Upload
kibria-prangon
View
234
Download
0
Embed Size (px)
Citation preview
7/30/2019 L8 Molding Sand
1/17
Prianka Binte ZamanAssistant ProfessorDept. of IPE, BUET
7/30/2019 L8 Molding Sand
2/17
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
7/30/2019 L8 Molding Sand
3/17
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.
7/30/2019 L8 Molding Sand
4/17
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
7/30/2019 L8 Molding Sand
5/17
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
7/30/2019 L8 Molding Sand
6/17
16/6
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.
7/30/2019 L8 Molding Sand
7/17
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
7/30/2019 L8 Molding Sand
8/17
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.
7/30/2019 L8 Molding Sand
9/17
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
7/30/2019 L8 Molding Sand
10/17
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
7/30/2019 L8 Molding Sand
11/17
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
7/30/2019 L8 Molding Sand
12/17
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
7/30/2019 L8 Molding Sand
13/17
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
7/30/2019 L8 Molding Sand
14/17
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
7/30/2019 L8 Molding Sand
15/17
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
7/30/2019 L8 Molding Sand
16/17
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.
7/30/2019 L8 Molding Sand
17/17