Core and core box making for casting process

Information on core making which finds its extensive use in foundry industries.

1. CORE
SY PROD-2017-18

2. CORE
•Usedformakingcavitiesandhollow
projections.
•Allsidesofcorearesurroundedbythemolten
metalandarethereforesubjectedtomuch
moreseverethermalandmechanical
conditionsandasaresultthecoresand
shouldbeofhigherstrengththanthe
mouldingsand.

3. Natural and Synthetic molding sand
Natural molding sand:
This is ready for use as it is dug from the ground. Good natural
molding sand are obtained from Albany, New york etc.
The following average compositions are seen
in natural molding sand:
.>5.5% silica grains, 21.7% clay content, 12.8% undesirable impurities.
>Too much clay content and other impurities fill up the gaps between
the sand grains.
> This will hinder the necessary passage of steam and other gases
during pouring of the mold.
>Synthetic molding sand Synthetic molding sand is made by mixing
together specially selected high quality clay free silica, with about 5%
of clay.
>They are tailor made to give most desirable results

4. Some of the advantages of synthetic
molding sand are:
1.Refractory grain sizes are more uniform,
2. Higher refractoriness (= 3000oF),
3. less bonding agent is required (about 1/3rd of
the clay percentage found in natural molding
sand),
4. More suitable for use with mechanical
equipment
Advantages of natural molding sand: 1.
moisture content range is wide,
2. molds can be repaired easily

5. Desired characteristics of a core
•Green Strength: A core made of green sand
should be strong enough to retain the shape till
it goes for baking.
•Dry Strength: It should have adequate dry
strength so that when the core is placed in the
mould, it should be able to resist the metal
pressure acting on it.
•Refractoriness: Since in most cases, the core is
surrounded all around it is desirable that the
core material should have higher refractoriness.

6. •Permeability: Gases evolving from the molten metal
and generated from the mould may have to go through
the core to escape out of the mould. Hence cores are
required to have higher permeability.
•Permeability Number: The rate of flow of air
passing through a standard specimen under a
standard pressure is termed as permeability
number.
•The standard permeability test is to measure time
taken by a 2000 cu cm of air at a pressure typically
of 980 Pa (10 g/cm
2
), to pass through a standard
sand specimen confined in a specimen tube. The
standard specimen size is 50.8 mm in diameter and
a length of 50.8 mm.

7. •Then, the permeability number, R is obtained by
Where V= volume of air = 2000 cm
3
H = height of the sand specimen = 5.08 cm
p = air pressure, g/cm
2
A = cross sectional area of sand specimen = 20.268 cm
2
T = time in minutes for the complete air to pass through
Inserting the above standard values into the expression,
we getVH
R
pAT
= 501.28
.
R
pT
=

8. PERMEABILITY NUMBER
•Calculate the permeability number of sand if it
takes 1 min 25 s to pass 2000 cm
3
of air at a
pressure of5 g/cm
2
through the standard
sonample.
Solution:2
5.0/
1min251.417min
501.28
70.75
51.417
pgcm
Ts
R
=
==
==


9. IES-2012
What is permeability? Permeability is more
important in the basic process of sand casting
than porosity. Give one important reason for
this feature.
[2 marks]

10. SOME OF THE PROPERTIES
•Collapsibility: At the time of cooling, casting shrinks,
and unless the core has good collapsibility (ability to
decrease in size) it is likely to provide resistance against
shrinkage and thus can cause hot tears.
•Friability: The ability to crumble should be a very
important consideration at the time of removal.
•Smoothness: Surface of the core should be smooth
for good finish to the casting.
•Low Gas Emission

11. CORE SANDS
•Used clay free silica sand.
•Binders used are linseed oil, core oil, resins, dextrin,
molasses, etc.
•Core oils are mixtures of linseed, soy, fish and
petroleum oils and coal tar.
•The general composition of a core sand mixture could
be core oil (1%) and water (2.5 to 6%).

30. Metal Casting
Dr.D. N. Raut