CASTING
Chapters 10, 11, 12
Group 7
Members
William Waller
James Ball
William Ramos
Christmas Trieu
Perry Barrow
2/22/06
What is Casting?
Casting is a process where molten metal is poured into
a patterned mold. The metal is allowed to solidify
into a part and is removed from the mold to be
manufactured.
Three Important considerations:
1. How the metal cools and solidifies in the mold
2. How the molten metal flows into the mold cavity
3. How the mold influences the metal
Solidification Of Metals
Pure Metals Vs Alloys
Pure Metals have very defined melting/freezing points
and solidifies at a specific temperature.
Pure Metals have a more uniform “columnar grain”
growth.
Solidification of Metals (cont.)
a. Columnar grains of a
pure metal
b. cast structure of solidsolution alloy
c. Non-columnar structure
Solidification Of Metals (Cont.)
Alloy solidification begins at some temperature TL and is
not completed until it reaches a temperature TS. In
between TL and TS, coined the “Mushy Zone,” both
liquid and solid exists. Here is where columnar
dendrites may form which contributes to detrimental
factors.
Effects of cooling rates on Alloys varies. The faster the
alloy cools the more columnar dendrites may form.
The slower the rate, the less columnar dendrites.
Solidification of Metals (Cont.)
Illustration of alloy solidification and temperature
distribution.
Fluid Flow
Fluid flow of molten metal is extremely important in
casting. Note how the liquid metal has to flow through
the system.
Fluid Flow (continued)
Bernoulli’s Theorem
Explains how the fluid behaves by relating pressure,
velocity and elevation of the fluid at any location within
the system.
Mass Continuity
Explains that for an incompressible liquid in a mold
with impermeable walls the volume rate of flow is the
same at one location as it is at another location.
Fluid Flow (continued)
Sprue Design
A tapered design or a choke is used to insure that the
fluid flow is sufficiently slowed to prevent aspiration.
Flow Characteristics
An important consideration of flow through a grate is
how much turbulence the fluid experiences. Turbulence
creates the formation of dross. The more that
turbulence is eliminated and laminar flow increases the
less dross is created.
Fluid Flow (continued)
Turbulent flow
Laminar flow
Fluidity of Molten Metal
Viscosity: The higher the value of viscosity the
lower the fluidity
Surface Tension: The higher the value of surface
tension of the liquid metal reduces the fluidity.
Inclusions: has adverse affects on fluidity. An
analogy is like oil with sand particles in it. This would
reduce the liquids fluidity.
Solidification Pattern of the alloy: The manner in
which solidification takes place influences fluidity.
Mold Design: influences how the fluid travels
through the mold.
Mold Material and Its Surface
Characteristics: How the mold conducts heat
and how rough its surface is influences fluidity.
Degree of Superheat: defined as the
increment of temperature of an ally above its
melting point, Superheat improves fluidity by
delaying solidification.
Rate of Pouring: The slower the rate of
pouring the lower the fluidity due to higher
cooling rates when pouring is slow.
Heat Transfer
Solidification Time explains how much time is
required for an object to solidify depending on its
volume and surface area. Depending on the
solidification time, the cavities of the mold begin to
form solidified metallic skin on it interior walls. Thus,
the interior of the mold becomes smaller.
Solidified skin on a steel casting.
Heat Transfer (continued)
Shrinkage
Due to thermal expansion, when a metal cools it tend to
shrink in size. Three events of this phenomena can
cause dimensional problems as well as cracking.
1.
2.
3.
Contraction of the molten metal as it cools prior ro its
solidification.
Contraction of the metal during phase change from liquid
to solid.
Contraction of the solidified metal as its temperature drops
to ambient temperature.
10.6 Defects
A- Metallic Projections: fins, flash, or
projections
B- Cavities-
consist of rounded/ rough
internal/ external cavities including blowholes,
pinholes, and porosity
C- Discontinuities-
cracks, cold or hot
tearing and cold shuts.
D- Defective surfaceand adhering sand layers.
surface folds, laps,
10.6 Defects
E- Incomplete casting: premature solidification, insufficient
volume of metal poured, pouring metal too slowly or molten
being at too low a temperature.
F- Incorrect dimensions/shapes: improper shrinkage
allowance, pattern mounting error, deformed pattern or warping
G- Inclusions- generally nonmetallic. Occurs during
melting, solidification, and molding. Regarded harmful b/c they
act as stress raisers which reduces the strength in casting.
Inclusions form during melting when molten metal reacts with
oxygen or mold material. Chemical reactions in molten metal
may also produce inclusions.
