|
Investment casting is another term for lost-wax casting which evolved from the original metal-forming techniques dating back thousands of years ago. Prior to the highly sophisticated waxes of today, beeswax was used for pattern forms. A wide range of refractory materials and specialty alloys have been introduced providing better performance castings with increased accuracy, versatility, and repeatability for a variety of metals.
Investment casting of an inlet-outlet cover of a valve for a nuclear power station
.
Investment casting is typically used for small components, but techniques have progressed allowing for routine castings on a much larger scale. Castings in steel of up to 300 kg and aluminum of up to 30 kg are now common. Although unit costs are more expensive than die or sand casting, equipment costs are lower. A benefit of the investment casting processes is that it can produce complicated shapes that would be impossible using die casting, and it requires little surface finishing with minor machining.
Ti squared technologies specializes in the investment casting of titanium alloys. With its factory focused on commercial and industrial applications, Ti Squared is capable of producing net-shape titanium components at prices competitive with stainless steel.
Applications
Investment-cast parts are found in automotive, medical, commercial and military applications. Firearms manufacturers are using investment casting to fabricate parts such as receivers, triggers, hammers, and other precision components at lower cost.
Investment casting works well for high volume products, particularly for small or highly complex components that cannot be machined or fabricated economically. Extremely good surface finish (CT4-CT6 class accuracy and Ra1.6-6.3 surface roughness) can be achieved with very little machining. Because of the many operations an processes involved, the costs are relatively high for simple shapes. As component become more complex however, investment casting can be the most cost effective method of manufacture.
Historical Uses
The process was originally used for idols, ornaments and jewelry, using natural beeswax for patterns, clay for the molds and manually operated bellows for stoking furnaces. Investment castings have been found in India's Harappan Civilization (2000 BC - 2500 BC) idols, Egypt's tombs of Tutankhamun (1333 – 1324 BC), in Mesopotamia, Mexico, and the Benin civilization in Africa. Detailed artwork cast in copper, bronze and gold have been found.
Sculptor and goldsmith Benvenuto Cellini (1500 - 1571) detailed in his autobiography the investment casting process he used for the Perseus and the Head of Medusa sculpture that stands in the Loggia dei Lanzi in Florence, Italy.
Dentists began using investment castings to make crowns and inlays, as described by Dr. D. Philbrook of Council Bluffs, Iowa in 1897. In 1907, a wax pattern compound and investment material having excellent properties was developed by Dr. William H. Taggart of Chicago along with an air-pressure casting machine.
In 1945, Robert Miller founded one of the first modern investment casting foundries, Precision Metalsmiths, to manufacture precision net shapes after World War II. The demand for specialized alloys that could not be machined or forged resulted from the war effort. Later, Sturm, Ruger, founded in 1949, rose to dominance in firearms manufacturing by using this new technology to reduce labor-intensive machining
A shell process using wax patterns known as the Investment X Process was developed in the United Kingdom. This method resolved the problem of wax expansion and shell cracking by enveloping a completed and dried shell in a vapor degreaser. The vapor permeated the shell to dissolve and melt the wax. Vapors of chlorinated hydrocarbons now known to be carcinogenic are no longer used for wax removal and the process has evolved over years into the current process of melting out the virgin wax in an autoclave.
The process
Step 1. Inject molten wax into a reusable aluminum mold to make an expendable wax pattern.
Step 2. Assemble the patterns into a cluster or tree.
Step 3. Dip the tree into a ceramic slurry.
Step 4. Apply a coating of sand (stucco) to add strength and provide sufficient surface area for adherence of subsequent dip coats .
Step 5. Apply multiple coats to build a thick shell of ceramic around the wax tree and provide sufficient backup to the primary coating.
Step 6. Remove the wax by flash melting in a steam autoclave.
Step 7. Kiln fire the ceramic mold to remove residual wax and sinter the ceramic to high hardness. 
Step 8. Melt and pour the molten metal into the ceramic mold.
Step 9. After cooling, remove the ceramic shell to reveal an exact replica of the wax tree.
Step 10. Remove the individual parts from the metal tree.
Step 11. Grind the attachment points (gates) flush to the finish part.
Step 12. Grit blast and upgrade as needed to finish the net-shape casting.
Step 13. The finished part is an exact replica of the wax pattern.
|
