Lecture # 1:
Lecture # 1 Contents
1. Introduction to Manufacturing Processes
4. Types of molds
5. Heating and pouring
Introduction to Manufacturing Processes:
Manufacturing is the term used to describe the making of products. Until the nineteenth century it
was largely an activity reserved for craftsmen. The industrial revolution during the second half of
the nineteenth century introduced manufacturing mechanization.
The use of machines for spinning and weaving in the textile industry is generally acknowledged
to be the beginning of modern manufacturing. During this same time, Bessemer (1855) in England
and William Kelly (1857) in the United States proposed methods for the mass production of steel.
This was followed by the Hall-Heroult process (1885) for melting aluminum. These processes
provided relatively cheap sources of the materials required to drive the industrial revolution.
One definition of manufacturing is the conversion of either raw or semi-finished materials into
finished parts. Such a definition serves to emphasize the importance of materials in manufacturing
operations. In fact, the choice of material for a given manufacturing situation may be the limiting
consideration. In general, a material must satisfy two criteria.
Manufacturing Operations Types:
Manufacturing operations can be generally classified into primary and secondary processes.
Primary Manufacturing Processes:
For metals, primary manufacturing usually refers to the conversion of ores into metallic materials.
The vast majority of pig iron produced from iron ores is processed by blast furnaces.
Steel making by Bessemer steel converter.
Secondary Manufacturing Processes:
Secondary manufacturing is generally understood to mean the conversion of the products from the
primary operation into semi-finished or finished parts. For example, the fabrication of automobile
engine blocks from a primary melt of iron or aluminum is said to be secondary manufacturing.
The conversion of primary products into secondary finished or semi-finished components can take
place by one of several alternative routes.
1. Mechanical Machining methods
2. Non-traditional machining processes
Mechanical Machining Methods:
1. Conventional Machining
a. Single point machining.
b. Turning, Shaping, Planning etc.
2. Multipoint Machining
Drilling, Milling etc.
3. CNC Machining
a. CNC Turning.
b. CNC Milling.
Non-Traditional Machining Processes:
1. Abrasive water jet Machining.
2. ECM (Electro-chemical machining.)
3. EDM (Electro-discharge machining.)
4. EBM (Electron Beam machining.)
5. LBM (Laser Beam machining.)
6. Rapid Prototyping.
Process in which molten metal flows by gravity or other force into a mold where it solidifies in the
shape of the mold cavity.
The term casting also applies to the part made in the process.
Steps in casting seem simple:
1. Melt the metal
2. Pour it into a mold
3. Let it freeze
Capabilities and Advantages of Casting:
1. Can create complex part geometries.
2. Can create both external and internal shapes.
3. Some casting processes are net shape; others are near net shape.
4. Can produce very large parts.
5. Some casting methods are suited to mass production.
Disadvantages of Casting:
1. Limitations on mechanical properties.
2. Poor dimensional accuracy and surface finish for some processes; e.g., sand casting.
3. Safety hazards to workers due to hot molten metals.
4. Environmental problems.
Parts Made by Casting:
1. Big parts: engine blocks and heads for automotive vehicles, wood burning stoves, machine
frames, railway wheels, pipes, church bells, big statues, and pump housings.
2. Small parts: dental crowns, jewelry, small statues, and frying pans.
3. All varieties of metals can be cast, ferrous and nonferrous.
Overview of Casting Technology:
Casting is usually performed in a foundry.
Foundry = factory equipped for making molds, melting and handling molten metal, performing
the casting process, and cleaning the finished casting.
Workers who perform casting are called foundrymen.
The Mold in Casting:
Contains cavity whose geometry determines part shape.
Actual size and shape of cavity must be slightly oversized to allow for shrinkage of metal during
solidification and cooling.
Molds are made of a variety of materials, including sand, plaster, ceramic, and metal.
Two Categories of Casting Process:
Expendable Mold Processes:
Uses an expendable mold which must be destroyed to remove casting.
Mold materials: sand, plaster, and similar materials, plus binders.
Permanent Mold Processes:
Uses a permanent mold which can be used many times to produce many castings.
Made of metal (or, less commonly, a ceramic refractory material.
Figure 1.2 - Two forms of mold: (a) open mold, simply a container in the shape of the desired part;
and (b) closed mold, in which the mold geometry is more complex and requires a gating system
(passageway) leading into the cavity.
