Selecting a Process
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 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 following checklist can make the task approachable.
When you are choosing an alloy, take note of the properties you are looking for. If you are choosing an alloy based 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 (.pdf file contains all charts and figures) shows the metals to which each casting process is best suited.
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, 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 high 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 slower solidification rate.
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.
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, followed by plaster and investment casting. If eliminating 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 high-end tolerances and surface finishes come at a cost.
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 highly 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. Post-casting 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 high temperature and high 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.
With varying processes and capabilities come varying costs. In general, the actual casting cost for most processes is fairly low, 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 lowest tooling cost, while investment and diecasting have the highest tooling cost. It is important to remember that quantity also will be a factor in tooling costs. If the component is a high volume job, tooling will be more expensive in order to handle the wear and tear of production. But, the higher the quantity, the more economical it becomes to front a higher 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 (.pdf file contains all charts and figures) 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 higher price because it is of higher 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.