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Casting

This article is about the manufacturing process. In entertainment, casting is a pre-production process for selecting a cast of actors and other talent for a live or recorded performance. See also Cast (computer science) for explicit type conversion and casting (sport).

Table of contents

1 Lost wax bronze-casting process 2 Other casting processes used in creating artworks 3 Casting in manufacturing 3.1 Expendable mold casting 3.1.1 Sand Casting 3.1.2 Plaster casting 3.1.3 Shell molding 3.1.4 Investment Casting 3.2 Nonexpendable mold casting 3.2.5 Permanent casting 3.2.6 Die Casting 3.2.7 Centrifugal casting 3.2.8 Continuous casting

Lost wax bronze-casting process

The Lost Wax bronze-casting process is an ancient practice that is still in widespread use today. The steps which are usually used in casting small bronze sculptures in a modern bronze foundry are as follows:

1. An artist creates an original artwork from wax, clay, or another material. Wax and oil-based clay are often preferred because these materials retain their softness.
2. A mould is made of the original sculpture. Most moulds are at least two pieces, and a shim with keys is placed between the two halves during construction so that the mould can be put back together accurately. Most moulds of small sculptures are made from plaster, but can also be made of fiberglass or other materials. To preserve the fine details on the original artwork's surface, there is usually an inner mould made of latex or vinyl, which is supported by the plaster part of the mould.
3. Usually, the original artwork is destroyed during the making and initial deconstruction of the plaster mould. This is because the originals are solid, and do not easily bend as the plaster mould is removed. Often long, thin pieces are cut off of the original and moulded separately. Sometimes, especially in the case of large original (such as life-size) sculptures, many moulds are needed to recreate the original sculpture.
4. Once the plaster and latex mould is finished, molten wax is poured into it and swished around until an even coating, usually about 1/4 inches think, covers the entire inner surface of the mould. This may be done in several layers.
5. This new, hollow wax copy of the original artwork is removed from the mould. As many copies as the artist desires may be produced this way, although normal wear and tear may limit the lifespan of any given mould. A common number of copies of small bronze artworks today is around 25.
6. Each hollow wax copy is then "chased," or all the marks which show the "parting line" (also known as "flashing") where the pieces of the mould came together are rubbed out using a heated metal tool. Any copies of pieces which were cut off and moulded separately can be reattached using heat to weld the wax pieces together just as they were in the original artwork. "Registration marks" are often used to help know where exactly to reattach pieces.
7. Once a wax copy is perfected in this way so it now looks just as the original artwork did, it is "sprued" onto a treelike structure, also made of wax. This structure usually consists of a wax cup, from which feeder tubes of solid wax attached to the bottom connect to the wax copy, and smaller vent tubes attach the uppermost parts of the sculpture back to the top of the cup. Much thought is required to design these structures, as will be explained further in step 10.
8. A completely "sprued" wax copy is then dipped into a ceramic slurry, and this wet object is further dipped into a mixture of powdered clay and sand. This is allowed to dry, and the process is repeated until a half-inch thick or thicker surface covers the entire piece. Only the inside of the cup is not coated. The flat top of the cup serves, coincidentally, as the base upon which the piece stands during this process.
9. Once several of these ceramic-coated sprued wax copies are dry, they are placed cup-down in a kiln and the wax inside them melts out. This is why the method is known as the Lost Wax process! Kiln-heating serves the dual purpose of hardening the ceramic coatings into a hard shell. Often, the melted "reclaimed" wax is collected and reused again and again. Now all that remains of the original artwork is the negative space, formerly occupied by the wax, inside the hardened ceramic shell. The feeder and vent tubes and cup are now hollow, also.
10. The ceramic shells are allowed to cool and are tested to see if water will flow through the feeder and vent tubes in the way that was predicted when the wax copy was being "sprued." Holes are sometimes drilled into the shell to test the thickness, and are patched over with thick ceramic paste. Any cracks or leaks in the ceramic shells are also patched.
11. The ceramic shells are reheated in the kiln, which hardens the ceramic patches. At the same time, bronze is being smelted in a crucible in a very hot furnace. When the bronze has reached the appropriate temperature, the ceramic shells are removed from the kiln and placed cup-upwards into a tub filled with sand, or stood upright in some other manner. Of course, workers involved in this part of the process must wear layers of protective gear against the potential of being burned. Carefully, the crucible filled with liquid bronze is lifted from its furnace and the metal is poured into the ceramic shells. It is important that the shells are also highly heated during the pouring, or the difference in temperatures would shatter the shells. The bronze-filled shells are allowed to cool.
12. Now the ceramic shell is "lost" as well as it is hammered and/or sand-blasted off of the bronze. The cup and sprue system, which are also faithfully recreated in bronze, are cut off. They will be remelted and become part of the next series of bronzes.
13. In a similar manner as the wax copies were "chased," the bronze copies are also worked on until the tell-tale signs of the casting process are removed, and the sculptures again look like the original artwork. Metal-chasing usually consists of filling any pits, which were air bubbles in the molten bronze, and recreating the original surfaces where feeder or vent tubes had to be attached.
14. When the bronze copies have been perfected, they are coloured to the artist's preference using heat and chemicals which change colour when they are painted onto the surface of the reheated bronze. This colouring is called patina, and is often green, black, white or brownish to simulate the surfaces of ancient bronze sculptures. (Ancient bronzes gained their patinas from oxidisation and other effects of being on Earth for many years. Yes, this may include chemical changes from pigeon droppings.) However, many artists prefer that their bronzes have brighter, paint-like colours. Today, these effects, too, can be achieved through the application of patina chemicals rather than painting the bronze. Patinas are less opaque, generally, than paint, and this allows the lustre of the metal to show through. After the patina is applied, a coating of wax is usually applied to protect the surface. Some patinas change colour over time because of oxidisation, and the wax layer slows this down somewhat.

