Containers

Abstract: A can body (40) drawn from a tinplate to comprise an end wall (41) and an integral side wall (42) which extends from the periphery of the end wall to a terminal portion defining a mouth, has a margin (48) of coating material, such as expoxy phenolic lacquer, applied only to an upper portion of the interior surface of the side wall (42) leaving the interior tin surface of the rest of the side wall and end wall (41) exposed. The side wall may be wall ironed from selected grade of tinplate. The advantages arising are protection of cold worked side wall material and control of the ammount of tin available to be picked up by a product in the can. Various methods are described for making the cans such as deep drawing and alternatively wall ironing.

Description: This invention relates to a can body drawn from a tinplate and a method of making the can body. For many years, white fruits and products containing tomato or tomato sauce have been packed in cans made from uncoated tinplate. The cans have a body comprising a cylindrical side wall which includes a longitudinal side seam and can end (called the maker's end) attached to one end of the side wall by a double seam, the combination of side wall and end wall being called an open top can. The open top can is filled with product, closed by double seaming a second end (called the packer's end) onto the other end of the side wall, and thermally processed to sterilise the contents. During thermal processing and subsequent storage the product takes up a certain amount of the tin coating, so preserving the organoleptic and visual properties of the product by minimising oxidisation of the product. Japanese Patent Publication Laid Open No. 52-37170 discusses attempts to lacquer locally the interior of tin plate can bodies to achieve a controlled area of tin available to the product and observes that this arrangement is not satisfactory. The specification describes can bodies made from tin free steel sheet laminated to a film of polymeric material which has a band of vapour deposited tin on the film so the interior surface of the can body presents a controlled amount of tin (the vapour deposited band) to the product whilst the rest of the internal surface of the can body is protected by polymeric film. Whilst this prior art can achieve control of the amount of tin presented to the product, the cans are expensive to make because they require vapour deposition of tin on a film and lamination of the film to a tinplate before manufacture of the can body comprising a tubular side wall closed at both ends by an end wall fixed by a double seam. British Patent No. 1575204 (nearest state of the art) addresses the problem of uneven or eared flanges arising when a cup comprising an end wall and an integral side wall is drawn from mild steel sheet. This earing problem is minimised by selection of a steel having a mean coefficient of normal plastic anisotropy within the range 1.3 to 1.6. When mild steel such as tinplate of chrome/chrome oxide coated steel (TFS) is used in a prelacquered form, it is desirable that the steel has a grain size finer than 3500 grains/mm 2 (preferably finer than 4000 grains/mm 2 ) to minimise surface roughening arising during drawing of the side wall so that the lacquered coating is not lifted or disrupted to spoil the complete continuity of the coating on the interior of the drawn can. Objectives of the present invention include: 1) Provision of a can body having a controlled amount of tin available to a product packed therein. 2) Avoidance of the expense of vapour deposition and laminated polymeric film. 3) An economical mass production method to achieve a can body giving a reliable rate of transfer into the product. 4) Avoidance of the side seam and one double seam associated with the traditional can body. According to a first aspect of the present invention, there is provided a can body for containing a product, the can body being drawn and, optionally, partially wall-ironed from a tinplate to comprise an end wall and an integral side wall which extends from the periphery of the end wall to a terminal portion defining a mouth of the body; in which the interior surface of the can body has a coating of organic material; characterised in that: the tinplate from which the can body is drawn has a surface roughness between 0.3µm and 0.64µm (12 and 25 microinches) CLA; and a margin of organic coating material is post-sprayed onto the can body and extends from the terminal portion along the interior surface of the side wall for an axial distance (H) less than the length of the side wall, the rest of the side wall having an exposed tin surface, the axial distance H being expressed by the formula: H = ½ CV πRT(1-I/100) - R where R is the radius of the side wall in mm V is the weight of the product in the can in g T is the weight of tin coating in gm -2 I is the % ironing reduction (if any) C is the chosen level of iron pickup limit in ppm According to a second aspect of the present invention, there is provided a can body for containing a product, the can body being drawn and, optionally, partially wall-ironed from a tinplate to comprise an end wall and an integral side wall which extends from the periphery of the end wall to a terminal portion defining a mouth of the body; in which the interior surface of the can body has a coating of organic material; and the tinplate from which the can body is drawn has a grain size greater than 4000 grains/mm 2 ; characterised in that: the