CHAPTER 3
MOLD MIXTURES
Ass.Pr.Dr. Nguyen Ngoc Ha
1. INTRODUCTION
• Mold Materials: Refractories, Binders, Additives
• Mold Mitures: Refractories + Binders + Additives
• Binders:
– Organic resins (e g , phenolic resins)
– Inorganic binders (e g , sodium silicate and phosphate
• Additives: are sometimes combined with the
mixture to enhance strength and/or permeability
1.1. Desirable Properties of Sand-Based
Molding Materials
• Inexpensive in bulk quantities
• Retains properties
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through transportation and
storage
• Uniformly fills flask or container
• Can be compacted or set by simple methods
• Sufficient elasticity to remain undamaged during
pattern removal
• Can withstand high temperatures and maintain
dimensional stability until solidification
• Sufficiently permeable to allow gases to escape
1.1. Desirable Properties of Sand-Based
Molding Materials
• Sufficiently dense to prevent metal
penetration
• Sufficiently cohesive to prevent wash-out of
mold material into the pour stream
• Chemically inert to the metal being cast
• Can yield to solidification and thermal
shrinkage, preventing hot tears and cracks
• Recyclable
1.2. Desirable Mold Properties
• Strength - to maintain shape and resist erosion
• Permeability - to allow hot air and gases to pass
through voids in sand
• Thermal stability - to resist cracking on contact
with molten metal
• Collapsibility - ability to give way and allow
casting to shrink without cracking the casting
• Reusability - can sand from broken mold be
reused to make other molds?
2. MOLD SAND
2.1. Introdution
• 80-98% mass of mold mixture
• Grain size: 0,016-2mm
• The most important properties:
- Chemical composition
- Size and shape of grains
- Melting temperature
- Thermal expansion
- Bulk density
2.2. Selection of sand
• Most sand casting operations use silica sand
(SiO2), because it is inexpensive and is suitable
as mold material because of its resistance to
high temperatures. There are two general
types of sand: naturally bonded and synthetic
sand. Because its composition can be
controlled more accurately most foundries
prefer synthetic sand.
2.2. Selection of sand
• Several factors are important in the selection
of sand for sand molds. Sand having fine,
rounded grains can be closely packed and
forms a smooth mold surface. Good
permeability of molds and cores allows gases
and steam evolved during casting to escape
easily.
• The selection of sand involves certain
tradeoffs with respect to properties. For
example, fine-grained sand enhances mold
strength, but the fine grains also lower mold
permeability.
2.2. Selection of sand
• Sand is typically conditioned before use.
Mulling machines are used to uniformly mull
(mix thoroughly) sand with additives. For
example clay 9bentonite) is used as a cohesive
agent to bond sand particles, giving the sand
strength.
• Zircon (ZrSiO4), olivine (Mg2SiO4), and iron
silicate (Fe2SiO4) sands are often used in steel
foundries for their low thermal expansion.
Chromate (FeCr2O4) is used for its high heat
transfer property.
2.3. Sand Parameters
• Grain Size – measured by sifting sand through sieves
• Moisture Content – measured with moisture meter
• Clay Content – measured by weighing a sample of
sand before / after washing
• Permeability – AFS permeability number measured
using “standard rammed sample”
• (Green) Compressive Strength – measure of mold
strength before pouring
• Hardness – resistance of packed sand to penetration
Grain Shapes of Sands
Effect of grain size and shape on
mould quality
Bigger grain size results in a worse
surface finish
Irregular grain shapes produce
stronger mold
Larger grain size ensures better
permeability
Effect of moisture, grain size and shape on
mould quality
2.4. Sand Properties and related Defects
• The characteristics of the sand granules can be
very influential in determining the properties
of the molding material. Round grains give
good permeability and minimize the amount
of clay required because of their low surface
area.
• Angular sands give better green strength
because of the mechanical interlocking of the
grains.
2.4. Sand Properties and related Defects
• Large grains provide good permeability and
better resistance to high temperature melting
and expansion, while fine-grained sands
produce a better surface finish on the final
casting.
• Uniform-size sands give good permeability,
while a wide distribution of sizes enhances
surface finish.
2.4. Sand Properties and related Defects
• When hot metal is poured into a silica sand
mold, the sand becomes hot, undergoes one
or more phase transformations, and has a
substantial expansion in volume.
• Because sand is a poor' thermal conductor,
only the sand that is adjacent to the mold
cavity becomes hot and expands.
2.4. Sand Properties and related Defects
• The remaining material stays fairly cool, does
not expand, and provides a high degree of
mechanical restraint. Because of this uneven
heating, the sand at the surface of the mold
cavity may buckle or fold.
• Castings having large, flat surfaces are more
prone to sand expansion defects since a
considerable amount of expansion must occur
in a single, fixed direction.
2.4. Sand Properties and related Defects
• Sand expansion defects can be minimized in a
number of ways. Certain sand geometries
permit the grains to slide over one another,
thereby relieving the expansion stresses.
• Excess clay can be added to absorb the sand
expansion, or volatile additives, such as
cellulose, can be added to the mix.
• As the sand becomes hot, the cellulose bums,
creating voids that can accommodate the sand
expansion.
