Investment
Casting
A Research paper by Laura Morford
In industry investment casting and the
lost wax process are terms used synonymously.
It is for very obvious and logical reasons that this process has been
given these titles. “It
is named the lost wax process because the wax from which the patterns are made
is lost or melted during the casting process (Graham,1981,
p. 231).”
The title investment casting comes from the
process deemed as investing. When a
pattern is invested it is encased in a flask or a shell made of slurry and
refractory material.
During
investment casting both the pattern and the mold are destroyed.
The pattern is melted out in order to pour the melted metal into the
cast. The cast is later destroyed
when it is broken away to release the artifact.
While this sounds wasteful it is also very beneficial to those who wish
to make one of a kind artifacts or objects with special specifications.
The history of investment casting is
very illusive. According to Johnson
(1955),
“Authorities
are not certain where or when it started, but both ancient Egypt and ancient
China have provided museum pieces that could only have been fabricated by this
method, thus giving authentic evidence that the process was used successfully
several thousand years ago (p.12).” Another author, Richard L.
Little (1977), however would argue that the lost wax
process began with the early settlers along the Mediterranean Sea (p.185).
Although both authors point toward the Mediterranean as being a point of
origin for this process, neither one could provide a specific date for its
birth.
Investment casting may be called the
lost wax process because of its unique way of creating artifacts, but it was
also for a long time a lost process. For
some unknown reason this process was unused and forgotten in the western
civilization until its rediscovery during the Italian Renaissance (Little,1977,
p.185). “It
existed in china for centuries, and Cellini employed a form of it in Italy in
the sixteenth century (Black, DeGarmo, and Kohser,1984,
p.287).”
From that point on investment casting was more commonly used for art and
small special tasks. “Dentists have also used the process since the turn
of the century. It did not,
however, come into common industrial use until World War II when high quality
jet turbine blades and supercharger buckets for aircraft engines were in great
demand (Wright,1999,
p. 67).” Since
then investment casting has become an important part of industry due to its
ability to create unique parts with complex shapes and extremely fine surface
details.
“Investment
casting involves the following steps:
2. From
the master pattern, produce a master die.
3. Produce
the wax patterns.
4.
Assemble
the wax patterns to a common wax sprue.
5.
Coat
the cluster with a thin layer of investment material.
6. Produce
the final investment around the coated cluster.
7. Vibrate
the flask to remove the entrapped air and settle the investment material
around the cluster.
8. Allow
the investment to harden.
9. Melt
or dissolve the wax pattern to permit it to run out of the mold.
10. Preheat
the mold preparatory to pouring.
11. Pouring
the molten metal.
12. Remove
the castings from the mold.
(Black, DeGarmo, and
Kohser,1984,
pgs. 287-288).”
The master pattern may be carved from wood, plastic, or even wax. The material chosen for the master pattern often depends on weather the final object will be a manufactured good or an original piece of art. Obviously if a one of a kind piece of art is the creators goal the master pattern would be created out of wax and step two would be eliminated. The master die may be created out of any material in which wax may be cast. Metal dies are often preferred.
Once the wax pattern are produced by injecting the wax into the master die the smaller patterns are attached to a common sprue, and larger patters, perhaps large sculptures, have a sprueing system created for them. “The first decision that must be made in planning a sprueing system is whether to use a direct or an indirect approach (Hitchcock,1985, p. 12-19).” The approach taken in attaching a sprueing system is very important to the desired outcome of the final artifact. “If the entire mold cavity can be filled from the top quickly and completely with little turbulence in the metal flow, then the direct approach is definitely better. However, the indirect approach has to be used if the interior surfaces of the mold are either (a) too irregular, which would cause the entering metal to splash and perhaps chill on the way down, or (b) too close together, creating spaces so narrow that the metal would solidify before completely filling the mold (Hitchcock,1985, p. 19).” Obviously the indirect approach requires more metal and has more metallic waste, but in some instances it is the only practical approach to casting a large artifact.
Once the sprueing system is designed and attached the pattern(s) will be invested in slurry and refractory particles. Once this has dried it will create a hard shell that will serve as the new die. During the burn-out stage two things will happen. The invested shell will harden and the wax will melt out of the mold. It is important to have a place for the wax to flow into as it melts out of the die. “Burn-out of wax should never be done over a direct flame, because one are of the mold and investment heats while the other areas remain at a lower temperature. This causes a breakdown in the investment and results in an incomplete wax burn-out (Choate,1966, p. 55).” When incomplete wax burn-out occurs the object cast in the mold will be misshaped and scrapped. The first eight steps will have to be repeated until a correct burn-out is achieved. “Correct burn-out eliminates all wax from the model and all moisture from the mold, thereby leaving a pattern chamber in the investment. It burns away any carbon residue of the wax remaining in the pattern chamber which would produce discolored castings. It also heats the mold and flask to the temperature necessary to receive the molten metal when casting takes place (Choate,1966, p. 55).” Correct burn-out usually occurs when the investment is heated evenly in an oven.
