Casting Terms
Casting Terms
• Mold consists of two halves:
– Cope = upper half of mold
– Drag = bottom half
• Mold halves are contained in a box, called a
flask
• The two halves separate at the parting line
• Mold cavity is formed by packing sand around
a pattern, which has the shape of the part
Casting Terms contd.
• When the pattern is removed, the remaining cavity has
desired shape of cast part
• The pattern is usually oversized to allow for shrinkage
of metal as it solidifies and cools
• Sand for the mold is moist and contains a binder to
maintain shape
• Through downsprue, metal enters a runner leading to
the main cavity
Casting Terms contd.
• At top of downsprue, a pouring cup is often
used to minimize splash and turbulence as the
metal flows into downsprue
• Riser is a reservoir in the mold which is a
source of liquid metal to compensate for
shrinkage during solidification
• The riser must be designed to freeze after the
main casting in order to satisfy its function
Types of Patterns
(a) Single Piece pattern
(b) split or two piece pattern
(c) match-plate pattern
(d) cope and drag pattern
Single Piece Pattern
• Made from one piece and does not contain
loose pieces or joints.
• Inexpensive.
• Used for large size simple castings.
• Pattern is accommodated either in the cope or
in the drag.
Split Pattern
• The upper and the lower parts of the split piece patterns are
accommodated in the cope and drag portions of the mold
respectively.
• Parting line of the pattern forms the parting line of the mould.
• Dowel pins are used for keeping the alignment between the
two parts of the pattern
Loose Piece Pattern
• Certain patterns cannot be withdrawn once
they are embedded in the molding sand. Such
patterns are usually made with one or more
loose pieces for facilitating from the molding
box and are known as loose piece patterns.
• The main body of the pattern is drawn first
followed by loose piece.
Loose piece pattern
Match Plate Pattern
• It consists of a match plate, on either side of
which each half of split patterns is fastened.
• The match plate with the help of locator holes can
be clamped with the drag.
• After the cope and drag have been rammed with
the molding sand, the match plate pattern is
removed from in between the cope and drag.
• Match plate patterns are normally used in
machine molding.
• By using this we can eliminate mismatch of cope
and drag cavities.
Cope & Drag pattern
• Each half of the pattern along with gating
system is fixed to a separate metal/wood plate.
• The two moulds of each half of the pattern are
finally assembled with the help of alignment
pins and the mould is ready for pouring.
• Cope and drag patterns are used for producing
big castings which as a whole cannot be
conveniently handled by one molder alone.
Sweep Pattern
• It is used for generating large shapes which are
axi-symmetrical.
• Making a sweep pattern saves a lot of time,
money and labour as compared to making a
full pattern
Sweep Pattern
Follow Board Pattern
• A follow board is a wooden board and is used
for supporting a pattern which is very thin and
fragile and which may give way and collapse
under pressure when the sand above the
pattern is being rammed.
Follow Board Pattern
Skeleton Pattern
• A skeleton of pattern is made with the help of
wooden strip. It is filled by packing sand.
Skeleton Pattern
Pattern Allowances
• A pattern is larger in size as compared to the
final casting, because it carries certain
allowances due to metallurgical and
mechanical reasons for example, shrinkage
allowance is the result of metallurgical
phenomenon where as machining, draft,
distortion, shake and other allowances are
provided on the patterns because of
mechanical reasons.
Shrinkage Allowance
• Liquid Shrinkage:- reduction in volume when
the metal changes from liquid state to solid
state at the solidus temperature. Riser is used
to accommodate
• Solid Shrinkage:- Reduction in volume caused
when metal loses temperature in solid state.
Shrinkage allowance is provided on Pattern.
The metal shrinkage depends upon:
1.
2.
3.
4.
5.
The cast metal or alloy.
Pouring temp. of the metal/alloy.
Casted dimensions(size).
Casting design aspects.
Molding
conditions(i.e.,
mould
materials and molding methods
employed)
Contraction Allowance
Machining Allowance
Machining Allowance is provided for
i. Castings get oxidized in the mold and during
heat treatment; scales etc., thus formed need to
be removed.
ii. It is the intended to remove surface roughness
and other imperfections from the castings.
iii. It is required to achieve exact casting
dimensions.
iv. Surface finish is required on the casting.
Machining allowance depends on
i. Nature of metals.
ii. Size and shape of casting.
iii. The type of machining operations to be
employed for cleaning the casting.
iv. Casting conditions.
v. Molding process employed
Machining allowance for various
metals
Draft or Taper allowance
• It is given to all surfaces perpendicular to
parting line
• It is given to avoid damage to mould cavity
while withdrawing pattern.
