AU2004201010A1 - Drawing tapered cans - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Applicant(s): BLUESCOPE STEEL LIMITED Invention Title: DRAWING TAPERED CANS The following statement is a full description of this invention, including the best method of performing it known to me/us: 2 DRAWING TAPERED CANS Technical Field The present invention relates to the manufacture of seamless, one piece, cup or dish-shaped, sheet metal articles, having a tapered cylindrical sidewall extending from the periphery of a substantially circular end wall.

The present invention relates particularly, although means exclusively, to the manufacture of seamless, one piece, cup or dish-shaped, sheet metal articles, having a tapered cylindrical sidewall extending from the periphery of a substantially circular end wall from thin, for example 0.16mm, double reduced tinplate or like material.

Background Art For descriptive convenience all such articles are referred to as "cans" or "precursors" hereinafter.

Furthermore, the end wall is referred to as the "floor" of the can or precursor irrespective of its orientation, and references to the depth of the can or precursor and like terms are to be construed accordingly.

Conventional drawing tools include a punch and die combination. Sometimes the punch and die have been distinguished one from the other on the basis that the punch moves relative to the apparatus as a whole whereas the die is stationary. Alternatively they have been distinguished on the basis that the punch is the male component of the combination, and the die is the female component. As relative movement between punch and die is all that is required for working co-operation therebetween, the latter convention is preferred and is consistently adopted hereinafter.

H \Bkrot\Keep\speci\Bluescope\Drawing Tapered Cans.doc 10/03/04 3 Thus, a conventional drawing tool may be said to comprise three co-axial components, namely a central punch, an annular die and an annular pressure pad. The die and pressure pad encircle the punch and the pressure pad is resiliently loaded towards an end face of the die.

In use, relative movement of the punch and die along their common axis causes workpiece material interposed between the punch and die to be drawn from between the pressure pad and the adjacent end face of the die into the wall of the can or precursor. The drawn material then conforms to the shape defined by the punch and die at the end of their operative stroke.

From the foregoing brief description of the prior art it will be appreciated that the flow characteristics of the workpiece material are critical for successful drawing operations. For preference, relatively ductile material, for example, single reduced tinplate, is used.

If it were not for its intractable hardness, thinner material, for example, double reduced tinplate, would be preferred, so as to reduce can weight and material cost.

The problems arising from using thin, hard material are exacerbated in the case of tapered cans.

In the conventional drawing of parallel walled cans, the clearance between the wall forming surfaces of the punch and die is very little more than the thickness of the sheet metal being drawn. This ensures that the metal remains unwrinkled and pre-coated surfaces remain undamaged. However, in the case of tapered cans there is necessarily a wider clearance between the punch and the die throughout most of their relative movement than is needed to just accommodate the thickness of the material being worked. This excessive clearance is conducive to wrinkling in the can wall, particularly with thin, less ductile material, leading to misshapen cans and coating H \Bkrot\Keep\speci\Bluescope\Drawing Tapered Cans.doc 10/03/04 4 damage.

Therefore, except for very shallow tapered cans, conventional practice in respect of non-tapered or so called parallel walled cans cannot be modified to produce a tapered can merely by substituting a tapered punch and die set for the conventional right cylindrical set.

Prior known processes for deep drawing tapered cans are well exemplified by US Patents 4,366,696 (Durgin), 4,503,702 (Bulso) and 6,038,910 (McClung).

Durgin discloses the production of a tapered can using a tapered punch and die set to form a tapered can precursor, wherein the clearance between the punch and die at the end of the operating stroke is larger than normal so as to avoid "coining" pressure on the workpiece material. The consequent wrinkling is initially tolerated and then suppressed by a reforming operation whereby the wall of the precursor is expanded while maintaining its taper. However, it has been found in practice, that undesirable residual wrinkling may still be discernable in the finished can unless the feed strip is relatively thick ductile material, such as the 0.21 mm thick material used by Durgin.

