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The Zinc Advantage
Coiled zinc strip and zinc sheet, produced for decades in relative obscurity, now provide positive results in numerous metalforming applications.
BY CARL DELSORBO
Most of us are familiar with the various alloys of steel, copper, brass and aluminum used in metalforming applications. Zinc, an alternative metal not normally considered, offers distinct advantages over these other materials.
Wrought-zinc-strip alloys (typically with at least 98-percent zinc content) can be stamped, deep drawn, rollformed, hydroformed, coined, impact extruded and perforated, usually on tooling originally designed for steel or other materials. Also, zinc strip can be soldered and welded without difficulty.
Very soft pure zinc strip, though not recommended for structural components, can be alloyed with copper or titanium to enhance mechanical properties significantly and improve tensile and yield strengths. Such alloying allows specification of zinc strip in many rigid applications, although its use in a load-bearing application should be reviewed on a case-by-case basis.
Due to its low recrystallization temperature, zinc strip does not workharden readily during forming operations. This gives zinc a self-annealing property that simplifies progressive forming operations such as deep drawing by eliminating the need for intermittent annealing steps. This self-annealing property makes zinc strip an attractive alternative in many forming operations.
When formed, zinc strip exhibits anisotropic behavior--longitudinal and transverse forming properties differ--due to rolling of the material and to its hexagonal crystal structure. In most applications, this is not an issue due to the metal’s high formability, but care should be taken during severe bending of the strip in the longitudinal direction, with the grain, along a tight radius.
Another positive forming property of zinc: its self-lubricating/non-abrasive nature. For most applications, only steel or hardened-steel tooling is needed. Long runs in more harsh environments, such as high-speed blanking, may require carbide tooling. In rare cases, where frictional heat can be extreme, zinc can adhere to steel tooling, creating a galling effect. However, with the use of carbide tooling, tool and die life has been extended far beyond that observed when running materials such as brass, copper, steel or aluminum in similar operations.
Zinc strip also boasts a high energy-absorption characteristic. This property makes zinc a good candidate for molding, trim or any other application requiring resistance to dents.
The property most notably attributed to zinc is its atmospheric-corrosion resistance. Upon exposure to atmospheric conditions, zinc forms a zinc carbonate film that provides long-term protection. An added bonus is the attractive nature of this film--a gray patina long considered as a decorative finish for roofing/raingoods applications in Europe. These attributes add to the durability and lifespan of zinc as a roofing/raingoods product.
Besides this natural film, zinc strip offers a myriad of finishing alternatives. It can be plated (chrome, copper, bronze, etc.), painted, lacquered, powder-coated or clear-coated with coatings exhibiting excellent adhesion and durability.
One more critical advantage of zinc strip is cost. Zinc is a lower-cost metal as compared to copper, aluminum, brass and some steels. Also, with the exception of aluminum due to its lower density, zinc strip can produce more pieces per unit weight than these other materials.
Greater Use in a Variety of Processes
With all these advantages, it is no surprise that zinc strip finds increasing use in a range of forming processes.
Rollforming—Zinc strip has long been rollformed for use in the roofing/raingoods market in the European community, creating an extensive database of processing knowledge. Rollforming applications for zinc strip include roofing panels, gutters, downspouts, control joints and window cames. The durability, flexibility and malleability of zinc strip allow architects to enhance and expand their creativity when employing it in designs. Zinc strip has also been used extensively for automotive trim, adding to rollforming knowledge.
When rollforming, consider the small amount of springback to be overcome due to the limited elastic region inherent in zinc strip. This means that little work is required to permanently deform the material. Secondly, expect minimal tool wear when rollforming zinc strip, resulting in long production runs with minimal die maintenance. Also, residual oil present on the strip provides sufficient lubrication for many zinc-strip rollforming applications, negating the need for added lube. Finally, consider the temperature of the metal. All metals become more brittle when they are cold and zinc is no exception. That makes it susceptible to brittle fracture. Therefore, perform rollforming on zinc strip that is at or slightly above room temperature.
Blanking/Coining—Zinc strip long has been used in high-speed blanking and coining operations with exceptional results. Using cold-heading-grade carbide blanking tools in a flat-blanking application for coining, tooling can muster 200 to 300 million hits prior to sharpening while consuming about 60-percent less lube than needed when using tool steel. This extremely long life cycle and reduced need for lubrication results in higher productivity, lower tooling maintenance costs and less residual oil than with other applicable materials. Using tool steel for blanking zinc strip, the life span between sharpening drops to 60 or 70 percent of that with carbide. Also, when employing tool steel, galling can occur due to friction welding of the zinc to the tool steel under the extreme temperatures, necessitating the use of more lube.
In coining, using zinc-based material increases die life six to 10 times over materials such as steel, stainless steel and aluminum-bronze. Long die life and the advantages gained when using carbide tooling make zinc an attractive alternative and result in longer production runs, less downtime and reduced production costs associated with manufacturing operations.
Want more? Zinc-based materials also exhibit excellent die fill with reduced press tonnage, thereby accommodating the modern design details often employed in coinage, tokens and medallions. Also, avoiding costly annealing processes means less energy per unit weight needed to produce zinc-based coinage as compared to other coin-blank materials.
Stamping—The tool-life advantages in zinc-strip blanking also hold true for flat to shallow-formed stampings. Zinc-strip stamping applications include plumbing hardware, automotive leaf-spring interliners and automotive trim. Zinc can be substituted in stainless-steel tool packages with slight modifications, but tool packages designed for brass often have heavier reductions that do not allow direct zinc-strip replacement. In brass dies, zinc presents difficulties at inside corners of sharp flanges on square or rectangular parts due to high stretch-forming percentages. That is why, when designing a tool package for zinc, look to preform corners early on in the progression with a generous radius and then restrike to size in later stations.
Deep Drawing—Zinc strip is a very attractive metal for deep-drawing applications as evidenced by its use for 50 years in the deep drawing of zinc-carbon battery-can liners for the dry-cell battery market. The self-annealing property of zinc results in little constraint on the possible number of successive draws. A blank-to-cup reduction of 35 percent, resulting in minimal earring, is suggested with subsequent draw reductions of 15 to 16 percent and a final reduction of seven to eight percent at the diametrical sizing station. To control body/shadow wrinkles, recommendations call for a slight sidewall ironing--a 0.0002-0.0003-in. reduction in thickness per station. Also, on battery cans, a common length-to-diameter ratio of 2.5:1 to 3.5:1 was maintained while employing zinc strip. Note that all of the information presented above is based on deep drawing with no draw sleeves, allowing for improved free flow of the material and the formation of a hoop skirt that is pinch-trimmed off in a later operation.
Expansion/Perforation--Zinc strip remains the material of choice for expanded metal mesh in markets such as cathodic protection, corner beads, expansion joints and diesel-engine oil filters, where it neutralizes potentially damaging corrosive combustion products. Here again, zinc strip—in thicknesses from 0.002 to 0.090 in.--replaces other materials with little adjustment to existing tooling and machinery.
Carl DelSorbo is process metallurgist with Zinc Products Co., Greeneville, TN, a division of Alltrista Corp.; tel. 800-251-7506; www.allzinc.com.