roughness curve, when the center line direction of the measured roughness curve is defined as X axis and the longitudinal direction (peak height direction of the measured roughness curve) is defined as Y axis, the roughness curve is given by the following formula:
y = f(x) and the center line average height (Ra) is given by the following formula:

R(a) = 1/e J ¦ f(x) ¦ dx (Ra is expressed by ~m.) When the center line average height of the surface roughness is more than 0.7 ~m, there are some cases where the adhesion of the laminated thermoplastic resin film to the aluminum sheet is deteriorated depending on the forming condition. Therefore, the upper limit of the center line average height of the surface roughness is defined as 0.7 ~m. On the other hand, the lower limit is defined depending not only on the performance of the aluminum sheet, but on the difficulty to produce an aluminum sheet having a center line average height of the surface roughness less than 0.05 ~m. In this respect, the lower limit is preferably 0.05 ~m.
Turning back to Fig. 1, the surface of the aluminum sheet 1 illustrated is treated and a coating 2 of hydrated chromium oxide is formed on it in order to have a good adhesion to the thermoplastic resin film 3.
The treatment can be a chemical treatment, an electrochemical treatment in a chromate solution or an anodic oxidization. The chemical treatments include chromate treatment, phosphate-chromate treatment and 2142~31 non-chromate treatment. The selection should be made in consideration of the forming condition, the process arrangement and the like. The weight of the coating is preferably 5 to 100 mg/m2 depending on the type of chemical treatment. The electrochemical treatment in a chromate solution or the anodic oxidization is preferably applied when a better adhesion is required.
The lubricant 4 which volatilizes at a high temperature and is coated on the thermoplastic resin film 3 plays an important part when the dry forming is carried out at a high production intensity and a high speed. The lubricant is preferably one of which more than 50% volatilizes when the formed can is heated for a few minutes at about 200C after forming. Such a lubricant can be liquid paraffin, synthetic paraffin, natural wax or a mixture thereof and is selected depending on the processing condition and the heating condition after forming. A lubricant having a melting temperature of 25 to 80C and a boiling temperature of 180 to 400C is preferably used. The weight of the lubricant coating which should be defined based on the surface of the can (inner or outer surface) to which the lubricant is applied, is preferably 5 to 100 mg/m2, more preferably 30 to 60 mg/m2. The coating weight should be defined based on the applied surface of a can (i.e. the inner or outer surface).
The aluminum sheet of the invention is suitable for producing a can having a can height about twice the length of the can diameter and a thin wall thickness from 40 to 70% of the original sheet thickness.
The ironing process will now be explained. By the application of a composite process comprising a redrawing process and an ironing process at the same time to the thinning of a can wall thickness, the objective of the present invention can be more effectively performed. Figures 2A-2E illustrate such a composite process which is suitable for producing a can having a thin wall and a high can height by dry forming from the resin laminated aluminum sheet of the present invention. Firstly, as shown in FIG. 2A, a blank 5 is punched out from the resin laminated aluminum sheet shown in FIG. 1. Then, it is drawn into a drawn can 6 (FIG. 2B), and redrawn into a redrawn can 7 (FIG. 2C) having a smaller diameter than that of the drawn can 6, and thereafter it is redrawn and ironed at the same time (i.e. the composite process) into a redrawn and ironed can 8 (FIG. 2D) having a further smaller diameter than that of the redrawn can 7. Subsequently, an upper edge portion 9 of the can 8 having a flange 10 is trimmed off and shaped into a trimmed can 11 (FIG.
2E), the upper edge portion of the can 11 is processed by a neck-in and a flange forming and then formed into the final can 12 shown in FIG. 4.
The composite process can be carried out using the tool arrangement illustrated in FIG. 3. The redrawn can 7 is held under pressure by a redrawing die 13 and a blank holder 14. A guide ring 15 is provided outwardly of the blank holder 14. A punch 16 is moved forward, in the direction indicated by the arrow at 17, to form a can having a high can height. At the same time, the can wall is ironed by an ironing die 18 thinning the wall to form a thin wall 19 as the punch 16 moves forward in the direction of arrow 17. By ironing with the imposition of an effective back tension at the ironed portion of the aluminum sheet, the resin film outside of a can is hardly damaged. The length L of the can wall 20 between the redrawing part of die 13 and the ironing part of die 18 is determined based on the gauge necessary for the next neck-in 21~2531 forming. Furthermore, it is preferred that the temperatures of the redrawing die 13 and the ironing die 18 be in the range from 25C to the glass transition temperature of the resin film.
FIG. 5 shows a typical profile of a wall thickness (the aluminum sheet alone with no resin film) in the can height direction of a trimmed can 11 produced from a thermoplastic resin laminated aluminum sheet having an original sheet thickness of 0.25 mm according to the process illustrated in FIG. 2. As shown in FIG. 5, the thickness of the can body is thinner (about 0.14 mm which is 56% of the original sheet thickness) and the upper portion of the can is thicker (about 80~ of the original sheet thickness), and the trimmed can 11 is thus suitable for the next neck-in forming process.
As can clearly be seen from FIG. 3, in the case where use is made of an ironing punch whose diameters at the part corresponding to the can body wall 19 and at that corresponding to the upper edge part 20 are the same, the stepped thickness difference between the body wall part 19 and the upper edge part 20 is more visibly formed on the outside of a can in contrast with the case of DWI having the step on the inside. FIG. 2A to FIG. 2E show the case where the step is formed on the outside. On the other hand, it goes without saying that in the case of using an ironing punch whose diameter at the part corresponding to the upper edge part 20 is smaller than that corresponding to the can body wall 19, the stepped thickness difference is formed on the inside of the can. The exterior of the can is scarcely affected by the step formed on the outside of the can, and the stripping performance of the ironing punch is scarcely affected by the step formed on the inside of the can. Then, 21425:3~

