Three piece cans

Three piece cans consisting of a can body and two end pieces, are used to seal heat-sterilized foods hermetically as well as for other food products like powders, syrups and cooking oils.

The can wall is formed by welding. Three piece welded cans are only constructed from steel as aluminum is not suitable for welding.

As the material for the can wall, lightly coated Sn-plated steel sheets and Ni-plated steel sheets may be employed.

Two main types of base steel are commonly used in can manufacturing type L and type MR. Type L is very corrosion-resistant and is used in canning of very corrosive products e.g., apple juice, berries, prunes and pickles.

Type MR is more suitable for canning moderately to mildly corrosive products, e.g. grapefruit, peaches, peas, and corn.

In both the two-piece can and the three-piece can the outside surface of the can is provided with printing, in order to appeal to consumers for commercial value of the canned goods.

On the other hand the inside surface of the can is coated with resin so as to ensure the corrosion resistance of the can body.

The coatings and ink are normally dried by passing the sheets through a thermally heated oven where the temperature is in the range 150–205 °C.

Three piece cans usually are delivered to the packer with one end in place and the other shipped separately. After filling, the second end is double-seamed by closing machines, which can change the headspace air to a vacuum or inert gas, if required.

Presently, the three-piece cans are being widely used and other cans like two-piece cans, aluminum cans, and other flexible containers are slowly replacing them.
Three piece cans

Two piece cans

Two piece cans is manufactured by press punching cups from sheets or coils of aluminum or steel. During this cupping operation, a blank is produced from the coil stock and in the same punch stroke, s formed into a shallow cup.

The cups are then forced though a series of rings to iron out and form a full-length can and to form the bottom dome.

Subsequent operations include trimming the can to a specified length, washing to remove drawing lubricants, decorating the exterior of the can with the desired brand level and applying coating to prevent contact of the food or beverage product with the metal substrate.

The product at this stage is a container with sides and a bottom representing one of the pieces of a two-piece can. The lid, secured and sealed after filling, is the second piece.

Since 1970, through the conversion of three to two piece cans and subsequent lightweighting, the weight of a 350 mL tinplate soft drink can has been reduced by 50% to 30g, and that of the inherently lighter two-piece aluminum can by 39% to 13 g.

It is evident that two-piece steel can would eliminate the long seam and one double seam and thus preclude two sources of potential leakage.

Steel is a cheaper metal than aluminum, but it is far more difficult to form. A two –piece steel can could offer the advantages of a two piece aluminum can, but at a lower price.

The two piece can has no side seam and only one double seam, which is more easily formed and controlled because of the absence of a side seam lap juncture.

Seaming by the filler at the top end is considerably easier owing to the absence of a junction of the side seam and the end seam.
Two piece cans