Moulding Sand
Moulding sand, also known as foundries sand, is a type of sand that is widely used to
create moulds. Although high-quality silica sand is mined, natural sand found on the
bed and banks of rivers provides a greater source. Chemically, the sand is Si02, silicon
dioxide in granular form. Aside from silica grains, ordinary river sand contains a
percentage of moisture,clay, non-metallic contaminants, and traces of magnesium and
calcium salts. This sand is used to make a mould once it has been properly treated.
Types of Moulding Sand
There are the 8 different types of moulding sands:
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Greensand
Dry sand
Loam sand
Facing sand
Backing sand
System sand
Parting sand
Core sand
Core
Cores are a compact mass of core sand that, when inserted in a mould cavity at the
desired location with proper alignment, prevents the molten metal from occupying
space for solidification in that area of the casting, resulting in hollowness. The
environment in which the core is placed differs significantly from that in which the
mould is placed. In fact, the core must endure the harsh action of hot metal that
surrounds it entirely. Green sand cores and dry sand cores are two types of sand cores.
As a result, the core must fulfil the following functions or purposes, which are listed
below.
 Hollowness in castings are caused due to core.
 It must be porous enough to allow gases to escape easily during pouring and
solidification.
 It could be a component of green sand mould.
 It could be used to improve the mold's surface.
 It may be used in casting to give external undercut characteristics.
 It can be put into the casting to create deep recesses.
 It can be used to make the mould stronger.
 It can be utilised to create a large-scale mould gating system.
Core Sand
The sand used to make cores is called core sand. Special considerations should be
taken when selecting core sand, keeping the above-mentioned objectives in mind.
These considerations include:
 The cores are exposed to a high temperature, so the core sand should be highly
refractory in nature
 The permeability of the core sand must be high in comparison to that of the
moulding sands, so that the core gases can escape through the limited area of
the core recesses created by core prints
 The core sand should not contain any materials that may produce gases while
in contact with the core
 The core sand should be collapsible by nature, i.e., it should crumble once the
metal solidifies, as this will make cleaning the casting easier.
Pure silica sand and a binder are the main components of the core sand. Because of its
high refractoriness, silica sand is preferred. Sands having a coarse grain size
distribution are employed to achieve better permeability ratings. The primary function
of the core binder is to keep the grains together, provide strength, and allow for proper
collapsibility. Aside from the qualities required in core sand, the binder should
produce the least amount of gases possible when molten metal is put into the mould.
Although both inorganic and organic binders are used, organic binders are preferable
for core manufacturing because they are flammable and may be dissolved by heat at
higher degrees, providing sufficient collapsibility to the core sand. The following are
some of the most popular binders used in the production of core sand:
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Cereal binder
Protein binder
Thermo setting resin
Sulphite binder
Dextrin
Pitch
Molasses
Core oil
Core Making
The process of core making is divided into four stages: core sand preparation, core
making, core baking, and core finishing.
1)Core Sand Preparation
Manual preparation of a suitable and homogeneous core sand combination is not
achievable. Core sands are generally mixed with the help of any of the following
mechanical means, namely roller mills and core sand mixers using vertical revolving
arm type and horizontal paddle type mechanisms, in order to achieve better and
uniform core sand properties using proper sand constituents and additives. The rolling
movement of the mulling machine, along with the turning over action induced by the
ploughs, results in a uniform and homogenous mixing in roller mills. The core sand
mixer is suited for all types of core binders, whilst roller mills are suitable for core
sands containing cereal binders. These machines perform the most complete mixing of
core sand elements.
2)Core Making Process
Core blowing, core ramming, and core drawing machines are used to automate the
core-making process, which is described in more detail below.
 Core blowing machines
The basic principle of a core blowing machine is to use compressed air to fill
the core box with core sand. The compressed air velocity is kept high to
provide a high velocity of core sand particles, assuring their deposit at the core
box's far corners. The shaping and ramming of core is carried out concurrently
in the core box after entering the core sand with high kinetic velocity.
 Core ramming machines
Cores can even be prepared by ramming core sands into core boxes with
machines that use the squeezing, jolting, and slinging methods. Jolting and
slinging are the most frequent core-making machines among these three.
 Core drawing machines
When the core boxes have deep draws, the core drawing is favoured. The core
box is put on a core plate supported on the machine bed after being rammed
with sand. A vibrating vertical plate creates a rapping movement on the core
box. This rapping motion assists in the removal of the core from the core box.
The core is pulled up after rapping the core box, leaving the core on the core
plate. To generate hollowness in the casting, the drew core is baked again
before being used in the mould cavity.
3) Core Baking
The cores will be baked in baking ovens or furnaces once they have been prepared.
The fundamental goal of baking is to remove moisture and harden the binder,
providing the core added strength.
4) Core Finishing
After baking and before being put in the mould, the cores are completely finished.
Rubbing or filing is used to remove fins, bumps, and other sand protrusion from the
surface of the cores. Dimensional examination of the cores is critical for successful
casting. To improve refractoriness and surface finish, cores are also coated with
refractory or protective compounds utilising brushing, dipping, and spraying methods.
Molten metal cannot enter the core because of the covering on the core.