Mold materials
1) A mold material is one, of which the mold is made.
2) A mold material should be such that the mold cavity retains its shape till the molten metal has solidified.
3) Casting can be made in –
a) Permanent molds – made up of ferrous metals and alloys. (Steel, gray cast iron etc.)
b) Temporary refractory molds – made up of refractory sands and resins.
Mold may also be made up of wax, plaster of Paris, carbon, ceramics etc.
4) Permanent molds are normally employed for casting low melting point materials.
5) Permanent molds are costly too.
6) For the above mentioned reasons most of the output of foundry industry has its castings produced using refractory mold materials chiefly refractory sands.
7) As compared to permanent molds, the refractory sand molds can cast high melting point materials and bigger objects whereas permanent molds produce small castings better quality and dimensional accuracy.
Refractory sands;
1) Silica sand
2) Magnesite
3) Zircon
4) Dolomite
5) Olivine
6) Sillimanite
7) Graphite / carbon
Molding sands:
Sources:
- River bed
- Sea
- Lakes
- Deserts
Types:
- Natural sand
- Synthetic sand
- Loam sand
Ingredients:
- Refractory sand grains
- Binders
- Water
- Additives
Properties of molding sands:
a) Flowability: is the ability of molding sand to get compacted to a uniform density. Flowability assists molding sand to flow and pack all around the pattern.
b) Green strength: the strength of the sand in the green or moist state. It helps in making and holding the molds.
c) Dry strength: the strength of molding sand in the dry condition.
d) Hot strength: it is the strength of sand (of mold cavity) above 212 degree F.
e) Permeability / porousness: ability of sand to allow gas to flow.
f) Refractoriness: it is the ability of sand to withstand high temperatures (experienced during pouring) without fusion, cracking, buckling or scabbing.
g) Adhesiveness : it is the property of molding sand owing to which , it
- Sticks with the wall of molding boxes
- Sticks with gaggers
- Thus makes it possible to mold cope and drag.
h) Collapsibility : is the property of the molding sand which determines the readiness with which the molding sand or mold
- Automatically gets collapse after the casting solidifies
- Breaks down in knock out and cleaning operations.
i) Fineness: resist metal penetration and produce smooth casting surface.
j) Bench life : it is the ability of molding sand to retain its properties during storage or while standing.(i.e. in case of any delay)
k) Coefficient of expansion: molding sand should posses low coefficient of expansion.
l) Durability: the molding sand should posses the capacity to withstand repeated cycles of heating and cooling during casting operations.
m) Should be chemically immune to molten metals.
n) Reusable
o) It should be easy to prepare and control the molding sand.
Types of sands used in molds:
1) Green sand
2) Dry sand
3) Loam sand
4) Synthetic sand
5) Core sand
6) Parting sand
7) Facing sand
8) Backing sand
9) Sharp sand
10) System sand
11) Heap sand
12) Oil and molasses sand
Buckle: crumple / collapse under pressure.
Core sand:
1) A core may be defined as any projection into the mold and (made up) up core sand. A core forms internal or external surface of castings.
2) Core sand is a suitably sand mixture employed for making core.
3) Cores are made separately, are baked and are suitably placed and positioned in the mould cavity.
Core sand properties:
1) Adequate green strength to retain its shape before baking.
2) High dry strength and core hardness after baking.
3) Adequate permeability for letting go the gases generated during pouring.
4) High refractoriness to withstand the effect of high temperature molten metal.
5) High collapsibility so that the core give away as the casting cools and shrinks. High collapsibility avoids the introduction of hot tears and cracks in the cast metals.
6) Core sand should be able to impart good smoothness and finish to the core surfaces.
7) Good friability so that core crumbles and falls apart when it must be removed from the casting.
8) Core sand should be such that it generates minimum amount of gases during the pouring of the casting.
9) Core sand should be able to retain its properties during storage.(bench life)
10) Core sand should be able to resist effect of molten metals like erosion, thermal shock etc.
Core sand ingredients; core materials
1) Granular refractories :
a) Clean, dry and pure silica sand.
b) Zircon
c) Olivine
d) Carbon
e) Chamotte
2) Core binders :
a) Organic binders
b) Inorganic binders
c) Other binders
3) Water
4) Special additives
Binders:
Produces cohesion between the sand grain and thus gives it the strength to retain its shape as mold cavity or core.
Sand control tests:
1) Tests on molding sand
a) Moisture content test
b) Clay content test
c) Grain fineness test
d) Permeability test
e) Strength test
1) Green and dry compression
2) Green tensile
3) Green and dry shear
4) Transverse (bending)
f) Hot strength test
g) Refractoriness test
h) Mold hardness test
2) Test on core sands
a) Green strength test
b) Permeability test
c) Hot strength test
d) Baked strength test
e) Retained strength test
f) Core hardness test
g) Moisture content test
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