• The gating system refers to all those elements which are concerned with the flow
of molten metal from ladle to the Mould Basin cavity.
• The various elements that comes under gating system are:
o Pouring basin/cup
o Sprue base well
o Runner extension
Requirements / function of the gating system
A gating system should,
• Fill the mold cavity completely before freezing.
• Introduce the liquid metal into the mold cavity with low
viscosity and little turbulence, so that mold erosion,
metal oxidation and gas pick up is prevented.
• Help to promote temperature gradient favorable for
proper directional solidification.
• Incorporate traps for separation of nonmetallic
inclusions which are either introduced with the molten
metal or are present in the gating system.
• The molten metal is poured into the a pouring
basin which acts as a reservoir from which it
moves smoothly into the sprue.
• The pouring basin may be cut into the cope
portion directly or a separate dry sand
pouring basin may be prepared and used.
• The molten metal in the pouring basin should
be full during the pouring operation to avoid
the atmospheric air and slag from entering
into mold cavity.
• The molten metal is not directly poured into
the mold cavity because it may cause the
• The pouring basin also stops the slag from
entering the mold cavity by means of
skimmer or skim core. It holds back the slag
or dirt which floats on the top and allows only
clean metal underneath it into the sprue. 6
A strainer core:
• It is a strainer or screen with many small holes.
• It is utilized to maintain the constant conditions of flow.
• The strainer restricts the flow of molten metal into the sprue, thus helps in quick
filling of the pouring basin and restricts the flow of slag into the mold.
• It allows only clean metal to enter into the sprue.
• Sprue is the channel through which the molten metal is brought to the parting
plane where it enters the runners and gates to ultimately reach the mold cavity.
• If the sprue is straight and cylindrical, then metal flow would not be full at
bottom, but some low pressure area would be created around the metal in the
sprue. Atmospheric air would be breathed into this low pressure area which
would be then carried to the mold cavity.
• Toeliminate this problem tapered sprue is used.
Sprue Base Well
• This is reservoir of the metal at the bottom of the sprue to reduce the
momentum of the molten metal.
• The molten metal gains velocity while moving down the sprue, some of which is
lost in the sprue base well by which the mold erosion is reduced.
• It is located in parting plane and connects the sprue to the in-gates.
• The runners are normally made trapezoidal in cross-section.
• The slag trapping takes place in the runner, when runner flows full.
If the amount of molten metal coming from sprue base is more than
the amount flowing through the in-gates.
• A partially filled runner causes slag to enter the mold cavity.
• While designing the runner system, care should be
taken to reduce the sharp corners or sudden
change of sections.
• From heat-loss factor circular cross-section runners
• Also runner is generally cut in cope and in-gate in
drag to trap the slag.
• It is also good practice to have half of the runner
in the cope and the rest in the
drag which effectively reduces the slag inclusion.
• The runner is extended little further after it encounters the in-gate.
• This extension is provided to trap the slag in the molten metal.
Gates or In-gates
• These are openings through which molten
metal enters the mold cavity.
• In this type of gate metal enters the cavity
• Cavity is filled very quickly. Therefore, top
gates are not advisable for those materials
which are likely to form dross (turbulence,
waste, slag, etc.).
• This type of gate is used when the molten
metal enters the mold cavity from bottom
of the cavity.
• It takes more time to fill the mold.
Gates or In-gates
• The metal enters the mold at the
parting plane when a part of the
casting is in the cope and a part of
the casting is in drag.
• They are used for heavy and large
• The molten metal enters mold
cavity through a number of in-
gates, which are arranged in
• The size of in-gates is normally
increased from top to bottom.
• This ensures the gradual filling of
the mold without mold erosion
and produces sound casting.
• Most of the foundry metals and alloys shrink during solidification, as a result of
volumetric shrinkage, the voids are likely to form in the casting.
• Additional molten metal is fed into these places which is termed as hot spots
since it remains hot till the end.
• Hence, a reservoir of molten metal is maintained from which the metal can flow
readily into the mold cavity when the need arises, this reservoir is called riser.
