Ch2 foundaryproc Erdi Karaçal Mechanical Engineer University of Gaziantep

02/11/14 CHAPTER 2 FOUNDARY
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CHAPTER 2
FOUNDRY PROCESSES
2.1 INTRODUCTION
ME 333 PRODUCTION PROCESSES II
Foundry processes consist of making molds, preparing and melting the metal
into the molds, cleaning the castings, and reclaiming the sand for reuse.
Founding, or casting, is the process of forming objects by putting liquid or
viscous material into a prepared mold or form. Generally solidification takes
place by cooling (metallic materials) but cooling may not be necessary (some
plastics).
A casting (döküm) is an object formed by allowing the material to solidify. So, the
casting is the product of the foundry. It may vary from a fraction of a gram to
several tons. All metals and alloys can be cast.
A foundry (dökümhane) is a collection of the necessary material and equipment
to produce a casting.

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Selection of castings of various materials, shapes, and sizes

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Casting technology involves the next steps:
Casting nomenclature
The figure in the right shows the nomenclature of mold and castings in sand casting.

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The pouring cup, downsprue, runners, etc., are known as the mold gating
system, which serves to deliver the molten metal to all sections of the mold
cavity.
Gating system in sand casting

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To understand the foundry process, it is necessary to know how a mold is made
and what factors are important to produce a good casting.
The elements necessary for the production of sound casting will be considered
throughout this chapter.
These include:
1. Mold
2. Pattern
3. Core
4. Molding Procedure
5. Sand
6. Properties of Cast liquid
7. Behavior of Cast Material

A mold (kalıp) is the container that has the cavity of the shape to be cast. It may
be made of metal, plaster, ceramics, or other refractory substances. Good
castings can not be produced without good molds
There are two types of molds:
1. Permanent mold: A mold used more than once. They are generally produced
from metallic materials such as; heat resisting (Ni-Cr) steels.
2. Expendable mold: A mold used only once and then destroyed to separate
the component. They are generally produced from sand. (for casting of ferrous
materials we have to use this type of mold, because melting points of ferrous
materials are very high).
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2.2 MOLDS

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There are plenty types of expendable molds, but we will deal with sand molds
only;
a) Green Sand Molds: The most common type consisting of forming
the mold from damp molding sand (silica, clay and moisture)
b) Skin-dried Molds: It is done in two ways; (1) The sand around
the pattern to a depth of about 1/2 in(10 mm). is mixed with a binder so that
when it is dried it will leave a hard surface on the mold. (2) Entire mold is
made from green sand, but a spray or wash, which hardens when heat is
applied, is used.
c) Dry Sand Molds: These molds are made entirely from fairly
coarse molding sand mixed with binders (linseed oil: bezir yağı or gelatinised
starch: nişasta). They baked before being used. A dry sand mold holds its
shape when poured and is free from gas troubles due to moisture.

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A mold should have the following characteristics:
i) The mold must be strong enough to hold the weight of the metal,
ii) The mold must resist the erosive action of the rapidly flowing
metal during pouring,
iii) The mold must generate minimum amount of gas when filled with
molten metal.
iv) The mold must be constructed in such a way that any gasses
formed can pass through the body of the mold itself (permeability).
v) The mold must be refractory enough to withstand the high
temperature of the metal.
vi) The mold must collapse easily after the casting solidifies.

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2.3 . PATTERNS
A pattern (model) is a form used to prepare and produce a mold cavity. It is
generally made from wood but it can be produced from materials like aluminium
alloys (low in density). (Disadvantage of wood is humidity absorption.)
The designer of a casting must look forward to the pattern to assure economical
production. The design should be as simple as possible to make the pattern easy
to draw from the sand and avoid more cores than necessary.
The pattern may be permanent, so that it may be reused repeatedly. Alternatively,
the pattern may be expendable (disposable), made up of a material that is melted
out before or burnt up during casting.
Pattern has some dimensional variations from that of the real component (i.e.
casting). These variations from the real component are called Pattern Allowances.

