Page 1.    About Casting

Other casting processes used in creating artworks: 

Sand-casting is mainly used for casting flat, relief-like sculptures. Aluminum is one material which is commonly used in sand-casting. The process starts with a tub filled with sand. The sand is wetted, and an object is pressed into the wet sand, or the sculptor uses his hands or tools to make the desired design in the sand, which is then dried. Molten aluminum is carefully poured into the depression and left to cool. Then the artist may choose to continue refining the object by "chasing" it or leave it with the roughened surface that is characteristic of sand-cast objects.

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 non-expendable 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 non-reusable 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, with a few days required for drying. Typically, sand casting is used for processing low-temperature steel and aluminum, 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. 

 

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 cast 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. 

About Casting    Page 2. of 2.

Non-expendable mold casting: 

Non-expendable 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 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 of 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-, aluminum-, 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 aluminum 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 aluminum 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 aluminum-, zinc-, magnesium, and copper-based alloys. 

 

This is the process used in the production of certain toys, notably that of model automobiles, see: Matchbox 

External link: die casting (http://www.diecasting.org/) 

 

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 aluminum and its alloys are relatively easy to process. The technique already is well-automated and is used to fabricate aluminum and copper alloys, but only on very special applications for iron.