Additives used for the molding sand:
1) Facing materials:
a) Sea coal
b) Pitch and asphalt:
1) Pitch is distilled from soft coal at about 600f and asphalt as a by-product of petroleum distillation.
2) They improve hot strength.
3) They are used to obtained fine surface finish on ferrous castings.
4) Pitch may be used up to 2%.
c) Graphite:
1) It may be natural or synthetic graphite.
2) It may be added from 0.2 to 2%.
3) It improves surface finish of the castings.
4) Graphite improves moldability of foundry sand mixture.
d) Coke
e) Silica flour
2) Cushion materials:
a) Wood flour, cereals hulls, cellulose etc.:
1) They minimize sand expansion defects.
2) They improved flowability and collapsibility of the sand.
3) They may be added to sand in amount ranging from 0.5 to 2.0%.
b) Perlite:
1) It serves the same purpose as wood flour.
2) In addition, perlite possesses insulating properties and thus is used as a riser insulator.
c) cereals
3) Other special additives
a) Cereals and finely ground corn flour.
b) Fuel oil:
1) It appears to improve moldability of sand.
2) It may be added in amount from 0.01 to 0.10%.
c) Iron oxide
d) Dextrin and molasses:
1) They increase dry strength of the sand.
2) They add to edge hardness of the sand.
3) Molasses resists the mold tendency to drying out.
e) Boric acid, sulphour and ammonium difluoride:
They prevent metal-mold reaction in case of magnesium castings.
f) Diethylene glycol:
1) When added to bentonite bonded sand, it decreases mold tendency to surface drying.
2) Improves casting surface finish.
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