The History of Investment Casting
Also known as lost wax casting, investment casting, a process used by ChinaSavvy, is one of the oldest metal-forming techniques. Starting at simple beeswax formed patterns 5 000 years ago, it is today a highly skilled process using complex technologies, specialists alloys and refractory materials.
The modern form of investment or lost was casting is known as Lost Foam Casting.
History of Casting Process
The history of the casting process is scattered across many ancient civilizations, all using the lost wax process to produce detailed work in bronze, gold and copper. Artists in Mesopotamia and ancient Egypt, as well as the Benin civilization of Africa and the Han Dynasty of China all used the process in order to create sculptures and other art.
Examples of this casting process can be seen in the idols made by the Harappan civilization (2500 - 2000 B.C) in Pakistan, as well as in Egypt, in the tomb of Tutankhamen (1333 - 1324 B.C). The history of casting process also has firms roots in pre-Columbian Mexico where Aztecs created elaborate pieces and jewelry using gold. While many pieces were destroyed by the conquistadors and melted in order to enrich the Spanish Treasury, some examples do still remain today, the most famous discovered at Monte Alban in 1930.
Many of the artifacts found here were decorated with wirework, which is presumed to be made by dipping threads into melted wax and then applying these threads to the beeswax pattern before casting took place.
Benvenuto Cellini (1500 - 1571), an Italian sculptor and seen as one of the most important artists of Mannerism, also used the lost wax process shortly after the dark ages in Europe. Learning the process from the writings of Theophilus Presbyter, who, in detail, described various medieval arts in Schedula diversarum artium (list of various arts).
Using the process to create the famous bronze statue, Perseus with the Head of Medusa, it was first revealed to public on the 27th of April, 1554. Today, it still stands at the Loggia dei Lanzi of the Piazza della Signoria in Florence, Italy.
In Benin, a city that is now a part of Nigeria, brass smiths still continue to use the process, passed down through generations, to produce lost wax casting. By looking at the process used by these brass casters today, examples of the earlier history of casting process is apparent.
The process as used in Benin by brass casters:
- Kneaded clay is used as the core. After shaping and sizing the clay into the approximate form and size needed for the article, they are placed in the sun and left to dry for up to several days.
- Next, the caster will create a pattern. This is done by covering one of the cores (made in the previous step) with beeswax and then carefully modeling it into the shape that is required for the item. This means that each and every casting made by these casters are unique and literally formed by hand.
- The wax form now made, it is covered in a layer of clay.
- The cores made can be either self supporting and, in other cases, the core is centered through the use of small pins. The first layer of clay is now applied, a thin roll of wax is added to serve as the channel and the pattern is fully sealed.
- More layers of clay is now added, in effect creating a mold that is left to air dry.
- Once these molds are ready, they are placed into a fire, allowing them to heat and the wax to melt. The wax is poured off and the molds are further heated to allow for the pouring of the molten metal. At the same time, pieces of brass are melted in crucibles on a forge fire.
- Immediately before pouring the molten metal, taken from the forge using long tongs, the molds are taken out of the fire and placed in an upright position.
- A crucible of brass is now poured into the mold, the pourer using a wooden stick on the edge of the crucible to ensure a smooth flow.
- After casting is complete and the molten metal has set, the mold is broken open and the item is cleaned, rough edges are filed and the item is polished.
Modern History and Developments
Investment casting was a technique very largely ignored by the industry until the late 19th Century when the dental profession started applying the process in order to produce inlays and crowns.
In 1897, a paper produced by Barnabas Federick Phillbrook of Council Bluffs, Iowa, first described the process of producing these crown and inlays, the process being further propelled in 1907 by William H. Taggart of Chicago and his paper on his development of the technique itself. Taggart also formulated invented an air-pressure machine, a wax pattern compound and developed the material used in the investment process for dentistry.
During the 1940s military demands due to the World War II, applied overwhelming pressure on the machine tool industry, allowing investment casting to step into the spotlight. Able to produce near-net shaped precision parts and allowing for the use of specialized alloys harder to shape using other methods, investment casting became a viable casting process.
Investment casting also offered the added benefits of a faster production rate, especially where ammunitions and aircraft were concerned, lesser secondary machining and the production of less waste (ideal for the more scarce and expensive metals).
After World War II, the investment casting process saw applications in both the commercial and industrial sectors where complex, metal parts were needed.
More generally used to produce smaller castings, the process has been used to produce aluminum castings of up to 30 kilograms (66 lbs.), steel castings ranging in a weight of up to 300 kilograms (660 lbs.) and even complete door frames for aircraft.
Today, developments within investment or lost wax casting address four main areas:
An increase in the range of materials than can be cast using this specific process. Developments are seeking to include a wider range of super alloys and is driven by the aerospace industry in need of higher operating temperature materials. Other developments is this area also seek to improve the life characteristics and fatigue of castings
Enabling the substitution of materials used in the process due to factors that include either environmental concerns or scarcity.
Addressing both the energy used during the process as well as the materials used. During the 1950s Silica-Sol fell out of favor as a chosen binder because of the faintly degraded finish it produced. Today it is widely used because it does not give off any ammonia or alcohol into the environment.
Improvements in both the process itself and the material used. Developments also seek ways to reduce costs as well as ways to increase the process' envelope of applicability. Technical development includes the adoption of photolithography to allows pattern makers to produce patterns from from CAD (Computer Aided Design) data.
From using natural beeswax, manually operated bellows and clay, the investment casting process today is used to produce precision components used in jet engines and spacecraft. The process itself has evolved, the industry transforming into a high-tech, specialized field applying the latest advances in counter-gravity techniques, computer technology and robotics.
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Further Suggested Reading:
- How precise is Investment Casting?
- Split Wax Molds for Hollow Cast Parts
- Water Soluble Waxes for Hollow Parts
- What Metals can be Investment Cast?