Ten Most Asked Questions About Investment Castings

1. What is an investment casting?

The term "investment" may bring dollar signs before the eyes of those who are not familiar with the 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 wax sprue and repeatedly dipped into a vat of ceramic and allowed to dry. After a shell thickness of appoximately 3/8" has been built, the molds are dewaxed by inserting them into a steam chamber. The hollow shells are then preheated 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 operations (cut off, grind, heat treat, straighten, blast, x-ray and etc.) are straightforward 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 an investment cast. Bimac pours most ferrous alloys such as carbon, tool and alloy steel, along with the 300, 400 and 15-5PH, 17-4PH and Duplex stainless steels. Bimac also pours most brass and bronze alloys. We do not pour aluminum.

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

Investment castings can be produced in all alloys, from a fraction of an ounce to over 2,000 pounds. At Bimac we stop at 300 pounds.

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

Typically, a linear tolerance of +/-.010in/in for the first inch and +/-.005in/in for each additional inch is standard for the industry. This can vary depending on the size and complexity of the part. A concerted initial effort between the customers 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 Microfinish is standard. Surface blemishes (positives, negatives) need to be discussed and agreed upon with the customer prior to release of the tooling order based on the function and cosmetic requirements of the part.

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

While investment castings are generally more expensive than forged parts or those produced by other casting methods, they make up for the higher cost through the reduction of machining achieved through the near net-shape and 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 the customer to discuss what can or cannot be cast in order to determine final finishing requirements and the potential cost savings.

7. How many pieces do I need for a practical investment casting?

Tooling amortization is a key factor in determining whether or not an investment casting is practical. The only sure way is to ask us for a quotation and then it will be obvious which way to go.

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, various combinations of aluminum, ceramic or soluble cores may be employed to yield the desired configuration.

9. What about the integrity of an investment casting? Will I have problems with porosity and shrinkage that is usually non-existant 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 six to 10 weeks is typical for tooling and sample casting and six to eight weeks for production.