there is no problem with the can quality and the forming process whether the step is formed on the inside of the can or on the outside of it.
The following non-limiting examples further illustrate the invention.

Six kinds of metal sheet whose properties are shown in TABLE 1 were heated to 240C, and laminated with thermoplastic resins as follows. Firstly, a biaxially oriented co-polyester resin film composed of 88 mole % of polyethylene terephthalate and 12 mole %
of polyethylene isophthalate (thickness : 25 ~m, orientation coefficient : 0.126 (on both sides of the film) and melting temperature : 229C. was laminated to one side of each metal sheet (defining the inside of a can), and a white colored biaxially oriented co-polyester resin film having the same chemical composition as the aforementioned film pigmented with titanium dioxide and a thickness of 15 ~m was laminated at the same time to the other side of the metal sheet (defining the outside of a can). The resin laminated metal sheets were immediately dipped into water and cooled off. After the lamination, the laminates were dried and coated on both sides with about 50 mg/m2 of paraffin based wax, then they were processed as follows. Firstly, they were punched out into blanks having a diameter of 160 mm, then drawn into drawn cans having a diameter of 100 mm. Next, they were redrawn into redrawn cans having a diameter of 80 mm, and then they were processed into drawn and ironed cans having a diameter of 66 mm in a composite process consisting of simultaneous redrawing and ironing. The composite process was carried out under the conditions where the distance between the redrawing part of the redrawing die and the ironing part of the ironing die (the upper edge of a can) was 20 mm, the corner radius of the redrawing die was one-and-a-half times as much as the sheet thickness, the clearance gap between the redrawing die and the punch was the same as the sheet thickness and the clearance gap between the ironing die and the punch was 50% of the original sheet thickness.
During the process, no water-based cooling nor lubricating agent were applied and dry forming was practiced in the process. In the composite process, the process proceeded in the direction of arrow 17 shown in FIG. 3, where the forming was completed to the stage where the flanged part was kept in the upper edge portion along the opposite direction of the can, and the processed can was then removed by pulling back the punch 16. The upper part of the can was thereafter trimmed off and processed by neck-in and flange forming to obtain the finished can illustrated in FIG. 4, having a high can height and a thin wall thickness. The formed cans were evaluated with respect to the break of the can wall, the appearance of the outside of the can, the metal exposure inside the can and the adhesion of the resin film to ECCS substrate based on the following standards:
l) the break ratio of the can wall (evaluated by the ratio of the number of cans whose walls were broken to that of all the formed cans) excellent : 0%, good : <10%, fair :>10% or <30%, bad :
>30%
2) the appearance of the outside of the can ~evaluated by the ratio of the number of cans whose outsides were damaged during the formation to that of all the formed cans) excellent: 0%, good: <10%, fair:>10% or <30%, bad: >30%

2I~2531 3) the metal exposure inside the can (evaluated by the enamel rater value (ERV:mA) wherein ERV was measured as follows:
Formed can was filed with sodium chloride solution, and current was measured in milliampere at a voltage of 6.3 V) excellent: 0 mA, or <0.05 mA, good:>0.05 mA or <0.5 mA, fair:>0.5 mA or <5 mA, bad: >5 mA

4) the adhesion of the laminated resin film after forming (evaluated by the peeling degree after neck-in forming) excellent: no peeling, good : slightly peeled off but no problem for the practical use, fair : visibly peeled off, bad : peeled off in the whole upper part of a can.

