Converting Aluminum Foil
Aluminum foil is readily formed (cut, folded, molded, embossed) on standard paper converting equipment. While this grouping of these separate operations may constitute a somewhat unconventional interpretation of the word forming, it is believed valid.
In metal working in general, the term forming implies one or more operations necessary to shape a metal to a desired pattern without intentional reduction of the metal thickness but, usually, with plastic flow or bending of the metal. A wide variety of products are formed out of aluminum sheet.
Because foil is much thinner than other forms of aluminum, it cannot be drawn to any significant degree. Therefore, very little flow of metal occurs in foil forming operations.
The term forming, as here applied to aluminum foil, means the processes which are commonly used to change the shape of a sheet or web of aluminum foil. These include the following principal operations, arranged in increasing amounts of elongation affected by the individual forming method:
- Shearing and Slitting
- Die Cutting
- Die Forming
- Scratch Brushing
Cutting Aluminum Foil
Cutting is the simplest method of forming aluminum foil into a new shape. In general, the cutting edges employed wear fully as long, and frequently longer, when cutting foil as when cutting paper, providing the edges are properly honed after sharpening. This comparatively long knife life is due to the fact that abrasive elements in aluminum foil are negligible, compared with those found in most papers and paperboards.
Standard paper cutting equipment is regularly used for all forms of foil cutting - slitting, sheeting, and die cutting. Cutting knives, too, are the same. Proper honing is essential in prolonging the life of the cutting edges.
Shearing and Slitting
Shearing or slitting operations are a necessary part of practically all foil converting operations. Slitting is required to produce narrow coiled widths, to trim the edges of coated or laminated stock and to produce rectangular pieces. Die cut designs by rotary or stationary die settings and shearing with flying knife cut off also are included as a shearing method.
Guillotine shears are standard equipment for square shearing. Plain foil is usually interleaved with thin paper (10 lbs. to 15 lb. tissue) before square shearing. This is done to facilitate separation of the thin foil sheets by eliminating interlocking or sticking edges, to add support and rigidity to the stacked sheets, and to minimize surface marring from handling the shipping.
Foil in the hard conditions and annealed foil coated or plain but mounted on paper or other supporting sheets, do not usually require interleaving for square shearing.
The most commonly used rotary or web method of shearing laminated coil material is the shear cut; however, this may also be accomplished by a score cut. The shear cut method of shearing is illustrated in here. The most common methods of shearing unmounted foil are the shear, burst type slitting, and razor blade methods. The latter two are shown in here.
Slitting and shearing equipment must be sturdy and true to produce clean, square cuts and uniform rolls of foil. Shear knives must be properly set, firmly fixed, sharp and otherwise in good condition. Knife edges are preferably lubricated with volatile, non-staining materials such as very lightweight white oils or kerosene.
Die Cutting and Scoring
Equipment and dies commonly used to die cut paper are also used for die cutting foil. In die cutting, the foil side of the sheet is usually up.
Sheets of laminated aluminum foil are most commonly die cut into intricate shapes with steel rule cutting dies. Depending upon the sheet size and that of the object being cut out, these dies are made to produce single units or several up with each impression. A multiple steel rule die is called a gang die. This type is used to cut one sheet at a time; scoring, to produce fold lines, is done at the same time.
The steel rule dies cut through the foil sheet or sheets against paper maker ready over a hard steel plate of cylinder. They are used in various types of platen and cylinder presses. Most of the foil is sheet fed to these presses, although a considerable quantity is also web fed in rotary printing and die cutting equipment, which utilizes machined or cast cutting dies.
Clean cutting of edges of most materials becomes a problem when too many sheets are die cut in a single stack. This is due principally to the relatively low mass of the knife blade to that of the stack of material. When such cutting is attempted and the die is forced through the stack, tearing of the material being cut results. This is particularly the case in the middle and lower layers.
Some ornament and novelty manufacturers use steel rule clicker dies in small punch presses or clicker machines for cutting a number of thicknesses of foil at a time. A clean cut can be obtained when die cutting foil by this method if heavy gauge, all steel clicker dies or ground punches are employed.
Die Forming Aluminum Foil
Die forming or die molding aluminum foil is used to produce rigid foil containers, trays and pans from bare or coated aluminum foil. The foil is web fed to a press tooled with a suitable combination die which incorporates three major functions in a single operation, or complete cycle. These are commonly referred to as blanking, drawing and curling. While some metal movement or displacement does occur, most of the drawing is actually slip, controlled release, or folding of the foil.
Tooling has been developed to achieve a high degree of manufacturing efficiency and reliability. Today's complex, rigid aluminum foil containers are available in various side-wall rib configurations. These along with similar sophisticated variations in controlled corner wrinkle patterns, and edge curl positions and configurations, have evolved from a relatively simple container design introduced about 1950.
Tooling design capability and die quality have kept pace with the container designers and a wide variety of die formed aluminum products is now available. Exact reproductibility is a major requirement of the process, as the high speed packaging machinery demands close tolerances in all dimensions. For example, the tolerance recommended for the shearing ring and the female die cavity for a typical aluminum foil frozen food container is .0005 in., when .005 in. foil is used.