Mar., 07 /Essen, Germany/ While there is a boom in plastics packaging, can sales are stagnant. What are the causes? The press repeatedly commented on the lack of innovative progress in the metal packaging industry. Cylindrical cans and old-fashioned style have practically become synonyms.

Can shaping is an additional operation in the canmaking process and cannot be had free of charge. Extra costs must, however, be proportionate to extra benefits. An additional process operation must not have a negative effect on capacity and efficiency of existing manufacturing lines, and the range of potential shapes must leave room to the creativity of packaging designers.

CAN-O-MAT produces shaped cans

SIG Cantec (previously Krupp) met this challenge by developing a broad spectrum of shaping processes for their CAN-O-MAT system, which ranges from die necking and spin-flow necking to spin-flow shaping. The newest and most extensive process from this series of developments is the die shaping process.

Die necking is a well-proven and established technology. First used for beverages, necked cans are nowadays very popular in the food industry, too. Advantages such as stackability and material savings for ends as well as esthetic aspects have boosted their success. Die necking is characterised by high process reliability. Hundreds of CAN-O-MAT die-neck systems are in operation world-wide. Customers can apply sophisticated technologies such as neck reforming, for example, to produce necked 3-piece cans with optimum mechanical parameters.

A necked 3-piece can typically has a single neck or a stepped multiple neck. Because of the welded side seam, a smooth multiple neck on 3-piece cans cannot be formed with the necessary process liability by die necking. For this reason, SIG Cantec has especially developed a (patented) spin-flow necking system for 3-piece cans. Spin-flow necking units have an adjustable degree of forming per rotation to adapt them to specific material properties and combine a maximum diameter reduction with an attractive neck geometry (smooth neck) for SR and - especially important - for DR material. Can diameters can thus be reduced by up to 13 mm.

A modified flow turning process forms the neck between two inner tooling components, which rotate together with the can body, and an outer shaping roller. The number of shaping rotations depends on diameter reduction and material. The tooling can be adjusted at infinitely variable increments up to a nominal rotary speed of up to 25 rotations per can. Specially developed high-precision clamping tools ensure that the can body securely follows these rotations. Upper and lower tool are centred relative to each other during the necking process so that radial forces can be transmitted without deformation. An accurately concentric neck geometry achieved without time-consuming mechanical adjustments is the result. 

Spin-flow shaping uses the same process and the same machine as spin-flow necking. The only difference is a specially designed tool which forms not a neck at one end of the body, but a bead-type shape in its middle section. When processing DR material, the number of rotations and the design of the inner tool are decisive for the formation of a wrinkle-free contraction of appropriate depth. Classical segment beading is no longer adequate for such shaping processes.

Spin-flow shaping is mainly applied to form simple, rotationally symmetric shapes. Several restrictions must be borne in mind, however: The tooling, for example, can only form a circumferential contraction. Other known shaping solutions also have specific restrictions, only reach low production speeds or cause high costs. During expansion, for example, the can body becomes polygonal and extreme local stretching weakens the can. Shaping with compressed air or high-pressure jets of water is expensive and its production capacity is restricted.

SIG Cantec has therefore developed the so-called die shaping process (patent pending). For this process, the bodies are welded to the smallest diameter of the shaped profile, or a little smaller. Depending on the desired geometry, i.e. whether the tools enter the body from one or from both sides, the can body can be enlarged at one or both ends. The geometry of the enlarging tools determines the shaped profile.

Die shaping (enlargement) offers the following advantages over spreading or expansion:

·        Nearly any shape can be formed by die shaping.

·        If a circular enlarged geometry is desired, the cross-section will really be circular and not polygonal. Die shaping can thus produce a "deeply necked can with smooth neck".

·        Elongation of the material along the circumference is regular, compared to spreading. This allows for a wider expansion with the same material elongation. There are no pronounced variations in height, although the body is shortened noticeably. Enlargement can take place along almost the whole length of the bodies.

·        With necked cans produced by die shaping, the desired uniform reduction in sheet gauge (down gauging) of approx. 5 % can be achieved in the cylindrical portion while the original sheet gauge remains at the necked portions. The axial performance of the can is improved as a result. Enlarging shortens the bodies by several millimeters.

·        Die shaping tools are simple, robust and easy to set up. Figure 6 shows the process sequence in 4 steps.

The open body is transported between the upper and lower strippers and positioned relative to the tooling axis. At this time the upper enlarging mandrel is at the upper dead centre while the lower enlarging mandrel is at the lower dead centre.

The lower and upper enlarging mandrels then move vertically upwards and downwards, respectively. The enlarging mandrels thus enter the body and enlarge it, forming it into the same shape as the profiled end of the mandrels.

Formation of the shape or "neck" has been completed when the two mandrels have reached their end positions. Then they return to their original positions, stripping the shaped body off at the strippers. The body with the finished shape now leaves the station and is then processed into a finished empty can by the usual operations of die necking, flanging, beading and seaming.

The possible degree of enlargement depends on the extensibility and plasticity of the material. Enlargements by 8-13 % are typical (e.g. from 45 mm to 49 mm for aerosol cans, from 59 mm to 65 mm for beverage cans, from 65 mm to 73 mm or from 73 mm to 83 mm for food cans). The actually required yield strength of the material is lower than the percentage of enlargement because the process takes material both out of the can height and out of the sheet thickness. Friction between enlarging mandrel and body, influenced by inside lacquering and side striping, also has a major effect.

SIG Beverages is an internationally leading manufacturer of plants and systems for the production of plastic packagings as well as complete solutions for the beverage industry. Since January 01, 2002, the newly created division of the SIG group is comprised of the former SIG Plastics (previously Krupp Kunststofftechnik) and the former SIG Simonazzi (previously the wet area business of Sasib SpA). With its approx. 9000 employees, the SIG group, which focuses its activities on packaging technology, achieved a turnover of approx. 2,400 million Swiss Francs in the financial year 2001.