10.6.1 Porosity
-Caused by shrinkage or gas or both.
-Thin sections in casting solidify quicker than thicker regions
RESULT: Molten metal flows into thicker regions before proper
solidification. Porous regions may develop at center b/c of
contraction as the surfaces of thicker regions begin to solidify first.
(Think of a funnel in the middle of casting) (cavities)
Micro Porosity- when liquid solidifies and shrinks between dendrites and
dendrite branches.
Chapter 11
Metal Casting Processes
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Expendable & Permanent mold
processes
Applications, advantages, and
limitations of common casting
Casting of single crystals
Inspection techniques
Foundries and their Automation
11.1 Introduction
Two trends on casting industry:
-Mechanization & Automation: led to significant changes in
equipment and labor. Advanced machine & automated
process-control systems replace traditional casting methods.
-Increasing demand for HIGH QUALITY CASTINGS
with close dimensional tolerances.
11.1 Introduction
1.
Major Categories of Casting Practices:
Expendable Molds: After casting solidifies,
mold broken up to remove casting
2. Permanent Molds: Used repeatedly and
designed so casting can be removed easily and
mold used for next casting.
3. Composite molds: Have expendable &
permanent portions. Used in various casting
processes: improve strength, cooling, and
economics of casting process
11.2 Expendable-Mold Casting Processes
11.2.1 Sand Casting
Casting consists of:
A) Placing a pattern in sand to make imprint
B) Incorporate gating system
C) Removing pattern and filling mold cavity with
molten metal
D) Allow metal to solidify
E) Break away sand mold
F) Remove casting
Types of Sand Molds
1) Green-Sand Molding- mix of sand, clay & water. “Green” refers to moistness
and dampness of mold while metal is being poured. LEAST EXPENSIVE and sand
easily recycled
Cold Setting Processes- bonding of mold w/o heat
2) Cold-Box Mold- Binders blended into sand to chemically strengthen grains. More
3)
Accurate but more Expensive than Green-Mold.
No Bake Mold- liquid resin mixed w/ sand and mixture hardens.
Patterns
-used to mold sand mixture into shape of casting
-b/c patterns are used repeatedly, strength & durability of
materials must reflect # of castings that mold will
produce.
Types of patterns:
-One-piece: simpler shapes and low
quantity production; inexpensive
-Split: two piece, complicated shapes
-Match-plate: picture
Patterns
Rapid Prototyping Machine- can fabricate a pattern at a fraction of time
and cost of machining a pattern.
*Pattern design is a critical aspect
of the total casting operation.
Design must provide for Shrinkage,
Ease of Removal from mold by
taper or draft, & proper metal flow
into mold cavity.
Sand Molding Machines
Vertical Flaskless
Molding-
Sandslingers- Fill under high pressure steam.
Used to fill large
flasks & operated by machine. High speeds cause sand to be
placed & rammed appropriately
Impact Molding- compacted by controlled explosion or
instantaneous release of compressed gas. Molds contain uniform
strength and good permeability
Vaccum Molding
Sand Casting Operation
Removal of Casting:
1)castings cleaned by blasting w/ steel shot
2)risers & gates cut off
3)Castings cleaned electrochemically to remove surface oxides,
may even be heat treated
Finishing Operations: involve machining, straightening,
forging with dies to obtain final dimensions
Final Step: Inspection to meet all design & quality control
requirements
11.2.2 Shell Molding
-Produces many types of castings w/ close dimensional
tolerances & good surface finish @ low cost.
-Unless molds are properly vented,
trapped air & gas can cause serious
problems in ferrous castings.
*High Quality of finished casting
reduces cleaning, machining, and
other finishing costs significantly.
Complex shapes - produced with
less labor, and easily automated
Precision Casting
11.2.3 Plaster-mold casting
11.2.4 Ceramic- mold casting
11.2.6 Investment Casting
High Dimensional
Accuracy & Good
Surface Finish
11.2.3 Plaster-Molding process
1) Made of Plaster of Paris (gypsum or calcium), talc, & silica flour
2) Mix w/ water:
Mixture poured over pattern.
3) Plaster sets and is removed, the mold dried at high temps to
remove moisture.
4) Mold halves assembled, preheated and molten metal is poured.
*Gases evolved during solidification cannot escape, so molten metal
is poured under a vacuum or under pressure.
Low Thermal Conductivity allows casting to cool slowly, allowing a
more uniform grain structure is obtained w/ less warpage
11.2.4 Ceramic Mold casting
-Similar to plaster mold process EXCEPT it uses refractory
metal materials suitable for HIGH TEMPS.
