Fundamentals of Modern Can Making and Materials

Development for Three-piece Can Manufacturing*

By M. SODEIK,** K. TAFFNER* * and F. WEBER* *

Synopsis In a comparison market review, it has been found that, in Europe, three-

piece cans are mostly produced as food cans with paper labels. Such cans are now almost exclusively made from tinplate with a welded side seam. In this article, the possibilities of improving the economics in the produc-

tion of the cans will be described taking into account technical limitations: i) Lower tin coating weight limits required by the welding operation.

Restrictions due to corrosion under today's lacquering conditions. Improving the tin coating uniformity to meet these requirements. ii) Using minimum possible metal thicknesses with a view to providing stability of can body and end under sterilization, distribution and storage conditions. The necessary material improvements will be

described. iii) Manufacture of cans with low body wall thicknesses by flanging. Special conditions to be observed, i f high-strength steel, such as DR material, is used. Current can specifications in Europe. iv) Current can specifications in Europe.

Key words; cold rolled fat steel; tinplate; can making; three-piece cans; seam welding; tin coating weight; can corrosion; can design.

I. Market Review

In a comparison of the modern can-making tech-nologies, as they have been developed in Europe and

particularly in Germany over the last years, with the progress of can making in Japan, it will be useful to first take a look at the can market situation in Ger-many-which approximately corresponds to that in Europe-and to see it against the situation in Japan.

If, in Fig. 1, the tonnages of tin mill products con-sumed in the two countries in 1984-figures which, according to our knowledge, should not have sub-

stantially changed by 1987-are compared, the follow-ing will be noticed :

(1) Whereas, in Japan, about 43 % of the ton-nage have been consumed for food and beverage cans, the same segment in Germany represents a definitely higher percentage of 58 %. In fact, this corresponds roughly to the percentages of the products supplied by Rasselstein.

(2) Furthermore, it is striking that, in Japan, the consumption for general line packagings of 54 % is much higher than that of only 25 % of the tonnage

produced in Germany. Presumably, the large share of 18-1 cans of 18 % in Japan is a major factor in this figure. Even though detailed data are not available, it must be assumed that, in Germany, a major part of these products is packed in 55-gallon drums which are

produced from sheet steel. (3) Another main difference is in the crown caps

-which, for Germany, have been listed together with closures for glass bottles and jars-which, in Japan, represent a considerably lower percentage of 3 % than that of 13 % in Germany. In our country, this per-centage presumably includes a major share of twist-off caps for fruit juices most of which are currently

packed in glass bottles, different from Japan, where-as will be discussed later-they are packed in cans. Looking at Fig. 2, showing a more detailed break-down for food packagings, i.e., food and beverage cans, with petfood included in the food cans, the following differences will be found :

(1) Food and petfood cans (including milk cans) in Germany hold 65 % of the total tonnage of food

Tonnage 1984

Fig. 1.

Tin mill products for can applications.

* Based on the paper presented to the Symposium on Material for Can Stock held at the 113th ISIJ Meeting , April 1987, The University of Tokyo in Tokyo. Manuscript received on January 18, 1988; accepted in the final form on March 1, 1988.

ISIJ * * Research and Quality Control Department , Rasselstein AG, D-5450 Neuwied 1, F. R. Germany.

A119, at

Q 1988

Research Article (663)

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packaging. In Japan, this is only 18 %. (2) Beverage cans in Germany take 35 % of the

food and beverage segment, but in Japan 82 %. (3) However, in Japan, a very large amount of

the beverage cans-66 % of the food can total-is used for non-carbonated beverages. Such cans are mostly pasteurized cans which should be compared to the glass containers for fruit juices used in Ger-many.

(4) Beverage cans in Germany are mainly cans for carbonated beverages, such as beer and lemonade.

II. General Trends of Development in Europe

The statistical review as described above leads us to the priorities in the general trends of development in Europe. They concentrate on food cans and on beverage cans for carbonated beverages, such as beer and lem-onade.