• Different materials have different shrinkages hence the risering requirements
vary for the materials.
e.g. Grey cast iron sometimes may have negative shrinkage. This happens
because with higher carbon and silicon contents, graphitization occurs which
increases the volume and counteracts the metal shrinkage. Thus risering may b
very critical in such situations.
For metals like aluminium and steel, the volumetric shrinkage being very high,
elaborate risering is required.
• The solidified metal in the riser is cut off from the main casting and melted for
• The higher the riser volume, the lower is the casting yield.
• The requirement of the riser depends on the type of metal poured and the
complexity of the casting.
• During solidification metal experience shrinkage which results in void formation
creation of hot spots.
• This can be avoided by feeding hot spot during solidification.
• Riser are used to feed casting during solidification.
• Riser must solidify after casting.
• Riser should be located so that directional solidification occurs from the
extremities of mold cavity back toward the riser.
• Thickest part of casting–last to freeze, riser should feed directly to these regions.
Types of Risers
1. Top Risers: They are open to atmosphere. They Tare most conventional and
convenient to make.
2. Blind Riser: are completely concealed inside the mold cavity. It loses heat slowly
since it is surrounded by the molding sand and thus would be more effective.
3. Internal Risers: They are enclosed on all sides by the casting. They are normally
used for the castings which are cylindrical in shape or have hollow portions.
Defects in Sand Casting
Defects occurring due to improper design of Gating System:
1. Oxidation of metal
2. Inclusion of slag, dross and other foreign matter
3. Cold shuts
4. Mold erosion
5. Rough surfaces
8. Entrapped gases
10. Penetration of liquid metal into mold walls
• The gating ration refers to proportion of the cross
sectional areas between the sprue, runner and in-gates
and generally denoted as:
Sprue area : runner area : in-gate area
• Depending on choke area, there can be two types of
gating systems: pressurized and non- pressurized
A pressurized gating system
• The in-gate area is smallest
• Back pressure is there
throughout the system
• Metal is more turbulent and
• Gating system flows full
• Straight sprue can be used
• Higher casting yield
• Used for ferrous castings
A non-pressurized gating system
• A non-pressurized gating system have choke area at the bottom of the sprue
base, total runner area and in-gate areas higher than the sprue area.
• In this system no pressure is existing in the metal flow system and thus it helps
to reduce turbulence.
• It is helpful for casting drossy metals and alloys such as aluminium and
• The gating system should be designed to see all the parts flow full. Otherwise
some elements of the gating system may flow partially allowing for air
• Tapered spruces
• Runners in drag
• Lower castingyield
• Solidification of casting occurs by losing heat from the surface and
amount of heat is given by the volume of the casting. The cooling
characteristics of a casting can be represented by surface-area-
to-volume ratio of the casting.
• Since Riser is also same in solidification behavior as casting, riser
cooling characteristics can also be specified by the ratio of its
surface area to volume.
• If this ratio of casting is higher than it is expected to cool faster.
• Solidification time of a casting is proportional to square of the
ratio of volume to surface area of the casting.
ts = K ( V / SA )2
• ts = solidification time
• K = mould constant depends on pouring temp., casting and mould
• V = Volume of the casting
• SA = Surface area
• The freezing ratio (X) of the mould is defined as the ratio of
cooling characteristics of casting to the riser.
X = (SA / V)casting / (SA / V)riser
• In order to feed the casting the riser should solidify last and hence
freezing ratio should
be greater than unity.
• Sphere has lowest SA to V ratio and hence that should be used as
riser. In sphere hottest material being at center, it is difficult to use
it for feeding the casting.
• The next best is the cylindrical type of riser.
Caine’s formula for freezing ratio
• Based on Chvorinov’s rule, Caine developed a relationship empirically for the freezing ratio as
X = a / (Y – b) – c
• Y = Riser volume / Casting volume
• The above equation when plotted will be shown as following graph. The line shows the locus of the
points that separates the sound castings and castings with shrinkages.