Bulso acknowledges Durgin and discloses a multistage process wherein the can is formed in shallow increments progressively from the floor up, using uniformly tapered punches operating in dies which increase in diameter from stage to stage, to thereby reduce the clearance space between punch and die in each stage sufficiently to avoid the formation of wrinkles. In the absence of any disclosure to the contrary it must be taken that Bulso is using conventional thick and ductile material. In tests similar to Bulso's process applied to thin (for example, 0.16mm), hard, double reduced tinplate, H\Bkrot\Keep\speci\Bluescope\Drawing Tapered Cans.doc 10/03/04 5 it was found that the tapered punch controlled the shape of the wall only near the bottom of the can, close to the punch nose. Most of the wall of the can then conformed to the parallel dies, rather than the uniform taper required.

McClung acknowledges both Durgin and Bulso.

McClung refers to thinner and harder (0.16 to 0.21 numm DR9) feed material. However in his preferred embodiment he utilises the thickest material in the range. He converts a blank of that material into a full depth parallel precursor, then in a second stage "re-draws" the precursor using a tapered die set "to impart a final taper to the sidewall without changing the final height thereof or the metal thickness". Notwithstanding his reference to no change in the metal thickness the expansion of the precursor into the tapered shape requires stretching of the metal at or near the mouth of the can and this cannot be effected without some thinning of that part of the can.

In his preferred embodiment the material at the top of the wall is stretched by more than 5% for an angle of taper of three degrees. Thus McClung's process imposes severe limitations on the height and taper angle of cans that could be produced. For example, if his preferred embodiment were modified to produce a taper angle of four degrees the material at the top of the wall would be stretched by more than Such a stretching of 0.16 nunmm DR9 material could be expected to result in some failures due to cans rupturing near the top of the wall.

Tapered cans are advantageous by comparison with parallel walled cans if the taper is sufficient to render them nestable, so that less space is required for the storage of empty cans and for their transport from the can-maker to the packager, and in that they facilitate the removal from the can of coherent masses such as, for example, cheese, processed fish or meat, and other soft but non-fluid materials, by the ultimate consumer of the H \Bkrot\Kee'p\speci\Bluescope\Drawing Tapered Cans.doc 10/03/04 6 canned material.

However, notwithstanding that it is desirable for strength and weight reduction to use double reduced tinplate, or an equivalent strong, hard, but relatively brittle material, such material has not been widely adopted hitherto in relation to deep, nestable, tapered cans, because of the problems associated with its use outlined above.

The above discussion of the background of the invention herein is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was published, known or part of the common general knowledge in Australia as at the priority date of any of the claims arising from this application.

Summary of the Invention An objective of the present invention is to provide a method and apparatus, as an alternative to the prior art, for the production of nestable, deep drawn, tapered cans from double reduced tinplate or like material.

The invention achieves that objective by a multistage process for the production of a can wherein the diameter at the top of the can wall may exceed the diameter at the bottom by, say but which ensures that the material is stretched by no more than 5% overall during the production process.

According to the present invention there is provided a method of manufacturing a tapered can comprising the steps of H:\Bkrot\Keep\speci\Bluescope\Drawing Tapered Cans.doc 10/03/04 7 drawing a first precursor from a substantially planar, sheet metal workpiece, which first.

precursor is a parallel walled cup that is shallower and larger in diameter than the required finished can, drawing a second precursor from the first precursor, which second precursor comprises a second parallel walled cup and a circumferential flange that is a remnant of the first precursor, wherein the second cup has a diameter substantially equal to the floor diameter required in the finished can and a wall height such that it may be expanded into a tapered cup with a taper angle substantially corresponding to that required in the finished can without stretching the wall at the top of the second cup by more than expanding the second cup in the manner set forth in step using a tapered punch, to form a third precursor comprising a tapered cup substantially conforming with a bottom part of the required finished can and a circumferential flange that is a remnant of the second precursor, and drawing the flange of the third precursor using a tapered die to form a can pre-form having a tapered wall extending upwardly as an extension of the wall of the tapered cup of the third precursor and substantially conforming with an upper part of the wall of the required finished can.

Preferably the method further comprises a step of press forming the can pre-form formed in step and perfecting the final shape of the can, and, as is frequently needed, to shape the rim of the can to adapt it to receive a top end wall or other closure.