After a short, more general survey, the main fea-tures of can development in Europe during recent

years will be discussed in this paper. The food can converted quickly and almost com-pletely from the can with a soldered side seam to the welded side seam can. In this connection, the devel-

opment of can welding as shown in Fig. 3 has been of major importance. The introduction of the weld-ing process for food cans was pushed ahead by the change-over from the butterfly weld to the WIMA and Super-WIMA weld. Even for food cans produced in large numbers, as they are used in the dimension of 73 mm dia. X 110 mm, the welded can has meanwhile been accepted as the standard can. Figure 4 shows the technologies available for mak-ing this can, i.e., the welded three-piece can, the deep-drawn can using the DRD process and the drawn and wall-ironed can by the DWI process. Figure 4 also states the relative can costs for the three processes. Even though the DWI can is produced at the lowest costs, it has been accepted in Europe for this can di-mension only to a limited extent, as, in the European countries, the numbers needed for an economical

production were insufficient and the problem of flexi-bly converting to other can dimensions has been of

greater importance than the costs. This cheapest food can with the dimension of 73 mm dia. X 110 mm has

previously been used only in England for petfood. Among the two-piece cans, the drawn DRD can in the dimension of 73 mm dia. X 58 mm has made its way in Europe and particularly in Germany for pet-

Tonnage 1984

Fig.

Tin

can

2.

mill products for

applications.

food and beverage

Fig . 3. Different welds.

Transactions ISIJ, Vol. 28, 1988 (665)

food products. This can is shown in Fig. 5. In ad-

dition, this figure represents a deep-drawn can with the dimension of 84 mm dia. X 42 mm, as it has been increasingly used for tuna also in Europe in the Medi-

terranean area. The cemented can is of practically no importance

in Europe. The beverage can, in Germany and Europe, for which the cemented design was leading

in the United States, has been directly introduced as DWI can.

As food can, only a special design developed by one

producer with a body score line is available. Evi-dently, this cemented can design does not provide a

price-worthy solution under European conditions. In view of the above considerations, it is obvious

that, for Europe, apart from some special develop-

ments, we must concentrate on studying the develop-ment of

-the three-piece food can with welded side seam,

which will be described in this publication, and -the two-piece can mainly used as DWI can for

beverages, on which we will give an extra paper in a later article.'

III, Three-piece Food Can

,1. Welded Food Can Bodies

The manufacture of a welded food can body is based on two essential operations which are influ-enced by the material: i) Rounding of a cylinder from a flat body blank,

and ii) Welding of a side seam using various materials

(different thicknesses, qualities, surface finish- ings).

For rounding the can body, roll forming operations are carried out which, as known from earlier times, react more subtly to variations of material properties than bending operations which are performed by the Wingform bodymaker. For the transport of the body blanks in the welder and for the actual welding operation, highly constant overrounding widths Bu are required. With chang-ing material properties and unchanged machine set-tings, variations will most likely result. In compre-hensive tests, we have tried to determine the essential

parameters influencing this rounding operation. It has been found that, as shown in Fig. 6, rounding de-

pends mainly on the yield point Ret of the material. The results of numerous material samples have been compiled in this figure. It will be noticed that there is a significant difference between batch annealed and continuously annealed material. Above all, it follows from our experiments as shown in Fig. 6 that it is almost impossible to mix batch annealed and con-tinuously annealed material, a finding which has been confirmed by actual practice. Up to date, it has not been possible to compensate for potential ma-terial differences by a defined flexing operation, as the flexer design could not yet be adjusted according to the current requirements. Work has been con-centrated on this feature.

Other main subjects of development in Europe have been the investigation of the surface parameters which

Fig. 4. Various can designs (73 mm~b X 110 mm).

Fig. 5. DRD cans.

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are of decisive influence for the resistance welding of tinplate. For some tin coating weight specifications of below 2.8 glm2 per side, welding is considered to be feasible only after preliminary tests have been made. At our laboratory, a high-speed can body side seam welder has been installed which has enabled us to carry out a vast number of tests.

In this connection, the possibilities of welding ma-terial with very low tin coating weights, especially lower than 2.8 g/m2 on one side, have been of interest. In welding can body side seams attention must be concentrated on a maximum setting range for the welding current. This is important in order to give the operator of the welder maximum freedom in set-ting his machine and to enable him to compensate for normal variations of the material properties. Looking at Fig. 7, we will see that, with decreasing tin coating weights, the available current ranges will become smaller. This effect has been similarly found by other authors.2~ For solving this problem, special materials were even developed which were then re-commended as packaging materials with thinner tin coatings suitable for welding. Figure 7 shows the secondary currents of the welder which are necessary or justifiable as functions of the tin coating weight. In addition, the welding loads were varied in our tests. It has been shown that, as compared to a weld-ing load of 400 N, a welding load of 700 N will almost duplicate the available current range. This extreme dependence of the available current range on the weld-ing load is described once more in Fig. 8 in the form customary in Japan. From these two diagrams it will be seen that materials with lower tin coating weights of considerably less than 2.8 g/m2 can be successfully welded into can bodies at correspondingly higher weld-ing loads. According to our experience, we consider 0.4 kA the necessary minimum value of the available secondary current range. In this connection, it should be mentioned that crushing of the weld overlap in the welder must be prevented by an adequate tool

guiding control with improved diabolo rolls. Such tools are now offered by the welder manufacturer as substitute parts.

