Die Casting’s Blog

lost wax casting sometimes called by the French name of cire perdue, is the process by which a bronze is cast from an artist’s sculpture. An ancient practice, the process today varies from foundry to foundry, but the steps which are usually used in casting small bronze sculptures in a modern bronze foundry are generally quite standardized.

lost wax casting process

1.Sculpting. 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.Moldmaking. A mold is made of the original sculpture. Most molds are at least two pieces, and a shim with keys is placed between the two halves during construction so that the mold can be put back together accurately. Most molds 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 mold made of latex, vinyl, or silicone which is supported by the plaster part of the mold. Usually, the original artwork is destroyed during the making and initial deconstruction of the plaster mold. This is because the originals are solid, and do not easily bend as the plaster mold is removed. Often long, thin pieces are cut off of the original and molded separately. Sometimes, especially in the case of large original (such as life-size) sculptures, many molds are needed to recreate the original sculpture.

3. Wax. Once the plaster and latex mold is finished, molten wax is poured into it and swished around until an even coating, usually about 1/8 inches thick, covers the entire inner surface of the mold. This must be done in several layers until desired thickness is reached.

4. Removal of wax. This new, hollow wax copy of the original artwork is removed from the mold. The artist may reuse the mold to make more wax copies, but wear and tear on the mold limit their number. For small bronze artworks, a common number of copies today is around 25.

5.Chasing. Each hollow wax copy is then “chased”: a heated metal tool is used to rub out all the marks which show the “parting line” or “flashing” where the pieces of the mold came together. The wax is then “dressed” to hide any imperfections. The way the wax looks at this stage, is what it will look like when it is cast. Wax pieces that were molded separately can be heated and attached; foundries often use “registration marks” to indicate exactly where they go. v

6. Spruing. Once the wax copy looks just like the original artwork, it is “sprued” with a treelike structure of wax that will eventually provide paths for molten bronze to flow, while allowing air to escape. The carefully-planned spruing usually begins at the top with a wax “cup,” which is attached by wax cylinders to various points on the wax copy.

7. Slurry. A “sprued” wax copy is dipped into a slurry of liquid silica, then into a sand-like “stucco”, or dry crystalline silica of a controlled grain size. The slurry and grit combination is called “ceramic shell” mold material, although it is not literally made of ceramic. This shell is allowed to dry, and the process is repeated until a half-inch thick or thicker dries coating covers the entire piece. The bigger the piece, the thicker the shell needs to be. Only the inside of the cup is not coated, and the cup’s flat top serves as the base upon which the piece stands during this process.

8. Burnout. The ceramic shell-coated piece is placed cup-down in a kiln, whose heat hardens the silica coatings into a shell, and the wax melts and runs out. The melted wax can be recovered and reused, although often it is simply combusted by the burnout process. 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.

9.Testing. The ceramic shell is allowed to cool, then is tested to see if water will flow through the feeder and vent tubes as necessary. Cracks or leaks can be patched with thick refractory paste. To test the thickness, holes can be drilled into the shell, then patched.

10. Pouring. The shell is reheated in the kiln to harden the patches, then placed cup-upwards into a tub filled with sand. Bronze is melted in a crucible in a furnace, then poured carefully into the shell. If the shell were not hot, the temperature difference would shatter it. The bronze-filled shells are allowed to cool.

11.Release.The shell is hammered or sand-blasted away, releasing the rough bronze. The spruing, which are also faithfully recreated in metal, are cut off, to be reused in another casting.

12. Metal-chasing. Just as the wax copies were “chased,” the bronze copies are worked until the telltale signs of casting are removed, and the sculptures again look like the original artwork. Pits left by air bubbles in the molten bronze are filled, and the stubs of spruing filed down and polished.

13.Patinating. The bronze is colored to the artist’s preference, using chemicals applied to heated or cooled metal. Using heat is probably the most predictable method, and allows the artist to have the most control over the process. This coloring 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.) However, with current artistic trends in the United States, many artists prefer that their bronzes have brighter, more stylized patinas. Patinas can be applied to replicate marble or stone. Depending on how the metal is prepared, either sandblasted or polished, the finish can be either opaque or transparent. After the patina is applied, a coating of wax, which is the most traditional type of sealer is usually applied to protect the surface. Many artists prefer to use lacquer as a sealer on some of the more unstable patinas.

This protects the piece more from ultraviolet rays. Some patinas change color over time because of oxidiation, and the wax layer slows this down somewhat.

Die casting is frequently referred to as the fastest route between raw material and finished product.

Because of the differences in the melting temperatures of various die casting alloys, two methods of inserting the molten metal into the die cavities are used. These are referred to as hot chamber and cold chamber machines.

Hot chamber

Hot chamber or plunger machines are used mainly for zinc alloys. With modern technology, this process is increasingly being used for magnesium. The hot chamber process is a preferred die casting method due to its high rate of productivity. However, it cannot be used for some high melting point alloys or for those alloys which attack the steel working parts of the machine.

Operating sequence for the hot chamber die casting process:

1. Die is closed and gooseneck cylinder is filled with molten metal.

2. Plunger pushes molten metal through gooseneck passage and nozzle and into the die cavity. Metal is held under pressure until it solidifies.

