CASTABILITY
Castability (Fluidity) is the ability of the molten metal to flow
easily without premature solidification is a major factor in
determining the proper filling of the mold cavity.
The higher the castability of a molten metal, the easier it is for
that molten metal to fill thin grooves in the mold and exactly
reproduce shape of mold cavity, there by successfully producing the
castings with thinner sections. Poor castability leads to casting
defects such as incomplete filling or misruns especially in thinner
sections of a casting. Because castability is dependent mainly upon
the viscosity of molten metal, it is clear that higher temperature
improves castability of molten metal and alloys, where as presence
of impurities and non metallic inclusions adverse it.
Castability is nothing but producing a casting with minimum
cost, defects and time. This can be done by the high compatibility
between the product requirement and process castability. This topic
reviews about the design of castability and some methods of
castability analysis.
Castability is the ease of forming a casting. Castability can be
thought of as how easy is it to cast a quality part. A very castable
part design is easily developed, incurs minimal tooling costs,
requires minimal energy, and has few rejections.
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In simple castability is the capability of molten metal to fill mold
cavities.
This consists of two basic factors.
1. Characteristic of molten metal
2. Casting properties
Characteristic of molten metal
a) Viscosity:As the viscosity increases the fluidity decreases since the
resistance to flow for the molten metal increases and the molten
metal cannot pass through the mold cavity successfully.
b) Surface Tension:High surface tension of the liquid metal reduces the castability.
Some times oxide films developed on the surface of the molten
metal have adverse effects on the castability since they also
increases surface tension. Example: oxide film on surface of pure
molten alluminium triples the surface tension.
c) Inclusions:Any Insoluble particles, inclusions decreases the castability.
Example: a fluid with impurities (sand inclusions) have high
resistance to flow thereby decreasing the castabilty.
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d) Freezing zone:Freezing zone is the range of temperature where solidification of
molten metal starts and temperature where it ends. Castability is
inversely proportional to the freezing zone.
Pure metals and eutectics have shorter freezing zones and high
castability.
Solid-solution alloys have long freezing zones and castability is low.
Casting properties:
a) Mold Design:Design and dimensions of the components such as sprue, runners
and risers influence the castability.
b) Degree of Superheat:Superheating is the improvement of temperature above the melt
point of the alloy. Superheat improves the fluidity by delaying
solidification.
c) Mold material and its surface characters:Mold material should have low thermal conductivity and fine
surface. Higher the thermal conductivity of mold and rougher the
surface the lower will be the castability.
Example: Dry sand mold has high castability than green sand
mold.
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d) Rate of pouring:Slower rate of pouring lower the castability because of high rate
of cooling.
e) Heat Transfer:- It directly affects the viscosity of liquid metal.
f) Pouring temperature of moulds are to be a little above the
melting temperatures of alloys along with a sufficient superheat
to account for cooling of molten metal from time it is tapped from
durnace till it is poured into mould.
Castability of different metals:
Ferrous casting alloys:
Grey cast iron:- It is the most fluid of all ferrous alloys and it has
the high castability of all other ferrous metals.
Cast steels:- Because of high temperature required to melt cast
steels up to about 16500c (30000 ) are difficult to cast. Selection of
molding material is a different task.
Stainless steels:- these have long freezing zone & high melting
temperature.
Non-ferrous castings alloys:Alluminium based alloys:- castability depends on oxides & alloying
elemnets in the metal. These have good thermal conductivity.
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Magnesium-based alloys:-same as the alluminium alloys.
Zinc-based alloys:- These have good castability & commonly used
in die casting.
The three major factors that influence castability are explained
in detailed as follows:
 Material Selection.
 Geometry Planning.
 Quality.
Material Selection:
The metals used for casting are characterized by their following
casting properties:
 Solidification capacity.
 Slag formation.
 Pouring temperature.
Geometry Planning:
Designing the geometry of the product is quite cost effective & it
depends upon the complexity of the process. It also depends on the
tools used.
Quality:
Quality of the casting product means free of defect or less defect.
The casted product should have a desired surface quality, a proper
dimension and internal reliability. Radiography and pressure testing
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are some of the methods by which the quality of a casted product
can be tested.
Castability process-friendly design:
A casting design is considered to be friendly if it has the following
characteristics:





Easy to manufacture.
Lower tooling cost.
Lower energy requirement.
Implying faster development.
Minimal rejections.
Some guidelines to improve the castability:
Parting Line:
Parting line is also one of the factors that affect the tooling cost. A
non-planner parting line must be avoided to have a proper parting.
Cored Features:
The cored holes should posses the following criteria's:

Minimum diameter.

Aspect ratio.

Location in thick sections.

Distance from edge and neighbor hole.
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Filling Characteristics:
The part must be designed in such a way that it reduces the
turbulence during filling and to promote complete filling of all
sections.
Solidificationprocess:
The Solidification process should have the following characteristics:

Minimizing isolated hot spots.

Promoting controlled progressive directional solidification.
Complexity of the part:
In order to improve the castability the shape, Complexity criteria
can be explained by 3 Different dimensionless equations:
Better castability is denoted by a larger number

Ratio of the part volume to the volume of its bounding box:
Where Vc is the volume of the casting and
Vb is the volume of the smallest box that the casting could fit in
Ratio of surface area of cube of equal volume to the surface
area of the cast part:

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Where Vc is the volume of the casting and Ac is the
surface area of the casting
Features:
Where nf is the number of features (holes, pockets, slots,
bosses, ribs, etc.)

Castability also depends upon melting point of metals.
Melting practices have direct effect on quality of castings. Factors
that effect melting are
1. Fluxes,which are inorganic compounds added to molten metal
to remove dissolved gases.
2. Type of furnace used
3. Foundary operations that is
a) Pattern
b) Mold making
c) Pouring of melt
d) Removal of cast items from molds.
For casting ceramics slip casting type is used where the slurry of
ceramics is poured to a cavity and allowed to solidify to get
required shape and after it again heated to increase the hardness.
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Test for castability:
Although there is no perfect test to determine the castability since it
is in the dependence of many parameters like viscousity, superheat,
impurities present in the molten metal, pouring temperature, rate of
pouring the molten metal and some other material properties.
A most commonly test is spiral fluidity test in which molten
metal is allowed to flow through the cahnnels or spirals provided for
it at a room temperature, the distance of the metal flow before it
solidifies and stops is a measure of metal’s castability.
The castability is affected by composition factor (CF)
Where
CF = %C + 0.25 * % Si +0.5 * % P
Fluidity or castability is givrn in terms of composition factor by
Castability = 37.846*CF + 0.228*T-389.6 cm
Where T= pouring temperature, 0C
Castability of Fibres:
Fibre
Silica fibre
melting point
10000C
Alumino silicate fibre
14000C
Alimina fibre
16000C
Zircon fibre
22000C
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castability
Castability of different metals
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References:
 Kogel, Jessica Elzea; Society for Mining, Metallurgy, and Exploration
(2006), Industrial minerals & rocks: commodities, markets, and
uses (7th ed.), SME, p. 1406, ISBN 9780873352338.
 Ravi, B. (2004), Metal Casting - Overview, IIT Bombay.
 Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2003), Materials
and Processes in Manufacturing (9th ed.), Wiley, ISBN 0-471-656534.
 Manufacturing process for engineering materials –serope kalpakjian
Steven R.schmid
 Manufacturing technology --------- P N RAO
 Process and design for manufacturing ----- sherif D El Wakil
 Metal casting (computer aided design & analysis) --- B.Ravi
 Manufacturing process – II -------- H S BAWA
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