2.4. Sand Properties and related Defects
• Castings can also contain voids that form
where the molten metal is held back by
trapped or evolved gas.
• These are usually attributed to low sand
permeability and/or large amounts of gas
evolution caused by high moisture or
excessive amounts of volatiles.
• If adjustments to the mold composition are
not sufficient to eliminate the voids, vent
passages may have to be cut, a procedure that
adds significantly to the mold-making cost.
2.4.Sand Properties and related Defects
• The molten metal can also penetrate between
the sand grains, causing the mold material to
become embedded in the surface of the
casting.
• Penetration can be the result of high pouring
temperatures (excess fluidity), high metal
pressure, or the use of high-permeability
sands with coarse, uniform particles.
• Fine-grained materials, such as silica flour, can
be used to fill the voids, but this reduces
permeability and increases the likelihood of
gas and expansion defects.
2.5. Sand Quality Tests
• Periodic tests are necessary to determine the
essential qualities of foundry sand. Various
tests are designed to determine the following
properties of molding sand.
a) Hardness Test (Mold Hardness): A spring
loaded (2.3 N) steel ball 5.08 mm in diameter
is pressed into the surface of the mold and
depth of penetration is recorded as hardness.
Medium hardness is about 75.
2.5. Sand Quality Tests
b) Fineness Test: It is
used to obtain
percentage distribution
of grain sizes in the sand.
Sand is cleaned and
dried to remove clay. It is
placed on graded sieves,
which are located on a
shaker. Standard sieve
sizes (mesh) are
6,12,20,30,40,50,70,100,
200 and 270. Shaking
time is 15 minutes
vibrator
6
12
270
2.5. Sand Quality Tests
c) Moisture Content: Measure the weight of the
given sand sample. Dry it around 1000C and then
weigh it again. Calculate the percentage.
d) Clay Content: A sample of sand is dried and
then weighed. Then clay is removed by washing
the sand with caustic soda which has absorbed
the clay. Sand is dried and weighed again. The
percentage gives the clay content.
e) Strength Test: Most common compressive test.
A universal strength tester loads a 50 mm long 50
mm diameter specimen by means of dead weight
pendulum with a uniform loading rate.
2.5. Sand Quality Tests
f) Permeability: It is
measured by the
quantity of air that
passes through a given
sample of sand in a
prescribed time under
standard pressures.
g) Refractoriness Test:
High temperature
withstanding ability of
sand is measured.
piston
2.6. Types of mold sand
• Silica (SiO2)
• Zircon (ZrSiO4)
• Olivine (Mg2SiO4)
• Iron silicate (Fe2SiO4)
• Chromate (FeCr2O4)
a. Silica Sand
• Silica:
- SiO2 - = 2,5-2,8 kg/dm
3
- Tmelting 1680 – 1713
0C
- Color: grey, yellow, black
• Felspat:
- MeO.Al2O3.6SiO2 (Me: K,Na) -Tmelting=1170-1550
0C
• Mica:
- K2O.3Al2O3.6SiO2.H2O - Tmelting= 1150-1400
0C
a. Silica Sand
• Ferrous Oxides: hematite (Fe2O3), magnetite
(FeO.Fe2O3), ilmenite (FeO.TiO2)
• Ferrous hydroxides
• Cacbonates:
- MgCO3, CaCO3
- Tm= 500-900
0C
• NaCl, KCl
• Clay
2.6. Types of mold sand
b. Zircon Sand
• ZrSiO4
• High Tmelting: 2400C
• High Density: 4.7
• Low thermal expansion
• High cost
c. Olivin Sand
• Mg2SiO4
• Tmelting= 1750-1830
0C
• Low thermal expansion
3. BINDERS USED WITH FOUNDRY SAND
• Sand is held together by a mixture of water
and bonding clay
– Typical mix: 90% sand, 3% water, and 7% clay
• Other bonding agents also used in sand
molds:
– Organic resins (e g , phenolic resins)
– Inorganic binders (e g , sodium silicate and
phosphate)
3.1. Mold Clay
a. Bentonite Clay
• The main mineral: Montmorillonite
(Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·nH2O
• Chemical composition:
(Na,Ca)(Al,Mg)6(Si4O10)3(OH)6.nH2O
• High swelling in water
• Sodium bentonite; Calcium bentonit
• Bentonite has been widely used as a foundry-
sand bond in iron and steel foundries. Sodium
bentonite is most commonly used for large
castings that use dry molds, while calcium
bentonite is more commonly used for smaller
castings that use "green" or wet molds
b. Kaolinite Clay
• The main mineral: Kaolinite
• Chemical composition: Al2O3.2SiO2.2H2O
• Kaolinite has a low shrink–swell capacity and a
low cation-exchange capacity (1–15
meq/100 g)
3.2. Water Glass
• Sodium silicate is the common name for
compounds with the formula Na2(SiO2)nO. A well
known member of this series is sodium
metasilicate, Na2SiO3.
• Also known as waterglass or liquid glass, these
materials are available in aqueous solution and in
solid form. The pure compositions are colourless
or white, but commercial samples are often
greenish or blue owing to the presence of iron-
containing impurities.
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