Rolling Aluminum Foil
A series of passes through rolling mills specifically designed for the production of foil is required to make a uniform, close-tolerance, thin-gauge material. The mill rolls that come in contact with the metal are called work rolls. These have finely ground and polished surfaces to assure flatness and bright finish.
The work rolls in each foil mill are paired with heavier rolls called backup rolls, which bear against the work rolls. These exert pressure and add stability to help hold product dimensions within tolerances. Backup rolls substantially prevent deflection of the work rolls, thus playing a major part in holding gauge across the web. Depending upon the roll width and diameter, slight crowning at the center to compensate for deflection is a precise detail of foil mill roll design.
In general, the reduction of metal in a rolling mill is the result of
- the vertical pressure applied through the rolls
- back and front web tension applied to the metal through speed and pinch adjustment at the payoff and rewind mandrels.
Within this system, reduction is regulated, as stated, by rpm of the rolls, and by the viscosity, quantity, and temperature of the rolling lubricants.
As mentioned earlier, the width of the re-roll stock from which the foil is produced establishes the maximum width of the ultimate product. Each pass through a rolling mill reduces the thickness of the metal and at the same time lengthens it, but the width remains practically the same. This is because rolling is, in one sense of the word, a form of extruding as depicted below.
The sketch shown illustrates what happens as an aluminum web of given thickness is passed between a pair of reducing rolls set with a gap (nip) of lesser dimension than that of the metal being fed to the rolls. The web is moving toward the right in Fig. 7 (note that the rolls rotate in opposite directions). As the aluminum is drawn into the nip, it is subjected to a squeezing force exerted downward and also in the web direction. The metal thus displaced or moved is extruded thru the gap between the rolls, always in the direction of web travel because this offers the least resistance to the flow of metal. Rolling oils are used to lubricate the work as well as to cool the rolls.
Roll gap, which determines both thickness and web length of the foil leaving the mill, is adjusted by raising or lowering the upper work roll. Any desired gauge can be preset by the operator, and is maintained to close tolerances automatically by a solid state electronic thickness sensing device, as rolling progresses.
Other operations routinely performed as foil passes through the mill include splicing web breaks if required; edge trimming for precise dimensioning of web width if mill trim widths is ordered; and rewinding. These and other foil production operations are discussed in later sections of this chapter.
Rolling produces two natural finishes on aluminum foil, bright and matte. The sides of foil in contact with the mill work rolls are polished to a bright, specular finish by the action of the smooth roll surfaces. When a single web of foil is run (normally, but not limited to, gauges of approximately 0.001 in. and thicker), both sides are bright.
When two webs of foil are pack rolled (usually in gauges thinner than 0.001 in.), they are passed through the nip together in intimate contact. The foil-to-foil side of each web is matte finish, having a satin-like appearance, and the other sides, polished by the rolls, are bright.
A wide variety of other mill finishes can be produced with special roll patterns or, more commonly, by the use of separate or tandem mechanical finishing units. When such finishes are desired, patterns produced may be any random or other free-hand, or geometric design. However, such effects usually are produced in converting operations and are not mill finishes.
Economical production of various size rolls of aluminum foil involves the use of occasional splices. These are made by various means to meet the demands of different fabricating and converting operations, with splice design and execution tailored to meet both mechanical and reliability requirements. For example, much of the light-gauge foil destined for web printing and laminating operations is spliced, when necessary, with a continuous ultrasonic weld seam, which provides the smooth surface and superior strength needed for continuous travel through the converting equipment. Average reliability of these splices is about 98% splice performance.
Commonly used types of splices for joining webs of plain foil and/or most backed foil include ultrasonic (for plain foil, in most instances), heat-sealing tape, pressure-sealing tape, knurled, and electric welded. Heat-sealed coated foil webs usually are spliced merely by lapping the ends and ironing.
The ultrasonic splice is a solid-state weld in the overlapped metal, made with an ultrasonic transducer welder. Excellent strength is obtained and tail ends are short.
Heat-sealing tape splicing employs various types of heat-seal tape, as appropriate to the application. One type of tape used chiefly to splice rolling mill foil breaks is placed between the lapped ends to be joined and the web ends are locked back onto the take-up with a few turns of the coil; full splice strength is later developed by the heat of the furnace in which the completed rolls is annealed. Another heat-sealing splice tape is a nylon type which is activated with a hot iron. Principal use is for making splices on webs in continuous slitting or shearing operations. A third type is a gutta percha heat sealing tape, which is also hot ironed, to make splices during some laminating runs.
The knurled-seam joint is a mechanical splice. It is made with a knurling wheel and an impressed or matting knurling surface, which is heated when an elevated temperature will improve the mechanical bond. While of relatively low strength, this splice may be found adequate for light foil gauges, depending upon such factors as presence or absence of a coating on the foil surface, and demands of subsequent converting operations.
