Green sand moulding – Steps , Advantages and Disadvantages
GREEN SAND MOULDING
A green sand mould is composed of a mixture of sand (silica sand SiO2), clay (act as binder), and water. The word green is associated with the condition of wetness or freshness and because the mould is left in the damp condition, hence the name “green sand mould”. This type of mould is the cheapest and has the advantage that used sand is readily reclaimed. But the mould being in the damp condition, is weak and cannot be stored for a longer period. Hence such moulds are used for small and medium sized casting.
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Principal Methods of Green-sand Moulding are:
- Open-sand method
- Turn-over method
It is simplest form of green sand moulding, particularly suitable for solid patterns. For convenience in working and pouring, the entire mould is made in the foundry floor or in a bed of sand above floor level. Moulding box is not necessary and the upper surface of the mould is open to air. After proper levelling the pattern is pressed in the sand bed for making mould. Pouring basin is made at one end of the mould, and the overflow channel cut at the exact height from the bottom face of the mould for giving necessary thickness.
In this method, the pattern is hammered down or pressed to bed it into the sand of the foundry floor or in a drag filled partially with sand to form the mould cavity. The sand should be rammed close to the pattern sand; a cope is placed over the pattern. The cope is rammed up, runners and risers are cut and the cope box is lifted. Now the pattern is withdrawn, the surfaces of drag and cope replaced in its correct position for completing the mould.
iii) Turn-over method
One pattern-halfis placed with its flat side on a moulding board, a drag is rammed and rolled over. The other pattern half and a cope box are placed in position. After ramming the cope is lifted off and the two pattern halves shaken and withdrawn. Now the cope is replaced on the drag for assembling the mould.
Steps involved in Green sand moulding
- First one half of the pattern is placed on the moulding board.
- The drag is placed with the dowel pins down.
- Moulding sand is filled in the moulding box to cover the pattern.
- The drag is completely filled with sand up to the top and rammed by the peen end of the hand rammer.
- Excess sand is levelled by a strike-off bar.
- The drag is tilled upside down.
- Parting sand is applied on the surface.
- The other half of the pattern is how placed correctly on the already placed half
- The cope is placed in position on the drag and aligned using dowel pins.
- The sprue pin is placed vertically for the purpose of pouring the molten metal.
- The risers are placed over the highest point of the pattern for the purpose of escaping the gases and identify the level of molten metal.
- Again the moulding sand filled in the cope box, and rammed.
- The riser and the sprue pin are removed.
- The funnel shaped opening called a pouring basin is cut at the top of the sprue pinhole.
- The cope is lifted, turned over and placed on the floor.
- The pattern pieces are carefully removed.
- The gate is cut that is connecting the sprue basin and the mould cluity.
- The cope is placed carefully over the drag.
- Pouring the molten metal.
Advantages of green sand Moulding :
- Green sand moulds are softer than dry sand moulds. This allows greater freedom in construction when the castings solidify and cool.
- Green sand moulds are quite strong for small depths, as the gases escape from them.
- Green sand moulds do not require any backing operations or equipment, but dry sand cores are to be used.
Disadvantages of green sand moulding :
- The green sand moulds cannot be stored for long time.
- The green sand moulds are not so strong as other moulds are liable to be damaged during handling or pouring.
- The surface finish of the casting obtained from green sand mould is not very smooth.
- The green sand mould lacks permeability and strength, which causes certain defects like blow holes etc.
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