green sand molding

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.