Sand casting

Asand castingor asand molded castingis a cast part produced by forming a mold from a sand mixture and pouring molten liquid metal into the cavity in the mold. The mold is then cooled until the metal has solidified. In the last stage the casting is separated from the mold. There are six steps in this process:

Place a pattern in sand to create a mold. Incorporate a gating system. Remove the pattern. Fill the mold cavity with molten metal. Allow the metal to cool. Break away the s[1]and mold and remove the casting.

There are two main types of sand used for molding."Green sand"is a mixture of silica sand, clay, moisture and other additives. The"air set"method uses dry sand bonded to materialsotherthan clay, using a fast curing adhesive. When these are used, they are collectively called "air set" sand castings to distinguish these from "green sand" castings. Two types of molding sand are natural bonded (bank sand) and synthetic (lake sand), which is generally preferred due to its more consistent composition.

With both methods, the sand mixture is packed around a master "pattern" forming a mold cavity. If necessary, a temporary plug is placed to form a channel for pouring the fluid to be cast. Air-set molds often form a two-part mold having a top and bottom, termed Cope and drag. The sand mixture is tamped down as it is added, and the final mold assembly is sometimes vibrated to compact the sand and fill any unwanted voids in the mold. Then the pattern is removed with the channel plug, leaving the mold cavity. The casting liquid (typically molten metal) is then poured into the mold cavity. After the metal has solidified and cooled, the casting is separated from the sand mold. There is typically no mold release agent, and the mold is generally destroyed in the removal process.[1]

The accuracy of the casting is limited by the type of sand and the molding process. Sand castings made from coarse green sand impart a rough texture on the surface of the casting, and this makes them easy to identify. Air-set molds can produce castings with much smoother surfaces. Surfaces can also be ground and polished, for example when making a large bell. After molding, the casting is covered in a residue of oxides, silicates and other compounds. This residue can be removed by various means, such as grinding, or shot blasting.

During casting, some of the components of the sand mixture are lost in the thermal casting process. Green sand can be reused after adjusting its composition to replenish the lost moisture and additives. The pattern itself can be reused indefinitely to produce new sand molds. The sand molding process has been used for many centuries to produce castings manually. Since 1950, partially-automated casting processes have been developed for production lines.

Alternative casting methodsAs a supplement to the sand casting other casting methods were successfully applied.

Modern casting production methods can manufacture thin and accurate molds—of a material superficially resembling papier-mâché, such as is used in egg cartons, but that is refractory in nature—that are then supported by some means, such as dry sand surrounded by a box, during the casting process. Due to the higher accuracy it is possible to make thinner and hence lighter castings, because extra metal need not be present to allow for variations in the molds. These thin-mold casting methods have been used since the 1960s in the manufacture of cast-iron engine blocks and cylinder heads for automotive applications. Increasingly in modern production, various automotive components are frequently made of aluminium, which for appropriately shaped components may be made either by sand casting or by die casting, the latter an accurate process that greatly reduces both materials use and machining and finishing costs. While the material and the processing setup is more expensive than the use of iron this is one of the most straightforward ways to reduce weight in a vehicle, important as a contributor to both fuel economy and acceleration performance. For front engine vehicles with rear wheel drive the improvement in weight distribution can improve both handling and traction. For all configurations weight saved in the engine is multiplied in that this enables use of lighter suspension components which in turn improves suspension response by reducing unsprung weightStarting in the early 1980s, some castings such as automotive engine blocks have been made using a sand casting technique conceptually similar to the lost wax process, known as the lost foam process. In this process, the pattern is made of polystyrene foam, around which the sand is packed, leaving the foam in place. When the metal is poured into the mold, the heat of the metal vaporizes the foam a short distance away from the surface of the metal, leaving the molding cavity into which the metal flows. The lost-foam process supports the sand much better than conventional sand casting, allowing greater flexibility in the design of the cast parts, with less need for machining to finish the casting. This technique was developed for the green sand mold casting of sculpture and was first adopted for large quantity commercial production by the Saturn Corporation. Vacuum molding process, popularly known as V-process, is a sand molding process, in which unbonded sand is held in place in the mold by a vacuum. In this process the pattern is covered by a tightly conforming thin sheet of plastic film which is applied with vacuum after being heated. The film, conforming to the shape of the pattern, may have a refractory coating applied which is dried before filling the flask with sand. A flask is placed over the plastic coated pattern, and is filled with free-flowing sand, with vibration for compaction. Another sheet of plastic is placed over the top of the sand in the flask and the flask is evacuated. The vacuum "hardens" (compacts) the sand so the pattern can be withdrawn, the vacuum holding the film to the pattern being released at this time. The other half of the mold is made the same way. After cores are set in place, the mold is closed and poured while still under vacuum. When the metal has solidified, the vacuum is turned off and the sand runs out freely, releasing the casting. The V-process is known for the high dimensional tolerances and good surface finish of the castings. Due to multiplicity of operations it is suitable for low to medium production volumes, depending on the amount of conveyorized equipment within the foundry. Because the sand never touches the pattern itself, there is almost no pattern wear. Minimal or zero draft allowance can be used on vertical surfaces. Shell molding process' principle is applied, when a heated (200 °C / 392 °F) metal pattern is covered with a mixture of sand and thermoset plastic (sometimes the sand is precoated with this mixture). This causes a skin of about 3.5 mm (0.125 in) of sand/plastic mixture to adhere to the pattern. This skin is removed from the pattern to form the "shell mold". The two halves of the shell mold are matched and secured together and the metal is poured in the shell to form the casting. Once the casting solidifies, the shell is broken and the sand can be regenerated. Shell molding process offers good surface finish, good dimensional tolerances, however the productivity is incomparable with the automatic green sand molding processes and fairly high capital investment is required.