Moulding Sand: Types and Constituents | Casting | Metallurgy

In this article we will discuss about:- 1. Definition of Moulding Sand 2. Types of Moulding Sand 3. Chief Constituents.

Definition of Moulding Sand:

The principal raw material used in moulding is the moulding sand because it provides several major characteristics that may not be obtained from other materials. Moulding 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, moulding sand differs considerably in different parts of the world. In nature, it is found on the bottom and banks of rivers and lakes. Moulding sand is classified into different categories according to the nature of its origin.

The principal constituents of moulding sands are as follows:

i. Silica (SiO₂)—86 to 90%,

ii. Alumina (Al₂O₃)—4 to 8%,

iii. Iron oxide (Fe₂O₃)—2 to 5% with smaller amounts of the oxides of Ti,

iv. Mn

v. Ca, and some alkaline compounds.

Natural Sand:

It is also called green sand and is collected from natural resources. It contains water as the only binder. It has the advantage of maintaining moisture content for a long time, having a wide working range of moisture content, permitting easy patching and finishing of moulds.

Synthetic Sand:

It is an artificial sand obtained by mixing relatively clay free sand, binder (water and bentonite) and other materials as required. It is a better moulding sand as its properties can be easily controlled by varying the mixture content.

Composition of green synthetic sand for steel castings is as under.

New silica sand — 25%, old sand — 70%, Bentonite — 1.5%, Dextrine — 0.25%, and moisture — 3 to 3.5%.

Composition of dry synthetic sand for steel castings is as under.

Net silica sand — 15%, old sand 84%, Bentonite — 0.5%, and moisture — 0.5%.

In addition to it, there are certain varieties of special sands such as Zirconite, Olivin etc. These special sands are more expensive than silica and are, therefore, used only where their use is justified.

Types of Moulding Sand:

Moulding sands may be classified, according to their use as under:

(i) Green Sand:

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

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

(ii) Dry Sand:

Dry sand moulding is employed for large castings. The moulds prepared in green sand are dried or baked to remove, almost, all moisture of the moist sand. The structure in the moulding boxes after drying becomes stronger and compact. Venting is therefore necessary but not to that extent, as in the case of green sand mould. For larger heavy moulds, cowdung, horse manure, etc. are mixed with the sand of coarser grains.

(iii) Loam Sand:

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

Loam sand contains upto 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 mould. 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.

(iv) 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 moulding sand to make mould facings.

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

(v) Backing Sand:

The old, repeatedly used mould­ing sand, black in colour due to addition of coal dust and burning or coming in contact with molten metal is known as backing sand or floor sand or black sand. It is used to fill in the mould at the back of facing layer. It is weak in bonding strength because the sharp edges of sand grain become rounded due to high temperature of molten metal and burn­ing of clay content.

(vi) System Sand:

This is used in machine moulding to fill the whole flask. Its strength, permeability, and refractoriness must be higher than those of backing sand.

(vii) Parting Sand:

The moulding boxes are separated from adhering to each other by spreading a fine sharp dry sand called ‘parting sand’. Parting sand is also used to keep the green sand from sticking to the pattern. It is clean clay- free silica sand. Burnt core sand could also be used for this purpose.

(viii) Core Sand:

It is used for making cores. It is silica sand mixed with core oil (linseed oil, rosin, light mineral oil and other binders). For the sake of economy pitch or flour and water may be used as core sand for large cores.

(ix) CO₂-Sand:

In CO₂ sand, the silica grains, instead of being coated with natural clay, are coated with sodium silicate. This mixture is first packed around the pattern and then hardened by passing CO₂ through the interstices for about a minute. The sand thus sets hard and produces a strong mould.

(x) Shell Sands:

Shell sands are synthetic sands coated with phenol or urea-formaldehyde resins and cured against a heated pattern to produce very strong, thin shell. No back up sand is required to provide support for the weight of the casting. Since, alloys solidify at high temperatures, the resins are not dissociated. But moulds disintegrate when casting has solidified due to breaking up of chemical bond by heat from solidifying casting.

(xi) Facing Sands:

Usually, facing sand is first applied on the pattern, so that only it comes in contact with the molten metal. This sand is refractory enough so as not to get fused and burnt on coming in contact with the metal.

(xii) Backing Sands:

These are applied as back up me­chanical support to facing sand. These are permeable to al­low gases to escape.

(xiii) Mould Washes:

These are slurries of fine ceramic grains. These are applied over the mould surfaces to minimise fusing of the facing sand grains. These also produce smoother surface on casting due to filling up of the inter­stices.

Chief Constituents of Moulding Sand:

Silica, clay (bond) and moisture are three chief constituents of the moulding sand. Silica in the form of granular quartz (itself a sand) is the chief constituent of moulding sand. Silica sand contains from 80 to 90% silicon oxide and is characterised by high softening temperature and thermal stability. It imparts refractoriness, chemical resistivity and permeability to sand.

It is specified according to size and shape. Sand grains could be fine, medium, or coarse as regards size, and could be rounded, semi-angular or compounded, as regards shape. Fine sand is desirable for small and intricate castings. As fine grains lie close, permeability is poor. Medium sized sand is used in bench work and light floor work. Coarse particles are used for large castings to permit gases to escape.

Grain size is determined by passing sand through screens of sieves. Rounded grains have least contact with one another and lack in strength; permeability is high. Sub- angular grains are comparatively less permeable than round grains. Angular grains having defined edges give more strength and less permeability. Compounded grains being hard lumps are not preferred.

Silicon oxide is obtained from quartz rocks or by the decomposition of granite (composed of quartz and felspar).

The felspar when decomposed, becomes clay which imparts plasticity to the moulding sand in the moist state, i.e. imparts necessary bonding action and strength in the presence of moisture and increases its strength after drying. Normally, the amount of clay found in silica sand varies from 6 to 10%.

Clay actually consists of fine silt (mineral deposit having no bonding property) and fine clay, which imparts necessary bonding strength to mould sand so that mould does not lose its shape after ramming. However, it decreases permeability. Too much of clay causes cracking of the mould after drying.

Oxides of iron magnesia, soda potash, lime and water are the other substances, which are found in the moulding sand. A good moulding sand contains impurities below 2 per cent. Green sand moulding is carried with sand of low moisture content (3 to 5%). In dry sand moulding, more water is present when making mould, as it is beneficial in promoting dry bond strength after storing.

Many times coal dust is also added which makes the sand more open and helps to cool the mould after metal is poured. It absorbs fairly high amount of heat, preventing the sand grains from overheating and fusing. It also releases CO₂ whose protective film helps to keep the metal and sand separated from each other.

Fig. 3.17 shows four type of sand grains. The success of a casting process depends to a great extent on compressive strength, permeability, (gas flow rate through sand specimens under specified differential pressure across it), deformation, flow ability, (ability of sand to flow around and over the pattern during ramming) and refractoriness of the moulding sand. Fig. 3.18 shows, how the moisture content in moulding sand (for a given sand-clay ratio) affects all these important properties of the moulding sand.