Patterns in sand casting are used to form the mold cavity. One major requirement
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is that patterns (and therefore the mold cavity) must be oversized:
(i) to account for shrinkage in cooling and solidification, and
(ii) to provide enough metal for the subsequence machining operation(s).
Types of patterns used in sand casting:
(a) solid pattern, (b) split pattern, (c) match-plate pattern, and (d) cope-and-drag pattern

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Solid pattern for a pinion gear Split pattern showing the
two sections together and
separated. Light-colored
portions are core prints.

1. Shrinkage Allowance: Shrinkage takes place in a volumetric way, but it is
given linearly. Each dimension is measured with a shrinkage rule, which
automatically gives shrinkage allowance. It is expressed as in/ft. When metal
patterns are to be cast from an original master pattern, double shrinkage must be
given.
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2.3.1 Pattern Allowances
Fig. 2.1. Pattern Allowances for a Cast Connecting Rod.

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Typical shrinkage allowances:
Cast Iron Steel Al Brass Bronze
In/ft 1/8 1/4 5/32 3/36 1/8-1/4
% 1.04 2.08 1.30 2.0 1.04-2.08

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2. Draft: It is the taper placed on the sides of the pattern on the parting line.
This allows the pattern to be removed from the mold without damaging the sand
surface. Draft is added to the dimensions on the parting line
Exterior dimensions: 1/8 - 1/4 (in/ft), 1.04 %- 2.08 %
Interior dimensions: As large as 3/4 (in/ft), 6.25 %
3. Machining Allowance: It is given on the working areas of the part where
further machining will be performed. In value, it is equal to shrinkage allowance.
4. Shake: Negative allowance is given by making the pattern slightly smaller to
compensate for the rapping of the mold.

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2.4 CORES
A core (maça) is a body of material, usually sand, used to produce a cavity in
or on a casting. A core must have sufficient strength to support itself and
should not fracture when liquid metal is approaching to it.
Cores may be classified as Green-Sand and Dry-Sand Cores. Green-sand
cores are formed by the pattern and made from the same sand as rest of the
mold. Dry-sand cores are made separately to be inserted after the pattern is
drawn but before the mold is closed. They are usually made of clean river
sand (40 parts) which is mixed with a binder (1 part) and then baked to give
the desired shape. The box in which cores are formed to proper shape is
called a CORE BOX. Generally, perforated pipe or wire frames are added to
give sufficient strength.

Fig. 2.2. Types of Cores.
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Most commonly used binder is Linseed oil. The oil forms a film around the sand
grain and hardens when baked at 180-2200C for 2 hours. Other binders are
wheat flour, dextrin, starch and several types of thermosetting plastics.

Cores serve to produce internal surfaces in castings In some cases, they have to
be supported by chaplets for more stable positioning:
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(a) Core held in place in the mold cavity by chaplets,
(b) chaplet design,
(c) casting with internal cavity

Cores are made of foundry sand with addition of some resin for strength by means
of core boxes:
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Core box, two core halves ready
for baking, and the complete
core made by gluing the two
halves together

Production sequence in sand casting
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Pattern making
Preparation Mold making
of sand
If necessary
core making
Raw
material
Melting Pouring
Solidification and
cooling
Removal of sand
mold
Cleaning &
Inspection
Finished casting

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2.5 MOLDING PROCEDURE
Procedure for making green sand molds;
A. Pattern on molding board ready to ram up drag

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B. Drag rolled over and pattern assembled ready to ram cope

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C. Mold complete with dry sand core in place

2.6 SAND
Silica sand (SiO2) is well suited for molding purposes because it can withstand a
high temperature without decomposition. This sand is low in cost, has longer life,
and is available in a wide range of grain sizes and shapes.
Pure silica sand is not suitable in itself for molding, since it lacks binding qualities.
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The binding qualities can be obtained by adding 8-15 % clay (kil).
Silica (SiO2) + Binders Þ Green Sand Mold
Moisture 5-10% (used in castings of Cast Iron
Clay 8-15% and Non-ferrous Alloys)
Silica (SiO2) + Binders Þ Dry Sand Mold
Linseed Oil (used in castings of Steels)
(40 part) (1 part)
Dry it first and then bake at 180-2200C for 2 hours

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Synthetic molding sands are composed of washed, sharp grained silica to which 3-5
% clay is added. Less gas is generated with synthetic sands, since less than 5 %
moisture is necessary to develop adequate strength.
The size of the sand grains will depend on the type of work to be molded. For small
and intricate castings fine sand is desirable so that all details of the mold are
brought out sharply. Sharp, irregular-shaped grains are usually preferred because
they interlock and add strength to the mold.