For heavier gauge foils (0.002 in. and thicker), either pressure-sensitive tape or electric welding are often employed to make splices, depending chiefly upon customer specification.
Trimming and Slitting
Trimming and slitting are essentially the same operation, the principal difference being in the location of the cut. Trimming refers to edges, while slitting involves intermediate cutting at intervals across the web. Both may be done with one of several arrangements of circular knives of various edge geometry, or with razor-like knives mounted so as to bear with positive pressure against the foil as it passes over slots in a steel roll.
Desired widths are obtained from mill rolls (Fig. 8) by appropriately locating and securing the knives in position across the web. The smaller webs or ribbons of foil thus produced are rewound onto cores of appropriate design and length. Slitting is described in more detail in Chapter III.
When pack rolled foil is produced, the duplex web is coiled on the mill rewind. Coils thus produced are transferred to a separator (Fig. 9) This is a unit which simultaneously separates the doubled webs of foil, and rewinds each on its own core. If desired, each web can be trimmed and slit into narrower rolls on a common core, or on individual small cores, at the same time it is separated. Any of the rolls can be wound to have either the bright or the matte side on the outside of the roll. Some separators also can be employed as primary slitters and rewinders for producing smaller rolls of single-web foil, if desired, but separate slitter/rewind units generally are employed.
As it comes from the mill, all foil is work hardened by rolling, as previously mentioned, because the reduction of aluminum re-roll stock to foil gauges is exclusively a cold rolling operation. Between pass annealing is required to restore workability. Final annealing (Fig. 10) achieves essentially the same purpose as earlier intermediate anneals: it relieves the internal stresses created by work hardening, leaving the alloy in its particular soft condition. In the annealed condition, each foil is at its optimum workability and dead-fold state, which is most advantageous for many packaging and other end uses, and for certain converting operations.
Household and institutional wrap stock, for example, must be fully annealed for maximum flexibility. Other foil used for food trays may also be ordered annealed for maximum workability in die-forming, which in turn will work harden the foil to some extent, rendering it more rigid, as appropriate for the application.
When decorative finishes appear on the opposite side of foil to which a heat seal coating has been applied, or when bake-in pans are decorated, the coatings must withstand the temperatures required for activating the heat seal coating, or baking temperatures, without becoming discolored, brittle or tacky. Film forming materials and colors of basically different characteristics may be required to obtain these distinctive film requirements.
When foil is ordered for applications requiring either an intermediate temper or the full hard condition, appropriate annealing and/or work hardening procedures are utilized to produce the final foil mill product.
Temperature and time required for annealing vary with the alloy. Effective annealing requires accurate control and temperature recording sensors in the annealing furnace are placed next to and/or at various locations within large coils to indicate when proper temperatures are reached, thus assuring that all of the metal is completely annealed.
Lubricants - The rolling lubricants used in producing aluminum foil vary in composition and volume, depending on gauge, alloy, and other conditions. The harder alloys in general require more lubrication than the softer ones. For some end uses, the presence of a thin, invisible film residue of lubricant is an asset either in subsequent converting operations, or in the end use itself. In other cases, the oils are not utilized but present no problem and so are not removed.
In many applications, such as coating, printing, anodizing, and others, little or no oil residue can be tolerated. Fortunately, annealing temperatures are sufficiently high to burn off all traces of foil, so that annealed foils are generally completely dry, or oil-free. Where harder tempers are required and annealing is not done, lubricant-free chemically cleaned foil is specified. When annealed foil with a slip surface is required, it is necessary to apply a lubricant in a separate, post-anneal operation.
Wettability Test for Dry Foil Determination
Different converting processes and end uses have been found to require varying degrees of dryness on the foil surface for satisfactory performance. A Dry Surface* is defined as, A foil surface substantially free from oily film, and suitable for lacquering, printing, or coating with water-dispersed adhesives.
The degree of dryness of a particular foil surface is often determined by a Wettability Test. Wettability* is defined as, The degree to which a metal surface may be wet to determine the absence of or the amount of residual rolling or added lubricants or deposits on the surface.
A typical wettability test method is given in the Appendix.
The finished foil rolls are inspected and packed for shipment. Mill packing of aluminum foil utilizes the most appropriate developments in packing materials, design and construction to help assure safe delivery of all foil products to the converter and other consumers. Several types of packing are employed to meet the special requirements of type and weight of the foil, and converting and/or end use processes and requirements, and to comply with applicable freight regulations.
Typical foil shipping containers, skids and pallets are shown in Chapter 1, Fig. 1 thru 10. Packing designs and weights are updated, as appropriate, by individual foil producers.
Storage - Recommended shipment, inspection and storage procedures also are discussed in detail in Chapter 1.