2. Can End Stability

In developing a welded food can with a view to low

costs, the possibilities of saving in the costs for the end material must, of course, also be carefully studied. Food cans are subjected to a high strain especially during the sterilization process. In this process, as a result of the inside pressure, the end may be subject to plastic deformation with " buckling ". This must be prevented, as a buckled can cannot be sold any more. In Fig. 9, the parameters influencing the oc-currence of buckling are shown. The diameter of the end is of major importance. The metal thickness, too, has a great influence. The third part of the picture indicates the effect of the metal hardness. Unfortunately, it must be said that the influence via the metal hardness is almost negli-

gible. As has been found in comprehensive investigations, most of which have been published,3~ buckling of the can end is only one factor among the stability require-ments. The flexibility of the end, i.e., the volume in-crease under the can inside pressure, is an additional

Fig. 6. Round ing behaviour.

Fig. 8. Available current

weights.

ranges as functions of tin coating

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and most important parameter. This is shown in Fig. 10. Figure 11 tries to describe the strain situation to which the end is subjected during sterilization. The hyperbola representing the behaviour of the canned

product has been calculated, with the filling assumed to consist of water and air. This assumption is ap-

plicable to most of the products. The curve has been determined for the temperature of 130°C, a retort outside pressure of 1.5 bar and a can headspace of 4%. The curve as shown for the can behaviour is rep-resentative of an end of 99 mm dia. on a can with a height of 119 mm, with the end provided with 3 beads. Furthermore, the behaviour of the end has been plot-ted for various end designs with different metal thick-ness and hardness values. Above the pressure-volume curve-the hyperbola for the product-, the end will not fail, whereas below the curve, buckling must be expected. It will be recognized that with a thickness of 0.24 mm-independent of the hardness of the end between 55 and 70 HR30T-a favourable behaviour

can be reliably expected. For metal thicknesses of

0.20 mm, it is obvious that, with lower hardnesses, the ends will, at any rate, fail, whereas with higher hard-

nesses-about 70 HR30T-presumably no problems

will be experienced. Detailed data on the relations described have been published by us.4~

Apart from the specifications for preventing failure

by buckling, other specifications have recently been requested by the can consumers. The specification

for the flip-back behaviour of the end has been stated recently, with the flip-back after pressure pre-loading

defined as the standard. This phenomenon is ex-

plained in Fig. 12. A certain value of the flip-back vacuum necessary to return to the original state is fixed at certain values of pressure pre-loading caused

by inside pressure. The curve marked " Specifica-tion " represents the situation as it is desired. In con-

nection with a minimization of the end metal thick-ness, a new material had to be developed because of

this new requirement. A continuously annealed ma-terial with a high aging capacity is now used. As,

today, ends for food cans for such extreme loads, mostly

petfood cans, are almost exclusively produced from special chromium coated EGGS material. They are

Fig. 9. Buckling.

Fig. 10. Pressure-volume diagram for food cans.

Fig. 11. Stability of ends.

Fig. 12. End flip b ack after pressure pre-loading.

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punched from not finally aged material which will allow the forming of narrow bead radii without cracks. If a water-soluble lining compound is used for the end edge, the heat which is necessary for drying the lining will simultaneously, without additional costs, complete the aging process of the end material. If other compounds are used, the can maker must pro-vide a separate heating operation.

