mp ch. 10 - lectures
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What is manufacturing?
the process of converting raw materials into products.
The word manufacturing is derived from the Latin manu factus, meaning madeby hand.
the conversion of stuff into things (by DeGarmon, 1998).
processing or making a product from raw materials, especially as a large scale
Operation using machinery (by Collin English Dictionary, 1998).
economic term for making goods and services available to satisfy customer
- (by T.Black, 1991).
INTRODUCTION TO MANUFACTURING
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- the making of products from raw materials using various processes,
equipments, operations and manpower according to a detailed plan.
- During processing, the raw material undergoes changes to allow it to become
a part of a product(s).
- Once processed, it should have worth in the market or a value.
- Therefore, it encompasses:
- The design of the product.
- The selection of raw materials.- The sequence of processes through which the product will be manufactured.
- Word production is often interchangeably with word manufacturing.
INTRODUCTION TO MANUFACTURING
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Manufacturing can be defined two ways:
1) Technology manufacturing is the application of physical and chemical
processes to alter the geometry, properties, and/or appearance of a given
starting material to make parts or products.Manufacturing also includes the assembly of multiple parts to make products.
2) Economic manufacturing is the transformation of materials into items of
greater value by means one or more processing involve. Therefore,manufacturing is added value to the material.
The processes to accomplish manufacturing involve a combination of
machinery, tools, power, and manual labor.
- Added value by changing the materials shape or properties or by
combining it with other materials that have been similarly altered.
INTRODUCTION TO MANUFACTURING
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Two ways models to define manufacturing:
1)As a technical process
Manufacturing
Process
Raw materials
Product
Profit
Machine
ry
Tooling
Power
Labor
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2)As an economic process.
ManufacturingProcess
Value
added
Starting
material
Material in
processing
Processed
material
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ME 311: MANUFACTURING PROCESSES - Introduction
Classification of
Manufacturing
Processes
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ME 311: MANUFACTURING PROCESSES - Fundamental of Metal Casting
MANUFACTURING
PURCHASINGMACHINERY & TOOLING
PROCESS PLANNING
MATERIALS
PRODUCT DESIGN
PRODUCTION CONTROL
SUPPORT SERVICES
SHIPPING
CUSTOMER SERVICE
MARKETING
SALES
MANUFACTURING INDUSTRY
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ME 311: MANUFACTURING PROCESSES - Fundamental of Metal Casting
FUNDAMENTALS OF METAL CASTING
1. Overview of Casting Technology
2. Heating and Pouring
3. Solidification and Cooling
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METAL CASTING PRODUCTS
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METAL CASTING PRODUCTS
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Solidification Processes
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:
Metals
Ceramics, specifically glasses Polymers and polymer matrix composites (PMCs)
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Classification of Solidification Processes
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Casting of Metals
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 castingalso 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
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Capabilities and Advantages of Casting
Can create complex part geometries
Can create both external and internal shapes
Some casting processes are net shape; others are nearnet shape
Can produce very large parts
Some casting methods are suited to mass production
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Disadvantages of Casting
Different disadvantages for different casting processes:
Limitations on mechanical properties
Poor dimensional accuracy and surface finish forsome processes; e.g., sand casting
Safety hazards to workers due to hot molten
metals
Environmental problems
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Parts Made by Casting
Big parts
Engine blocks and heads for automotive
vehicles, wood burning stoves, machine frames,railway wheels, pipes, church bells, big statues,
pump housings
Small parts
Dental crowns, jewelry, small statues, fryingpans
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Overview of Casting
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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
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The Mold in Casting
Contains cavity whose geometry determines part shape
Actual size and shape of cavity must be slightly
enlarged to allow for shrinkage of metal duringsolidification and cooling
Molds are made of a variety of materials,
including sand, plaster, ceramic, and metal
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Open Molds and Closed Molds
Two forms of mold: (a) open mold and (b) closed mold for
more complex mold geometry with gating system leading
into the cavity
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Two Categories of
Casting Processes1. Expendable mold processes use an expendable
mold which must be destroyed to remove casting
Mold materials: sand, plaster, and similar
materials, plus binders
2. Permanent mold processes use a permanent mold
which can be used to produce many castings
Made of metal (or, less commonly, a ceramic
refractory material
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Advantages and Disadvantages
More intricate geometries are possible with expendable
mold processes
Part shapes in permanent mold processes are limited
by the need to open the mold
Permanent mold processes are more economic in high
production operations
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Sand Casting Mold
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2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e
Terminology for Sand Casting Mold
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 theparting line
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Forming the Mold Cavity in Sand Casting
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 of
the packed sand has desired shape of cast part
The pattern is usually oversized to allow for shrinkage
of metal during solidification and cooling
Sand for the mold is moist and contains a binder to
maintain its shape
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Use of a Core 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
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Gating System
Channel through which molten metal flows into cavity from
outside of mold
Consists of a downsprue, through which metal enters a
runnerleading to the main cavity
At the top of downsprue, apouring cup is often used to
minimize splash and turbulence as the metal flows into
downsprue
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2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e
Riser
Reservoir in the mold which is a source of liquid metal to
compensate for shrinkage of the part during
solidification
The riser must be designed to freeze after the main
casting in order to satisfy its function
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Heating & Pouring
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Heating 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
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where H= total heat required to raise the temperature of the metal to
the pouring temperature, J ;
= density, g=cm3
Cs= weight specific heat for the solid metal, J/g C;
Tm = melting temperature of the metal, C;
To = starting temperatureusually ambient, C;
Hf= heat of fusion, J/g;
Cl= weight specific heat of the liquid metal, J/g C;Tp = pouring temperature, C;
= volume of metal being heated, cm3
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Assume ambient temperature in the foundry is 25C
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Discuss the assumptions of the problem
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Pouring the Molten Metal
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
Pouring temperature
Super Heat
Pouring rate Turbulence
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Pouring and Gating System
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Functions of Gating System
Fill the mould cavity completely before freezing
Minimizing turbulence
Avoiding Erosion
Removing inclusions
Regulate flow of molten metal
Consume least metal - less scrap
Establish directional solidification.