tinplate has a surface roughness between 0.3µm and 0.64µm (12 and 25 microinches) CLA; and organic coating material is precoated onto the tinplate as an annulus, prior to drawing the can body, such that the organic coating extends as a margin from the terminal portion of the can body formed therefrom, along the interior surface of the side wall for an axial distance (H) less than the length of the side wall, the rest of the side wall having an exposed tin surface, the axial distance H being expressed by the formula: H = ½ CV πRT(1-I/100) - R where R is the radius of the side wall in mm V is the weight of the product in the can in g T is the weight of tin coating in gm -2 I is the % ironing reduction (if any) C is the chosen level of iron pickup limit in ppm The benefits arising from this container body are a seamless body that can be relied upon not to leak, and that presents a calculated amount of tin to the product. The organic coating material may be chosen from a group consisting of epoxy phenolic lacquer, epoxy amine lacquer, acrylic resin lacquer and vinyl lacquer, with or without a pigment. However, several problem arise from the deformation occurring during deep drawing and/or wall ironing of can bodies. Whilst a can body may be drawn from a precoated tinplate blank, the incorrect choice of too large a grain size of the steel substrate of the tinplate will give rise, during drawing, to surface disturbance called the "orange peel" effect at the risk of disruption of a preapplied coating or lacquer to expose the metal substrate of the blank. Typically a can drawn in a single drawing operation would be made from a tinplate having a grain size finer than (i.e. greater than) 4000 grains/mm 2 , a surface roughness between 0.3 µm and, of the order of, 0.64 µm (12 and 25 microinch) CLA, and a weight of tin in a range between 3 to 10 gm -2 . A differentially coated tinplate may be used if desired, preferably with the heavier tin weight inside the can. Deeper drawn articles made by redrawing a shallow uncoated cup may suffer extensive unacceptable localised redistribution of the tin coating. Severe wall ironing reductions of side wall thickness thin the tin at risk of exposing tin iron alloy present on flow brightened tinplates, or steel on a non-flow brightened tinplate. In a second embodiment of the drawn can, the side wall has been passed through at least two drawing dies, and the tinplate from which the can was drawn had a grain size finer than 4000 grains/mm 2 , a surface roughness between 0.3 and 0.64 µm (12 and 25 microinches) CLA, and an initial tin weight greater than 5.6 grams/m 2 that becomes the interior surface of the can body. The last die may, if desired, be of a shape to impose an ironing reduction of the order of 10% so the side wall becomes thinner than the end wall. The organic coating is chosen from epoxy phenolic lacquer or epoxy based lacquer. In this embodiment, after the first drawing stage, the drawn can is mounted on a punch and pushed through a redrawing die which makes with the punch a clearance smaller than the thickness of the tinplate so that the side wall of the drawn cup is reduced in diameter and thickness, and the side wall length is increased. It is convenient to apply coating material as a spray from a nozzle directed onto the redrawn side wall as the can body is rotated. An epoxy phenolic lacquer or epoxy based lacquer may be used. Similar tinplates may be used for redrawn cans as are used for drawn cans, but with a final grain size finer than 4000 grains/mm 2 . More tin weight may be needed if the side wall is ironed. It is possible to estimate the tin coating weight required by means of the formula: T = CV πR (2H + R)(1-I/100) and the height H of base tin along the side wall from the end wall to the lacquer margin is expressed by the formula: H = ½ CV πRT(1-I/100) - R as noted above. The invention is particularly useful for the packing and storage of tomato based products. Various embodiments of the invention will now be described, by way of example, and with reference to the accompanying drawings, in which: Figure 1a is a sectioned side view of a fragment of matt tinplate; Figure 1b is a sectioned side view of a fragment of flow brightened tinplate; Figure 2a is a surface roughness trace of the large grain size tinplate of figure 1a; Figure 2b is a surface roughness trace of the tinplate of figure 1a after drawing and redrawing to become a side wall of a can; Figure 2c is a surface roughness trace of the tinplate of figure 1a after drawing and redrawing through a punch/die gap to achieve a small degree of ironing reduction in wall thickness; Figure 3a is a surface roughness trace of the fine grain size tinplate of figure 1b; Figure 3b is a surface roughness trace of the tinplate of figure 1b after drawing and redrawing to become the side wall of a can; Figure 3c is a surface roughness trace of the tinplate of figure 3b after drawing and redrawing and redrawing through a punch/die gap to achieve a small degree of ironing reduction in wall thickness; Figure 4 is a diagrammatic sketch of the steps of a first method in which a precoated blank is drawn to a shallow can; and Figure 5 is a diagrammatic sketch of the steps of a second method in which a plain tinplate blank is drawn in a first die and redrawn and ironed in a second die. In order to manufacture cans it is necessary to: (A) select appropriate tinplate to permit deep drawing to a hollow article but which minimises roughening of the cup wall; (B) select