Application Review: Investment Casting

Process:
Investment Casting

Applicable Industries:
Aerospace, Medical, Dentistry, Defense, Jewelry, Automotive, Sports Equipment (golf club heads)

Types of Masters Used:
CNC or Conventionally Machined Patterns
LOM (Laminated Object Manufacturing)
SLA (Stereolithograph Apparatus)

Types of Molds Used & Number of Parts:
LOM Mold 20 to 100
SLA Mold 20 to 100
RTV Mold 50 to 300+
Epoxy Mold 150 to 1000+
Spray Metal 1000 to 3000+

Procedure:
Investment casting, which dates back 5000 years, is the oldest form of casting. Historically known as the lost wax process, products manufactured in the 16th century included cannons, urns, jewelry and statues. Dentistry began utilizing the process in 1897, but it was not until the production of the jet engine near the end of World War II that its impact on industry was realized. The modern version of the lost wax process has come to be known as precision investment casting.

There are a number of steps involved in the investment casting process. In general, this process is accomplished by:

Multiple wax patterns are produced from RTV, Epoxy or Spray metal tooling.
Patterns are gated to a central sprue.
Pattern clusters are dipped in ceramic slurry.
Refractory grain is sifted onto the coated patterns, steps 3 and 4 are repeated (several times) to obtain desired shell thickness.
Hot molds are filled with metal by gravity, pressure, vacuum or centrifugal force.
Mold material is broken away from castings.
Castings are removed from sprue and gate stubs are ground off.

In 1988, shipments of steel investment castings reached 55,000 tons, with a value of more than $1.4 billion. Analysts predict a 5% growth rate per year for steel investment castings through 1995. Nonferrous castings in 1988 totaled 1.8 million tons and were valued at $6 billion. According to industry analysts, aluminum castings sold to machinery, automotive and aerospace OEMs accounted for 70% of nonferrous casting tonnage. Aluminum castings will continue to dominate the nonferrous sector and grow at a rate of 2.8% through 1995.

Investment casting has a number of benefits. It is economically feasible when a part's geometry requires complex shapes or thin sections, that could not otherwise be efficiently reproduced. The dimensional tolerances are excellent, typically .005 to .010 inch/inch. Post-cast machining is usually eliminated (referred to as netcast) or greatly reduced (near netcast). Surface finish is smooth and uniform. Efficiency is achieved when automation is utilized via robotics and assembly lines in high production environments. The most time consuming and expensive portion of investment casting is the creation of the wax patterns, tooling has to be produced to create complex shapes. Changes in allowance for shrink factors, ECOs, and gating design result in long delays and expense. Rapid Prototyping has an important impact on investment casting. The inherent pattern-making capability of rapid protoyping affects the most expensive and time consuming aspect of investment casting.