An illustration of a pattern shown below having
proper draft allowance. Here, the moment the pattern
lifting commences, all of its surfaces are well away
from the sand surface. Thus the pattern can be
removed without damaging the mold cavity.
Taper amount depends on
i. Shape and size of pattern in the depth
direction in contact with the mould cavity.
ii. Moulding methods.
iii. Mould materials.
iv. Draft allowance is imparted on internal as
well as external surfaces; of course it is
more on internal surfaces.
Draft values for patterns
Distortion or cambered allowance:

A Casting will distort if
 It is of irregular shape,
 All it parts do not shrink uniformly i.e., some parts
shrinks while others are restricted from during so,
 It is u or v-shape,
 The arms possess unequal thickness,
 It has long, rangy arms as those of propeller strut
for the ship,
 It is a long flat casting,
 One portion of the casting cools at a faster rate as
compared to the other.
Shake Allowance
• A Pattern is shaked or wrapped to take it out of
mould. This in turn enlarges the mould cavity.
Hence a –ve allowance is given to pattern.
• If the draft angle is provided, shake allowance
reduces.
Pattern Material
• Factors affecting selection of material:-
No. of castings to be produced.
Metal to be cast.
Dimensional accuracy & surface finish.
Shape, complexity and size of casting.
Casting design parameters.
Type of molding materials.
The chance of repeat orders.
Nature of molding process.
Position of core print.
Desirable Properties of Pattern
Material
1.
2.
3.
4.
5.
Easily worked, shaped and joined.
Light in weight.
Strong, hard and durable.
Resistant to wear and abrasion.
Resistant to corrosion, and to chemical
reactions.
6. Dimensionally stable and unaffected by
variations in temperature and humidity.
7. Available at low cost.
Material for pattern Making





Wood
Metal
Plastic
Plaster
Wax.
Wood
These are used where the no. of castings to be produced
is small and pattern size is large.
Advantages:
Inexpensive
Easily available in large quantities
Easy to fabricate
Light in weight
They can be repaired easily
Easy to obtain good surface finish
Wood cont.
Limitations:
Susceptible to shrinkage and swelling.
Possess poor wear resistance.
Abraded easily by sand action.
Absorb moisture.
Cannot withstand rough handling.
Life is very short.
Metal Patterns
These are employed where large no. of castings have
to be produced from same patterns.
Advantages:
Do not absorb moisture
More stronger
Possess much longer life
Do not wrap, retain their shape
Greater resistance to abrasion
Accurate and smooth surface finish
Good machinability
Metal Patterns cont.
Limitations:
Expensive
Require a lot of machining for accuracy
Not easily repaired
Ferrous patterns get rusted
Heavy weight , thus difficult to handle
Plastic Pattern
Advantages:
 Durable
 Provides a smooth surface
 Moisture resistant
 Does not involve any appreciable change in size or shape
 Light weight
 Good strength
 Wear and corrosion resistance
 Easy to make
 Abrasion resistance
 Good resistance to chemical attack
Plastic Cont.
Limitations:
Plastic patterns are Fragile
These may not work well when subject to
conditions of severe shock
Plaster Pattern
Advantages:
It can be easily worked by using wood
working tools.
Intricate shapes can be cast without any
difficulty.
It has high compressive strength
Wax Pattern
Wax patterns find applications in Investment casting
process.
Advantages:
Provide very good surface finish.
Impart high accuracy to castings.
After being molded, the wax pattern is not taken out of
the mould like other patterns;
rather the mould is inverted and heated; the molten
wax comes out and/or is evaporated.
Thus there is no chance of the mould cavity getting
damaged while removing the pattern.
Pattern Color code
The patterns are normally painted with contrasting
colors
The color code used is,
1. Red or orange on surface not to be finished and
left as cast
2. Yellow on surfaces to be machined
3. Black on core prints for unmachined openings
4. Yellow stripes or black on core prints for
machined openings
Moulding Material
• Hot strength:- when sand reaches to high temp.
its strength to retain shape of cavity.
• Permeability:-ability of sand to allow gases to
escape from the mould
Properties of Moulding Material
• Refractoriness:-Ability
to
withstand
high
temperature of the molten metal and avoid fusion.
• Green Strength:-Sand with moisture is green sand
and its property to retain shape of mould is green
strength.
• Dry strength:- Sand without moisture is dry sand.