3. Die opens and cores, if any, retract. Casting stays in ejector die. Plunger returns, pulling molten metal back through nozzle and gooseneck.

4. Ejector pins push casting out of ejector die. As plunger uncovers inlet hole, molten metal refills gooseneck cylinder

Cold chamber

Cold chamber machines minimize contact between the alloy to be cast and steel machine parts which allows the processing of high melting temperature alloys. Its primary use is for aluminum, brass, and larger magnesium die castings.

Operating sequence for the cold chamber die casting process:

1. Die is closed and molten metal is ladled into the cold chamber cylinder.

2. Plunger pushes molten metal into die cavity. the metal is held under high pressure until it solidifies.

3. Die opens and plunger follows to push the solidified slug from the cylinder. Cores, if any, retract.

4. Ejector pins push casting off ejector die and plunger returns to original position.

Aluminum Die Cast Parts produced by Kinetic Die Casting, Inc. can be stronger than steel. Kinetic Die Casting Company is a part diecaster of lower priced, better quality aluminum die cast parts, sometimes also known as castings, aluminum part castings, high pressure part die castings, or aluminum diecastings. Kinetic Die Casting has Low Quantity Die Casting Production or High Quantity Die Casting Part Production.

High pressure die casting is a process we use to make aluminum die cast parts. We inject molten aluminum alloy metal under pressure into a steel die to produce aluminum parts. Diecasting is a very inexpensive aluminum part manufacturing process.

Primarily we produce aluminum die casting, but we also make some diecastings in zinc alloys. Kinetic Die Casting Company’s die cast design consultants will assist you with die casting aluminum part design, aluminum die casting prototypes, die cast tooling, machining of die cast parts and finish the surface of many kinds of die cast aluminum parts.

Die casting component parts, decorative trim, and/or finished products offer many features, advantages and benefits to those who specify this manufacturing process.

Die casting provides complex shapes within closer tolerances than many other mass production processes.

Die castings are produced at high rates of production. Little or no machining is required.

Die castings can be produced with thinner walls than those obtainable by other casting methods … and much stronger than plastic injection moldings with the same dimensions.

Die casting provide parts which are durable, dimensionally stable, and have the feel and appearance of quality.

Die casting dies can produce thousands of identical castings within specified tolerances before additional tooling may be required.

Zinc castings can be easily plated or finished with a minimum of surface preparation.

Die castings can be produced with surfaces simulating a wide variety of textures.

Die cast surfaces, as cast, are smoother than most other forms of casting.

Holes in die castings can be cored, and made to tap drill sizes.

External threads on parts can be readily die cast.

Die castings provide integral fastening elements, such as bosses and studs, which can result in assembly economies.

Inserts of other metals and some non-metals can be die cast in place.

Corrosion resistance of die casting alloys rates from good to high.

Die castings are monolithic. They combine many functions in one, complex shaped part. Because die castings do not consist of separate parts, welded or fastened together, the strength is that of the material, not that of threads or welds, etc.

Die casting is an efficient, economical process which, when used to its maximum potential, replaces assemblies of a variety of parts produced by various manufacturing processes at significant savings in cost and labor.

COMPARISONS WITH OTHER PRODUCTS

Plastics injection moldings

Compared with plastic injection moldings, die castings are stronger, stiffer, more stable dimensionally, more heat resistant, and are far superior to plastics on a properties/cost basis. They help prevent radio frequency and electromagnetic emissions. For chrome plating, die castings are much superior to plastic. Die castings have a high degree of permanence under load when compared to plastics, are completely resistant to ultra-violet rays, weathering, and stress-cracking in the presence of various reagents. Manufacturing cycles for producing die castings are much faster than for plastic injection moldings. Plastics, however, may be cheaper on a unit volume basis, have color inherent properties which tend to eliminate finishing, are temperature sensitive, and are good electrical insulators.

Sand castings

Compared with sand castings, die castings require much less machining; can be made with thinner walls; can have all or nearly all holes cored to size; can be held within much closer dimensional limits; are produced more rapidly in dies which make thousands of die castings without replacement; do not require new cores for each casting; are easily provided with inserts die cast in place; have smoother surfaces and involve much less labor cost per casting. Sand castings, on the other hand, can be made from ferrous metals and from many non-ferrous alloys not suitable for die casting. Shapes not producible by die casting are available in sand castings; maximum size can be greater; tooling cost is often less and small quantities can be produced more economically.

Permanent mold castings

Compared with permanent mold castings, die castings can be made to closer dimensional limits and with thinner sections; holes can be cored; are produced at higher rates with less manual labor; have smoother surfaces and usually cost less per die casting. Permanent mold casting involves somewhat lower tooling costs; can be made with sand cores yielding shapes not available in die casting.

Forgings

Compared with forgings, die castings can be made more complex in shape and have shapes not forgeable; can have thinner sections; be held to closer dimensions and have coring not feasible in forgings. Forgings, however, are denser and stronger than die castings; have properties of wrought alloys; can be produced in ferrous and other metals and in sizes not suitable for die castings.

