Investment Casting FAQs
By Hank Harvey, Tech Cast Inc., Myerstown, Pennsylvania
The term "investment" refers to the ceramic materials that are used to build a hollow shell into which molten metal is poured to make the castings. This term is derived from the solid mold process where a plaster-type material is poured, or invested, into a container that holds a clustered tree of small plastic patterns that are identical to the casting being produced. After the plaster has set, the disposable patterns are burned out leaving a hollow cavity into which the metal is poured.
The same method is used in the investment casting process (also known as the lost wax process). Wax is injected into an aluminum die to produce a pattern that is an exact replica of the part to be produced. The method is similar to diecasting, but with wax used instead of molten metal.
For every investment casting to be produced, a wax pattern must be created. These patterns then are assembled in clusters around a coated sprue to create the casting tree, which is repeatedly dipped into an agitated vat of ceramic, and allowed to dry. After a shell thickness of approximately 0.375 in. (9.525 mm) has been formed, the molds are dewaxed by either flash firing at high heat [1,400F (760C)] or autoclaving (pressure and steam), and the wax is drained and recycled back into the process. The hollow ceramic shells then are preheated to 800-2,000F (427-1,093C) depending on the alloy to be poured, and the molten metal is cast immediately into the hot shell. The casting process either is performed by gravity-pour or vacuum methods (gravity or counter-gravity).
After cooling, the ceramic is vibrated and blasted off the metal parts and discarded. The cast components are broken off the tree and run through cleaning/finishing processes, which are similar to those of other casting processes, such as cut off, grinding, heat treatment, straightening and blasting.
What Alloys Can Be Poured?
Generally all ferrous and nonferrous materials can be investment cast. For ferrous materials, carbon, tool and alloy steel along with the 300, 400, 15-5PH and 17-4PH stainless steel alloys are most commonly poured. Also, the rise in ductile iron casting demand has increased the use of the metal for investment casting. For nonferrous applications, most aluminum, magnesium, copper-base and other nonferrous materials can be cast, with aluminum as one of the most common.
Additionally, certain applications require the use of specialized “exotic” alloys used primarily in harsh environments. These alloys, such as titanium and vanadium, meet the additional demands that might not be achieved with standard aluminum alloys. For example, titanium alloys often are used to produce turbine blades and vanes for aerospace engines. Cobalt- and nickel-base alloys (with a variety of secondary elements added to achieve specific strength-, corrosion- and temperature-resistant properties), are additional types of exotics. Some examples are hastelloys, which can endure steam and acids found at chemical manufacturing plants, and monels, which have good weldability.
As with any casting process, investment casting alloys can be specifically made for a customer depending on the materials at the metalcasting plant. But unlike other processes, the ceramic molds in investment casting can be brought to more than 1,500F (816C), thus increasing their strength. This allows for high-temperature pours. Ductile iron and steel pour in the 2,600-2,900F (1,427-1,593C) range and monels pour at more than 3,100F (1,704C). These temperatures ensure the most efficient pouring procedures and allow for smoother cooling methods when cast in the ceramic shell.
What Are the Size Limits of Investment Cast Components?
Investment castings can be produced in all alloys from a fraction of an ounce, for dental braces, to more than 1,000 lbs. (453.6 kg) for complex aircraft engine parts. Smaller components can be cast at hundreds per tree, 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 casting plant. 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 shrink will end up in the tree, not the casting.
What As-Cast Dimensional Tolerances Can Be Achieved?
Typically, a linear tolerance of ±0.005 in/in (0.127 mm/in.) is standard for investment castings (Table 1). For example, if a 1-ft. component were cast, it likely would have a tolerance of ±0.06 in. (1.52 mm). This would vary depending on the size and complexity of the part. Post-casting procedures, such as straightening or coining, often allow for tighter tolerances to be maintained on several specific dimensions.
Table 1. Normal and Premium Linear Tolerances for Investment Castings
|Dimensions (in.)||Normal Tolerances (+/- in.)||Premium Tolerances (+/- in.)|
|Up to 0.25||0.005||0.003|
|Up to 0.5||0.005||0.004|
|Up to 1||0.005||0.005|
|Up to 2||0.01||0.01|
|Up to 3||0.015||0.013|
|Up to 4||0.019||0.015|
|Up to 5||0.022||0.017|
|Up to 6||0.025||0.02|
|Up to 7||0.028||0.022|
|Up to 8||0.031||0.024|
|Up to 9||0.034||0.026|
|Up to 10||0.037||0.028|
By working with an investment casting firm’s engineering staff, design engineers can produce an investment casting drawing for a part that substantially reduces or completely eliminates the previous machining requirements to produce an acceptable component.
What Kind of Surface Finishes Are Produced?
Because the ceramic shell is assembled around smooth patterns produced by injecting wax into a polished aluminum die, the final casting finish is excellent. 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 facility staffs and design engineers/customers will discuss these capabilities before the tooling order is released. Certain standards depend on a component’s end-use and final cosmetic features.
Are Investment Castings Expensive?
Due to the costs and labor with 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 of this 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 0.020-0.030 finish stock.
Further, investment castings do not need taper to remove the components because the ceramic shells are cracked off the part. This can allow castings with 90-degree angles to be designed with no shrink build included and no additional machining to obtain those angles.
How Many Parts Are Needed to Make Investment Casting Practical?
This number depends on how many casting runs will occur. If there is a run size of 50 that will be produced 10 times per year, the tooling costs for the wax patterns likely will be well paid off after the 10th run. Further, no additional repairs need to be made to the tool because wax does little to cause wear defects to the tool material. Generally, machine toolmakers say that the logical part count to pay off a tool is at least 25 castings. Once a tool is purchased, it likely needs only one or two pieces at a time for replacements.
There are a number of investment casting facilities, mainly in the automotive industry, that produce more than 100,000 parts per month. The high part numbers are dependent on how much a metalcasting facility is willing to expand its capacity to produce such high volumes. For standard orders, the bulk of investment castings produced fall in the range of 100-10,000 pieces per year.
What Tooling and Pattern Equipment Is Necessary?
To produce the wax mold patterns, a split-cavity aluminum die (with the counter shape of the final casting) will need to be made. Depending on the complexity of the casting, various combinations of aluminum, ceramic or soluble cores may be employed to allow for the desired configuration. Most tooling for investment casting falls between $1,000-$10,000.
Rapid prototypes (RP), including stereolithography (SLA) models, also can be used. The RP models can be created in hours and take on the exact shape of a part. The RP parts then can be coated in ceramic slurry and burnt out allowing for a hollow cavity to obtain a prototype investment cast component. If the casting is larger than the build envelope, multiple RP parts can be made, assembled into one part and cast to achieve the final prototype component. Using RP parts 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.
Are There Porosity and/or Shrinkage Defects with Investment Castings?
This depends on how well a metalcasting facility degasses a melt and how fast the parts solidify. As mentioned earlier, a properly built tree will allow porosities to be trapped in the tree, not the casting, and a 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 and must meet definite soundness criteria. The integrity of an investment casting can be far superior to parts produced by other methods.
What Are the Typical Leadtimes?
Like most casting processes, leadtimes with investment casting vary due to 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 hrs. In addition, by using RP processes, engineered cast metal components can be delivered only days after accepting a final CAD model. AFS