Sand casting is a metal casting process characterized by using sand as the mold material. The term “sand casting” can also refer to an object produced via the sand casting process. Sand castings are produced in specialized factories called foundries. Over 70% of all metal castings are produced via a sand casting process.
Sand casting is relatively cheap and sufficiently refractory even for steel foundry use. In addition to the sand, a suitable bonding agent (usually clay) is mixed or occurs with the sand. The mixture is moistened, typically with water, but sometimes with other substances, to develop strength and plasticity of the clay and to make the aggregate suitable for molding. The sand is typically contained in a system of frames or mold boxes known as a flask. The mold cavities and gate system are created by compacting the sand around models, or patterns, or carved directly into the sand.
Sand casting is used to produce a wide variety of metal components with complex geometries. These parts can vary greatly in size and weight, ranging from a couple ounces to several tons. Some smaller sand cast parts include components as gears, pulleys, crankshafts, connecting rods, and propellers. Larger applications include housings for large equipment and heavy machine bases.
Sand casting is also common in producing automobile components, such as engine blocks, engine manifolds, cylinder heads, and transmission cases.
Centrifugal casting is a casting technique that is typically used to cast thin-walled cylinders. It is noted for the high quality of the results attainable, particularly for precise control of their metallurgy and crystal structure. Unlike most other casting techniques, centrifugal casting is chiefly used to manufacture stock materials in standard sizes for further machining, rather than shaped parts tailored to a particular end-use.
In centrifugal casting, a permanent mold is rotated continuously about its axis at high speeds (300 to 3000 rpm) as the molten metal is poured. The molten metal is centrifugally thrown towards the inside mold wall, where it solidifies after cooling. The casting is usually a fine-grained casting with a very fine-grained outer diameter, owing to chilling against the mould surface. Impurities and inclusions are thrown to the surface of the inside diameter, which can be machined away.
Features of centrifugal casting are as follows:
● castings can be made in almost any length, thickness and diameter;
● different wall thicknesses can be produced from the same size mold;
● eliminates the need for cores;
● resistant to atmospheric corrosion, a typical situation with pipes;
● mechanical properties of centrifugal castings are excellent;
● only cylindrical shapes can be produced with this process;
● size limits are up to 3 m (10 feet) diameter and 15 m (50 feet) length;
● wall thickness range from 2.5 mm to 125 mm (0.1 – 5.0 in);
● tolerance limit: on the OD can be 2.5 mm (0.1 in) on the ID can be 3.8 mm (0.15 in);
● surface finish ranges from 2.5 mm to 12.5 mm (0.1 – 0.5 in).
Die casting is a metal casting process that is characterized by forcing molten metal under high pressure into a mold cavity. The mold cavity is created using two hardened tool steel dies which have been machined into shape and work similarly to an injection mold during the process.
Most die castings are made from non-ferrous metals, specifically zinc, copper, aluminium, magnesium, lead, pewter and tinbased alloys. Depending on the type of metal being cast, a hot- or cold-chamber machine is used.
The casting equipment and the metal dies represent large capital costs and this tends to limit the process to high volume production. Manufacture of parts using die casting is relatively simple, involving only four main steps, which keeps the incremental cost per item low. It is especially suited for a large quantity of small to medium sized castings, which is why die casting produces more castings than any other casting process. Die castings are characterized by a very good surface finish (by casting standards) and dimensional consistency.
Advantages of die casting:
● excellent dimensional accuracy (dependent on casting material, but typically 0.1 mm for the first 2.5 cm (0.005 inch for the first inch) and 0.02 mm for each
● additional centimeter (0.002 inch for each additional inch);
● smooth cast surfaces;
● thinner walls can be cast as compared to sand and permanent mold casting (approximately 0.75 mm or 0.030 in);
● inserts can be cast-in (such as threaded inserts, heating elements, and high strength bearing surfaces);
● reduces or eliminates secondary machining operations;
● rapid production rates;
● casting tensile strength as high as 415 megapascal;
● casting of low fluidity metals.