Sand Casting

A sand csting is a cast part, which is produced by forming a mold out of a sand mixture and pouring a casting liquid (often molten metal) into the mold. The mold is then air-cooled untill the metal solidifies, and the mold is removed. Thre are two main type of sand of molding. "Green sand", clay, moisture and some other additives. The "Air set" method uses dry sand that is bonded to other materials other than moist clay, using a fast curing adhesive. When these chemicals are used, they are collectively called "air set" and castings to distinguish these from "green sand" castings. Many chemicals and mixtures have been designed for this use.

With both methods, the sand mixture is packed around a master "pattern" in order to form a mold cavity. If necessary, a termporary plug is placed to form a channel for pouring the fluid to be molded. AIr-set molds often form a two-part mold having a top and bottom. The sand mixture istamped-down as it is added, and the final mold assembly is sometimes vibrated in order to compact the sand and fill any unwanted volds in the mold. Then the pattern is withdrawn along with the channel plug. Then the casting liquid (typically hot molten metal) is poured into the mold cavity left by the pattern. After the metal has solidfied and cooled, the casting is separated from the sand mold. The mold is often designed to be single-use. There is typically no mold release agent, and the mold is generally destroyed in the removal process.

The accuracy of the castng 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 recognize. Air-set molds can produce castings with much smoother surfaces. Surfaces can also be ground and polished 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.

From the design, provided by an engineer or designer, a skilled pattern maker builds a pattern of the object to be produced, using wood, metal, or plastic; other materials such as expanded polystyrene. Sand can even be ground, swept or even stickled into shape. The metal to be cast will contract during solidification, and this may be non-uniform due to uneven cooling. Therefore, the pattern must be slightly larger than the finished product, a difffrence known as contraction allowance. Pattern-makers are able to proudce suitable patterns using 'Contraction rules' (these are sometimes called "shrink allownace rulers" where the ruled markings are eliberately made to a larger spacing according to the percentage of extra length needed). Different scaled rules are used for different metals because different metals / alloys contract by differing amounts. Patterns also have core prints; these create registers within the molds, into which are placed sand cores. Such cores, sometimes reinforced by wires, are used to cereate under cut profiels and cavities which cannot be molded with the cope and drag, such as the interior passages of valves or cooling passages in motor blocks.

Paths for the entrance of metal, during the pouring (casting) process into the mold cavity cnstitute the runner system and include the spur, various feeders which maintain a good metal 'feed' and 'runners', and in-gates which attch the runner system to the casting cavity. Gas and steam generated during casting exit through the permeable sand or via the riser, are added either in the pattern itself, or as separate pieces.

Cope & drag (top and bottom halves of a sand mold), with cores in place on the drag.