In preferred embodiments step comprises the H:\Bkrot\Keep\speci\Bluescope\Drawing Tapered Cans.doc 10/03/04 -8following steps, namely: drawing the flange of the third precursor using a parallel die and parallel punch to form a fourth precursor having a parallel wall extending upwardly as an extension of the wall of the tapered cup of the third precursor, and drawing the fourth precursor using a tapered die to form a fifth precursor comprising a tapered wall extending upwardly as an extension of the parallel wall of the fourth precursor and substantially conforming with an upper part of the wall of the required finished can.

Preferably the method described in the preceding paragraph further comprises press forming the fifth precursor using a tapered punch to bring the fifth precursor into accurate conformation with the body of the required can.

In preferred embodiments the tapered punch of step the parallel punch and parallel die of step and the tapered die of step (ii) are each parts of a single punch and single die, of which each has a tapered leading portion and a parallel trailing portion.

Under those circumstances the punch and die constitute a single tool at one station of a multi-station drawing apparatus, wherein the tapered portion of the punch coacts with the tapered portion of the die to expand the second cup, the parallel portions of the punch and die coact to draw the parallel wall extension of the fourth precursor and the tapered portion of the die co-acts with the parallel portion of the punch to draw the tapered wall extension of the fifth precursor. In that event steps (c) and and (ii) may be effected consecutively by one operating stroke of the tool.

H \Bkrot\Keep\speci\Bluescopie\Drawing Tapered Cans.doc 10/03/04 9 Preferably the method comprises a still further step, being a conventional pressing operation, and producing a non-planar floor in the finished can, for example a centrally recessed floor or one with circular corrugations to render it axially flexible so that manual pressure on the floor may partly express the contents of the can.

Preferably the sheet metal workpiece is double reduced tinplate or like material.

According to the present invention there is also provided a deep drawing tool for effecting the said steps and (ii) of the kind comprising a punch, die and pressure pad characterised in that both the punch and die have tapered leading portions and parallel trailing portions.

According to the present invention there is also provided a multi-station drawing apparatus that comprises the deep drawing tool described in the preceding paragraph at one station of the apparatus.

Brief Description of the Drawings.

By way of example, an embodiment of the invention is described in more detail hereinafter with reference to the accompanying drawings.

Figures 1 to 6 are sectional elevations taken on a vertical centre plane of a workpiece at progressive stages of its transformation from a blank to a can.

Figure 4 is drawn to a larger scale than the other figures in this group.

Figure 7 is a sectional elevation on a vertical H \Bkrot\Keep\speci\Bluescope\Drawing Tapered Cans.doc 10/03/04 10 centre plane of a conventional tool as used to convert a planar blank into a parallel walled, cup shaped precursor.

The figure is divided along a vertical centre line, with the left hand half of the figure showing the disposition of the components at the start of an operating stroke and the right hand half showing the disposition at the end of the stroke.

Figure 8 is a view similar to figure 7 of a drawing tool at a first station of a multi-station apparatus for effecting the method of the invention.

Figure 9 is a view similar to figure 7 of a drawing tool at a second station of the multi-station apparatus, wherein the right hand half shows the disposition part way through the operating stroke.

Figure 10 is a view similar to figure 9 of the drawing tool of that figure, wherein the left hand half shows the disposition at the part way position and the right hand half shows the disposition at the end of the stroke.

Figure 11 is a view similar to figure 8 of a press-forming tool at a third station of the apparatus.

Best Mode of Putting the Invention into Effect.

Figure 7 illustrates a drawing tool for conversion of a circular workpiece or blank 12 (see figure typically double reduced 0.16mm tinplate, into a first precursor 13 (see figure 2) comprising a floor 14 and a parallel wall 15. The floor 14 is larger in diameter than the floor 16 of a required finished can 17 (see figure 5) and the wall 15 is considerably less tall than the tapered wall 18 of the can 17.

H\Bkrot\Keep\peci\Bluescope\Drawing Tapered Cans.doc 10/03/04 11 The tool of figure 7 and its operation are quite conventional, and could be used to effect the first step in a conventional drawing operation for forming, say, a parallel walled can. The tool comprises a parallel punch 19, a parallel die 20, an annular pressure pad 21 encircling the die orifice, a knock out 23 and conventional means including cushion pins 22 resiliently urging the pressure pad 21 towards a reaction surface constituted by the annular end face of the die 20. The cylindrical wall forming surfaces of the punch 19 and the die 20 are separated by a clearance space that is very little larger than the thickness of the material of the blank 12. In view of its conventional nature, neither the tool of figure 7 nor its operation needs to be described in further detail herein.

