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 materialsthat 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 solidmold 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 holds 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 in to 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 ash ring at high heat (1400 degrees F) or autoclaving (pressure and steam). The hollow shells are then preheated to 1800-2000 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 o the metal parts and discarded. The balance of the cleaning operations (cut o, grind, heat treat, straightening, blast) are straight forward and quite similar to the other casting processes.

Generally all ferrous and non-ferrous materials can be investment cast. On the ferrous side, carbon stell, stainless steel, duplex stainless steel, super alloys, Nickel alloys, Cobalt alloys , 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 specic strength, corrosion and temperature resistant properties.

Investment castings can be produced in all alloys from a fraction of a few grams (such as a dental brace for a tooth) to over a few grams to 100 Kgs.

Typically, a linear tolerance of +/- .010 for the rst inch of geometry and +/-.005 for each inch thereafter 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 specic dimensions.

Because the ceramic shell is built around smooth patterns produced by injecting wax into a polished aluminum die, the resultant casting nish is excellent. A 125 micro-nish is standard and even ner nishes (63 or 32) are not uncommon on aircraft engine castings.

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 tight tolerances that can be held as cast. Many parts that require milling, turning, drilling, and grinding to nish can be investment cast with only .020 – .020 nish stock.