appropriate lacquers or coatings that will adhere to the drawn side wall surface of cans produced; (C) select a manufacturing procedure appropriate to the size of can produced; (D) perform tests to make sure the cans actually release appropriate amount of tin into product packed in the can. These matters are discussed in this order with reference to the drawings. (A) SELECTION OF TINPLATE Figure 1a shows diagrammatically a section through a matt tinplate M, so called because the tin layer (a) is in the as electroplated condition on the rolled steel (b) matrix. In figure 1a the steel matrix comprises elongated grains extending in a direction left to right called the rolling direction "R". The surface of the steel matrix is not smooth because the mill rolls impose their surface finish on the steel as shown in figure 2a. A range of surface roughness is available from the rolling mills ranging typically from 8 microinch CLA to 100 microinch CLA. In figure 1a the tin layer "a" can be seen approximately to follow the steel surface. A thin oxide layer "c" will develop on the tin layer and the mills usually cover this with a protective film such as dioctyl senacate (not shown in figure 1) before sale. Figure 1 shows a tinplate characterised by the following parameters: Grain size 2500-3500 grains/mm 2 Surface roughness 35 microinch CLA Tensile strength 320-380 N/mm 2 Tin weight 10-15 g/m 2 Steel of this type is used to make cans by blanking a disc, drawing a cup from the disc, redrawing the drawn cup, and then wall ironing the redrawn cup. The surface roughness of 35 microinch is chosen to provide surface contours to hold lubricant and the plate thickness and tin weight are chosen so that after an appropriate wall ironing reduction, a tin coating of at least about 6 grams/m 2 will remain on the side wall. Figure 2c shows the roughness of the side wall after partial or slight wall ironing: the flattened peaks, created by ironing, are clearly visible and illustrate the risk of iron exposure. Figure 1b shows diagrammatically a section through a flow brightened tinplate (FB), so called because the electroplated tin layer has been melted to create a visually bright finish on the outer layer of tin. A layer of an intermetallic compound (D) of tin and iron joins the outer tin layer to the rolled steel inside. As in figure 1a, an oxide layer covers the exterior surface of the tin layer. This thin oxide layer may be modified by a passivation treatment such as a chromate treatment. As depicted in figure 1b, the parameters characterising this flow brightened tinplate are: Grain size 40,000-50,000 grains/mm 2 Surface roughness 35 microinch CLA UTS 370-430 N/mm 2 Tin weight 10-15 g/m 2 This steel is used to make cans by blanking a disc, drawing a cup from the disc and, if required, redrawing the drawn cup to reduced overall diameter and increased height. This steel is also used to produce DWI cans but since finer grain size leads to less surface roughening during drawing, the final thickness of the can body tin coating is more uniform and steel exposure is reduced. In comparison with cans produced from steel as depicted in figure 1a, when steel as depicted in figure 1b is used, the same can characteristics re steel exposure can be achieved from a lower plate tin coating. The lower tin weight of 8 grams/m 2 is tolerable because little disruptive ironing forces are applied to the tinplate surface. This flow brightened smooth finish 0.3 µm (12 microinch) CLA surface is satisfactory for deep drawing but a rougher surface finish will assist lubrication of deeper cups and cans during wall ironing, say 0.89 µm (35 microinch) CLA, although a lower roughness is preferred for drawn or redrawn cans. During contraction of the periphery of a blank in a drawing die, compressive hoop strains develop to compress the steel so that large grain size steels develop surface roughening called the "orange peel" effect. The surface roughness of the side wall developed during redrawing is shown in figure 2b. Figure 3a shows the surface of a fine grain size tinplate (e.g. 12 microinch CLA) before drawing or ironing. Figure 3b shows that much less roughening arises during drawing of a cup that arose in figure 2b. Figure 3c serves to show the flattening effect of wall ironing. Coatings and lacquers are known which can be applied to tinplate before drawing to cup shaped articles and their adhesion permits manufacture of quite deep cups before the roughening disrupts them but wall ironing puts such coatings at greater risk of disruption. Lacquer continuity can be improved by use of a steel grain size greater than 4000 grains/mm 2 as is discussed in GB-1575204. For our purpose it is concluded that steel behaviour limits the use of preapplied lacquer or coatings to articles which are drawn to cups not exceeding a depth to diameter ratio of 1.5 to 1. For greater depths of cup formed by deep drawing or wall ironing of drawn cups, our lacquered side wall margin is best achieved by redrawing or ironing the plain tinplate and then applying a suitable lacquer. (B) SELECTION OF LACQUERS AND COATING If a precoated tinplate blank is to be used, it is preferable that the tinplate surface is passivated by immersion in a sodium dichromate solution, before subjection to drawing and redrawing operations. This sort of chromate treatment improves the retention or continuity of lacquer film during the drawing operations. If the tinplate is to be drawn, redrawn and wall ironed, the presence or absence of a chromate