Advantages and Disadvantages:
1. More intricate geometries are possible with expendable mold processes.
2. Part shapes in permanent mold processes are limited by the need to open mold.
3. Permanent mold processes are more economic in high production operations.
Figure 1.3 (b) Sand casting mold
Sand Casting Mold Terms:
1. Mold consists of two halves:
a. Cope = upper half of mold
b. Drag = bottom half
2. Mold halves are contained in a box, called a flask.
3. The two halves separate at the parting line.
Forming the Mold Cavity:
Mold cavity is formed by packing sand around a pattern, which has the shape of the part.
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.
Cores in the Mold Cavity:
The mold cavity provides the external surfaces of the cast part.
In addition, a casting may have internal surfaces, determined by a core, placed inside the mold
cavity to define the interior geometry of part.
In sand casting, cores are generally made of sand.
Channel through which molten metal flows into cavity from outside of mold.
Consists of a down sprue, through which metal enters a runner leading to the main cavity.
At top of down sprue, a pouring cup is often used to minimize splash and turbulence as the metal
flows into down sprue.
Reservoir in the mold which is a source of liquid metal to compensate for shrinkage during
The riser must be designed to freeze after the main casting in order to satisfy its function.
Heating and Pouring the Metal:
Heating furnaces are used to heat the metal to molten temperature sufficient for casting.
The heat required is the sum of:
1.Heat to raise temperature to melting point.
2.Heat of fusion to convert from solid to liquid.
3. Heat to raise molten metal to desired temperature for pouring.
For this step to be successful, metal must flow into all regions of the mold, most importantly the
main cavity, before solidifying.
Factors that determine success:
1. Pouring temperature.
2. Pouring rate.
Starting work material is either a liquid or is in a highly plastic condition, and a part is created
through solidification of the material.
Solidification processes can be classified according to engineering material processed:
2. Ceramics, specifically glasses.
3. Polymers and polymer matrix composites (PMCs).
Solidification of Metals:
Transformation of molten metal back into solid state.
Solidification differs depending on whether the metal is a pure element or an alloy.
A pure metal solidifies at a constant temperature equal to its freezing point (same as melting
Figure 1.4 - Cooling curve for a pure metal during casting
Solidification of Pure Metals:
Due to chilling action of mold wall, a thin skin of solid metal is formed at the interface immediately
Skin thickness increases to form a shell around the molten metal as solidification progresses.
Rate of freezing depends on heat transfer into mold, as well as thermal properties of the metal.
Figure 1.5 - Characteristic grain structure in a casting of a pure metal, showing randomly oriented
grains of small size near the mold wall, and large columnar grains oriented toward the center of
Most alloys freeze over a temperature range rather than at a single temperature.
Figure 1.6 - (a) Phase diagram for a copper-nickel alloy system and (b) associated cooling curve
for a 50%Ni-50%Cu composition during casting.
Figure 1.7 - Characteristic grain structure in an alloy casting, showing segregation of alloying
components in center of casting.
Solidification takes time.
Total solidification time TST = time required for casting to solidify after pouring.
TST depends on size and shape of casting by relationship known as Chvorinov's Rule.
where TST = total solidification time; V = volume of the casting; A = surface area of casting; n =
exponent usually taken to have a value = 2; and Cm is mold constant.
Mold Constant in Chvorinov's Rule:
Cm depends on mold material, thermal properties of casting metal, and pouring temperature relative
to melting point.
Value of Cm for a given casting operation can be based on experimental data from previous
operations carried out using same mold material, metal, and pouring temperature, even though the
shape of the part may be quite different.
What Chvorinov's Rule Tells Us:
A casting with a higher volume-to-surface area ratio cools and solidifies more slowly than one
with a lower ratio.
To feed molten metal to main cavity, TST for riser must greater than TST for main casting.
Since riser and casting mold constants will be equal, design the riser to have a larger
volume-to-area ratio so that the main casting solidifies first.
This minimizes the effects of shrinkage.
Figure 1.8 - Shrinkage of a cylindrical casting during solidification and cooling: (0) starting level
of molten metal immediately after pouring; (1) reduction in level caused by liquid contraction
during cooling (dimensional reductions are exaggerated for clarity in sketches).