Ten Most Asked Questions 
About Investment Casting

1. What is an "Investment Casting"?

An investment casting is a near net shape part made by the "lost wax" process that features cast-in detail, tight tolerances, and excellent surface finish. 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. Originally, a plaster type material was poured or "invested" into a container that held a clustered tree of small wax patterns, which were identical to the desired casting. After the plaster was set, the disposable patterns were melted out, leaving a hollow cavity into which the metal was poured. While some non-ferrous castings are still made this way, most investment castings are now made by the shell method.

Every casting requires a pattern that is made by injecting wax or plastic material into a steel or aluminum die. If the finished part is to have complex internal configurations, a soluble or ceramic core is inserted in the die at this stage. Once the wax has hardened, the machine operator opens the die and removes the pattern. The completed patterns are then wax welded on a cylindrical shape for further processing.

Assembled patterns are dipped in a prime coat of agitated ceramic slurry and then coated with a stucco of fine sand to build a good surface finish. The process is repeated until a thickness of approximately 3/8" surrounds each part. Once the shell has dried, the wax is melted out in a pressurized steam autoclave or by flash firing at high temperatures (1400°F.) The hollow shells are preheated to 800-2000°F, depending on the alloy to be cast and then immediately filled with molten metal.

When the metal has cooled, operators remove the shell or investment material from the cast "tree," cut off the castings, and grind off any of the gate, which remains. Depending on your requirements, the parts may be shipped "as cast" or may be heat-treated, plated, or otherwise processed before shipment.

PMI has been and continues to be a pioneer in investment casting technology. We strive to maintain our early technical leadership in the industry with a continuing investment in both research and development and state-of-the-art equipment.

2. What alloys can be poured as investment castings?

Generally, most ferrous and non-ferrous alloys can be cast. Among ferrous metals, the most commonly poured are carbon, tool, and alloy steel, along with the 300, 400, 15-5 PH and 17-4 PH stainless steels. Of the non-ferrous metals, aluminum and copper-based are the most common alloys used in investment castings.

In addition, there are super alloys used primarily in the gas turbine industry to produce blades and vanes. These alloys are usually composed of nickel and cobalt with a variety of secondary elements added to achieve specific strength, corrosion, and temperature-resistant properties. Often, a vacuum process is used in casting the super alloys.

PMI specializes in air-melt alloys. In our 50 year history, we have successfully cast nearly 200 alloys, including ductile iron and austempered ductile iron. PMI is the only investment foundry that is a member of and certified by the Ductile Iron Society.

Process	Ductile Iron	Tool Steel	Steel	Stainless Steel	Aluminum/ Magnesium	Copper Bronze Brass	Titanium	Super Alloys*
Investment Casting
Die Casting
Forging
P/M High Density
Stamping
Weldments
Extrusion
Permanent Mold
Roll Forming

*includes nickel and cobalt.

3. What size range of parts can be produced by the investment casting process?

Depending on the foundry, investment castings can be produced in all alloys from a fraction of an ounce (such as for a dental brace for a tooth) to over 1,000 pounds (complex aircraft engine parts.) Of the approximate 300 commercial investment casting foundries nationwide, most cast parts in the ounces to 20 pound range. Many foundries are increasing their capacity to pour larger parts, so that pieces in the 20-120 pound range are becoming quite common.

PMI, which produces parts from fractions of an ounce to 10 pounds, specializes in parts under three pounds.

4. What are the "as cast" dimensional tolerances I can expect?

Typically, a linear tolerance of + .005 in./in. is standard for investment casting. This varies depending on the size and complexity of the part. Subsequent straightening or coining procedures often enable even tighter tolerances to be held on one or more specific dimensions.

By working closely with the investment caster's engineering staff, you can often get an as-cast part that is within 90% or more of the requirements for your finished part. As a result of good casting design, the as cast part should require minimal or no machining.

5. What type of surface finish can 1 expect from an investment casting?

Because the ceramic shell is built around smooth patterns produced by injecting wax into a metal die, the resultant casting finish is excellent. A 125 microfinish is standard and even finer finishes (63 or 32) are not uncommon on aircraft engine castings. In general, lower melting point alloys will produce better as-cast surface finishes than higher melting point alloys, while non-ferrous alloys will result in better surface finishes than ferrous.

Each foundry has its own standards for surface blemishes (positives, negatives), which it should discuss with you prior to constructing tooling.