passivation layer is less important because the severe redrawing and wall ironing reductions will disturb it. The presence of a chromate passivation layer on the base may slow down take up of tin into the product. Lacquers suitable for application to tinplate before deep drawing include epoxy amino resin varnishes, epoxy esters and epoxy phenolic lacquers. These lacquers are applied to tinplate sheets by the usual roller coating apparatus and stoved before use. Lacquers and coating suitable for spraying onto the side wall of drawn, redrawn or wall ironed cans include vinyl lacquers, vinyl organosols and epoxy amine, which are adjusted to the required viscosity for accurate spraying onto each can: the word "lacquer" indicating generally unpigmented material and the word "coating" indicating pigmented materials. In a preferred embodiment, the necessary firm adherence of the lacquer margin to the tinplate is achieved by an epoxy phenolic lacquer formulation adjusted to the desired viscosity and stoved for 2 minutes at 200°C. (C) SELECTION OF MANUFACTURING PROCESS Figure 4a shows a fragment of a sheet of tinplate 1 to which has been applied an annulus of lacquer 2. The periphery of the annulus defined the periphery of a blank 3 of a size to form the shallow cup shown in figure 4c. Typically the blank has a thickness T in the range between 0.004" (0.1 mm) and 0.009" (0.22 mm), typically 0.17 mm. This method of forming the can combines the steps of stencil coating a sheet of tinplate with an array of annuli of lacquer, stoving the tinplate sheets to dry the lacquer, stamping out the blanks of thickness "T" shown in figure 4b and drawing each blank to a cup 4 such that the annulus of lacquer forms a margin 5 of lacquer on the interior side wall surface 6 of each cup as can be seen in figure 4c. The bottom wall 7 of the tinplate cup is in the as plated condition and a lower, unlacquered, part of the side wall is not significantly different. Both side wall 6 and end wall 7 are substantially of thickness "T", but the side wall surface has been compressed in the hoop direction. Any disturbance of the upper part of the side wall metal, by the act of drawing, is covered by the lacquer margin, so that the available tin, presented to a product packed in the cup, is defined by the bare tin surface area presented to the product. We have observed that tin on matt tin plate surfaces tends to go into solution more rapidly than does flow brightened tin. Figure 5a shows a fragment 10 of a sheet of tinplate after blanking out of the circular blank 11 shown in side view in figure 5b. The blank has a thickness "T" typically in a range 0.004" to 0.009" (0.10 mm to 0.22 mm). The blank is drawn to a cup 12 shown in figure 5c to have a side wall 13 of thickness "T" substantially equal to the thickness of the blank and to the thickness of the end wall 14. The taller cup/can 15 shown in figure 5d has been redrawn by pushing, with a punch, the cup of figure 5c through a die that makes with the punch a clearance less than the metal thickness "T" so that the cup of figure 5c has been reduced in overall diameter, increased in height, and the thickness of the side wall 16 has been reduced to a thickness "t", typically between 0.0035" and 0.008" (0.09 mm to 0.20 mm). This redrawing process in which the wall thickness is only slightly reduced, is sometimes called "partial wall ironing" (PWI) because only a modest percent reduction of wall thickness is imposed. The dashed line "RD" indicates the shortage in height of cup that would have been achieved by redrawing with a punch/die clearance greater than T so the height advantage of about 10% available by a reduction in wall thickness of about 10% would not be achieved. However, this ironing force does abrade the tin surface of the side wall. Figure 2c shows the surface finish of the side wall of the can of figure 5d. It will be noticed that the roughness peaks of the internal surface of the can are flat topped which indicates that some of the tin layer has been moved into the troughs and steel exposure is a serious risk, particularly towards the top of the can. In order to repair this surface damage, a lacquer is sprayed from a nozzle 17 to cover the more severely worked upper margin of the side wall. The axial extent of the lacquer margin 18 is chosen to control the area of tin exposed on the interior of the rest of the side wall and the bottom wall and also to protect the worked surface of the side wall. A feature of the partial wall ironing process is that a smaller blank diameter is needed than would be required in a draw/redraw process to achieve that same height of can but the PWI side wall will suffer more cold working, giving a risk of electrode potential difference between end wall and side wall. A typical tin plate specification for these partially wall ironed cans is: Grain size greater than 4000 grains/mm 2 Surface roughness 0.3 µm to 0.64 µm (12 to 25 microinch) CLA Tin weight greater than 5.6 g/m 2 Suitable spray lacquers include epoxy phenolic or epoxy amino lacquers which are formulated to a suitable viscosity and applied by spraying onto the can interior. The cans are then stoved for 2 minutes at 200 °C. (D) PACK TESTS Data for pack tests for drawn and wall ironed cans is provided in parent European patent application no. 91311393.2 (EP-B-0492870). From that data it was noted that the tin coating weight in the body side wall and the height of the base tin margin are chosen to satisfy shelf life and tin pickup requirements.