Metal sheets A and E whose properties are shown in TABLE 1 were heated to 240C, and laminated with thermoplastic resins as follows. A biaxially oriented co-polyester resin film composed of 88 mole %
of polyethylene terephthalate and 12 mole % of polyethylene isophthalate (thickness : 6 ~m, orienta-tion coefficient : 0.126 (on both sides of the film) and a melting temperature : 229C) was laminated to one side of each metal sheet (defining the inside of a can), and a white colored biaxially oriented co-polyester resin film having the same chemical composition as the aforementioned film pigmented with titanium dioxide and a thickness of 8 ~m, was laminated at the same time to the other side of the metal (defining the outside of a can). The resin laminated metal sheets were immediately dipped into water and cooled off. After the lamination, the laminates were dried and coated on both sides with about 50 mg/m2 of paraffin based wax and were processed under the same 2142~31 conditions in Example 1, and then the formed cans were evaluated in the same manner as in Example 1.

Metal sheets A and C whose properties are shown in TABLE 1 were heated to 235C, and laminated with thermoplastic resins as follows. A biaxially oriented double layered co-polyester resin film comprising an outer layer composed of 88 mole % of polyethylene terephthalate and 12 mole % of poly-ethylene isophthalate and having a thickness of 15 um,a melting temperature of 229C and an orientation coefficient of 0.123 and an inner layer composed of a mixed resin comprising 45 weight % of co-polyester resin- composed of 94 mole % of polyethylene terephthalate and 6 mole % of polyethylene isophthalate and 55 weight % of polybutylene terephthalate, and having a thickness of 5 um, a melting temperature of 226C and an orientation coefficient of 0.083 was laminated to one side of these metal sheets (defining the inside of a can), and a white colored biaxially oriented co-polyester resin film having the same chemical composition as the afore-mentioned film pigmented with titanium dioxide and a thickness of 15 ~um was laminated at the same time to the other side ~defining the outside of a can). The resin laminated steel sheets were immediately dipped into water and cooled off. After the lamination, the laminates were dried and coated on both sides with about 50 mg/m2 of paraffin based wax and were processed under the same conditions as in Example 1, and then the formed cans were evaluated in the same manner as in Example 1.

Metal sheets A and E whose properties are shown in TABLE 1 were heated to 240C, and laminated with thermoplastic resins as follows. A biaxially oriented co-polyester resin film comprising 88 mole %
of polyethylene terephthalate and 12 mole % of polyethylene isophthalate and having a thickness of 25 ,um, an orientation coefficient of 0.126 (on both sides of the film) and a melting temperature of 229C, precoated with an epoxy-phenol based primer (dry weight: 0.5 mg/m2), was laminated to one side of each metal sheet (defining the inside of a can), and a white colored biaxially oriented co-polyester resin film having the same chemical composition as the aforementioned film pigmented with titanium dioxide and having a thickness of 10 ,um was laminated at the same time to the other side (defining the outside of a can).
The resin laminated metal sheets were immediately dipped into water and cooled off. After the lamination, the laminates were dried and some of the laminates were coated on both sides with about 50 mg/m2 of paraffin based wax, while others were not coated with any wax,and were processed in the same manner as in Example 1, and then the formed cans were evaluated in the same manner as in Example l.
The results of the evaluation are shown in TABLE 2 and TABLE 4. As it is apparent the thermoplastic resin laminated steel sheet of the present invention is suitable for the production of a can by dry forming, which can has a high can height and a thin wall thickness.