3. Can Body Stability

The manufacture of economical, welded food can bodies will also require looking in more detail at the minimum body metal thickness that is practicable. The weld of the Super-WIMA type permits a much easier forming of can body beads as compared to the side seam of a soldered can. In the frequently used continuous sterilization process in the hydrostatic re-tort, the can body is subjected to a high load due to the hydrostatic outside pressure. In order to make the thin can body material resistant to the outside

pressure, it is provided with beads. The panelling pressure due to the outside pressure will increase with the bead depth, but the axial load, which is impor-tant for stacking the cans during storage, will decrease. If a certain panelling pressure PR and a certain axial load F have been specified, the optimum bead depth for the minimum metal thickness can be determined by a calculation4 which has been confirmed by ex-

periments carried out at our laboratory. In Fig. 13, the bead depth tR has been plotted as a

function of the body metal thickness SR at constant axial loads and constant panelling pressures. From the specification for an axial load of F=4 kN and a

panelling pressure of PR =1 bar, a bead depth of 0.45 mm with a body metal thickness of 0.18 mm will be obtained for the can of 99 mm dia. X 119 mm. So, a single-reduced steel with a yield point of Ref= 350 N/mm2 was selected. By increasing the strength of the body metal, only the axial load, but not the panelling pressure can be

raised, the latter depending solely on the Young's modulus. If a reduction of the metal thickness by increasing the strength of the metal is intended-e.g., by the conversion to DR material-, the stability char-acteristics can be maintained only, if the bead depth is increased. This has been shown in Fig. 13 for a yield point increase from 350 to 520 N/mm2. The metal thick-ness could be reduced from 0.18 to 0.16 mm, if the bead depth were increased from 0.45 to 0.57 mm. But this can only be realized, if the higher-strength material still provides sufficient forming capacity for making such deeper beads. This may eliminate the conversion to DR material for the can body.

4. Flanging of Can Bodies

The use of can body materials of ever higher strengths with low metal thicknesses will eventually lead to problems in flanging the can bodies, especially if the influence of the heat during the welding opera-tion is taken into account. Small can diameters will raise additional problems. Principally, it can be said that a spin flanging operation, as it is shown in a basic diagram in the upper right corner of Fig. 14, will

provide superior results as compared to the die flang-ing method shown on the left. Spin-flanging, of course, requires more expensive machinery. For studying these problems, we have developed a method of overflanging. A number of cans is tested by overflanging beyond the dimension of 2.5 mm which is needed in practical operation, and on the basis of 100 samples, the accumulative frequency of the cans cracked during flanging is stated.

The relative elongations of the flange edge are shown in Fig. 14 for various can diameters. From Fig. 14 it will also be seen that the elongation values obtained by the tensile test are obviously unsuitable, as this test would have indicated values which would not have been sufficient even for a 99 mm dia, can. Figure 15 gives an example of overflanging tests.

Fig . 13. Stability of beaded can bodies. Fig. 14. Flangin g of the can edge.

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The influences of the parameters such as the flanging

process, the position of the rolling direction, the an-nealing process and the can diameter are shown for the can diameters of 66, 73 and 99 mm.

5. Can Corrosion

At the time of high tin prices, we too, similar to

Japan, carried out numerous investigations in order to find out, if the tin coating weight could be reduced below 2.8 g/m2 on one side.

The reductions of the tin coating weight resulting from the elimination of the soldering method have al-ready been discussed. Work in Europe has then been concentrated, above all, on such tin coating weight reductions that could be used without employing other

processing steps, such as the nickel deposition. In this regard, the cost situation as shown in Fig.

16 should be considered in more detail. Here, we see -based on the German price-list-tinplate prices in-dicated for different tin prices. In addition, the dia-

gram states the costs for pre-nickel coating of packag-ing material before the deposition of the tin coating. According to Fig. 16, at the today's tin prices, pre-nickel coating will be as expensive as an additional tin coating of about 1.0 g /m2.

In view of this new situation caused by the decline of the tin prices, it has been obvious to concentrate on investigating only tin-coated materials with coating weights from 1.0 to 2.8 g/m2. Figure 17 presents the results of inside corrosion tests. For the pack tests, a test product which was developed at our laboratory consisting of a special carrot pulp has been used. The evaluation basis is the iron pick-up by the product after 3 months' storage at 50°C. Cans were used in the tests with an epoxy

phenol resin lacquer coating of 5 g/m2 on one side. A tin coating weight of 2.8 g/m2 was used as a com-

parison basis. In the tests, this coating weight in-dicated iron pick-up values of less than 20 mg Fe per kg of product. Tinplate from our production with only 1.0 g/ m2 tin coating showed somewhat lower val-ues in the tests, i.e., 20 to 30 mg Fe per kg. The other materials which we tested and which had been developed in Japan-Nos. 1 to 4 in different coatings -could not indicate better iron pick-up results in

our tests. The coatings are described in the upper left of the figure. In Fig. 18, more pack test results are shown which

were obtained under the same conditions as those of the previous figure. It will be seen that decreasing tin coating weights will reach critical iron pick-up values in the product only at coating weights of less than 1.0 g/ m2. An investigation with natural prod-

Fig. 15. Influence in over-flan ging welded cans.