Claims: A can body (15) for containing a product, the can body (15) being drawn and, optionally, partially wall-ironed from a tinplate (10) to comprise an end wall and an integral side wall (16) which extends from the periphery of the end wall to a terminal portion defining a mouth of the body (15); in which the interior surface of the can body (15) has a coating of organic material (18); characterised in that: the tinplate (10) from which the can body (15) is drawn has a surface roughness between 0.3µm and 0.64µm (12 and 25 microinches) CLA; and a margin or organic coating material (18) is post-sprayed onto the can body (15) and extends from the terminal portion along the interior surface of the side wall (16) for an axial distance (H) less than the length of the side wall (16), the rest of the side wall (16) having an exposed tin surface, the axial distance H being expressed by the formula: H = ½ CV πRT(1-I/100) - R where R is the radius of the side wall in mm V is the weight of the product in the can in g T is the weight of tin coating in gm -2 I is the % ironing reduction (if any) C is the chosen level of iron pickup limit in ppm A can body (4) for containing a product, the can body (4) being drawn and, optionally, partially wall-ironed from a tinplate (1) to comprise an end wall (7) and an integral side wall which extends from the periphery of the end wall (7) to a terminal portion defining a mouth of the body (4); in which the interior surface (6) of the can body (4) has a coating of organic material; and the tinplate (1) from which the can body (4) is drawn has a grain size greater than 4000 grains/mm 2 ; characterised in that: the tinplate (1) has a surface roughness between 0.3µm and 0.64µm (12 and 25 microinches) CLA; and organic coating material is precoated onto the tinplate (1) as an annulus (2), prior to drawing the can body (4), such that the organic coating extends as a margin (5) from the terminal portion of the can body (4) formed therefrom, along the interior surface (6) of the side wall for an axial distance (H) less than the length of the side wall, the rest of the side wall having an exposed tin surface, the axial distance H being expressed by the formula: H = ½ CV πRT(1-I/100) - R where R is the radius of the side wall in mm V is the weight of the product in the can in g T is the weight of tin coating in gm -2 I is the % ironing reduction (if any) C is the chosen level of iron pickup limit in ppm A can body according to claim 1 or claim 2, in which the organic coating material is chosen from a group consisting of epoxy phenolic lacquer, epoxy amine lacquer, acrylic resin lacquer and vinyl lacquer, with or without a pigment. A can body according to any one of claims 1 to 3, in which the can body (4, 15) is drawn without any partial wall ironing from tinplate (1, 10) having a tin weight between 3 gm -2 and 10 gm -2 . A can body according to claim 4, wherein the tinplate (1, 10) has a tin weight greater than 5.6 gm -2 . A can body according to claim 4 or claim 5, wherein the tinplate (1, 10) is a differentially coated tinplate and the heavier tin coating is on the interior surface of the can body. A can body according to claim 6, wherein the differential tinplate is D15/2.8, the 15 gm -2 heavier tin coating being on the interior surface of the can. A can body according to any one of claims 1 to 7, in which the tinplate (1, 10) is flow brightened.

Inventors: MONRO STUART ALEXANDER, PARKER MARY ANNE, ROSE SIMON PAUL, ROTHWELL GORDON, TURNER TERENCE ARTHUR, MONRO, STUART ALEXANDER, PARKER, MARY ANNE, ROSE, SIMON PAUL, ROTHWELL, GORDON, TURNER, TERENCE ARTHUR
Applicants: METAL BOX PLC, CARNAUDMETALBOX PLC
Application year: 1991
Main classification: B21D51
IPC codes: B21D22, B21D51, B65D25