Sand Casting

Castings vs Forging Comparison Chart

castings vs forging comparison chart

Sand Casting is typically used to produce larger products utilizing Steel, Iron, Brass, and Aluminum. Molten metal is then poured into a mold cavity formed out of sand. The cavity in the sand is formed by using a pattern which is an approximate duplication of the actual part. Patterns are usually made out of wood. The cavity is contained within the sand which is housed in a box called the flask. Within the mold, one can have various elements to allow for various internal features, such as ……A Core which is a sand shape inserted into the mold to produce the internal features of the part such as holes or internal passages; A Core Print is the area added to the pattern, core or mold that is used to locate and support the core within the mold; A Riser is an extra void created in the mold to contain excessive molten material. The purpose for this is to feed the molten metal to the mold cavity as the molten metal solidifies and shrinks, and thereby prevents voids in the main casting.

In a two part mold, which is typical of sand castings, the upper half, including the top half of the pattern, flask, and core is called the cope and the lower half is called the drag. The parting line or the parting surface is the surface that separates the cope and drag. The drag is first filled partially with sand, and the core print, the cores, and the gating system are placed near the parting line. The cope is then assembled to the drag, and the sand is poured on the cope half, covering the pattern, core and gating system. The sand is compacted by vibration and mechanical means. Next, the cope is removed from the drag, and the pattern is carefully removed. The object is to remove the pattern without breaking the mold cavity.

 The molten metal is then poured into the pouring cup, which is part of the gating system that supplies the molten material to the mold cavity. The vertical part of the gating system connected to the pouring cup is called the sprue, and the horizontal portion is called the runners. From there it goes to the multiple points where it is introduced to the mold cavity, and these are called gates. Additionally, there are extensions to the gating system called vents that provide the path for the built up gases and the displaced air to vent to the atmosphere.

The mold cavity is usually made oversize to allow for the metal contraction as it cools down to room temperature. This is achieved by making the pattern oversize. To account for shrinking, the pattern must be made oversize by these factors, on the average. These are linear factors and apply in each direction. These shrinkage allowances are only approximate, because the exact allowance is determined by the shape and size of the casting as well as the material being used.

Sand castings generally have a rough surface sometimes with surface impurities, and surface variations. A machining (finish) allowance is usually added to account for this.