Its ability to retain shape of cavity and withstand
metallostatic force is dry strength.
Moulding Sand
Silica Sand (SiO2)
Clays are binding agents to provide strength
Moisture
Types of sand
Facing Sand:-This sand is used next to pattern
to obtain cleaner and smoother casting surfaces
Mould Wash:-Carbonaceous material applied
on the inner cavity after the pattern is
withdrawn. This is done to prevent metal
penetration and prevent sand fusion.
Core & Core Prints
Core & core prints
Core is a Body made of refractory material
which is set into prepared mould before
closing and pouring it.
Core should be able to collapse after the metal
is solidify
Chaplets are used to support cores if the core
are very big in size. They are made with same
metal or alloys being casted.
Type of Cores
 Green Sand core:-it is formed by the pattern
itself using moulding sand.
 Dry Sand Core:- They are made separately
and positioned in the mould after the pattern
is taken out.
Backing sand:-Foundry sand which is used for
ramming after facing sand is applied to pattern.
Parting sand:-Sand which is sprinkled on the
parting surfaces of mould before they are
prepared.
Classification of casting Processes
Casting processes can be classified into following
FOUR categories:
1. Conventional Molding Processes
a.
Green Sand Molding
b. Dry Sand Molding
c.
Flask less Molding
2. Chemical Sand Molding Processes
a.
Shell Molding
b. Sodium Silicate Molding
c.
No-Bake Molding
3.
a.
b.
Permanent Mold Processes
Gravity Die casting
Low and High Pressure Die Casting
4.
a.
b.
c.
d.
e.
Special Casting Processes
Lost Wax
Ceramics Shell Molding
Evaporative Pattern Casting
Vacuum Sealed Molding
Centrifugal Casting
Shell Moulding process
Procedure
• Sand and additives are mixed in a muller for
1min and then resin is added and heated for
another 3 min.
• Only metal patterns can be used as it is
subjected to 200-350 C.
• Coated sand is allowed to be in contact with
metallic pattern and is heated.
• Pattern is removed and clamped together and
ready for pouring.
Cont.
Advantages:
a. Dimensionally accurate as well as tolerance
of 0.03 – 0.13mm can be achieved.
b. Smoother surface can be achieved.
c. Lower draft angle required.
d. Thin sections can also be made.
e. Small amount of sand is used.
Limitations:
a. Patterns are expensive, so only used for large
scale production
b. Size of casting is limited
c. Highly complicated shapes can not be
prepared
d. Sophisticated equipments are required for
handling
Precision Investment casting
• A wax pattern is prepared by injecting molten
wax at pressure of 2.5MPa in metallic die
• Pattern is ejected and cluster of wax patterns
are attached to gating system by application of
heat.
• Prepared pattern is dipped into a slurry made
by suspending fine ceramic materials
Cont.
• When sufficient thickness is attained, the
mould is reverted and is heated to take wax out
of it (Removal of pattern).
• Now, the pattern is preheated and molten metal
is poured in it
Advantages
a. Since the pattern is withdrawn by melting , so
shapes which are difficult to produce by any
other method are possible to fabricate.
b. Very fine details can be incorporated.
c. Dimensional accuracy and good surface
finish can be achieved.
d. Casting is ready for use as very little or no
machining is required.
Cont.
• Controlled mechanical properties can be
achieved.
Limitations
a. Size is limited.
b. Expensive due to larger manual labour
involved.
Die Casting
• Die casting involves injecting molten metal at
high pressure into a metallic die
• It involves one stationary half and a moving
half.
• Moving half is used to withdraw casting.
• Lubricant is sprayed on inner surface of
metallic die in order to avoid fusion.
• Casting is solidify under pressure.
• Hot chamber die casting: Furnace is integral
with die.
• Cold Chamber die casting:Furnace is not integral
with casting
Die Casting
Casting defects
•
•
•
•
•
Gas defects
Shrinkage cavities
Molding material defects
Pouring metal defects
Metallurgical defects
Gas Defects
These defects are mainly caused by lower gas
passing tendency of the mould.
• Blow holes and open blows:Spherical,
flattened
or
elongated cavities caused by moisture left in
the core.
On the surface they are called open blows and
inside they are called blow holes
• Air Inclusion:the atmospheric and other gases absorbed by
molten metal in furnace, laddle or during flow
in mould, if not allowed to escape will weaken
it.
