Sand Casting

INTRODUCTION:

A sand casting or a sand molded casting is 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: (see sketch below):

1. Place a pattern in sand to create a mold.
2. Incorporate a gating system.
3. Remove the pattern.
4. Fill the mold cavity with molten metal.
5. Allow the metal to cool.
6. Break away the sand 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 materials other than 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.

   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.

Types of sand casting molds:-

1: Green Sand

These molds are made of wet sands that are used to make the mold's shape. The name comes from the fact that wet sands are used in the molding process.

2: Cold Box

Uses organic and inorganic binders that strengthen the mold by chemically adhering to the sand. This type of mold gets its name from not being baked in an oven like other sand mold types. This type of mold is more accurate dimensionally than green-sand molds but are more expensive.

3: No Bake Molds

No bake molding is a type of molding used for the casting of molten metals. Like sand casting it is an expendable mold that is made up of sand. The primary difference is that it keeps its form from having a liquid resin mixed with the sand at room temperature to help keep its form. Because no heat is involved it called a cold-setting process. This type of molding also produces a better surface finish than other types of sand molds, and due to the binder does not need to be baked in an oven. Common flask materials that are used are wood, metal, or plastic. Common metals cast into no bake molds are brass, ferric, and aluminium.

Patterns:

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 a plastic such as expanded polystyrene. Sand can be ground, swept or even strickled 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 difference known as contraction allowance. Pattern-makers are able to produce suitable patterns using 'Contraction rules' (these are sometimes called "shrink allowance rulers" where the ruled markings are deliberately made to a larger spacing according to the percentage of extra length needed). Different scaled rules are used for different metals because each metal and alloy contracts by an amount distinct from all others. Patterns also have core prints that create registers within the molds into which are placed sand 'cores. Such cores, sometimes reinforced by wires, are used to create under cut profiles and cavities which cannot be molded with the cope and drag, such as the interior passages of valves or cooling passages in engine blocks.

Paths for the entrance of metal into the mold cavity constitute the runner system and include the sprue, various feeders which maintain a good metal 'feed', and in-gates which attach the runner system to the casting cavity. Gas and steam generated during casting exit through the permeable sand or via risers, which are added either in the pattern itself, or as separate pieces.

Molding box and materials:

A multi-part molding box (known as a casting flask, the top and bottom halves of which are known respectively as the cope and drag is prepared to receive the pattern. Molding boxes are made in segments that may be latched to each other and to end closures. For a simple object—flat on one side—the lower portion of the box, closed at the bottom, will be filled with prepared casting sand or green sand—a slightly moist mixture of sand and clay. The sand is packed in through a vibratory process called ramming and, in this case, periodically screeded level. The surface of the sand may then be stabilized with a sizing compound. The pattern is placed on the sand and another molding box segment is added. Additional sand is rammed over and around the pattern. Finally a cover is placed on the box and it is turned and unlatched, so that the halves of the mold may be parted and the pattern with its sprue and vent patterns removed. Additional sizing may be added and any defects introduced by the removal of the pattern are corrected. The box is closed again. This forms a "green" mold which must be dried to receive the hot metal. If the mold is not sufficiently dried a steam explosion can occur that can throw molten metal about. In some cases, the sand may be oiled instead of moistened, which makes possible casting without waiting for the sand to dry. Sand may also be bonded by chemical binders, such as furane resins or amine-hardened resins.

Cores:

To produce cavities within the casting—such as for liquid cooling in engine blocks and cylinder heads—negative forms are used to produce cores. Usually sand-molded, cores are inserted into the casting box after removal of the pattern. Whenever possible, designs are made that avoid the use of cores, due to the additional set-up time and thus greater cost.

With a completed mold at the appropriate moisture content, the box containing the sand mold is then positioned for filling with molten metal—typically iron, steel, bronze, brass, aluminium, magnesium alloys, or various pot metals alloys, which often include lead, tin, and zinc. After filling with liquid metal the box is set aside until the metal is sufficiently cool to be strong. The sand is then removed revealing a rough casting that, in the case of iron or steel, may still be glowing red. When casting with metals like iron or lead, which are significantly heavier than the casting sand, the casting flask is often covered with a heavy plate to prevent a problem known as floating the mold. Floating the mold occurs when the pressure of the metal pushes the sand above the mold cavity out of shape, causing the casting to fail.

After casting, the cores are broken up by rods or shot and removed from the casting. The metal from the sprue and risers is cut from the rough casting. Various heat treatments may be applied to relieve stresses from the initial cooling and to add hardness—in the case of steel or iron, by quenching in water or oil. The casting may be further strengthened by surface compression treatment—like shot peening—that adds resistance to tensile cracking and smooths the rough surfa