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TABLE 1 - Continued Alloy Chemical Composition of Aluminum Alloy Code No. si Fe Cu Mn Mg Cr Zn other elementA1 dual total JIS 30040.30 0.7 0.25l.0-l.5 0.8~1.3 -- 0.25 -- O.l5 residue JIS 50520.25 0.40 0.10 0.10 2.2~2.80.15~0.35 0.10 0.05 0.15 residue 21~2531 Q Q
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Example No. Example 2 Example 3 Metal No. A E A C

discrimination of Example Comparative Example Example examples Example resin laminated inside and inside and inside and inside and surface outside outside outside outside break of can wall good bad excellent excellent resin damage of good bad excellent excellent outside metal exposure of good good excellent excellent inside outside excellent excellent excellent excellent adhesion inside excellent excellent excellent excellent Example No. Example 4 Metal No. A A E E

discrimination of Example Example Comparative Comparative examples Example Example resin laminated inside and inside and inside and inside and surface outside outside outside outside lubricant coated uncoated coated uncoated break of can wall excellent good fair bad resin damage of excellent good fair bad outside metal exposure of excellent excellent excellent excellent inside outside excellent excellent excellent excellent adhesion inside excellent excellent excellent excellent

Claims

1. A resin laminated aluminum sheet for producing a drawn and ironed can produced by dry forming, said sheet comprising an aluminum sheet made of aluminum or of an alloy thereof and having a yield strength of 15 to 50 kg/mm2, a tensile strength of 15 to 55 kg/mm2, a center line average surface roughness height of 0.05 to 0.7 µm, and a thickness of 0.15 to 0.50 mm, said aluminum sheet being laminated on both sides with a thermoplastic resin film having a thickness of 5 to 50 µm and coated with a dry lubricant having a volatilizing temperature below the melting temperature of said thermoplastic resin.

2. A resin laminated aluminum sheet according to claim 1, having a crystal grain size of 10 to 50 µm, and a yield ratio between 0.7 and 1Ø

3. A resin laminated aluminum sheet according to claim 1, wherein said thermoplastic resin is a crystalline polyester resin.

4. A resin laminated aluminum sheet according to claim 3, wherein said crystalline polyester resin is a polyethylene or polybutylene terephthalate resin.

5. A resin laminated aluminum sheet according to claim 1, wherein said thermoplastic resin is a co-polyester resin.

6. A resin laminated aluminum sheet according to claim 5, wherein said co-polyester resin is mainly composed of ethylene terephthalate units.

7. A resin laminated aluminum sheet according to claim 1, 2, 3, 4, 5 or 6, wherein said thermoplastic resin has a melting temperature of about 180 to about 260°C

8. A resin laminated aluminum sheet according to claim 1, 2, 3, 4, 5 or 6, wherein said thermoplastic resin film has a thickness of about 10 to about 30 µm.

9. A resin laminated aluminum sheet according to claim 1, wherein said thermoplastic resin film is a biaxially oriented double layered polyester resin film having an innermost layer with a planar orientation coefficient of 0.00 to 0.05 and an outermost layer with a planar orientation coefficient of 0.01 to 0.10.

10. A resin laminated aluminum sheet according to claim 1, wherein said thermoplastic resin is a bisphenol A polycarbonate resin, a nylon-type polyamide resin or a polyethylene naphthalate resin.

11. A resin laminated aluminum sheet according to claim 10, wherein said nylon-type polyamide resin is selected from the group consisting of nylon 6, nylon 66, copolymer nylon 666, nylon 610, nylon 7 and nylon

12.

12. A resin laminated aluminum sheet according to claim 1, wherein said lubricant has a melting temperature of about 25 to about 80°C and a boiling temperature of about 180 to about 400°C.

13. A resin laminated aluminum sheet according to claim 1, wherein said lubricant is liquid paraffin, synthetic paraffin, natural wax or a mixture thereof.

14. A resin laminated aluminum sheet according to claim 1, 12 or 13, wherein the lubricant coating comprises about 5 to about 100 mg/m2 of said lubricant.

15. A resin laminated aluminum sheet according to claim 1, 12 or 13, wherein the lubricant coating comprises about 30 to about 60 mg/m2 of said lubricant.

16. A resin laminated aluminum sheet according to claim 1, wherein a layer of adhesive is disposed between the aluminum and the thermoplastic resin film.

17. A method for manufacturing a can, comprising the steps of:
a) drawing a resin laminated aluminum sheet as defined in claim 1 to form a drawn can having a predetermined diameter;
b) redrawing said drawn can to form a redrawn can having a diameter smaller than the diameter of said drawn can;
c) simultaneously redrawing and ironing said redrawn can to form a redrawn and ironed can having a diameter smaller than the diameter of said redrawn can, a height about twice the diameter of said redrawn and ironed can, and a can wall thickness from 30 to 70% of the thickness of said resin laminated aluminum sheet prior to drawing; and d) heating said redrawn and ironed can to a temperature over the volatilizing temperature of said lubricant.