Fig . 16. Price/cost tin mill p

comparison of

roducts.

various

Fig. 17. Iron pick-up after 3 months' storage of 50°C.

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ucts over extended periods is presented in Fig. 19. In this study, cans with a greater number of various

products were tested for iron pick-up by the product after 18 months' storage at 37°C. This test corre-sponds to a storage time of 3 years at room tempera-ture, As a standard for inside lacquered cans, the soldered can with a tin coating weight of 2.8 g/m2 per side was used.

Among the cans of most recent development, welded can bodies with tin coating weights of 2.0 g/m2 and 1.5 gJm2 with a well covered side seam, mostly by

powder, were tested. The products consisted of vari-ous vegetables and even some pickles. The iron pick-up results of these cans will be seen from Fig. 19 in-dicating that, even with low tin coating weights, de-finitely improved values are obtained due to the obviously superior covering of the side seam of a welded can, and this mainly in the cross seam area. So, the change-over to the welded can resulted in a reduced tin coating weight with a simultaneous im-

provement of the can quality. The question of outside corrosion resistance for cans with tin coatings below 2.8 g/m2 could not be com-pletely clarified. In this connection, it should be noted that, normally, can bodies are not lacquered on the outside, but only provided with a paper label. According to our knowledge, test method providing sufficient evidence are not yet available so that relia-ble information, especially on the influence of the sur-face roughness, the porosity, the passivation and other surface parameters could be given.

The successful results of our pack and corrosion tests could, naturally, only be achieved due to the fact of modifications that we had made in our tinning

processes, particularly with regard to obtaining a more uniform coating.

This development is represented in Figs. 20 and 21. Edge overcoating was noticeably reduced, and addi-tionally, lower tolerances across the strip width could be achieved by a careful anode installation. Besides, the growth of the FeSn2 layer was reduced by mea-sures taken during flow-melting.

Therefore, we do feel that, with a view to the con-ditions prevailing in Germany, the fabrication of tin-

plate with low tin coating weights has, indeed, been the right decision.

This is underlined by the fact that our tinplate shipments in 1987 included already more than 32 %

Fig. 18. Iron pick-up after 3 months' storage at 50°C

.

Fig. 19. Quality design.

comparison of food cans of new and old

Fig . 20. Tin coating weight distribution across strip width.

Fig. 21. Tin coating weights with nominal

coating.

2.0 g/m2 tin

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with tin coating weights on one side of less than 2.8 g/m2. This figure does not include the material for DWI cans.

6. Standard Cans From the data which have been presented above, the following standard specifications as shown in Table 1 can be stated for the cans most frequently used in Europe : For the can of 99 mm dia. X 119 mm, the standard specification is : Can body 0.195 mm-T 65, D 2.0/1.0 Can end 0.23 mm-T 65, E 2.82.8, or ECCS. For the can of 73 mm dia. X 110 mm, the standard specification is: Can body 0.155 mm-DR 8, D 2.0/1.0 Can end 0.20 mm-T 65, E 2.8/2.8, or ECCS.

IV, Conclusions

Based on the market situation in Europe as com-

pared to that in Japan, the major developments of the three-piece can making technology in Europe have been described :

(1) Such developments are marked by a complete elimination of the soldered side seam can within only few years.

(2) For the food can, the welded side seam has been introduced.

(3) This has permitted the design of cans with optimally low body wall thicknesses, after the basic data for evaluating the loads to which the can willl be subjected and its stability had been developed.

(4) Improvement of tinning and can lacquering technologies eventually resulted in the production of cans at lower tin coating costs and with still superior corrosion resistance.

Table 1. Food can specifications.

REFERENCES

1) M. Sodeik, K. Taffner and F. Weber: Trans. Iron Steel Inst.

Jpn., 28 (1988), 672. 2) W. Waddell, D. E. Thomas and N. T. Williams: Meta Construction, (1986), March, 156. 3) M. Sodeik and R. Sauer: Proceedings of the Third Inter- national Tinplate Conference, International Tin Research

Inst., London, (1984), 152-167. 4) M. Sodeik and R. Sauer: Verpackungs-Rundschau, 36 (1985),

No. 6 Technical Scientific Supplement, 35-42.