High-temperature resistance of refractory molding materials
allows for casting of high temp alloys, stainless steel, and
tool steels.
*Expensive Process but castings have good dimensional
accuracy and surface finish
11.2.5 Evaporative Pattern Casting (lost-foam)
-Mold produced for every casting
(polystyrene pattern evaporates upon contact with molten metal)
*Polymers require considerable energy to degrade, so large thermal
gradients are present. The degradation products are vented into
the surrounding sand.
Advantages:
1) Simple process: Design flexibility
2) Inexpensive flasks are satisfactory
3) Plastic is inexpensive; easily processed into
complex shapes, sizes, and fine surface
detail
4) Casting requires minimal finshing and
cleaning
11.2.6 Investment Casting (lost-wax process)
-Pattern made of wax then dipped repeatedly into a slurry of
refractory material and allowed to dry.
-Mold heated and held UPSIDE-DOWN to allow melt out of
wax.
*the wax can be recovered and reused
Molten metal then is then poured.
TREE- A number of patterns joined to make one
mold
11.2.6 Investment Casting (lost-wax process)
-Mold materials and labor costly
-Suitable for high-melting point alloys with good surface finishes and close
dimensional tolerances; few or no finishing operations cut costs of
casting
Ceramic-Shell Investment Casting
-Uses same type of wax pattern. The pattern is dipped into coarsergrained silica to build up coatings and develop proper thickness so
pattern can withstand the thermal shock due to pouring
-rest of process similar to investment casting
-Used for precision casting of steels and high-temperature alloys.
Investment casting of an integrally cast rotor for a gas turbine. (a) Wax pattern assembly.
(b) Ceramic shell around wax pattern. (c) Wax is melted out and the mold is filled, under a
vacuum, with molten superalloy. (d) The cast rotor, produced to net or near-net shape.
Permanent-Mold Casting
Two halves of a mold are made from materials with high
resistance to erosion and thermal fatigue (ex iron, steel,
graphite)
Part weight can range from .1 kg to 300 kg but typically
weigh less than 25 kg
Medium quality surface finish, porosity, shape complexity
High dimensional accuracy
Good to average cost for equipment and labor
Can produce 5-50 parts/mold hour
Typically requires 1000 parts before being used
Permanent-Mold Casting
Bronze mold dated about 1400 B.C.
Permanent-Mold Casting
Vacuum Casting
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Used for thin walled (0.75 mm) complex shapes with uniform
properties ex gas-turbine components from superalloys. Process
uses slight vacuum (2/3 atm) also reduces porosity. Metal only
about 55C above liquidus temperature. Automated, and
production costs similar to green-sand casting.
 CLA – metal melted in air, parts are made at a high volume
and low cost
 CLV - Metal melted in vacuum, involve reactive metals such
as aluminum, titanium, zirconium, and hafnium
Permanent-Mold Casting
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Slush casting – used for small production runs for
ornamental and decorative items and toys from low melting
point metals (lead, zinc, and tin alloys) Similar operation used
in making hollow chocolate shapes.
Pressure casting - uses pressurized gas to force metal up
into the mold, pressure is maintained until metal has solidified
Permanent-Mold Casting
Die Casting
Product weight <0.01 to 50 kg
 Good surface finish, low porosity, capable of
high complexity parts
 Excellent dimensional accuracy +/- 0.001
tolerance
 Takes several weeks before first product output
 Produces 2-200 parts/mold hour
 Requires approximately 10,000 parts for use

Permanent-Mold Casting
Die Casting
 Hot chamber process - Uses a piston to trap molten metal
and forces it into die cavity thru gooseneck and nozzle. Used
for low melting point alloys (zinc, magnesium, tin, and lead)
 Cold-chamber process – Same as hot chamber, except
chamber not heated, used for high melting point alloys.
(aluminum, magnesium, and copper)
 Die to part weight is about 1000 to 1 … i.e. a 2 kg part
requires a 2000 kg die
Permanent-Mold Casting
Die Casting
Permanent-Mold Casting
Centrifugal Casting
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Used for hollow cylindrical parts like pipes, gun barrels, posts
Due to differences in density, lighter elements collect in
center, therefore properties of casting vary through its
thickness.
Results in forces upward of 150 g for thick - walled parts
Can produce parts over 5000 kg, but takes months for first
part is produced.
Permanent-Mold Casting
Squeeze Casting
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Results in parts made to near-net shape and fine surface
detail for both ferrous and non ferrous material.
