Make a Selection Choosing metalcasting is just the start. This article will help you navigate the casting process palette and find the optimal one for your part. Design engineers must choose among several manufacturing methods to find the best fit for a particular component. This fit takes into account manufacturability, required properties, time and cost. Every method, including fabricating, forging, machining, and powder metallurgy has unique advantages, but metalcasting has the ability to manufacture the widest range of engineered components Shannon Kruse, Associate Editor by alloy, size and geometry. Much of metalcasting s appeal comes from the geometries achieved through shaping molten metal. Design engineers can use geometry to attain better properties from their chosen metal because shape controls stress points. While you re considering metalcasting, you ll need to whittle down the list of available casting processes for the one that best fits your part. With the wide range of casting choices available, selecting a process can be daunting, but running through the foling checklist can make the task approachable. Castability When you are choosing an alloy, take note of the properties you are looking for. If you are choosing an alloy based Choose an alloy and correlating casting process Compare properties of casting processes and narrow your choice 26 Engineered Casting Solutions http://www.afsinc.org/files/selectingaprocess.pdf
Fig. 1. Use this chart to compare the main casting processes by part specification. Metal Iron Steel Aluminum Copper High Alloy Steel Magnesium Zinc Titanium Sand Plaster Permanent Centrifugal Lost Foam Investment Die Thickness (in.) down to 0.025 down to 0.03 down to 0.1 down to 0.125 Tolerances excellent good fair Surface Finish (RMS) 300-200 200-100 less than 100 Draft (degrees) 2 1 0.5 0 Complexity complex simple on structural considerations, be aware that elongation and yield strength can be traded off with geometry. Perhaps you can build that structural piece with aluminum rather than a heavier steel using the geometric freedom that metalcasting provides. The metal you choose and its castability will narrow your metalcasting options. Some processes, such as sand casting or investment casting, are flexible enough to accommodate almost any type of metal. However, other casting processes, such as plaster, permanent mold, lost foam and diecasting, work best with a handful of metal choices. The chart in Fig. 1 shows the metals to which each casting process is best suited. Compare lead times and cost tooling cost tooling lead time casting cost eliminated machining medium tooling cost tooling lead time casting cost reduced machining Microstructue In some part designs, the effect of an alloy s microstructure on the properties of the cast component is a major factor in producing a successful part, particularly for iron and aluminum parts. The rate of solidification can either positively or negatively affect the metal s desired properties. Aluminum gains strength from small, Shop for quotes http://www.afsinc.org/files/selectingaprocess.pdf For Design Engineers & Purchasers 27
Fig. 2. Use this chart to compare casting processes by cost and size. Sand Plaster Permanent Centrifugal Lost Foam Investment Die Casting Size large medium small very small Tooling lead time Tooling cost Casting lead time Casting Cost Finishing Cost Economical quantities very large large medium small tight dendrites in its microstructure. The quicker the metal solidifies, the smaller the dendrites and the stronger the aluminum. Conversely, this same rapid solidification alters iron s microstructure to adversely affect its machinability. Molding processes with a thermal gradient, usually from metal tooling, such as with permanent molding, are well-suited for aluminum structural parts. But iron components perform better with sand molding due to the ser solidification rate. Size Size matters when you re choosing a casting process. For instance, if you plan on designing a 1,000-lb. part, investment casting is a less likely candidate. Although fairly large investment castings do exist, the investment casting process has complexities that are best suited to very aggressive net shape requirements and/or tight specifiations for solidification integrity and surface finish. Other molding processes offer aspects of investment casting capabilities. Frequently, one of those alternative processes can meet part requirements by matching specific capabilities to specific component functional needs. For example, that 1,000-lb. part might have its needs met with precision air set molding with carefully engineered cores and chills. Another part might have its needs met in the diecast, lost foam, permanent mold or resin shell processes. Before you start piling stacks of casting process books on your desk, check the files on the opposite page. Each casting process is listed with a photo of a representative casting, features of the process, such as achievable dimensions and surface finish, and a description of suitable jobs. This quick reference is a good place to Dimensional Requirements After you ve narrowed down your list of casting process candidates based on metal and size, you can dive into the dimensional requirements and surface finish you are seeking. Because there is such a wide variety of casting processes, you have the ability to tailor a process and metal to fit your needs. If you are looking for a smooth surface, diecasting will be your best bet, foled by plaster and investment casting. If elimi- Narrow Your Choices start when you are trying to narrow down your choices. If you re still struggling to make a choice, call up a metalcaster from each process. Ultimately, they will have the best knowledge of a process capabilities, and often, they will be able to show you additional tricks that will help you achieve your goals. ECS 28 Engineered Casting Solutions http://www.afsinc.org/files/selectingaprocess.pdf
