Green sand system: Simple, Complex,
Effective.
Simple: in terms of additives and control;
Complex: in the amount of variables
possible and the need for constant vigilance.
Effective:
most cost-effective method for high volume production.
History
The
history of green sand molding is 8 to 10 thousand years old. Presently there
are 40 to 50 thousand foundries in the world. World annual production is about
90 million tons of castings worth about 250 billion US$. About 2 million people
are working in the foundries globally. First foundry was set up in China during
the Shang dynasty about 3500 years ago. First cast iron foundry was established
in England by Abraham Darby in 1709 AD. John Wilkinson of England obtained a
Patent on cupola furnace in the year 1794. Castings were made in the Indus
Valley Civilization in the middle ages. Presently there are about 4500
foundries in India. India’s annual production exceeds about 7 million tons of
castings. India ranks fourth in the world in terms of annual production. India’s
annual production is about 15 billion US$. Global average casting productivity:
34 tons/man/year. Casting productivity in Japan: 97 tons/man/year. Casting
productivity in India: 16 tons/man/year.
Green sand:
The
term “green sand” is known principally because of the moisture content within
the sand. The sand undergoes a mulling process in which various clay and
chemical additives that act as binder are blended with the sand, which results
in a compound which is suitable for the sand molding process.
Green
sand moulds are not dried and metal is poured in them in the wet condition.
Why we use
Green Sand Molding process?
•
Reasonable
•
Environmental friendly.
•
High productivity (Economical).
•
Easily adaptable to manual, semi – auto and
automatic molding Machines.
Green sand
testing equipments
1)
Mould hardness tester
2)
Permeability meter
3)
Universal
sand tester
4)
2 x 2 sand rammer
5)
AFS sieve shaker
6)
Clay tester, etc
GENERAL PROPERTIES OF MOLDING SANDS
1.
Green strength. The green sand, after
water has been mixed into it, must have adequate strength and plasticity for
making and handling of the mold.
2.
Dry strength.
As a casting is poured, sand adjacent to the
hot metal quickly loses its water as steam. The dry sand must have strength to
resist erosion, and also the metal static pressure of the molten metal, or else
the mold may enlarge.
3) Hot strength.
After the moisture has
evaporated, the sand may be required to possess strength at some elevated
temperature, above 100 °C. Metallostatic pressure of the liquid‐metal bearing
against the mold walls may cause mold enlargement, or if the metal is still
flowing, erosion, cracks, or breakage may occur unless the sand possesses
adequate hot strength.
4. Permeability.
Heat from the casting causes a
green‐sand mold to evolve a great deal of steam and other gases. The mold must
be permeable, i.e. porous, to permit the gases to pass off, or the casting will
contain gas holes.
5. Thermal
stability.
Heat from the casting causes rapid expansion of the sand
surface at the mold‐ metal interface. The mold surface may then crack, buckle,
or flake off (scab) unless the molding sand is relatively stable dimensionally
under rapid heating.
6. Refractoriness.
Higher pouring temperatures, such as those for ferrous
alloys at 2400 to 3200 F, require greater refractoriness of the sand.
7. Flow ability.
The sand should respond to molding processes.
8. Produces good casting finish.
9. Collapsibility.
Heated sand which
becomes hard and rocklike is difficult to remove from the casting and may cause
the contracting metal to tear or crack.
10. Is reusable.
11. Offers ease of sand preparation and control.
12. Removes heat from the cooling casting.
This list by no means includes all the properties which
might be desirable. Obviously, the most important characteristic of molding
sand is that it facilitates the economic production of good castings.
Sand Additives
•
SAND (SiO2)
•
CLAYS (Bentonite)
•
WATER
•
CELLULOSE (Wood flour, Cob flour)
•
OIL‐CHEMICALS (Soda ash, Polymers)
•
REFRACTORIES (Alumina, Silica flour)
•
SOLUBLES (Corn flour, Corn sugar)
Green sand components
• Silica
Sand
• Bentonite
• Additives
• Water
• Dead
Clay
Silica sand grain shapes
Effects of sand
grains and influence
• Principal
ingredient - Silica Sand Grains.
• Casting Surface finish.
• Mold
Permeability.
• Sand
Strength.
• Refractoriness.
• Expansion
characteristics
• Sand
Grain contour of mold cavity
• Fine
grains – smooth wall at the metal interface.
Effect of clay
• Bentonite
is one of the Most Important component of Green sand
• Water
is necessary to activate the Clay. (2 to
5 percent.)
• With
suitable water content, it is the principal source of the strength and
plasticity of the molding sand.
Clay and sand strength
• For
given clay type and content, there is optimum water content.
• The
effect of the clay on DRY and HOT strengths is quite important.
• Too
low a dry strength permits washing of the sand by the metal, and dirt in the
castings.
• Too
low or too high a hot strength is also undesirable.