Topics covered: Sand Casting of Metals Design and manufacture of gating system Pouring basin Sprue Runners In-gates Riser

1. Sand Casting of Metals
Prof. Ms. Amruta A. Rane
Assistant Professor,
DJSCE, University of Mumbai,
Mumbai.

2. Sand Casting of Metals
Topics covered: Sand Casting of Metals
• Designing and manufacturing of gating system
• Pouring basin
• Sprue
• Runners
• In-gates
• Riser

3. Gating System
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4. Gating System
• 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
o Sprue base well
o Runner
o Runner extension
o In-gate
o Riser
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5. 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 nonmetallicminclusions which are either
introduced with the molten metal or are present in the gating system.

6. Pouring Basin
• 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
mold erosion.
• 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

7. Pouring Basin
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.
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8. Pouring Basin/Cup
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9. 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.
• To eliminate this problem tapered sprue is used.
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10. 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.
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11. Runner
• 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.
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12. Runner
• 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 are preferable.
• 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.
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13. Runner Extension
• The runner is extended little further after it encounters the in-gate.
• This extension is provided to trap the slag in the molten metal.
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14. Gates or In-gates
• These are openings through which molten
metal enters the mold cavity.
Top Gate:
• In this type of gate metal enters the cavity
from top.
• Cavity is filled very quickly. Therefore, top
gates are not advisable for those materials
which are likely to form dross (turbulence,
waste, slag, etc.).
Bottom Gate:
• 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.
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15. Gates or In-gates
Bottom Gates

16. Gates or In-gates
Parting Gate:
• 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.
Step Gate:
• They are used for heavy and large
castings.
• The molten metal enters mold
cavity through a number of in-
gates, which are arranged in
vertical steps.
• 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.
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17. Gates or In-gates
• The gates are generally made wider comparing to depth, up to a ration of 1:4.
• The gates should be placed near the core-print or chill
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18. Riser
• 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 reuse.
• 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.
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19. Riser
• 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.
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20. Riser
The shrinkage occurs in three stages,
1. When temperature of liquid metal drops from pouring to freezing temperature.
2. When the metal changes from liquid to solid state, and
3. When the temperature of solid phase drops from freezing to room temperature.
• The shrinkage for stage 3 is compensated by providing shrinkage allowance on pattern,
while the shrinkage during stages 1 and 2 are compensated by providing risers.
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21. Riser
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.
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22. Chills
• The chills are used to provide progressive solidification and to avoid the
shrinkage cavities.
• Chills are large heat sinks.
• When the geometry of the molding cavity prevents solidification from occurring
naturally, a chill can be strategically placed to help promote it.
• Whenever the thickness of the walls of the casting is unequal, the chills are
placed close to the wall with larger thickness, so that the heat is quickly absorbed
by the chill from larger mass making the cooling rate equal to that of thin
sections.
• It does not permit the formation of shrinkage cavity.
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23. Materials of Chills
• Chills can be made of many materials, including iron, copper, bronze, aluminium,
graphite and silicon carbide.
• Sand materials with higher densities, thermal conductivity or thermal capacity
can also be used as a chill.
• For example, chromite sand or zircon sand can be used when molding
with silica sand.
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24. Types of Chills
Internal chills:
• They are pieces of metal that are placed inside the molding cavity.
• When the casting cavity is filled, part of the chill will melt and ultimately become
part of the casting, thus the chill must be made of the same material as that of
the casting.
External chills:
• They are masses of material that have a high heat capacity and thermal
conductivity.
• They are placed on the edge of the molding cavity and effectively become part of
the wall of the molding cavity.
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25. 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
6. Shrinkage
7. Porosity
8. Entrapped gases
9. Misruns
10. Penetration of liquid metal into mold walls

26. Gating Ratio
• 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

27. 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 magnesium.
• 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 aspiration.
• Tapered spruces
• Runners in drag
• Lower casting yield

28. A pressurized gating system
• The in-gate area is smallest
• Back pressure is there throughout the system
• Metal is more turbulent and dross formation
• Gating system flows full
• Straight sprue can be used
• Higher casting yield
• Used for ferrous castings

29. Caine’s Method
• 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.

30. Chvorinov’s Rule
• 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 thermal characteristics
• V = Volume of the casting
• SA = Surface area

31. Freezing Ratio
• 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.

32. Caine’s formula for freezing ratio
• Based on Chvorinov’s rule, Caine developed a relationship empirically for the freezing ratio as
follows:
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.

33. References
• Principles of Metal Casting, R W Heine, C R Loper, P. C. Rosenthal.
• Metal Casting, T. V. Ramana Rao.
• Manufacturing Technology, P. N. Rao.
• Foundry Engineering, P. L. Jain.

34. Thank You!