The next step in the process, which may be effected by a conventional drawing tool at a first station of the multi-station apparatus, as illustrated by figure 8, is the drawing of the first precursor 13 into a second precursor 24 (see figure 3) comprising a parallel walled cup 25, having a floor 26 substantially equal in diameter to or slightly smaller than the floor 16 of the can 17, and a circumferential flange 27 being the remnant of the first precursor 13. The height of the wall of cup 25 is such that the cup may be expanded into a tapered cup with a taper angle equal to that required in the finished can to render the can readily nestable. Cans with walls tapered greater than 30 can be made successfully, providing the maximum overall expansion is kept at no more than 5% by adjusting the intermediate stages determined by the taper angle and height of the can wall.

Again, because of its conventional nature the tool of figure 8 requires no detailed description. It is sufficient to say the tool comprises a parallel punch 28, H \Bkrot\Keep\speci\Bluescope\Drawing Tapered Cans.doc 10/03/04 12 a parallel die 29, a knock out 30, a pressure pad 31 and cushion pins 32.

The next three steps in the exemplary embodiment of the invention now being described, which may be effected by a single operative stroke of a tool in a second station of the multi-station apparatus illustrated in figures 9 and 10, are as follows; expanding the cup 25 into a tapered cup substantially conforming with a bottom part of the required finished can 17, to momentarily form a third precursor comprising that tapered cup and a circumferential flange, drawing a part only of the circumferential flange of the third precursor into a parallel walled upward extension of the tapered wall of the tapered cup of the third precursor, and drawing the remainder of that circumferential flange into an upward tapered extension of the parallel walled extension, which may substantially conform with an upper part of the wall of the required finished can 17.

That second station tool includes a novel punch 33 comprising a tapered leading portion 34 and a parallel trailing portion 35, and a novel die 36 comprising a tapered leading portion 37 and a parallel trailing portion 38. The second station tool also includes conventional components, namely a pressure pad 39, knock out 40, and cushion pins 41.

The operation of the second station tool may now be described. Turning firstly to figure 9 it will be seen that the leading end face of the die 36 advances to H,\Bkrot\Keep\speci\Bluescope\Drawing Tapered Cans.doc 10/03/04 13 contact the flange 27 of precursor 24 and draws the parallel walled cup 25 of that precursor over the tapered portion 34 of the punch 33. This expands the mouth of the cup to produce a tapered wall and convert the cup 25 into a tapered cup substantially conforming with a bottom portion of the required can 17. The right hand half of figure 9 shows the disposition of the punch and die at the end of this expansion.

The left hand side of figure 10 shows the die 36 at the same position as it appears in the right hand side of figure 9. As the die 36 moves further down, its parallel portion 38 reaches the parallel portion 35 of the punch 33. The clearance between the parallel portions is very little more than the thickness of the workpiece material.

Thus, still further movement draws material from the flange 27 into conformity with the tapered portion 37 of the die and then into the clearance between the two parallel portions, until the end of the stroke is reached.

The end result of the operation of the second station tool is the production of a precursor 41 (see figure 4) comprising a floor 16, a generally cylindrical wall and a circumferential flange 42, being a final remnant of flange 27, wherein the wall of precursor 41 comprises three zones, namely zone H2 that conforms to the tapered portion 34 of punch 33, zone H5 that conforms with the clearance space between the overlapping parts of the parallel portions 35 and 38 of the punch 33 and die 36 respectively at the end of the tool's stroke, and zone H4 that conforms to the tapered portion 37 of the die 36. It will be apparent that the dimensions of the punch 33 and die 36 and the length of the stroke are selected so that the precursor 41 conforms approximately with the shape and dimensions of the required can, especially in respect of H.\Bkrot\Keep\epeci\Bluescope\Drawing Tapered Cans.doc 10/03/04 14 the diameter of the floor and the positions and angles of taper of zones H2 and H4.

The next, and in many instances the final, step in the process is to finish form the precursor 41 into the can 17. This may be done by a conventional press tool in a third station of the apparatus as shown in figure 11.

