Diecast Zinc Alloys
While traditionally focused in automotive, hardware and plumbing markets, diecast zinc alloys have become an alternative material of choice in consumer product industries such as communications, electronics and home appliances.
Often at the expense of thermoplastic parts, this growth is due primarily to three reasons:
• the ability to produce complex shapes—The complex shapes produced with diecast zinc alloys are the closest any metallic alloy system can come to that which can be molded in thermoplastics. This ability far exceeds that of other manufacturing processes, such as sheet stamping, extrusion or machining. In addition, extra shape complexity comes with a small cost penalty when compared to other processes;
• fine surface finish—The surface finish on a diecast zinc component is unsurpassed by any other diecast metal. In addition, electroplated and painted finishes can equal or exceed the aesthetic qualities of similar finishes on molded thermoplastics while providing an engineering structure only possible with metal;
• low cost and high mechanical properties when compared to competing materials—Pound for pound, zinc is not an inexpensive material, but the diecasting process uses only a minimum amount of metal to produce components. In addition, the ability to produce complex shapes allows for the consolidation of several components into a single zinc die casting, eliminating product and assembly costs down the road. Zinc die castings also can be made to a 0.02-in. wall thickness, which allows for a conversion from thermoplastic parts that has a reduced weight and cost and a superior structure. In addition, zinc die casting dies have a lower cost than those used for other alloys because zinc produces less wear and tear due to its lower heat content per unit volume and its lower casting temperature.
Zinc alloys most often are diecast—a process in which molten metal is injected into metal molds at high pressure—because the process provides the material with optimum properties of surface finish, ability to produce complex parts and reduced cost. The alloys also can be sand cast to produce a prototype component; however, alloy control is critical in achieving similar properties as those achieved via diecasting. Zinc components range in size from less than an ounce (termed miniature zinc casting) up to 6 lb.
Zinc alloys are assigned to three alloy groups. The first group includes the alloys commonly known as Numbers 2, 3, 5 and 7. All of these alloys have 4% aluminum as the primary alloying constituent with 0.099% or less magnesium to control intergranular corrosion. The difference between all of these alloys (except for Number 7) is the percentage of copper as the second alloying element. The alloys with the highest copper have the highest hardness but the lowest impact strength. Number 7 alloy achieves improved properties through higher purity and 0.005-0.02% nickel.
The second alloy group consists of ZA-8, ZA-12 and ZA-27, in which the number represents the percentage of aluminum in the alloy. The ZA alloys have superior hardness, wear and creep resistance that increase with the aluminum content.
The third alloy group consists of a single alloy, ACuZinc, which has copper as the primary alloying element. This alloy is patented by General Motors. ACuZinc has superior mechanical properties as compared to the other zinc alloys. It has greater hardness (118 Brinell), tensile strength (59 ksi) and even a higher modulus (14.5x106 psi). But its greatest superiority is in its creep resistance (710 hr. to fracture at 3,600 psi and 300F). It is nearly seven times as creep resistant as the ZA-8 alloy.
Creative design of a diecast zinc component (or any cast component) must begin with a clear statement that precisely defines the product functions to be performed. After product function has been defined, a configuration compatible with the diecasting process and the selected alloy must be developed. Alloy selection is based primarily on the required mechanical, physical and chemical properties of the component.
For a product configuration optimized for diecasting, three factors must be considered:
• the ability to fill completely with metal;
• the ability to solidify quickly and without defects;
• the ability to eject from the die tooling.
These factors can best be achieved by applying six principles when designing diecast zinc component walls and sections and establishing tolerances.
1. There are no hard and fast rules governing maximum and minimum limits for wall thicknesses. They should be as consistent as possible throughout the component and, where variations are required, transitions should be provided to avoid abrupt changes. Diecasters who use high-technology equipment and techniques routinely produce castings with maximum and minimum wall thicknesses and with variations that were impossible until recently. This capability should be utilized only as necessary to achieve performance or economic advantages. Uniform wall thicknesses are otherwise preferred.
2. Intersections of features, such as walls, ribs and gussets, should blend with transition sections and generous radii. This practice promotes metal flow and structural integrity and rarely creates a conflict between casting requirements and product integrity.
3. Draft angles may be minimized where metal content is critical, such as thin sections oriented parallel to die draw. Casting to zero draft may be specified in some cases to eliminate finish machining operations. These capabilities may be utilized as necessary to gain an economic advantage or to reduce weight. In all other cases, standard draft must be specified to facilitate ejection from the die and reduce die maintenance.
4. Sharp exterior corners can be specified on appearance surfaces when crisp styling features are desired. Otherwise, sharp corners should be broken with radii or chambers to reduce die maintenance.
5. Undercuts should be avoided whenever possible because they require additional machining operations or additional die members, such as retractable core slides. When core slides are used, the design should allow them to be located in the die parting plane.
6. Dimensions with critical tolerances should relate to only one die member—either the ejector die half or the cover die half. Critical tolerances across the parting line are difficult to maintain.
--Leo Baran, American Foundry Society, and Henry Bakemeyer, Die Casting Design and Consulting
This article was adapted from various bulletins issued and The Zinc Die Casting Process published by the North American Die Casting Assn. (NADCA).