Stampings

Compared with stampings, one die casting can often replace several parts. Die castings frequently require fewer assembly operations; can be held within closer dimensional limits; can have almost any desired variation in section thickness; involve less waste in scrap; are producible in more complex shapes and can be made in shapes not producible in stamped forms. Stampings, on the other hand, have properties of wrought metals; can be made in steel and in alloys not suitable for die casting; in their simpler forms, are produced more rapidly; and may weigh less than die castings.

Screw machine products

Compared with screw machine products, die castings are often produced more rapidly; involve much less waste in scrap; can be made in shapes difficult or impossible to produce from bar or tubular stock; and may require fewer operations. On the other hand, screw machine products can be made from steel and alloys which cannot be die cast; they have the properties of wrought metals; and they require less tooling expense. cnc machining parts

Investment casting dates back thousands of years. Its earliest use was for idols, ornaments and jewellery, using natural beeswax for patterns, clay for the moulds and manually operated bellows for stoking furnaces. Examples have been found in India¡¯s Harappan Civilisation (2000 BC - 2500 BC) idols, Egypt¡¯s tombs of Tutankhamun (1333 ¨C 1324 BC), in Mesopotamia, Mexico, and the Benin civilization in Africa where the process produced detailed artwork of copper, bronze and gold.

The earliest known text that describes the investment casting process (Schedula Diversarum Artium) was written around 1100 A.D. by Theophilus Presbyter, a monk who described various manufacturing processes, including the recipe for parchment. This book was used by sculptor and goldsmith Benvenuto Cellini (1500 - 1571), who 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.

Investment casting came into use as a modern industrial process in the late 19th century, when dentists began using it to make crowns and inlays, as described by Dr. D. Philbrook of Council Bluffs, Iowa in 1897. Its use was accelerated by Dr. William H. Taggart of Chicago, whose 1907 paper described his development of a technique. He also formulated a wax pattern compound of excellent properties, developed an investment material, and invented an air-pressure casting machine.

In the 1940s, World War II increased the demand for precision net shape manufacturing and specialized alloys that could not be shaped by traditional methods, or that required too much machining. Industry turned to investment casting. After the war, its use spread to many commercial and industrial applications that used complex metal parts. For example, Sturm, Ruger, founded in 1949, based much of its manufacturing on the then newly-adopted technology, rising to dominence in the firearms manufacturing world through the elimination of labor-intensive machining of firearms as had been common practice in the firearms industry.

Modern investment casting techniques stem from the development in the United Kingdom of a shell process using wax patterns 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. This process has been evolved over years into the current process of melting out the virgin wax in an autoclave.

A sand casting is a cast part, which is produced by pouring liquid metal a mold made of a mixture of silica sand with addition of clay, moisture and some other additives (green sand) packed around an impression cavity shaped by a pattern). After the molding sand mixture has been packed and compacted around it, the pattern is withdrawn and molten metal is poured into the mold cavity which remains. After the metal has solidified and cooled, the casting is separated from the sand mold. The sand castings are easily recognized due its sand-like texture imparted by the sand mold. As the accuracy of the casting is limited by the green sand making process, there is always some sand residues on the surface of the castings to be removed. It is usually done by shot blasting process, where the casting surface is exposed to a stream of steel or iron shots, blasting its surface until the skin of oxides, silicates and other compounds will be removed. The molding sand going back into the sand processing plant will be remoistured and new additives mixed in. This means that the molding sand is reusable after adjusting its composition, replacing the elements lost in the thermal casting process. The pattern itself may be reused to produce new sand molds. This manual sand molding process that has been known for many centuries opened in the fifties of the twentieth century possibilities of automating the casting production process.

Sand castings may also be produced with molds made of sand that is bonded with materials other than moist clay. Many chemicals and mixture have been designed for this use. When these chemicals are used, they are collectively called "air set" sand castings to distinguish these from "green sand" castings.

Investment casting, also called lost-wax casting, is one of the oldest known metal-forming techniques. From 5,000 years ago, when beeswax formed the pattern, to today’s high-technology waxes, refractory materials and specialist alloys, the castings allow the production of components with accuracy, repeatability, versatility and integrity in a variety of metals and high-performance alloys.

The process is generally used for small castings, but has produced complete aircraft door frames, steel castings of up to 300 kg and aluminium castings of up to 30 kg. It is generally more expensive than die casting or sand casting, but can produce complicated shapes that require little rework or machining.

Investment casting offers high production rates, particularly for small or highly complex components, and extremely good surface finish (CT4-CT6 class accuracy and Ra1.6-6.3 surface roughness) with very little machining. The drawbacks include the specialized equipment, costly refractories and binders, many operations to make a mould, and occasional minute defects.

Investment casting is used in the aerospace and power generation industries to produce single-crystal turbine blades, which have more creep resistance than equiaxed castings. It is also widely used by Sturm, Ruger among other firearms manufacturers to fabricate firearm receivers, triggers, hammers, and other precision parts at low cost. Other industries that use standard investment-cast parts include military, medical, commercial and automotive