Paper-casting is a method whereby paper slurry is couched onto a plaster or other porous surface from a screen where the paper fibers are caught. The plaster absorbs the water remaining in the paper fibers, and when dry, the paper retains the shape of the plaster object over which it was formed.

Casting in manufacturing

Casting is a process by which a fluid melt is introduced into a mold, allowed to cool in the shape of the form, and then ejected to make a fabricated part or casing. Four main elements are required in the process of casting: pattern, mold, cores, and the part. The pattern, the original template from which the mold is prepared, creates a corresponding cavity in the casting material. Cores are used to produce tunnels or holes in the finished mold, and the part is the final output of the process.

Substitution is always a factor in deciding whether other techniques should be used instead of casting. Alternatives include parts that can be stamped out on a punch press or deep-drawn, items that can be manufactured by extrusion or by cold-bending, and parts that can be made from highly active metals.

The casting process is subdivided into two distinct subgroups: expendable and nonexpendable mold casting.

Expendable mold casting

Expendable mold casting is a generic classification that includes sand, plastic, shell, and investment (lost-wax technique) moldings. All of these involve the use of temporary and nonreusable molds, and need gravity to help force molten fluid into casting cavities.

Sand Casting

Sand casting requires a lead time of days for production at high output rates (1-20 pieces/hr-mold), and is unsurpassed for large-part production. Green (wet) sand has almost no part weight limit, whereas dry sand has a practical part mass limit of 2300-2700 kg. Minimum part weight ranges from 0.075-0.1 kg. Sand in most operations can be recycled many times and requires little additional input.

Preparation of the sand mold is fast and requires a pattern which can "stamp" out the casting template in a few days. Typically, sand casting is used for processing low-temperature steel and aluminium, magnesium, and nickel alloys. It is by far the oldest and best understood of all techniques. Consequently, automation may easily be adapted to the production process, somewhat less easily to the design and preparation of forms. These forms must satisfy exacting standards as they are the heart of the sand casting process - creating the most obvious necessity for human control.

Plaster casting

Plaster casting is similar to sand molding except that plaster is substituted for sand. Plaster compound is actually comprised of 70-80% gypsum and 20-30% strengthener and water. Generally, the form takes less than a week to prepare, after which a production rate of 1-10 units/hr-mold is achieved with a capability to pour items as massive as 45 kg and as small as 30 g with very high surface resolution and fine tolerances.

Once used and cracked away, normal plaster cannot easily be recast. Plaster casting is normally used for nonferrous metals such as aluminium-, zinc-, or copper-based alloys. It cannot be used to cast ferrous material because sulfur in gypsum slowly reacts with iron. Prior to mold preparation the pattern is sprayed with a thin film of parting compound to prevent the mold from sticking to the pattern. The unit is shaken so plaster fills the small cavities around the pattern. The form is removed after the plaster sets.

Plaster casting represents a step up in sophistication and required skill. The automatic functions easily are handed over to robots, yet the higher-precision pattern designs required demand even higher levels of direct human assistance.

Shell molding

Shell molding is also similar to sand molding except that a mixture of sand and 3-6% resin holds the grains together. Set-up and production of shell mold patterns takes weeks, after which an output of 5-50 pieces/hr-mold is attainable. Aluminium and magnesium products average about 13.5 kg as a normal limit, but it is possible to cast items in the 45-90 kg range. Shell mold walling varies from 3-10 mm thick, depending on the forming time of the resin.

There are a dozen different stages in shell mold processing that include:

1. initially preparing a metal-matched plate
2. mixing resin and sand
3. heating pattern, usually to between 505-550 K
4. investing the pattern (the sand is at one end of a box and the pattern at the other, and the box is inverted for a time determined by the desired thickness of the mill)
5. curing shell and baking it
6. removing investment
7. inserting cores
8. repeating for other half
9. assembling mold
10. pouring mold
11. removing casting
12. cleaning and trimming.

