l8 molding sand

7/30/2019 L8 Molding Sand

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Prianka Binte ZamanAssistant ProfessorDept. of IPE, BUET


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Molding Sand

The primary function of any molding material is to maintain the

shape of the mold cavity until the metal solidifies.

Molding sand is the principal raw material used in molding because it

possesses several major characteristics required for molding.

Molding Sand CompositionThe main ingredients of any molding sand are:

Base Sand: Have the necessary refractory properties to withstand

the intense heat of the molten metal

Binder or Bonding Material: To hold grain together

Moisture or Water: To coalesce the grains and binder into a plastic

molding material


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Base SandSilica sand is most commonly used base sand. Other base sands that

are also used for making mold are zircon sand, Chromite sand, andolivine sand. Silica sand is cheapest among all types of base sand and it

is easily available. It is reusable and capable of giving good detail.

BinderBinders are of many types such as:

1.Clay binders,2.Organic binders and

3.Inorganic binders

Clay binders are most commonly used binding agents mixed with the

molding sands to provide the strength. The most popular clay types are:Kaolinite or fire clay (Al2O

32 SiO

22 H

2O) and Bentonite (Al

2O

34

SiO2

nH2O). Of the two the Bentonite can absorb more water which

increases its bonding power.


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MoistureClay acquires its bonding action only in the presence of the required

amount of moisture. When water is added to clay, it penetrates themixture and forms a microfilm, which coats the surface of each flake of

the clay. The amount of water used should be properly controlled.

This is because a part of the water, which coats the surface of the clay

flakes, helps in bonding, while the remainder helps in improving the

plasticity.

Molding Sand Constituent Weight Percent

Silica sand 92Clay (Sodium Bentonite) 8

Water 4

Table: A Typical Composition of Molding Sand


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Advantages of synthetic Sand over natural sand:More uniform grain size

Higher refractoriness

Improved permeability

Requiring less binder

Easier control of properties

Semi-skilled workers can work on machine molding

The need of less storage space since one kind of sand may suffice for different

kinds of casting

The molding sand is classified into two categories according to the nature of its

origin.Natural or Green sand: It is collected from natural resources like river beds or isdug from pits. It contains the only binder as water. It has the advantages of

maintaining moisture content for a long time, wide working range of moisture

content and permits easy patching and finishing of molds.

Synthetic sand: It is an artificial sand prepared in the foundry by mixing clay freesand, binder, other additive materials as required and water. Its properties can beeasily controlled by mixture content.

Types of Molding Sand


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Types of Molding SandThe molding sands are classified according to their use in the following categories:

Green sand: The sand in its natural condition with moisture to give it enough strength is formedas green sand.

Dry sand: The green sand mold is not very suitable for large castings. So they are dried in somesuitable oven to evaporate the excess moisture and to give then extra strength.

Facing sand: The sand which remains around the pattern is the facing sand. So it forms the faceof the mold and comes in direct contact with the molten metal.

Parting sand: The cope and drag are placed over each other after the patterns are placed in therespective haves of the flask and the necessary operations of mold making done. There is every

possibility that the sand in the cope half and drag half may stick together and with the pattern. To

eliminate this possibility parting sand is sprinkled over the parting surface of the cope and drag

and the pattern.

Core sand: The sand used for making cores is called core sand. This sand has high silica contentand is mixed with selected binders.


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Properties required in molding materials:

Refractoriness

Green Permeability

Green StrengthDry Strength

Hot Strength

CollapsibilityProperties are determined by:

Grain Fineness

Grain ShapeClay Content

Moisture Content

Properties of Sand


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RefractorinessIt is the property by virtue of which the sand can withstand high temperatures

without fusing. This property is very important because always molten metals

with high temperature are poured in it and poor refractoriness would cause

fusion of the sand. As a result slag will form which will come on the surface of

the casting to spoil it. The refractoriness of the silica sand is highest.

Green PermeabilityPermeability is also called porosity. It is the property by virtue of which themolding sand permits the escape of gasses and steam through it. As the hot

molten metal is poured in the cavity, steam and gasses are formed due to the

heat of the metal. These gasses must escape to atmosphere otherwise if either

the mold may burst or blow holes in the casting will be formed.

Grain fineness is measured by passing sand through standard sieves,each with a certain number of openings per inch. Grain size of sand is

designed by a number that indicates the average sizes as well as the

proportions of smaller or larger grains in the mixture.


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Finer grains in a mold impart a smoother finish to a casting. Permeability

decreases as the grains and the void between them becomes smaller. The

same conditions results from a large proportions of fine grains in a mixture.

For large castings that require coarse sand for high permeability.

Shapes of grains: Angular and Rounded. Sharp angular grains cannot packtogether as closely and give a higher permeability than rounded grains.

Figure: Relationship

between

Permeability of

molding sand withmoisture content

and grain shape


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Type and amount of binder effects on the permeability of sand.

Over a wide range of moisture content, bentonite was found to give

more permeability than fire clay.

Figure: Permeability of two kinds of clay for molding sand with various

amounts of moisture


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Green Strength or cohesivenessGreen strength represents the ability of the sand to hold to the shape of the

mold when metal is poured into the mold. The molding sand that contains

moisture is termed as green sand. The green sand particles must have the ability

to cling to each other to impart sufficient strength to the mold. The green sand

must have enough strength so that the constructed mold retains its shape.

The finer the sand grains, the larger

the surface area of a bulk, and the

larger the amount of binder needed to

cover the area. The contact and the

bonds between grains are more

numerous, and thus green strength is

higher with finer grains. Figure shows

that, the grain size becomes larger, the

green strength decreases under

normal conditions.Figure: Green strength of molding sand inrelation to the size of the grains


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Round grains packs together much more closely than the angular grains

and as a result are bonded together with a higher green strength thanangular grains.

Figure: Green strength of two molding sand with different clay

binders at different moisture levels


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Some binders provide higher green strength than the others. The green

strength increases in proportion to the amount of binder in a molding sand,but too much binder is detrimental to permeability.

Figure: Green strength of molding sand to moisture content and grain shape


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The effect of the moisture on green strength is similar to the effect on

permeability. Green strength increases with the first additions of moisture,

reaches a maximum strength and then starts to decrease. An excess amount

of moisture has a weakening effect, even nullify the influence of grain size.

Figure: Green strength of molding sand varies for several sizes of grains


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Dry StrengthDry strength is the strength of sand that has been dried or baked. In

general, dry strength varies in the same way as green strength with grainfineness, grain shape and moisture content. Different binders can

affect dry strength and green strength differently.

When the molten metal is poured in the mold, the sand around the

mold cavity is quickly converted into dry sand as the moisture in thesand evaporates due to the heat of the molten metal. At this stage the

molding sand must posses the sufficient strength to retain the exact

shape of the mold cavity and at the same time it must be able to

withstand the metallostatic pressure of the liquid material.

FlowabilityThe ability for the sand to flow into intricate details and tight corners

without special processes or equipment


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Hot StrengthAs soon as the moisture is eliminated, the sand would reach at a high

temperature when the metal in the mold is still in liquid state. The

strength of the sand that is required to hold the shape of the cavity at

high temperature is called hot strength.

CollapsibilityThis is the ability of the sand to be easily stripped off the casting after ithas solidified. Sands with poor collapsibility will adhere strongly to the

casting. When casting metals that contract a lot during cooling or with

long freezing temperature ranges a sand with poor collapsibility will

cause cracking and hot tears in the casting. Special additives can be

used to improve collapsibility.

Besides these specific properties the molding material should be

cheap, easily available, reusable and should have good thermal

conductivity.


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l8 molding sand

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