Once the wax is completely melted out of the mold it is ready to be cast. “Best casting results are obtained when the mold is used hot, just from the oven, and the metal poured into it relatively cold. The plaster mold is a much poorer conductor of heat than a sand mold and for this reason, if the metal is too hot, large and undesirable crystal development may occur because of the slow heat loss (Clarke,1948, p. 96).” In the cases where it is not possible to cast the artifact right away the mold is allowed to cool. “The hot mold should not be chilled suddenly, for this may cause thermal cracking (Clarke,1948, p. 96).” When thermal cracking occurs the mold is ruined and a new one must be made. The mold should be allowed to gradually and evenly cool until it is ready for use at a later time. When it is time to cast the artifact the mold must be gradually and evenly heated to a proper temperature for receiving melted metal.
When the metal is poured it travels through the sprues and to the pattern. The metal then cools and hardens inside of the mold. “After the molds have been poured and have cooled, the molds are broken to remove the castings (Repp,1994, p.305).” The sprues are removed and the artifact is polished and finished.
“In the early 1950’s,
investment casting began to be categorized as either investment flask casting or
investment shell casting (Little,1977,
p.187).”
Although the patterns for the flask and shell
castings are created the same way and out of the same materials, the difference
lies in the way in which it is invested in the slurry (Little,1977,
p.189).
With flask casting the pattern in invested upside down in a canister
filled with a refractory ceramic material, which is allowed to harden around the
pattern. In shell casting the
pattern is dipped into slurry and then sprinkled with particles of refractory
material (Little,1977,
p.189).
There are many reasons for using investment casting. “Investment molds are used for making precision castings (Johnson, 1955, p. 304).” When details and accuracy are essential no other metal casting process is as accurate as the lost wax process. “Not only can complex shapes be cast which could not be cast by any other process or be made by machining, but, very thin sections – down to 0.38 mm (0.015 inch) – can be cast (Black, DeGarmo, and Kohser,1984, p.288).” Although there are many steps in the lost wax process it does cut down on the amount of work needed for the finishing steps. “Investment molding can produce complex castings with great precision and with very smooth surfaces. This reduces or eliminates the need for machining and finishing operations on many castings (Repp,1994, p.305).” The reduced amount of finishing operations needed on invested items is a wonderful incentive to use this process. The extra work required to cast the artifact is almost made up for by the reduction of attention needed on the tedious task of finishing.
When trying to
determine if investment casting is appropriate or required to create the desired
artifact it is important to remember the following. “Investment
casting is usually limited to small parts weighing under one pound but the
process has no specific size limitations. The
process is complex and expensive and, therefore, is used primarily when:
1. The
shape is complex.
2. Accuracy
is critical.
3. Material
is difficult to machine.
4. Fine
surface finish is needed.
5. The
part has thin or delicate cross-sections which would break under other
molding processes.
(Wright,
1999,
p.70).”
Casting
– “An object that is produced by pouring liquid metal into a mold
(Graham,1981,
p.539).”
Sprue
– “The passage way or hole in the gating system that allows the molten
metal to flow down the parting line (Graham,1981,
p.543).”
Mold
– “A hollow form in which molten metal is cast (Repp,1994,
p. 693).”
Black, J.T., DeGarmo, E.P.,
Kohser, R.A. (1984). Materials and
Processes in Manufacturing (Sixth Edition).
New York: Macmillan Publishing Company.
Choate, S. (1966).
Creative Casting: Jewelry, Silverware, Sculpture.
New York: Crown Publishers, Inc.
Clarke, C.D., Ph.D. (1948). Metal Casting of Sculpture.
Butler, MO: The Standard Arts Press.
Graham, G.S., Ed.D. (1981).
Metalworking: An Introduction. Belmont,
California: Wadsworth, Inc.
Hitchcock, H. (1985).
Out of The Fiery Furnace. Los
Altos, California: William Kaufmann, Inc.
Johnson, I.H. (1955).
The Lost Wax Process and its Use in Industrial Arts.
Unpublished doctoral dissertation, Graduate School of Bradley University.
Little, R.L. (1977).
Metalworking Technology. New
York: McGraw-Hill, Inc.
Repp,
V.E., Ed.D. (1994). Metalwork, Technology and Practice (Ninth Edition).
New York: McGraw-Hill, Inc.