6. Aren't investment castings expensive? If so, how can they save me money?

Investment castings can offer a total lower cost for precision metal parts. The process builds many features into the cast part that translate into benefits for you. While the piece price of an investment casting is gener-ally more than that of a forging or of other types of casting, investment castings can make up for the possibly higher piece price by requiring less machining due to their near net shape tolerances. Many parts are usable as cast with no finishing required, while parts that require milling, turning, drilling, and grinding to finish can be investment cast with only .020 - .030 finished stock.

Again, communication between you and the casting engineers early in the process is crucial to determining what can be cost-effectively cast vs. what should be achieved through after cast machining.

7. How many pieces do I need to purchase to make an investment casting practical?

The bulk of investment castings that are produced fall in the 100-10,000 annual piece requirement range. However, since tooling cost is reasonable, investment casting might be a practical choice for even smaller quantities. For example, in the machine tool industry, investment castings are often specified on 25 pieces of a new part, with subsequent orders being for only a piece or two. On the opposite end of the spectrum, companies that provide investment castings for automotive users might produce 100,000 parts per month.

Your projected annual volume will help you determine if investment casting is the best process for you. Investment casting is most price competitive at medium to high volume, but companies vary according to their effective order size and minimum order requirements. There are companies which offer small quantities of prototype or replacement parts, while other firms are particularly equipped for higher volume work.

8. What type of tooling or pattern equipment is necessary?

Investment casting patterns are produced in a die machined from a block of aluminum or steel. Depending on your volume requirements, the die can include one or more cavities. When the two halves of the die are locked together, wax is injected into the cavity. The hardened wax is an exact replica of the as-cast part, plus "gates," wax tabs which will direct the flow of the metal from the pouring chamber to the individual part.

Because the die is built to withstand wax injection rather than metal injection, investment casting tooling is usually much less expensive than tooling for other "hot" casting methods. The American Foundrymen's Society estimates that most investment casting tooling falls in the $1,000-10,000 price range.

9. What about the integrity of an investment casting? Will I have problems with porosity and shrinkage that are usually non-existent in bar stock or forgings?

The integrity of an investment casting is often far superior to that of parts produced by other methods. Porosity can be controlled in the casting process, while good tooling design and proper selection of pattern materials can compensate for other normal occurrences such as shrinkage. Although some porosity will not affect the mechanical properties of the casting, you can specify x-ray and other NDT tests to verify that the part meets soundness criteria for critical applications where minimal porosity is required.

10. What lead times can 1 expect when ordering an investment casting?

Lead times vary according to part complexity and foundry capacity. Since investment castings are custom-made to your specifications, 8-14 weeks is typical for tooling and sample casting and 8-14 weeks for produc-tion orders.

To make lead times more livable in a business climate that demands Just-In-Time ordering, many companies offer stocking programs. By determining your projected use of a part over a given time period (such as a year), by communicating this need to your supplier, and by committing to purchase the projected quantity upon release, you can shortcut the normal lead time. Advance planning on your part will enable your investment casting supplier to have parts available when you need them.

In summary, investment casting can offer you a near net shape part with cast-in detail, tight tolerances, and excellent surface finish. Because PMI and other investment casting companies can design so much of what you need into the actual casting, you save money in machining, welding, assembly, and other secondary operations. When a part is designed as an investment casting or converted to one from another process, you can receive a finished part at total lower cost.

By communicating your exact specifications and performance requirements to the casting engineer, you will receive the most castable part possible, thus assuring the best value for your money.

PMI is committed to offering you quality metal parts that are easy to buy, ready to use, and available when you need them. Our value-added services include:

• Prototyping using our patented Proto-CastŪ system
• EDI and Barcoding 
• CAD transfer of your print
• Stocking programs 
• Machining, surface finishing, and heat treating

Our sales engineers and field sales agents will be pleased to further discuss with you the particular advantages that the investment casting process can offer for your application, as well as show you how PMI can maxi-mize those advantages. For further information about the process or to get the name of our sales agent nearest you, please call us at (216) 481-8900.

Much of the information in this brochure was provided courtesy of the Investment Casting Division of the American Foundrymen's Society. For further information about their programs and literature, please contact them at (800) 537-4237.

PRECISION METALSMITHS, INC.  1081 East 200th St. Cleveland, OH 44117 (216) 481-8900 FAX (216) 481-8903