When to Choose Investment Casting
The investment casting process is used to produce metal castings found in automotive, aircraft, spacecraft, dental and machinery applications in addition to art and jewelry. Two types of molds can be used in this process: solid and shell. Today, the shell process is used more predominately.
The process offers an engineer more design freedom than most methods of metal forming because of the near-net-shape advantages that can be attained in a wide variety of alloys. This near-net-shape allows metals not readily machined to be investment cast.
Designers also do not have the same concern for parting lines as they do with other casting processes. Another benefit of investment casting is almost all configurations can be cast; however, not all configurations may economically fit the process. When evaluating or comparing different metal forming processes, the component’s total cost (including machining and finishing) must be considered to determine if investment casting is a good fit. The investment casting process can eliminate some costly machining operations by incorporating detail not readily attainable from other less expensive metal forming methods (Fig. 1). However, a design intended for one manufacturing method must be redesigned for investment casting to take full advantage of the process’ benefits. Table 1 compares the investment casting process with other metal forming processes.
Investment casting begins with the production of wax replicas of the desired castings. These replicas, called patterns, are formed by injecting liquid wax into metal dies. A pattern must be manufactured for each casting to be produced. Next, the patterns are attached to a central wax sprue to form a casting cluster called an assembly. After initial dipping into a solution that cleans the wax, the assemblies are immersed into a liquid ceramic slurry and then into a bed of extremely fine sand to form a shell surrounding (investing) the assembly (Fig. 2.). This investing process is repeated several times. Each layer is allowed to dry before another is added, and enough layers must be applied to build a shell strong enough to withstand subsequent operations. After the shell is completely dry, the wax is melted out in a high-pressure steam autoclave, leaving a hollow void within the mold that matches the shape of the wax pattern and sprue assembly. Prior to casting, the shells are fired in an oven to burn out any remaining wax residue and preheat the mold for the molten metal.
Investment castings can be produced in all alloys from a fraction of an ounce, for parts such as orthodontic (dental) braces, to more than 1,000 lbs. (453.6 kg) for complex aircraft engine parts. Smaller components can be cast in quantities of hundreds per sprue assembly, while heavier castings often are produced with an individual tree. The weight limit of an investment casting depends on the mold handling equipment at the metalcasting facility. The majority of U.S. facilities cast parts up to 20 lbs. (9.07 kg). However, many domestic facilities are increasing their capability to pour larger parts, and components in the 20-120-lb. (9.07-54.43-kg) range are becoming common.
A ratio often used in designing for investment casting is 3:1—for every 1 lb. (0.45 kg) of casting, there should be 3 lbs. (1.36 kg) to the tree, depending on the necessary yield and the size of the component. The tree always should be significantly larger than the component, and the ratio ensures that during the casting and solidification processes, the gas and shrinkage will be located in the tree, not in the casting.
Because the ceramic shell is assembled around smooth patterns produced by injecting wax into a polished aluminum die, the final casting finish is excellent (Fig. 4). A 125 microfinish is standard and even finer finishes (63 or 32) are not uncommon on aircraft engine castings. The size of shot particles used also factors into the final surface finish.
Individual metalcasting facilities have their own standards for surface blemishes, and the metalcaster, design engineer and customer will discuss these capabilities before the tooling order is released. Certain standards depend on a component’s end-use and final cosmetic features.
Because labor and material costs when fabricating the molds, investment castings generally have higher costs than forged parts or sand and permanent mold casting methods. However, they make up for the higher cost through the reduction of machining achieved through as-cast near-net-shape tolerances. One example is innovations in automotive rocker arms, which can be cast with virtually no machining necessary. Many parts that require milling, turning, drilling and grinding to finish can be investment cast with only 0.02-0.03 in. (0.05-0.076 cm) finish stock.
Rapid Prototyping Reduces Lead Times
Rapid prototypes (RP), including 3D printed patterns, also are used (Fig. 5) to speed up the time to market. The RP models can be created in hours and duplicate the exact shape of a part. If the casting is larger than the printer’s build envelope, multiple pattern pieces can be made, assembled into one final pattern and cast to achieve the final prototype component. Using 3D printed patterns is not ideal for high production, but can help a design team examine a part for accuracy and form, fit and function before submitting a tool order.
Like most casting processes, lead times with investment casting vary because part complexity and casting plant capacity. Generally, 6-8 weeks is typical for tooling and sample castings and 8-10 weeks for production. Once a wax pattern is created, a component can be produced in seven days; much of this time is spent with the coating and drying of the ceramic slurry. Several investment casting facilities have quick drying capabilities for ceramic molds to produce parts in 24 hours. In addition, by using 3D printing processes, engineered cast metal components can be delivered only days after accepting a final computer-aided design (CAD) model.