• Pin hole Porosity:when the molten metal solidify, its solubility
with gases decreases. If hydrogen is present, it
will expell from molten metal and will cause a
long pin hole shape cavity. High pouring
temperature is main reason for that defect.
Shrinkage cavity
• These are caused by liquid shrinkage occurring
during the solidification of the casting. To
compensate for this, proper feeding of liquid
metal is required.
Moulding material defects
These defects are caused by characteristics of
molding material
• Cuts & Washes:Appears as rough spots and area of excess metal
due to erosion of moulding sand by the
flowing molten metal.
Caused by poor strength of moulding sand or
molten metal flowing at high velocity.
• Metal penetration:When the molten metal enters the gap between
sand grains resulting in rough casting surface.
This could be due to coarse grain size or no
mould wash is applied to mould cavity.
• Fusion:fusion of sand grains with molten metal resulting
in brittle, glassy appearance. Reason may be
lower refractoriness of clay in moulding sand
or higher pouring temperature.
• Runout:A runout is caused when the molten metal leaks
out of the mould. Caused by faulty mould
making or faulty flask.
• Rat tails and buckles:caused by compression failure of the skin of
mould cavity. Due to excessive heat, sand
expands resulting in a small line on casting
surface. Buckles are rat tails that are severe.
Reasons may be poor expansion properties of
sand or high pouring temperature.
• Swell:Under the metallostatic force mould wall my
move back causing a swell in the dimension.
This may result in increased feeding
requirement.
• Drop:The dropping of loose moulding sand or lumps
from the cope surface into the mould cavity.
Caused by improper ramming of the cope
flask.
Pouring metal defects
Pouring metal defect
• Misrun & Coldshut:Misrun is caused when the metal is unable to fill
the mould cavity completely.
Coldshut is caused when two metal streams
while meeting in the mould cavity do not fuse
together properly.
These are caused due to lower fluidity of molten
metal or section thickness is too small.
Metallurgical defects
• Hot tears:Since metal has low strength at higher
temperature, any unwanted cooling stress may
cause rupture of the casting.
• Hot spots:These are caused by chilling of the casting.
CASTING SYSTEM DESIGN
Aim of an effective gating system are as follows:-
• Mould should be completely filled in smallest
possible time.
• Metal should flow smoothly in cavity without
turbulence.
• Unwanted material should not be allow to
enter mould cavity.
• Entry of metal should be properly controlled
so that atmospheric air can be released.
Cont……..
• A proper thermal gradient should exist in
casting so that it can be cooled without any
shrinkage cavity or distortion.
• Metal flow should not cause gating or mould
erosion.
• Adequate supply of molten metal is ensured.
• Gating design should be economical, simple
and easy to remove after solidification.
Pouring Basin
• Main function of PB is to reduce the
momentum of flowing liquid.
• Molten metal if poured directly in cavity can
cause erosion. Molten metal is poured in
pouring basin and act as a reservoir.
• It also separates dirt and slag which float on
the surface, with the help of a skim core.
Sprue
• Sprue is the channel through which molten
metal is brought into parting line.
• While moving downward, metal velocity
increases, hence requires smaller cross section
area.
A1V1 =A2V2
So a straight sprue will result in air inclusion in
mould. Hence a tapered sprue is always used.
Runner
• Located in horizontal plane, runner connects
the sprue to ingates.
• For ferrous metal, runner is in the cope and the
in-gate is in drag in order to trap slag and
dross.
• Runner should always be full in order to trap
slag.
Gates or In-gates
Opening through which molten metal enters the
mould cavity.
• Top gate:molten metal enters the cavity from the top.
Advantages:
a. Mould cavity is filled very quickly
b. Favorable temperature gradient exist
Disadvantages:a. As metal directly falls into the mould through
height, it is likely to cause mould erosion.
b. It may cause turbulence in mould cavity.
c. Not suitable for nonferrous alloys.
Application:Suitable for ferrous alloys and simple casting
shapes
Bottom gate
Metal enters the mould cavity from bottom.
Advantages:a. It would not cause mould erosion
b. Very useful for deep moulds.
Limitations:
a. Cause unfavorable temperature gradient.
b. Side risers are to be used with these.
Parting gate
Most widely used gates, metal enters the cavity
at the parting plane.
For cope it is bottom gate and for drag it is top
gate.
Advantages:a. Easiest and most economical to prepare.
b. Incorporate advantages of both top and
bottom gate.
Limitations:
Not useful if drag part is too deep.
Step gate
• Molten metal enters the cavity through number
of vertical gates. Such gates are used for heavy
and large casting.