History of the Process
Investment casting, the process used at Hitchiner® Manufacturing to produce intricately detailed near-net-shape castings, is one of the oldest and also the most modern of the metallurgical arts. Hitchiner® employs the latest scientific advances in CAD-CAM design and manufacturing, automated systems, conveyorization, robotics, countergravity casting techniques and other innovations to produce the highest quality investment cast parts available today.
Hitchiner® castings are at work in the fiery combustion chambers of jet aircraft, in the sub-zero vacuum of space, on the world's streets and highways and in a myriad of other applications. Yet, the root of this technology, the cire perdue or "lost wax" process dates back many thousands of years. The artists and sculptors of ancient Egypt and Mesopotamia, the Han Dynasty in China, and the Benin civilization in Africa used the lost wax method of casting to produce their intricately detailed artwork of copper, bronze and gold.
It is remarkable that the art of lost wax casting was so widely known in ancient times. It is not an easy process and calls for considerable skill in its execution. The Aztec gold-smiths of pre-Columbian Mexico used the lost wax process to create much of their elaborate jewelry. Unfortunately, few examples of this work survived the plunder of the conquistadors. Countless masterpieces were melted down into gold bars to enrich the Spanish treasury. The quality of the few pieces which have survived demonstrate a mastery of the process which must have taken many years of trial and error to develop. Accounts of the methods used are provided in the book by Friar Bernardino de Sahagun, who spent 60 years in an intensive study of Aztec Mexico. Each step in the process, which is described and illustrated in his writings, was told to him by the Indians themselves.
Some of the finest remaining examples of pre-Columbian Mexican casting were discovered in the 1930s at Monte Alban, the sacred mountain of the ancient Zapotecs near the city of Oaxaca. Many gold artifacts found there were decorated with wirework presumably made by dipping threads into melted wax and applying them to the beeswax pattern prior to casting.
In the city of Benin, now a part of Nigeria, brass smiths continue to produce lost wax castings using a method passed down through the ages from one generation to the next. A study of their methods provides a living example of the early history of the investment casting process.
The brass casters at Benin begin with a core of clay kneaded into a mass. They shape the clay into the approximate size and shape of the article to be made. These cores are then allowed to dry thoroughly in the sun for several days.
The brass smith creates a pattern for the casting by covering one of these cores with beeswax and carefully modeling it into the exact shape desired. Thus, each casting is a unique hand formed work. When the wax form is finished to the artist's satisfaction, it is covered in a thick coating of clay. Sometimes the cores are made to be self supporting, in other cases small pins are used to keep the core centered. The first layer of clay is applied as a very fine slip. Before the pattern is fully sealed in this coating, a thin roll of wax is added to form a channel into which the molten metal will be poured. Subsequent layers of a thicker clay are added, gradually investing (covering) the form completely, creating a mold. This mold is allowed to air dry thoroughly.
When a batch of molds have been created and are ready for casting. they are placed in a fire and heated so that the wax will melt and can be poured off. The clay molds are further heated to a point where they are sufficiently fired to permit the pouring of the molten metal without causing the shell to burst. Meanwhile, pieces of brass are melted in crucibles on a nearby forge fire. The fire on the forge is stoked by a manually operated bellows.
Immediately prior to the pour, the molds are taken from the fire and placed upright in spaded earth. A crucible of metal is taken from the forge with long tongs and the molten brass is poured into the open mold. The brass smith holds a wooden stick in his other hand during the pour and places it on the edge of the crucible to help insure a smooth flow of metal into the shell. Soon after casting, the molds are broken open, the shell knocked off and the final object is cleaned, filed and polished. Benin lost wax castings can be found in museums throughout the world.
Shortly after the dark ages in Europe, the industrious sculptor and goldsmith, Benvenuto Cellini began to make use of the lost wax method of casting. He learned this process from the writings of the monk Theophilus Presbyter (circa 1100) whose Schedula Diversarum Artium is the earliest known foundry text. In Cellini's autobiography, considered to be one of the classics of literature, he describes in great detail the casting of his famous Perseus and the Head of Medusa. This three and a half ton statue was completed in 1554 and was unveiled at the Loggia dei Lanzi in Florence, Italy, where it stands to this day.
The investment casting process has been used in the arts by many civilizations for countless centuries. Not only was this process in use, but it was developed to a high degree of excellence, as is attested to by many beautiful and finely detailed statues, jewelry and artifacts from antiquity.
This technique was largely ignored by modern industry until the dawn of the twentieth century, when it was "rediscovered" by the dental profession for producing crowns and inlays. The first authenticated record of the use of investment castings in dentistry appears in a paper written by Dr. D. Philbrook of Council Bluffs, Iowa in 1897. However, the true significance of this process was not realized until the research of Dr. William H. Taggart of Chicago was published in 1907. Dr. Taggart not only developed and described a technique, he formulated a wax pattern compound of excellent properties, developed an investment material and even invented an air pressure casting machine.
During World War II, with urgent military demands overtaxing the machine tool industry, the art of investment casting provided a shortcut for producing near net shape precision parts and allowed the use of specialized alloys which could not be readily shaped by alternative methods. The investment casting process was found practical for many wartime needs--and during the postwar period it expanded into many commercial and industrial applications where complex metal parts were needed. It was in this period that the Hitchiner® Manufacturing Company was founded at the Amoskeag Millyards of Manchester, NH.
The solid mold technique was first utilized because a technology to successfully remove the wax patterns from a shell without causing it to collapse, crack or burst had not yet been devised. In the solid mold technique, a wax sprue was placed in a steel casing and surrounded by a setting slurry. The drawbacks of the solid mold technique were extremely long pre-heat, size limitations and poor dimensional tolerances.
The first successful shell technology was the Mercast Process, which used solidified mercury as a pattern material. Mercury patterns were very heavy but extremely accurate. This was a very difficult process as all pattern production and shell building had to be done at temperatures below minus 39 degrees Celsius--the melting temperature of mercury! This process is no longer used due to high costs and the health hazards involved in handling this toxic element.
The first shell process utilizing wax patterns was developed in England and was known as the Investment X Process. This method resolved the problem of wax removal by enveloping a completed and dried shell in a vapor degreaser. The vapor permeated the shell to dissolve and melt the wax. Some measure of success was also found using flash firing methods. Hitchiner® developed a wax elimination tank containing hot liquefied reclaimed wax into which a shell would slowly lowered. The wax would melt out due to the transfer of heat to the solidified pattern material. The advent of the steam autoclave for dewaxing shells made these earlier techniques obsolete. In the autoclave, highly pressurized steam rapidly melts the surface of the wax patterns before the bulk of the wax is heated and can expand.
One of the recent major advances in casting technology occurred at Hitchiner Manufacturing Co. Inc., when, in the early seventies, the first of its exclusive countergravity casting processes was patented.
Over 4,000 years ago, between the Tigrus and Euphrates Rivers in a land known as Mesopotamia, ancient artisans produced idols and ornaments using natural beeswax for patterns, clay for molds and manually operated bellows for stoking furnaces. Today, precision components for spacecraft and jet engines are investment cast using the latest advances in computer technology, robotics and countergravity casting techniques. The future of the investment casting process is very bright, in part due to the research and development commitment of Hitchiner Manufacturing Co., Inc. and Metal Casting Technology, Inc., a wholly owned R&D facility of Hitchiner Manufacturing Co., Inc.