Three Piece Can
Three Piece Can
The three piece can is so called because it consists of a cylindrical body and two ends.The first modern can design had a soldered side seam on the body but in recent years this has almost totally been replaced by a can having a welded side seam body.
- Three-piece can - soldered side seam
The first stage in making a soldered three-piece can is the forming of the cylinder which constitutes the body. Flat sheets of tinplate are slit into blanks. These are notched at each corner of one edge, slits being cut a little way from the ends at the outer edge; the edges are then hooked over, one up and one down, at 28° to the blank. Flux is applied to these hooked edges by contact with a revolving wheel partly immersed in flux.
On the bodymaker, the blank is folded over a mandrel so that the hooked edges engage, whereupon the mandrel expands to tighten the joint and form a cylinder. This lock seam, located on the underside of the can, is next hammered down flat. In the soldering station, the cylinders pass over a gas heater, to pre-heat the seam, which then slides in tangential contact with the top of the rotating roll immersed to its centre line in a bath of molten solder. Extra heating afterwards helps the solder to flow through the seam and a wiper removes excess solder. A splash eliminator ensures that wipings do not contaminate the can following the one being wiped.
A 98% lead-2% tin solder was commonly used.However, for certain products other solder compositions, even those including pure tin side seam solder, have been used.When a fully lacquered interior is required, the side seam may be protected by a side stripe of lacquer, which is applied immediately downstream of the bodymaker. On a soldered can, the heat of soldering is sufficient to cure the side stripe.Further operations to complete the manufacture of a three-piece soldered can are identical to those used for a welded can and are described in the next section.
- Three-piece can - welded side seam
Because of its greater integrity and total absence of solder, which may have contained lead, welding is now the most accepted method of creating the side seam of a three-piece can. The technique of welding side seams on can bodies involves producing a series of overlapping weld nuggets along the length of the seam, using a suitable alternating current supply and copper electrode wheels. A problem with welding tinplate with copper electrodes is that as the tin melts it tends to alloy with the copper, thus altering the electrical characteristics. This has been overcome by using a copper wire as an intermediate electrode material.
The wire is threaded over the rims of two electrode wheels and being slightly proud of the surface of the electrode makes direct contact with the tinplate surface; as the tin melts it is carried away on the wire and fresh copper is available for further welding. Immediately prior to the welding operation, the wire is flattened to a rectilinear cross section so as to provide a greater surface area.
On the bodymaker line, a tinplate blank is formed into a cylinder with suitable overlaps, approximately 0.4 mm, and firmly held in position by calibration rollers during passage through the unit.The welding electrodes (wheels carrying copper wire as described above) are situated one above the other. The lower electrode is smaller in diameter than that of the an body and the cylinder moves in such a way that the lower electrode is inside it; the seam thus lies between the two electrodes. When the current is passed between the electrodes, a series of spot welds is produced as the container moves, these constituting a continuous weld. The upper electrode, which is outside the can, is mechanically loaded so that the two layers of overlapping tinplate are squeezed together as they pass between the two electrode wheels.
The result of this is a mechanical weld with total thickness equivalent to approximately 1.4 thicknesses of body metal. After the welding process has been completed a layer of oxide will form over the weld on both the inside and outside of the can. If it is necessary to provide a lacquer coating over the side seam weld and to later form beads in the wall of the can, it is usually necessary to prevent the build-up of a heavy oxide layer on the weld by providing an inert gas shield for the welding process. Failure to do this may result in lacquer breaking away from the weld area during the subsequent beading operation.
The quality of the weld may be continuously checked by an in-line electronic weld monitor device.This checks that the electrical current conditions for each nugget of the weld are within the parameters set by the operator for production of a satisfactory weld. The device may be set to alarm or automatically remove cans from the line when the weld conditions fall outside the set parameters.
Many tinplate cans are protected on their internal surface by a lacquer system; the types of lacquer and number of coatings applied depend on the intended contents of the can and the specific processing requirements.