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Foundry sands
The typical foundry sand is a mixture of fresh and recycled sand, which contains 90%
silica (SiO2), 3% water, and 7% clay.
The grain size and grain shape are very important as they define the surface quality of
casting and the major mold parameters such as strength and permeability:
Bigger grain size results in a worse
surface finish
Irregular grain shapes produce
stronger mold
Larger grain size ensures better
permeability

2.7 SAND QUALITY TEST
Periodic tests are necessary to determine the essential qualities of foundry sand.
Various tests are designed to determine the following properties of molding sand.
a) Hardness Test (Mold Hardness): A spring loaded (2.3 N) steel ball 5.08 mm in
diameter is pressed into the surface of the mold and depth of penetration is
recorded as hardness. Medium hardness is about 75.
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vibrator
6
12
270
b) Fineness Test: It is used to obtain
percentage distribution of grain
sizes in the sand. Sand is cleaned
and dried to remove clay. It is
placed on graded sieves, which
are located on a shaker. Standard
sieve sizes (mesh) are
6,12,20,30,40,50,70,100,200 and
270. Shaking time is 15 minutes.

c) Moisture Content: Measure the weight of the given sand sample. Dry it around
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1000C and then weigh it again. Calculate the percentage.
d) Clay Content: A sample of sand is dried and then weighed. Then clay is
removed by washing the sand with caustic soda which has absorbed the clay.
Sand is dried and weighed again. The percentage gives the clay content.
e) Strength Test: Most common compressive test. A universal strength tester
loads a 50 mm long 50 mm diameter specimen by means of dead weight
pendulum with a uniform loading rate.
f) Permeability: It is measured
by the quantity of air that
passes through a given
sample of sand in a
prescribed time under
standard pressures.
g) Refractoriness Test: High
temperature withstanding
ability of sand is measured.
piston

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2.8 PROPERTIES OF CAST LIQUID
The properties of the castings depend on foundry skin as well as other material
properties. Under similar foundry conditions, the properties will be affected by:
a) Viscosity of the liquid metal: It is a function of superheat that is the degree of
overheating above the melting temperature. Since the pouring process is
essentially a problem of fluid flow, lower viscosity is beneficial.
b) Surface Tension: It affects the wetting of inclusions and also limits the minimum
radius that can be filled without pressure (typically to 0.1 mm in cavity casting).
c) Oxide Films: Surface of the liquid metal quickly oxidizes and metals act as if it is
flowing in an envelope. Aluminum produces many problems due to quick
formation of strong oxides.
d) Fluidity: It is material plus mold property. It is the ability to fill the cavity in the
mold.

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Fluidity is a measure of the capability of a metal to flow into and to fill the mold
before freezing. It defines to the great extend the quality of casting.
Factors affecting fluidity:
1. OE Pouring temperature
2.  Metal composition
3. Ž Heat transfer to the surroundings
4.  Viscosity of the liquid metal
In the foundry practice, test for
fluidity is carried out for each
ladle just before pouring the
molten metal into the mold

