1. What is the Investment Casting Process?
The term “investment” may bring dollar signs before the eyes of those who are not familiar with the process. Actually, it refers to the ceramic materials that are used to build a hollow shell into which molten metal is poured to make the castings. The origin of the term investment comes 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 hold true for the investment or “lost wax” casting process. Wax is injected into an aluminum die to produce a pattern that is an exact replica of the part to be produced. For every casting, a wax pattern must be manufactured. The patterns are then clustered around a coated sprue and repeatedly dipped into an agitated vat of ceramic and allowed to dry. After a shell thickness of approximately 3/8″ has been built, the molds are dewaxed by either flash firing at high heat (1400 degrees F) or autoclaving (pressure and steam). The hollow shells are then preheated to 800 – 2000 degrees F depending on the alloy to be poured and the molten metal cast immediately into the hot shell. After cooling, the ceramic is vibrated and blasted off the metal parts and discarded. The balance of the cleaning operations (cut off, grind, heat treat, straightening, blast) are straight forward and quite similar to the other casting processes.
2. What alloys can be poured as an investment casting?
Generally, all ferrous and non-ferrous materials can be investment cast. On the ferrous side, carbon, tool and alloy steel along with the 300, 400, 15-5PH and 17-4Ph stainless steels are most commonly poured. most aluminum, copper base, and other non-ferrous materials can be cast. In addition, there are the “exotic” alloys used primarily in the aircraft engine industry to produce blades and vanes. These alloys are primarily composed of nickel and cobalt with a variety of secondary elements added to achieve specific strength, corrosion and temperature resistant properties.
3. What size range of parts can be produced by the investment casting process?
Castings can be produced in all alloys from a fraction of an ounce (such as dental brace for a tooth) to over 1,000 pounds (complex aircraft engine parts). Of the approximately three hundred foundries nationwide, most cast parts fall in the ounces to 20 pound range. Presently, a larger number of foundries are increasing their capability to pour larger parts, and pieces in the 20-120 pound range are becoming quite common.
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 two specific dimensions. A concerted initial effort between the customer’s and the foundry’s engineering staff can often result in an investment casting drawing for a part that substantially reduces or completely eliminates the previous machining requirements to produce an acceptable part.
5. What type of surface finish can I expect from an investment casting?
Because the ceramic shell is built around smooth patterns produced by injecting wax into a polished aluminum die, the resultant casting finish is excellent. A 125 micro finish is standard and even finer finishes (63 or 32) are not uncommon on aircraft engine castings. Each foundry has its own standards for surface blemishes (positives, negatives). These are discussed and agreed upon with the customer based on the function and cosmetic requirements of the part prior to release of the tooling order.
6. Aren’t investment castings expensive? And if so, how can they save me money?
While castings are generally more expensive than forged parts or those produced by other casting methods, they make up for higher cost through the reduction of machining achieved through the near net shape tight tolerances that can be held as cast. Many parts that require milling, turning, drilling, and grinding to finish can be investment cast with only .020-.030 finish stock. Again, it is imperative for the engineering staff of the foundry and customer to get together and discuss what can or cannot be cast to determine final finishing requirements and the potential cost savings.
7. How many pieces do I need to make buying an investment casting practical?
Not as many as you think! Tooling amortization is a key factor in determining whether or not an investment casting is practical. The machine tool industry will often specify an investment casting on 25 pieces of a new part. This is bought once and after that only one or two pieces at a time for replacements. Conversely, quantities exceeding 100,000 parts per month are produced by some foundries largely for automotive use. The bulk of investment castings produced fall in the 100 – 10,000 piece annual range.
8. What type of tooling or pattern equipment is necessary?
Typically, a split cavity aluminum die is manufactured that is the “female” mold from which the “male” wax patterns are produced. Depending on the complexity of the casting, carious combinations of aluminum, ceramic or soluble cores may be employed to yield the desired configuration. Most tooling for investment castings falls in the $1,000 – $10,000 category.
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? Investment castings are used for many critical applications that require the parts to be x-rayed and meet definite soundness criteria. The integrity of an investment casting can be far superior to parts produced by other methods.
10. What are the lead times I can expect when ordering an investment casting?
Nothing varies more than lead times depending on part complexity and foundry capacity. Generally 6-12 weeks is typical for tooling and sample casting and 6-12 weeks for production.