The exterior may be protected by a decorative system which may be a combination of a clear size coat, or solid base coat, plus printing inks and over varnish. Lacquers are formulated to give corrosion and process resistance in conjunction with adhesion, flexibility, non-toxicity and freedom from taint. Film thicknesses generally lie in the range 1.5-10 µm. The essentials of the lacquering processes are described in standard texts.
Many tinplate cans have a plain exterior, with paper labels applied to identify the products. However, where the exterior of three-piece cans are to be decorated, the tinplate sheets are printed in the flat, prior to being slit into body blanks. Similarly, internal lacquer coatings are applied by roller coating to the flat sheets, normally in a stencil pattern to leave the seam margins free from lacquer.Side seam lacquering on the welded can is carried out immediately after making the welded joint. It may be done using either a liquid or a powder coating material. In each case the coating is then cured by a gas fired heating device.
Following body forming, the subsequent operations are identical for both soldered and welded side seam cans.When the height of a three-piece can is equal to or less than its diameter it is relatively unstable when being fed at high speed through a bodymaker with the axis of the can horizontal. For this reason, it is common to initially form a seamed can body which is two or three times the height of a single can. After the soldering or welding operation this long body is then passed through a parting machine which separates the long body into two or three short bodies, as appropriate. From this point onwards the can bodies are then conveyed with their axes in the vertical position.
Three-piece beverage can bodies are very often necked in at both top and bottom, so that the diameter over the seam is equal to or less than that of the body itself. There are a number of reasons why this is done; the uninterrupted body wall allows for easier handling of cans at high speed, particularly during collating of filled cans into cardboard trays, and permits the cans to be packed into a smaller volume; also the reduced flange diameter permits smaller diameter and, sometimes, thinner ends to be fitted with a consequent reduction in metal usage and therefore cost.
For food cans, inward necking of the top (opening end) of the can body may be done if the product consistency is such that emptying the can is not made difficult by the reduced diameter of the seam. In some cases the top of a food can may be necked outwards to facilitate product removal, particularly when a full aperture easy-open end is fitted. Inward necking, of the bottom end only, of a three-piece food can is done to permit easy stacking of filled cans.
The next step is to flange the top and bottom edges of the cylinder so that a rim is formed at 90° to the vertical axis of the cylinder; the condition of the base steel, in particular freedom from inclusions and the correct temper, as well as rolling direction of the steel, are important if flange cracking is to be avoided.
The use of spin flanging as opposed to die flanging is also important in this respect.The bottom end is fixed on to the body at the double seaming machine, where the closure is attached by the operation of two-stage seaming rolls whilst the can is spinning about its vertical axis. For beverage cans it is usual for the canmaker to fix the easy open end to the can at this point, leaving the plain end to be fixed by the can filler.
This sequence of events permits the easy open end to be pressure tested after it is fixed to the can and as a routine part of the can making operation. Manufacture of can ends is described in a later section of this Chapter.For food cans, which will be passed through a heat processing cycle after filling with product, the side wall must have sufficient strength to withstand the considerable external forces acting on it during the sterilisation process.
This may be achieved by either making the wall of sufficiently thick material that it will withstand these forces as a plain cylinder, or reinforcing the wall by addition of numerous circumferential beads and then utilising a much lower gauge of material. For economic reasons the latter route is the more common way of making processed food cans today.For non-beaded cans that may require further body shaping operations, such as bulging or embossing, the appropriate operation will generally be introduced at this point in the process.
Finally, the can’s integrity is checked in a leak tester, typically, a rotating carousel arrangement on which cans are mounted; testing occupies one full turn of the carousel and consists of electronically monitoring the rate of pressure loss when the open end of the can is sealed by a pad and compressed air introduced.Defective cans are rejected whilst sound cans are either fed directly to the filling lines or to a palletising station for onward shipment to a can filler.
- Source: The International Tin Association (formerly ITRI Ltd)