That press tool comprises a fully tapered punch 43 and a parallel die 44 and associated pressure pad 45 and knock out 46. The leading end face of the die is stepped and the punch is shouldered so as to convert the flange remnant 42 of precursor 41 into a rim flange 46 of the can 17.

The height of zone H5 of the precursor 41 depends on the length of the overlap between the parallel portions of the punch 33 and the die 36 at the end of their operative stroke.

The maximum heights of the respective zones are selected in accordance with the invention having regard to the need to ensure that the total strain imposed on the material of each zone during its formation does not exceed the 5% limit required by the invention. For preference the heights of zones H2 and H4 are the same, whereas zone H5 may be of the same height as the others or may be of a lesser height if desired.

As mentioned earlier, if desired the can 17 may be press-formed in a separate operation to provide a recessed floor 47 or otherwise profiled floor, as may be desired in the end product.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described.

All such variations and modifications are to be considered H \Bkrot\Keep\speci\Blueecope\Drawing Tapered Cans.doc 10/03/04 15 within the scope of the present invention the nature of which is to be determined from the foregoing description.

In the preceding summary of the invention, except where the context required otherwise due to express language or necessary implication, the word "comprising" is used in the sense of "including", that is the features specified may be associated with further features specified may be associated with further features in various embodiments of the invention.

H\Bkrot\Keep\speci\Bluescope\Drawing Tapered Cans.doc 10/03/04

Claims (11)

1. 2. The method defined in claim 1 comprises a further step of press forming the can pre-form formed in step (d) H\Bkrot\Keep\speci\Bluescope\Drawing Tapered Cans.doc 10/03/04 17 and forming the final shape of the can including shaping the rim of the can to adapt it to receive a top end wall or other closure.
2. 3. The method defined in claim 1 wherein step (d) comprises the following steps: drawing the flange of the third precursor using a parallel die and parallel punch to form a fourth precursor having a parallel wall extending upwardly as an extension of the wall of the tapered cup of the third precursor, and drawing the fourth precursor using a tapered die to form a fifth precursor comprising a tapered wall extending upwardly as an extension of the parallel wall of the fourth precursor and substantially conforming with an upper part of the wall of the required finished can.
3. 4. The method defined in claim 3 further comprises press forming the fifth precursor using a tapered punch to bring the fifth precursor into accurate conformation with the body of the required can. The method defined in claim 3 or claim 4 wherein the tapered punch of step the parallel punch and parallel die of step and the tapered die of step are parts of a single punch and single die, of which each has a tapered leading portion and a parallel trailing portion.
4. 6. The method defined in claim 5 wherein the punch and die constitute a single tool at one station of a multi-station drawing apparatus, wherein the tapered portion of the punch co-acts with the tapered portion of the die to expand the second cup, the parallel portions of H.\Bkrot\Keep\speci\Bluescope\Drawing Tapered Cans.doc 10/03/04 18 the punch and die co-act to draw the parallel wall extension of the fourth precursor and the tapered portion of the die co-acts with the parallel portion of the punch to draw the tapered wall extension of the fifth precursor.
5. 7. The method defined in claim 6 comprises carrying out steps and and consecutively by one operating stroke of the tool.
6. 8. The method defined in any one of the preceding claims further comprises a pressing operation to produce a non-planar floor in the finished can.
7. 9. A deep-drawing tool for effecting the said steps and of the kind that comprises a punch, die and pressure pad characterised in that both the punch and die have tapered leading portions and parallel trailing portions.
8. 10. A multi-station drawing apparatus that comprises the deep drawing tool defined in claim 9.
9. 11. A method of manufacturing a tapered can substantially as hereinbefore described with reference to the accompanying drawings.
10. 12. A deep drawing tool substantially as hereinbefore described with reference to the accompanying drawings.
11. 13. A multi-station drawing apparatus substantially as hereinbefore described with reference to the accompanying drawings. Dated this 10th day of March 2004 BLUESCOPE STEEL LIMITED By its Patent Attorneys GRIFFITH HACK Fellows Institute of Patent and Trade Mark Attorneys of Australia H,\Bkrot\Keep\speci\Bluescope\Drawing Tapered Cans.doc 10/03/04