Properties of moulding sands
The important properties are:
1. Strength:
· The sand should have adequate strength in its green, dry and hot states
· Green strength is the strength of sand in the wet state and is required for making possible to prepare and handle the mould.
·
If the metal is poured into a green mould the sand adjacent to the
metal dries and in the dry state it should have strength to resist
erosion and the pressure of metal.
· The strength of the sand that has been dried or basked is called dry strength
·
At the time of pouring the molten metal the mould must be able to
withstand flow and pressure of the metal at high temperature otherwise
the mould may enlarge, crack, get washed or break
· Strength of the moulding sand depends on:
1. Grain size and shape
2. Moisture content
3. Density of sand after ramming
·
The strength of the mould increases with a decrease of grain size and
an increase of clay content and density after ramming. The strength also
goes down if moisture content is higher than an optimum value.
2. Permeability:
·
The moulding sand must be sufficiently porous to allow the dissolved
gases, which are evolved when the metal freezes or moisture present or
generated within the moulds to be removed freely when the moulds are
poured. This property of sand is called porosity or permeability.
3. Grain size and shape:
·
The size and shape of the grains in the sand determine the application
in various types of foundry. These are three different sizes of sand
grains.
1. Fine
2. Medium
3. Coarse
·
Fine sand is used for small and intricate castings. Medium sand is used
for benchmark and light floor works. If the size of casting is larger
coarse sand is used
· Sand having fine, rounded
grains can be closely packed and forms a smooth surface. Although
fine-grained sand enhances mould strength.
4. Thermal stability:
·
The sand adjacent to the metal is suddenly heated and undergoes
expansion. If the mould wall is not dimensionally stable under rapid
heating, cracks, buckling and flacking off sand may occur.
5. Refractoriness:
·
Refractoriness is the property of withstanding the high temperature
condition moulding sand with low refractoriness may burn on to the
casting
· It is the ability of the moulding
material to resist the temperature of the liquid metal to be poured so
that it does not get fused with the metal. The refractoriness of the
Silica sand is highest.
6. Flowability:
·
Flowability or plasticity is the property of the sand to respond to the
moulding process so that when rammed it will flow all around the
pattern and take the desired mould shape. High flowability of sand is
desirable for the sand to get compacted to a uniform density and to get
good impression of the pattern in the mould.
· Flowability is also very important in machine moulding
· Flowability of sand increases as clay and water content are increased.
7. Sand texture:
· As mentioned earlier the texture of sand is defined by its grain size and grain size distribution.
·
The texture chosen for an application should allow the required
porosity, provide enough strength and produce the desired surface finish
on the casting.
8. Collapsibility:
·
The moulding sand should collapse during the contraction of the
solidified casting it does not provide any resistance, which may result
in cracks in the castings. Besides these specific properties the
moulding material should be cheap, reusable and should have good thermal
conductivity
9. Adhesiveness:
·
It is the important property of the moulding sand and it is defined as
the sand particles must be capable of adhering to another body, then
only the sand should be easily attach itself with the sides of the
moulding box and give easy of lifting and turning the box when filled
with the stand.
10. Reusability:
·
Since large quantities of sand are used in a foundry it is very
important that the sand be reusable otherwise apart from cost it will
create disposal problems
11. Easy of preparation and control:
· Sand should lend itself to easy preparation and control by mechanical equipment
12. Conductivity:
· Sand should have enough conductivity to permit removal of heat from the castings.
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