SAND CASTING PLASTER CASTING PERMANENT MOLD most types of metal (except titanium) or simple complexity large, medium or small casting sizes (less than 1 lb. to 10,000s of lbs.) quantities in the large, medium or small ranges base tolerances of +/- 0.4-1 in. for green sand and +/- 0.01-0.5 in. for resin-coated; surface finish of 120-350 RMS; minimum draft of 0.25-5 degrees; to tooling lead times; tooling cost ($800-$4,000); to casting lead times; to casting cost; to finishing cost. CENTRIFUGAL CASTING aluminum, copper, magnesium and zinc alloys simple to complexity medium or small casting sizes (ounces to 200 lbs.) quantities in small or medium ranges thicknesses down to 0.03 in.; base tolerances of +/- 0.005-0.01 in.; surface finishes between 63 and 125 RMS; draft of 0.5-2 degrees; tooling lead times; to tooling cost ($3,000 to $15,000) casting lead time; to casting cost; finishing cost. INVESTMENT CASTING aluminum, copper-base, magnesium and zinc alloys simple complexity small to medium casting sizes (ounces to more than 100 lbs.) quantities in medium to large ranges base tolerance of +/ -0.015 in.; surface finishes between 150-250 RMS; draft of 2-4 degrees; tooling lead times; tooling cost ($5,000- $20,000); to casting lead times; to casting cost; finishing cost. LOST FOAM CASTING steel, aluminum, -alloy steel and copper-base alloys simple complexity (must be cylindrical) small to large casting sizes (3-55 in. in diameter; 1-20 ft. long) quantities in the medium range thicknesses down to 0.1 in.; base tolerances of 0.1-0.15 in.; surface finishes between 100-300 RMS; draft of 0 1 degree; to tooling lead time; tooling cost ($10,000-$20,000); casting lead time; casting cost; to finishing cost. DIECASTING iron, steel, aluminum, copper-base, -alloy steel, magnesium and titanium alloys to complexity very small to medium casting sizes (ounces to 50 lbs.) quantities in the small to medium range thicknesses down to 0.025 in.; base tolerances of +/- 0.003-0.008 in.; surface finishes between 63-125 RMS; draft of 0-1 degree; to tooling lead time; to tooling cost ($3,000-$10,000); casting lead time; to casting cost; to finishing cost. aluminum, magnesium and zinc alloys simple to complexity very small to medium casting sizes (ounces to 30 lbs.) quantities in the medium to very large range thicknesses down to 0.025 in.; base tolerance of +/- 0.002 in.; surface finishes between 32-90 RMS; draft of 0.5-3 degrees; tooling lead time; tooling cost ($5,000-$500,000); to casting lead time; casting cost; finishing cost. iron and aluminum alloys simple to complexity small to medium casting sizes (ounces to 1,000 lbs.) quantities in the small to medium range base tolerance of +/- 0.005 in.; surface finishes between 100-300 RMS; draft of 1 degree; tooling lead time; to tooling cost ($1,000-$200,000); to casting lead time; to casting cost; to finishing cost. http://www.afsinc.org/files/selectingaprocess.pdf For Design Engineers & Purchasers 29
nating machining is a chief concern, know that the sand processes are going to reduce the machining less than others. Similarly, if you require thicknesses down to 0.025 in., minimal draft and excellent tolerances, diecasting and investment casting are the top choices. However, these -end tolerances and surface finishes come at a cost. Special Considerations Depending on the requirements of your part, other considerations might affect your final casting choice. For instance, the flexibility to use cores when needed to create internal passageways or the ability to create cavities without cores may be a deciding factor. Most permanent molds are not well-suited for cored-parts, but many shops use semi-permanent molding (metal tools, sand cores) in order to produce the necessary geometry of a part. On the other hand, investment casting and lost foam can produce ly complex parts without the use of cores at all. Secondly, the type of rapid prototyping you use may naturally steer you to a certain casting process. Stereolithography patterns lend themselves to investment casting because the prototype can be attached directly to an investment tree and used to produce the metal product. But fused deposition or laminated object manufacturing rapid prototypes are rigid enough to be used as patterns for sand molds. Metalcasting holds a few other quirks that can help you optimize your part s design. Investment casting shells can be hot when the molten metal is poured, which improves the castability of the alloy. Postcasting processes, such as hot isostatic pressing (HIP), can lend additional or improved properties to a casting. HIP can heal defects in solidified metal by using temperature and pressure to squeeze the part. Titanium castings that are HIP ed, for instance, have isotropic structural properties that can be preferable to anisotropic forged titanium properties. Semi-solid casting, in which the metal poured isn t completely liquid, results in less solidification shrinkage and entrained porosity. Cost With varying processes and capabilities come varying costs. In general, the actual casting cost for most processes is fairly, depending on the part you are designing. A large portion of your initial cost will come from the tooling for the mold and finishing the component. Sand casting generally comes with the est tooling cost, while investment and diecasting have the est tooling cost. It is important to remember that quantity also will be a factor in tooling costs. If the component is a volume job, tooling will be more expensive in order to handle the wear and tear of production. But, the er the quantity, the more economical it becomes to front a er tooling cost for a speedier casting process. At first glance, a casting process might seem too pricey for the part you re designing, but a more expensive process can cut your total manufacturing costs in the end. Remember to factor in tooling cost and the cost of the final assembled part (including machining assembly, etc.), as well as the total value of the casting. Fig. 2 shows the guidelines for economical quantities for each process. The final value of the part also should factor in weight savings and quality. If you are able to sell the product at a er price because it is of er quality, spending a little more in casting production might be worthwhile. When reducing weighting is an important design factor, castings are the most powerful form of engineered metal component. ECS 30 Engineered Casting Solutions http://www.afsinc.org/files/selectingaprocess.pdf