Investment Casting

Investment casting (lost-wax process) yields a finely detailed and accurate product. After a variable lead time, usually weeks, 1-1000 pieces/hr-mold can be produced in the mass range 2.3-2.7 kg. Items up to 45 kg and as light as 30 g are possible for unit production.

The process starts by creating an injection die to the desired specifications. This die will be used to inject wax to create the patterns needed for investment casting. The patterns are attached to a central wax sprue, creating an assembly, or mold. The sprue contains the fill cup where the molten metal will be poured into the assembly.

The wax assembly is now dipped multiple times in a ceramic slurry, depending on the shell thickness desired. A layer of fine sand (usually Zircon) is added on top of each ceramic layer. This process will be repeated until the desired shell is created.

After the shell is created to the specifications desired, the wax must be removed. This is where the name "lost-wax process" comes from. This leaves an impression of the desired castings, which will be filled with metal. Before being casted, however, the shells must be heated up in a furnace so they don't break during the casting process.

Next, the desired metal is poured into the hot ceramic shell. The metal fills each part on the assembly, and the central sprue cavity and fill cup. The individual parts will be removed after the mold cools and the shell is removed. The shell is generally removed with water-blasting, although alternate methods can be used. What remains is the casted metal parts, but they are still attached to the sprue assembly. The individual parts are removed by cold-break (dipping in liquid nitrogen and breaking the parts off with hammer and chisel) or with large cutoff saws. Now all that remains is finishing.

First the gate, or the place where the part was connected to the sprue, must be removed. The gate is ground off to part specifications. Parts are also inspected to make sure they were casted properly, and if not are either fixed or scrapped. Depending on the investment casting facility and specifications, more finishing work can be done on-site, sub-contracted, or not done at all.

Investment casting yields exceedingly fine quality products made of all types of metals. It has special applications in fabricating very high-temperature metals, especially those which cannot be cast in metal or plaster molds and those which are difficult to machine or work.

Nonexpendable mold casting

Nonexpendable mold casting differs from expendable processes in that the mold need not be reformed after each production cycle. This technique includes at least four different methods: permanent, die, centrifugal, and continuous casting.

Permanent casting

Permanent casting requires a set-up time on the order of weeks, after which production rates of 5-50 pieces/hr-mold are achieved with an upper mass limit of 9 kg per iron alloy item (cf., up to 135 kg for many nonferrous metal parts) and a lower limit of about 0.1 kg. Hot molds are coated with refractory wash or acetylene soot before processing to allow easy removal of the workpiece. Permanent molds have a life of 3000 castings after which they require redressing. Permanently cast metals generally show 20% increase in tensile strength and 30% increase in elongation as compared to the products of sand casting.

The only necessary input is the coating applied before each casting. Typically, permanent mold casting is used in forming iron-, aluminium-, magnesium-, and copper-based alloys. The process is highly automated.

Die Casting

In die casting fluid is injected into a mold at high pressures. Set-up time for dies is 1-2 months, after which production rates of 20-200 pieces/hr-mold are normally obtained. Maximum mass limits for magnesium, zinc, and aluminium parts are roughly 4.5 kg, 18 kg, and 45 kg, respectively; the lower limit in all cases is about 30 g. Die injection machines are generally large (up to 3 × 8 m) and operate at high pressures - 1000 kg/cm² and higher, although aluminium usually is processed at lower pressure. A well-designed unit produces over 500,000 castings during the production lifetime of a single mold. The major production step is die construction, usually a steel alloy requiring a great deal of skill and fine tooling to prepare. Only non-ferrous materials are die cast, such as aluminium-, zinc-, and copper-based alloys.

Centrifugal casting

Centrifugal casting is both gravity- and pressure-independent since it creates its own force feed using a temporary sand mold held in a spinning chamber at up to 90 g. Lead time varies with the application. Semi- and true-centrifugal processing permit 30-50 pieces/hr-mold to be produced, with a practical limit for batch processing of approximately 9000 kg total mass with a typical per-item limit of 2.3-4.5 kg.

Continuous casting

Continuous casting, much like centrifugal molding, produces sheets or beams which may undergo further fabrication. Continuous casting involves forcing a melted metal through an open-ended mold. Heat is extracted and metal exits the mold as a solid fabricated sheet. Molds are commonly made of graphite.

Molds or "dies" last several weeks, after which graphite must be reworked to original specifications. Metal melting points impose severe restrictions on mold design. Consequently, iron is difficult while aluminium and its alloys are relatively easy to process. The technique already is well-automated and is used to fabricate aluminium and copper alloys, but only on very special applications for iron.
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