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Aluminum Must Do More for Less in Beverage Can Competition
09-20-02
The dominance of aluminum as a preferred beverage can material continues to be challenged by competitive materials. Maintaining and growing the light metal's share will depend on improved material and design capabilities coupled with lower cost production methods, according to one recent view.
This perspective was provided in a keynote paper presented by Paul Butler, Director of Materials and Packaging Technology, Crown Cork & Seal Co, Inc. at the recent International Conference on Aluminum Alloys in Cambridge, UK. While noting that aluminum has had a very successful application track record in the US (although it holds a minority position in the European market), Butler states that "the aluminium industry is in danger of finding itself caught in a pincer movement between steel and plastics when packaging material choices are being made". With this in mind, he discusses areas for improving aluminum's competitive position. The materials and process technology areas discussed are relevant to those of us in the aluminum industry. Two of these, formability and continuous casting, will be the focus of this article.
One trend in packaging has been to add "shape" to aluminum beverage containers to provide brand differentiation. Shaping processes such as rheoforming, which utilizes internal water pressure to force the can into a mold, and blowforming, involving air pressure in concert with axial compression, require more of the can material from a formability perspective. Typically, an additional 8-10% strain is imparted to the formed can by a shaping process, and the strains tend to involve tensile components as compared to the compressive components that predominate in the case of the draw and ironing process used to make a typical can. As a result, the shaping process is more sensitive to the presence of sources of fracture initiation, and features such as non-metallic inclusions that are non-critical in the draw and iron process become sites for fracture in the shaping process. Thus, from a metallurgical standpoint, shaped cans put a premium on cleaner can sheet products with fewer non-metallic inclusions.
Another desire in the packaging field is to shorten the time for introduction of new packaging concepts. This puts an increased emphasis on rapid prototyping and computer-aided design tools. Worldwide efforts to develop and enhance such capabilities are underway. With respect to formability, suitable modeling of the forming process is needed to enable virtual evaluation of new designs. Finite element analysis is one technique well suited to visualizing the stress and strain conditions of metal cans of various designs. As with all modeling, keys to success are an effective modeling framework coupled with reliable material property and constitutive data. One key technical aspect cited as important in forming process modeling is the high strain rate utilized, especially in the draw and ironing process, and the influence that forming rate has on the tensile limit strain.
Coupled with the desire for enhanced formability performance is the push for reduced cost of the raw material. Butler noted in his presentation that the aluminum raw material accounts for 79% of the formed can cost, and hence any progress that can be made in this area will enhance aluminum's competitive position vis a vis steel and plastic. He noted that a prime opportunity for cost reduction would be the utilization of continuous casting in place of the current ingot casting plus hot rolling process. Information presented at the conference contrasted approximate costs and production capability as shown in the table below:
| |
Conventional Processing |
Minimill
(Belt/Block Caster) |
Micromill |
Capital expense
(million $) |
300-1000 |
180 |
50 |
Capacity
(thousands of tonnes/yr) |
150-450 |
100 |
35 |
| Cost ratio |
1.0 |
0.8 |
0.73 |
Cited as evidence that continuous casting was a viable process for producing can stock was experience that Crown Cork & Seal had with the San Antonio mill, from which 1 billion cans were successfully produced.
Butler noted, however, that the forming performance of the continuous cast metal was different than that of conventionally produced product. Hence, careful attention to the details of the alloy microstructure and subsequent formability characteristics will be very important in obtaining technical success, especially in light of the increased forming demands being placed on the metal by concepts such as the shaped can as discussed above. This latter area of structure-property-formability relationships in continuous cast materials is a very active one both in the academic and industrial communities, as evidenced by a number of presentations made on the subject at ICAA8.
The source information for this article is contained in the paper "Current & Future Challenges for Aluminium as a Packaging Material", Materials Science Forum, v.396-400 (2002), pp. 9-14.
Article courtesy of Secat, Inc. - Research Resource for the Aluminum Industry
www.secat.net
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