Unit VI casting

Part I

I. Read the text and circle the information that is not mentioned in the text.

1) a definition of casting

2) main categories of casting

3) disadvantages of casting

4) metal defects

5) mold materials

6) sand casting

7) shell molding

8) different methods of casting

9) modern casting

10) equipments used for casting

In metalworking, casting involves pouring a liquid metal into a mold, which contains a hollow cavity of the desired shape, and then is allowed to solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process. . Casting can be referred to as products made out of melted iron molecules. The casting properties depend on the foundry practice, on the cooling rate. Iron contains huge carbon concentrations. This content facilitates melting, simplicity of casting, and machining. The end product attains a great reputation for its strength and durability. Tensile strength ranges between 20,000 psi and over 60,000 psi. Castings are utilized extensively in machinery, automotive, and agricultural industry.

Casting is most often used for making complex shapes that would be difficult or uneconomical to make by other methods. Parts such as turbine and pump housings, dynamometer bases, compressors are made out of iron castings. Some manufacturers provide finishing services like galvanizing, painting, and machining as well. In general, iron castings include alloy casting, ductile castings, and malleable castings. Cast parts range in size from a fraction of an inch and a fraction of an ounce (such as the individual teeth on a zipper), to over 30 feet and many tons (such as the huge propellers and stern frames of ocean liners). Casting has marked advantages in the production of complex shapes, parts having hollow sections or internal cavities, parts that contain irregular curved surfaces (except those made from thin sheet metal), very large parts and parts made from metals that are difficult to machine. Because of these obvious advantages, casting is one of the most important of the manufacturing processes.

The casting process is subdivided into two main categories: expendable and non-expendable casting. It is further broken down by the mold material, such as sand or metal, and other material and pouring method, such as gravity, vacuum, or low pressure or high pressure.

Casting is a solidification process, which means the solidification phenomenon controls most of the properties of the casting. Moreover, most of the casting defects occur during solidification, such as gas porosity and solidification shrinkage.

Solidification occurs in two steps: nucleation and crystal growth. In the nucleation stage solid particles form within the liquid. When these particles form, their internal energy is lower than the surrounded liquid, which creates an energy interface between the two. The formation of the surface at this interface requires energy, so as nucleation occurs the material actually undercools, that is it cools below its freezing temperature, because of the extra energy required to form the interface surfaces. It heats back up to its freezing temperature, for the crystal growth stage. Note that nucleation occurs on a pre-existing solid surface, because not as much energy is required for a partial interface surface, as is for a complete spherical interface surface. This can be advantageous because fine-grained castings possess better properties than coarse-grained castings.

Expendable mold casting is a generic classification that includes sand, plastic, shell, plaster, and investment moldings. This method of mold casting involves the use of temporary, non-reusable molds.

Non-expendable mold casting differs from expendable processes because 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 mold casting is metal casting process that employs reusable molds (“permanent molds”), usually made from metal. The most common process uses gravity to fill the mold, however gas pressure or a vacuum are also used. A variation on the typical gravity casting process, called slush casting, produces hollow castings. Common casting metals are aluminum, magnesium, and copper alloys. Other materials include tin, zinc, and lead alloys and iron and steel are also cast in graphite molds. Permanent molds, while lasting more than one casting still have a limited life before wearing out.

The die casting process forces molten metal under high pressure into mold cavities (which are machined into dies). Most die castings are made from nonferrous metals, specifically zinc, copper, and aluminum based alloys, but ferrous metal die castings are possible. The die casting method is especially suited for applications where many small to medium sized parts are needed with good detail, a fine surface quality and dimensional consistency. 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.

Small art pieces such as jewelry are often cast by this method using the lost wax process, as the forces enable the rather viscous liquid metals to flow through very small passages and into fine details such as leaves and petals. This effect is similar to the benefits from vacuum casting, also applied to jewelry casting.

Continuous casting is a refinement of the casting process for the continuous, high-volume

production of metal sections with a constant cross-section. Molten metal is poured into an open-ended, water-cooled copper mold, which allows a ‘kin’of solid metal to form over the still-liquid centre. The strand, as it is now called, is withdrawn from the mold and passed into a chamber of rollers and water sprays; the rollers support the thin skin of the strand while the sprays remove heat from the strand, gradually solidifying the strand from the outside in. After solidification, predetermined lengths of the strand are cut off by either mechanical shears or travelling torches and transferred to further forming processes, or to a stockpile. Continuous casting is used due to the lower costs associated with continuous production of a standard product, and also increases the quality of the final product.