The presence of porosity and/or shrinkage defects depend on how well a metalcasting facility degasses the melt and how quickly the parts solidify. As mentioned earlier, a properly built tree will allow porosities to be trapped in the tree, not the casting, and the high-heat ceramic shell allows for better cooling. Also, vacuum-investment cast components rid the molten metal of gassing defects as air is eliminated. Investment castings are used for many critical applications that require x-ray inspection and must meet definite soundness criteria. The integrity of an investment casting can be far superior to parts produced by other methods.
In investment casting, tolerances may be affected by a number of variables, including wax or plastic temperature, injection pressure, die temperature, shell composition, backup coat, firing temperature, rate of cooling and position of the component on the casting tree. While each bears directly on the tolerances required in investment casting, the amount of tolerance required to cover each process step depends on the size and shape of the casting and will vary by casting facility. This is because one company may specialize in thin-walled components, another in mass production requirements and another in high-integrity aerospace applications.
As a general rule, linear tolerances on investment castings are: ± 0.01 in. (0.025 cm) with dimensions up to 1 in. (2.5 cm), and ± 0.003 in./in. (0.008cm/cm) for each additional inch of dimensions up to 10 in. (25.4 cm) (Table 16-2). For dimensions exceeding 10 in., allow a tolerance of ± 0.005 in./in. (0.013 cm). Investment casters also can offer premium linear tolerances, which require secondary controls and processes to comply with print specifications. During the design stage, close tolerance requirements in non-critical areas should be avoided. Generous tolerances in these areas will result in lower casting and inspection costs.
Investment casting provides designers many ferrous and nonferrous options. It is the design engineer’s decision to determine which alloy characteristics—strength, corrosion resistance, fatigue properties, machinability, hardness, etc.—are primary concerns.
Ferrous—in investment casting, carbon and low alloy steels comprise a large portion of the ferrous production. Tensile strengths in these alloys vary from 50 ksi to more than 230 ksi, depending on alloy and heat treatment.
Other ferrous alloys that are readily investment cast include tool and stainless steels and specialty steels. (Fig. 6). Investment casting is a logical choice for tool steels because of the inherent difficulties in forging and machining. Austenitic stainless steels (300 series) are corrosion resistant, not hardenable and possess tensile strengths in the area of 60-70 ksi. This family of stainless steels is typically used by the food machinery and processing markets, as well as for various pump housings and parts. Martensitic steels (400 series) are heat treatable and offer hardness in the range of Rockwell C 4058. This family of stainless steels offers improved properties in most applications but lacks the high degree of corrosion resistance found in 300 series stainless.
Additional ferrous alloys that are investment cast include ductile iron, cobalt-base alloys, nickel-base alloys, gray iron and magnetically soft and hard alloys.
Nonferrous—nonferrous alloys typically investment cast include aluminum- and copper-base alloys. The ease of casting and machining nonferrous alloys by many processes reduces the number of applications for nonferrous investment cast components. However, configurations to be die cast in production can frequently be pre-engineered and introduced at low volumes faster and at a lower cost as investment castings. After the design has been finalized and the manufacturing operations have been proven, the more expensive die casting tooling can be produced with a reduced chance of additional costly changes. Converting configurations that are sand cast and machined to an investment casting also can be cost-effective. Weight reduction potential and the elimination of some machining operations can reduce the total manufacturing cost.
The most widely used aluminum alloys for investment casting are A356, C355 and A357. A356 provides good strength and corrosion resistance and may be heat treated in several ways. C355 has greater strength than A356 (especially at elevated temperatures) but is less ductile. A357 is similar to A356, but stronger.
Investment cast copper-base alloys include the silicon and red brasses, aluminum and manganese bronzes and beryllium coppers. These alloys provide various strengths, conductivities and excellent resistance to many corrosive environments.
Vacuum alloys—when melted in air, certain alloys become contaminated. To prevent the alloy deterioration associated with this contamination, these highly reactive alloys must be melted and cast in an inert or vacuum atmosphere. These alloys are known as vacuum alloys, of which superalloys are the largest group produced.
Vacuum cast nickel—and cobalt-based superalloy parts, used primarily in the hottest sections of jet engines, often operate at temperatures exceeding 2,000F (1,010C). Stress rupture testing is commonly used to evaluate the physical properties of these alloys.
Superalloys presently provide the best combination of strength, oxidation resistance, service life at high temperature and cost effectiveness of any alloys. The high content of titanium, aluminum and other reactive elements in these materials makes vacuum casting mandatory for cleanliness and high quality.
This article is based on an excerpt of the textbook Design & Purchasing Metal Castings available at www.afsinc.org/estore.
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