Zinc die castings can hold tight tolerances. But their abilities in this area have gone largely unappreciated in recent years. Aluminum or magnesium alloys have often been the choice even where high strength, toughness, and load-bearing capabilities may have made zinc or ZA (zinc-aluminum) appropriate material candidates.
Zinc’s casting capabilities may have been handicapped by the very standards used in describing casting tolerances. The published standards from the North American Die Casting Association (NADCA) do not provide the latest and striking results obtained by the International Lead Zinc Research Organization (ILZRO) Inc. ILZRO reported in 1995 that for linear dimensions of less than an inch, most zinc castings can maintain a tolerance of ±0.0005 in., a better value than the published tolerance of ±0.002 in.
One illustration of how zinc castings can beat traditional aluminum is in a 10-in.-long lubrication-meter assembly housing. The part was originally made by machining the meter and valve areas formed from two joined aluminum die castings. Replacing the two aluminum castings with a single zinc housing eliminated secondary machining and produced an assembly with zero draft. The precision within the meter chamber permits free rotation of the metering gears. The dimensional precision and stability of the zinc castings reduced scrap and rework for the manufacturer.
As a dynamic manufacturing process, zinc die casting offers many advantages over competing processes using aluminum and magnesium alloys. The auto industry accounts for approximately 30% of the zinc die casts produced, the rest goes into builders hardware, machinery, electrical/electronics, and appliances. Besides providing stronger, tougher, and more ductile properties, zinc die-casting alloys are completely recyclable. Annually, 30% of the zinc consumed is from recycled material.
Zinc alloys can be die cast larger, smaller, and have greater complexity than their counterparts. Molten zinc alloys form a finished product in a single operation and provide net or near net-shape parts. Hot-chamber die casting produces castings with intricate detail and excellent surface finishes at high production rates. Casting cycle times for large parts are commonly 100 to 300 shots/hr. For extremely small castings (up to a few ounces) cycle rates on the order of 2,000 to 3,500 shots/hr are common. The machines used can produce flash-free, zero-draft and close-tolerance parts eliminating secondary trimming and machining operations. Zinc-cast parts have smooth surfaces and are more easily painted or plated. ZA castings offer good wear resistance compared to aluminum and magnesium alloys.
Zinc die casting is the most accurate of the metal-casting processes. A review of Nadca product specification standards shows widely used industry guidelines may not be the most accurate when specifying casting tolerance. The tightest (Precision) standard defined for die casting is in linear dimensions for features formed in the same die part. The Precision casting tolerance for linear dimensions is ± 0.002 in for the first inch and ± 0.001 for each additional inch.
Most zinc die castings easily meet the current Precision tolerance specification. But it may be this tolerancing guideline that knocks zinc castings out of contention for many designs. Work done at ILZRO shows that the dimensional repeatability of die casting is much better than that indicated by the NADCA standards. The broad range and easily met tolerances of the standard do not give designers the plain facts about zinc die casting. ILZRO researchers would like to see a tightening of the basic linear dimension tolerance for dimensions of less than an inch to ±0.0005 in. for the Precision Standard category.
The tight tolerance capability of zinc casting arises from two factors: The tight dimensional tolerances that the machined steel die provides during casting, and zinc’s natural properties. Zinc alloys have low melting temperatures (718 to 728°F for Zamak #3) and good flow qualities which improve dimensional control in castings. These properties also help minimize die wear commonly seen with other materials.
In addition, surface finish can be tailored for different applications. Three basic surface coatings can be deposited for better cosmetics or performance. The coatings are either chemical, metallic, or organic. Each has its own advantages in appearance, corrosion or wear resistance, and electrical properties.
Chemical finishes are commonly applied by immersion or spray. they react with the surface of the casting to form a complex conversion coating system. Typical formulations include zinc chromate or phosphate formations as well as chlorate and molybdate salts.
Metallic coatings are primarily electrodeposited, but other methods include vapor deposition or the electroless process. There are a myriad of metallic coatings available.
Organic finishing systems use finely dispersed pigment and resin (film-forming media) in organic solvents or water. The mixture easily coats the casting and the solvent or water evaporates or bakes off, leaving a dried barrier film on the casting surface.
Information for this article was contributed by Michael Sheehan, INTERZINC, Cleveland, Ohio.