1. CHAPTER 2 FOUNDRY PROCESSES 2.1 INTRODUCTION
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CHAPTER 2
FOUNDRY PROCESSES
2.1 INTRODUCTION
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:
Mold
Pattern
Core
Molding Procedure
Sand
Properties of Cast liquid
Behavior of Cast Material
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6. There are two types of molds:
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2.2 MOLDS
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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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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8. A mold should have the following characteristics:
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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.
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10. to account for shrinkage in cooling and solidification, and
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Patterns in sand casting are used to form the mold cavity. One major requirement is that patterns (and therefore the mold cavity) must be oversized:
to account for shrinkage in cooling and solidification, and
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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11. Solid pattern for a pinion gear
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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.
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2.3.1 Pattern Allowances
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.
Fig Pattern Allowances for a Cast Connecting Rod.
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13. Typical shrinkage allowances:
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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
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14. Exterior dimensions: 1/8 - 1/4 (in/ft), 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 % %
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.
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Fig Types of Cores.
Most commonly used binder is Linseed oil. The oil forms a film around the sand grain and hardens when baked at C for 2 hours. Other binders are wheat flour, dextrin, starch and several types of thermosetting plastics.
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Cores serve to produce internal surfaces in castings In some cases, they have to be supported by chaplets for more stable positioning:
Core held in place in the mold cavity by chaplets,
chaplet design,
casting with internal cavity
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Cores are made of foundry sand with addition of some resin for strength by means of core boxes:
Core box, two core halves ready for baking, and the complete core made by gluing the two halves together
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19. Production sequence in sand casting
Pattern making
Mold making
Preparation 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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20. 2.5 MOLDING PROCEDURE Procedure for making green sand molds;
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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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21. B. Drag rolled over and pattern assembled ready to ram cope
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B. Drag rolled over and pattern assembled ready to ram cope
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22. C. Mold complete with dry sand core in place
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C. Mold complete with dry sand core in place
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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. The binding qualities can be obtained by adding 8-15 % clay (kil).
Silica (SiO2) Binders  Green Sand Mold
Moisture 5-10% (used in castings of Cast Iron
Clay 8-15% and Non-ferrous Alloys)
Silica (SiO2) Binders  Dry Sand Mold
Linseed Oil (used in castings of Steels)
(40 part) (1 part)
Dry it first and then bake at C 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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25. Bigger grain size results in a worse surface finish
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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
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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.
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.
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c) Moisture Content: Measure the weight of the given sand sample. Dry it around 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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28. 2.8 PROPERTIES OF CAST LIQUID
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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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29. Factors affecting fluidity: Œ Pouring temperature  Metal composition
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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:
ΠPouring temperature
 Metal composition
Ž Heat transfer to the surroundings
 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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30. 2.9 HEATING THE METAL Heat energy required Heat to rise Tm
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2.9 HEATING THE METAL
Heat energy
required
Heat to rise Tm
Heat to fusion
(solid→liquid)
Tpouring = +
where
: Total heat required, Btu (J)
: Density, lbm/in2 (g/cm3)
: Weight specific heat for solid, Btu/lb-°F (J/g- °C)
: Volume of metal, in3 (cm3)
: Melting temperature, °F (°C)
: Room temperature, °F (°C)
: Heat of fusion, Btu/lb-°F (J/g- °C)
: Weight specific heat for liquid, Btu/lb-°F (J/g- °C)
: Pouring temperature, °F (°C)
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31. * 1 2.10 POURING ANALYSIS Sum of the energies from Bernoulli eqn.
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2.10 POURING ANALYSIS
Sum of the energies from Bernoulli eqn.
Head + Press.+ Kinetic E. + Fric.
Atmospheric pressure 1 2 *
Neglected
Base (Datum) point
Speed at the beginning of pouring
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Volume rate of flow remains constant
For continuity law
(Volumetric flow rate)
MFT = Mold filling time (sec)
V = Volume (cm3)
Q = Volumetric flow rate (cm3/sec)
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33. The Chvorinov’s Rule is used to calculate the riser’s dimensions.
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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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34. 2.11 RISER (FEEDER) DESIGN Chvorinov’s rule:
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2.11 RISER (FEEDER) DESIGN
Chvorinov’s rule:
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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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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EXAMPLE
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.
5 cm
15 cm
10 cm
Sol’n: →
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2.12 CASTING QUALITY
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.
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
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