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Pouring Cups
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Engineering Analysis of Pouring
An important relationship that governs the law of liquid
metal through the gating system in the mould.
After simplifications and taking assumption, this can be
used to measure the flow velocity
V=Velocity of the liquid at the base of the sprue
h= Height of the sprue
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Engineering Analysis of Pouring
Another important relationship during pouring is Continuity Law
V=Velocity of the LiquidA = Cross-Section area of the Liquid
The time required to fill a mould cavity of volume as
TMF= mould filling time, sec;
V= volume of mould cavity, cm3;
Q= volume flow rate, cm3/s.ME 311: MANUFACTURING PROCESSES - Fundamental of Metal Casting
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Engineering Analysis of Pouring
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Fluidity Fluidity is a measure of the capability of a metal to flow into
and fill the mould before freezing.
Spiral mould test is usually used to assess the fluidity
Factors affecting fluidity include
Pouring temperature relative to melting point,
Metal composition,
Viscosity of the liquid metal,
Heat transfer to the surroundings. The best fluidity is obtained by metals that freeze at a
constant temperature (e.g., pure metals and eutectic alloys)
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Solidification and Cooling
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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
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Cooling Curve for a Pure Metal
A pure metal
solidifies at a
constant
temperature equalto its freezing point
(same as melting
point)
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Solidification of Pure Metals
Due to chilling action of mold wall, a thin skin of solid
metal is formed at the interface immediately after
pouring
Skin thickness increases to form a shell around themolten metal as solidification progresses
Rate of freezing depends on heat transfer into mold, as
well as thermal properties of the metal
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Characteristic grain
structure in a casting of a
pure metal, showing
randomly oriented grainsof small size near the
mold wall, and large
columnar grains oriented
toward the center of the
casting
Solidification of Pure Metals
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Solidification of Alloys
Most alloys freeze over a temperature range
Phase diagram for a copper-nickel alloy system and
cooling curve for a 50%Ni-50%Cu composition
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Characteristic grain
structure in an alloy
casting, showing
segregation of alloyingcomponents in center
of casting
Solidification of Alloys
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Solidification Time
Total solidification time TTS = time required for casting
to solidify after pouring
TTS depends on size and shape of casting by
relationship known as Chvorinov's Rule
where TTS = total solidification time; V= volume of the
casting;A = surface area of casting; n = exponent withtypical value = 2; and Cm is mold constant.
n
TS m
VT C
A
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Mold Constant in Chvorinov's Rule
Mold constant Cm depends on:
Mold material
Thermal properties of casting metal
Pouring temperature relative to melting point
Value ofCm 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
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What Chvorinov's Rule Tells Us
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 the main cavity, TTS for
riser must be greater than TTS for main casting
Since mold constants of riser and casting 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
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Shrinkage during Solidification and Cooling
(0) starting level of molten metal immediately after pouring;
(1) reduction in level caused by liquid contraction during
cooling
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Shrinkage during Solidification and Cooling
(2) reduction in height and formation of shrinkage cavity
caused by solidification; (3) further reduction in volume
due to thermal contraction during cooling of solid metal
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Typical linear shrinkage values for different casting metals
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Typical linear shrinkage values for different casting metals
due to solid thermal contraction.
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Solidification Shrinkage
Occurs in nearly all metals because the solid phase has
a higher density than the liquid phase
Thus, solidification causes a reduction in volume per
unit weight of metal Exception: cast iron with high C content
Graphitization during final stages of freezing
causes expansion that counteracts volumetric
decrease associated with phase change
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Shrinkage Allowance
Patternmakers correct for solidification shrinkage and
thermal contraction by making the mold cavity
oversized
Amount by which mold is made larger relative to finalcasting size is calledpattern shrinkage allowance
Casting dimensions are expressed linearly, so
allowances are applied accordingly
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Directional Solidification
To minimize effects of shrinkage, it is desirable for
regions of the casting most distant from the liquid metal
supply to freeze first and for solidification to progress
from these regions toward the riser(s) Thus, molten metal is continually available from
risers to prevent shrinkage voids
The term directional solidification describes this
aspect of freezing and methods by which it iscontrolled
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Achieving Directional Solidification
Directional solidification is achieved using Chvorinov's
Rule to design the casting, its orientation in the mold,
and the riser system that feeds it
Locate sections of the casting with lowerV/A ratiosaway from riser, so freezing occurs first in these
regions, and the liquid metal supply for the rest of
the casting remains open
Chills - internal or external heat sinks that causerapid freezing in certain regions of the casting
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Achieving Directional Solidification
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External chill to encourage rapidfreezing of the molten metal in a
thin section of the casting
the likely result if the externalchill were not used.
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Riser Design
Riser is waste metal that is separated from the casting
and re-melted to make more castings
To minimize waste in the unit operation, it is desirable
for the volume of metal in the riser to be a minimum Since the shape of the riser is normally designed to
maximize the V/A ratio, this allows riser volume to be
reduced to the minimum possible value
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Types of Riser
Side Riser
Top Riser
Open Riser
Blind Riser
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