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2.9 HEATING THE METAL
Heat energy
required
Heat to rise
Tm
Heat to fusion
(solid→liquid)
Heat to rise
Tpouring
= + +
{ ( ) ( )} s m o f e p m H = rV C T -T + H +C T -T
where
H : Total heat required, Btu (J)
r : Density, lbm/in2 (g/cm3)
s C : Weight specific heat for solid, Btu/lb-°F (J/g- °C)
V : Volume of metal, in3 (cm3)
m T : Melting temperature, °F (°C)
o T : Room temperature, °F (°C)
f H : Heat of fusion, Btu/lb-°F (J/g- °C)
e C : Weight specific heat for liquid, Btu/lb-°F (J/g- °C)
p T : Pouring temperature, °F (°C)

h + P + V + = + + +
Speed at the beginning of pouring
2
2
h V 2
1 Þ = Þ =
2 2
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2.10 POURING ANALYSIS
2
2
2 2
1 2
2
1 1
1 2 2
F
g
F h P V
g
r r
0 1 2 P = P = Atmospheric pressure
0 1 2 F = F = Neglected
0 2 h = Base (Datum) point
0 1 V =
1
2
*
V gh *
g
Sum of the energies from Bernoulli eqn.
Head + Press.+ Kinetic E. + Fric.

MFT = V
Volume rate of flow remains constant
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For continuity law
1 1 2 2 Q =V A =V A (Volumetric flow rate)
Q
MFT = Mold filling time (sec)
V = Volume (cm3)
Q = Volumetric flow rate (cm3/sec)

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2.11 RISER (FEEDER) DESIGN
Several riser designs are used in practice as shown in the figure. The riser must
remain molten until after the casting solidifies.
The Chvorinov’s Rule is used to calculate the riser’s dimensions.
Possible types and positions for risers in sand casting

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2.11 RISER (FEEDER) DESIGN
Chvorinov’s rule:
n
TST = Cm(V A)
TST : Total Solidification Time (min)
Cm : Mold Constant (min/cm2)
V : Volume (cm3)
A : Surface area (cm2)
n : Exponent (n=2)

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Tf
TST
Tp
Tm
Liq.
V/A ↑ TST ↓
TSTcasting<TSTriser
Lower V/A located away from risers
So that: riser remains liquid until after the casting solidity

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= p = p
V D h D r
=p + p = p
A Dh 2 D D r
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EXAMPLE
5 cm
15 cm
10 cm
A cylindrical riser with dimensions of D=h
must be designed. Previous observations
show TST=1.6 min. for casting. Determine
dimension of riser.
TSTriser=2 min. suggested as.
Sol’n:
n
TST = Cm(V A)
V 15 10 5 750cm3 c = ´ ´ =
A 2(15 10 15 5 10 5) 550cm2 c = ´ + ´ + ´ =
1.6 (750 550)2 m = C
→ C 0.86min cm2 m =
2 3
4 4
6
2 2
4
4
4
3 D
D
p
r = =
2
D
6 4 6
V
A
r
p
2 = 0.86(D 6)2
D = 9.15cm h = 9.15cm

There are numerous opportunities in the casting operation for different defects to
appear in the cast product. Some of them are common to all casting processes:
Misruns: Casting solidifies before completely fill the mold. Reasons are low pouring
temperature, slow pouring or thin cross section of casting.
Cold shut: Two portions flow together but without fusion between them. Causes are
similar to those of a misrun.
Cold shots: When splattering occurs during pouring, solid globules of metal are
entrapped in the casting. Proper gating system designs could avoid this defect.
Shrinkage cavity: Voids resulting from shrinkage. The problem can often be solved
by proper riser design but may require some changes in the part design as well.
Microporosity: Network of small voids distributed throughout the casting. The defect
occurs more often in alloys, because of the manner they solidify.
Hot tearing: Cracks caused by low mold collapsibility. They occur when the material
is restrained from contraction during solidification. A proper mold design can solve
the problem.
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2.12 CASTING QUALITY
Some defects are typical only for some particular casting processes, for instance,
many defects occur in sand casting as a result of interaction between the sand mold
and the molten metal. Defect found primarily in sand casting are gas cavities, rough
surface areas, shift of the two halves of the mold, or shift of the core, etc.

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THE END

Ch2 foundaryproc Erdi Karaçal Mechanical Engineer University of Gaziantep

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Ch2 foundaryproc Erdi Karaçal Mechanical Engineer University of Gaziantep