Molding Sand - Types of Properties and its Classification

Definition of Moulding Sand:

The principal raw material used in molding is molding sand because it provides several major characteristics that may not be obtained from other materials. Molding sand is defined as granular particles resulting from the breakdown of rocks, due to the action of natural forces, such as frost, wind, rain, heat, and water currents. Rocks have a complex composition and sand contains most of the elements of the rocks.

Due to this reason, molding sand differs considerably in different parts of the world. In nature, it is found on the bottom and banks of rivers and lakes. Molding sand is classified into different categories according to the nature of its origin.

Molding Sand - Types of Properties and its Classification

Molding is the process of making a cavity or mold out of the sand by means of a pattern. The molten metal is poured into the molds to produce the casting.

Properties of molding sand.

1: Porosity or permeability

It is the property of sand that permits the steam and other gases to pass through the sand mold. The porosity of sand depends upon its grain size, grain shape, moisture, and clay components are the molding sand. If the sand is too fine, the porosity will be below.

2: Plasticity

It is that property of sand due to which it flows to all portions of the molding box or flask. The sand must have sufficient plasticity to produce a good mold.

3: Adhesiveness

It is the properties of sand due to it adheres or clings to the sides of the molding box.

4: Cohesiveness

It is the property of sand due to which the sand grains stick together during ramming. It is defined as the strength of the molding sand.

5: Refractoriness

The property enables it to resist the high temperature of the molten metal without breaking down or fusing.

Classification of Moulding sand according to their use:

1: Greensand

When sand is in its natural (more or less moist) state, it is referred to as green sand. It is a mix­ture of silica sand, with 18 to 30% clay and 6 to 8% water. The clay and water give bonding strength to green sand.

It is fine, soft, light, and porous. Being damp, it retains the shape given to it under pressure during squeezing.

As the mold becomes dense by ramming, the structure is made porous by venting. Sharp edges are avoided in green sand molding, because these being weak, break when hot metal is poured.

Greensand is generally used for casting small or medium-sized molds. Larger output can be obtained from a given floor space as the cost and delay involved in drying the molds are saved. Coal dust is mixed in green sand to prevent defects in castings.

2: Dry Sand

The green sand molds when baked or dried before pouring the molten metal are called dry sand molds. The sand of this condition is called dry sand. 

The dry sand molds have greater strength, rigidity, and thermal stability. 

These molds are used for large and heavy casting.

3: Loam Sand

It is a mixture of clay and sand milled with water to a thin plastic paste, from which, molds are built upon a backing of soft bricks.

Loam sand contains up to 50% clay and dries hard. It also contains fire clay. It must be sufficiently adhesive to hold on to the vertical surfaces of the rough structure of the mold. Chopped stray and manure are commonly used to assist in binding. The moisture content is from 18 to 20%.

Loam is dried very slowly and completely before it is ready for casting. It is used for casting larger regular-shaped castings like chemical pans, drums, etc.

4: Facing Sand

It is used directly next to the sur­face of the pattern and it comes into contact with the molten metal. Since it is subjected to the most severe conditions, it must possess high strength and refractoriness. It is made of silica sand and clay, without the addition of used sand.

Different forms of carbon known as facing materials, (e.g., plumbago powder, Ceylon lead, or graphite) are used to prevent the metal from burning into the sand. Sometimes they are mixed with 6 to 15 times fine molding sand to make mold facings.

The facing sand layer in a mold usually ranges from 20 to 30 mm. Facing sand comprises 10 to 15% of the whole amount of mold sand.

5: Backing or Floor Stand

Sand used to back up the facing sand and not used next to the pattern is called backing sand. The sand which has been repeatedly used may be employed for this purpose. It is also known as black sand due to its color.

6: System Sand

Sand employed in mechanical sand preparation and handling system is called system sand. This sand has high strength, permeability, and refractoriness.

7: Parting Sand

Sand employed on the faces of the pattern before the molding is called parting sand. The parting sand consists of dried silica sand, sea sand, or burnt sand.

8: Core Sand

The cores are defined as sand bodies used to form the hollow portions or cavities of desired shape and size in the casting. Thus the sand used for making these cores is called core sand. It is sometimes called oil sand. It is the silica sand mixed with linseed oil or any other oil as a binder.

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