SAND CONDITIONING
Natural sands are generally not well suited for casting purposes. On
continuous use of molding sand, the clay coating on the sand particles gets
thinned out causing decrease in its strength. Thus proper sand conditioning
accomplish uniform distribution of binder around the sand grains, control
moisture content, eliminate foreign particles and aerates the sands. Therefore,
there is a need for sand conditioning for achieving better results.
The foreign materials, like
nails, gaggers, hard sand lumps and metals from the used sand are removed. For
removing the metal pieces, particularly ferrous pieces, the sand from the
shake-out station is subjected to magnetic separator, which separates out the
iron pieces, nails etc. from the used sand. Next, the sand is screened in
riddles which separate out the hard sand lumps etc. These riddles may be manual
as well as mechanical. Mechanical riddles may be either compressed air operated
or electrically operated. But the electrically operated riddles are faster and
can handle large quantities of sand in a short time. The amount of fine
material can be controlled to the maximum possible extent by its removal
through exhaust systems under conditions of shake out.ย
at required proper proportion and mixed thoroughly. Next, the whole mixture is mulled suitably till properties are
developed. After all the foreign
particles are removed from and the sand
is free from the hard lumps etc., proper amount of
pure sand, clay and required additives are added to for the
loss because of the burned, clay and
other corn materials. As the
moisture content of the returned sand known, it is to be tested and after knowing
the moisture the required amount of
water is added. Now theseย things are mixed
thoroughly in a mixing muller
(Fig)
Sand mixing muller |
The main objectives of a mixing
muller is to distribute the binders, additives and moisture or water
content uniformly all around each sand grain and helps to develop the optimum
physical properties by kneading on the sand grains. Inadequate mulling makes the sandย mixture weak which can only be compensated by
adding more binder.ย
adequate mulling economizes the use of binders. There are two methods of adding
clay and waterย to sand. In the
first method, first water is added to sand follow by clay, while in the other method, clay addition is followed water. It has been suggestedย thatย
theย bestย orderย
ofย adding ingredients to clay
bonded sand is sand with water followed by the binders. In this way, the clay is more quickly and uniformly
spread on to all the sand grains. An additional advantage of this mixing order
is that less dust is produced during the mulling operation.ย
consists of a cylindrical pan in which two heavy rollers; carrying two ploughs, and roll in a circular path.
While the rollers roll, the ploughs scrap the sand from the sides and the
bottom of the pan and place it in front of For producing a smearingย ย action in the sand, the rollers are set
slightly off the true radius and they move out ofย ย the rollers can be moved up and down
without difficulty mounted on rocker arms.
Afterย the mulling is completed sand
can be discharged through a door.ย
mechanical aeratorsย are generally used
for aerating or separating the sand grains by increasing the flowability
through whirling the sand at a high speed by an impeller towards the inner
walls of the casting. Aerating can
also be done by riddling the sand mixture oil on a one fourth inchย ย mesh screen or by spraying the sand over the
sand heap by flipping the shovels.ย The aeration separates the sand grains and
leaves each grain free to flow in the direction of ramming with less friction.
The final step in sand conditioning is the coolingย ofย
sandย mixture because of the fact
that if the molding sand mixture is hot, it will cause molding difficulties.
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