Particularly good for overcoming feeding difficulties that arise
from casting metals with a long freezing range.
Permanent-Mold Casting
Semi-solid Metal forming
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Working at extremely high temperatures can reduce die life.
When the metal enters the die it is a combination of both
solid and liquid. As the metal is agitated, it’s viscosity is
lowered, this process is called thixoforming.
Advantages
 Parts are homogenous with high strength
 Thin and thick parts are produced
 Parts can subsequently be heat treated
Disadvantage
 Higher costs than die casting
Quality Assurance
Inspections
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Controlling each stage during casting is important in
maintaining good quality.
Nondestructive testing – visual and optical
inspection of outer surface for defects, can also
involve radiographic testing for vital components.
Destructive testing - various specimens are
removed from various sections to test for strength.,
durability, porosity, and any other defects.
Pressure testing - usually performed on cast
components such as pumps, valves, and pipes to
test for seal leakage.
The melting practice is
important because it affects the
quality of the castings. When
melting your metal stock in a
furnace, you would add flux and
other slag-forming constituents
to refine your metal. The type of
furnace you use also affects the
quality of castings as well.
•Electric arc: found in many foundries for it’s high rate of melting, low
pollution and ability to hold the metal for alloying purposes
•Induction: (coreless induction): great for mixing characteristics for
alloying and adding a new charge of metal or (core or channel furnace):
great for superheating, holding and duplexing
•Crucible: most commonly used furnace because it can be heated using
various fuels such as commercial gases, fuel oil, fossil fuels or electricity.
Can be stationary, tilting or movable.
•Cupolas: Furnaces that can be operated continuously. They have high
melting rates so produce large amounts of metal. They are currently
starting to be replaced by induction furnaces because of the high costs
involved.
•Levitation melting: Involves magnetic suspension of the molten metal
where an induction coil simultaneously heats, stirs and confines the melt
eliminating the need for a crucible. This leaves the castings free of
refractory inclusions and of gas porosity, and gives it a uniform fine grain.
Melting Practice and Furnaces
Electric Arc Furnace
Induction Furnace
Melting Practice and Furnaces
Crucible Furnace
Cupolas Furnace
Levitation melting furnace
Video clip of an electric arc furnace:
http://www.matter.org.uk/steelmatte
r/steelmaking/eaf.htm
Multi-melt inductor furnace
Foundries (from the Latin word fundere meaning
melting and pouring) are where all casting
operations are carried out. Foundries required a lot
of manual labor in the past, but at present many
of the operations are now automated. Automation
varies from foundry to foundry depending on what
and how much is needed to be produced. If a
foundry requires production in the hundreds of
thousands then most work is automated. But, if a
foundry only has short production runs then less
automation is required.
http://www.grede.com/foundry_terms/foundry_t
erms_frameset.shtml good site for terms about
foundries
Inside of a
foundry
This chapter describes general design considerations and
guidelines for metal casting, ways of avoiding defects in metal
casting, characteristics of alloys commonly cast together, and
outlines the economics involved in casting operations.
METAL CASTING (CH 12)
Design, Materials, and Economics
 Design Considerations
 Guidelines for successful
casting
 Characteristics and
Applications of metals
 Economic considerations
Design Considerations
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Corners, angles and section thickness
Flat areas
Shrinkage
Design Considerations
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Draft
Dimensional Tolerances
Lettering and Markings
Finishing Operations
Casting Alloys
Non Ferrous
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Aluminum
Magnesium
Copper
Zinc
Tin
Lead
TABLE 12.2
Type of alloy
Aluminum
Copper
Ductile iron
Gray iron
Magnesium
Malleable iron
Nickel
Steel (carbon and
low alloy)
Steel (high alloy)
White iron
Zinc
Application
Pistons, clutch housings, intake
manifolds
Pumps, valves, gear blanks,
marine propellers
Crankshafts, heavy-duty gears
Engine blocks, gears, brake disks
and drums, machine bases
Crankcase, transmission housings
Farm and construction machinery,
heavy-duty bearings, railroad
rolling stock
Gas turbine blades, pump and
valve components for chemical
plants
Die blocks, heavy-duty gear
blanks, aircraft undercarriage
members, rail-road wheels
Gas turbine housings, pump and
valve components, rock crusher
jaws
Mill liners, shot blasting nozzles,
railroad brake shoes, crushers and
pulverizers
Door handles, radiator grills,
*E, excellent; G, good; F, fair; VP, very poor; D, difficult.
Castability*
E
Weldability*
F
Machinability*
G–E
F–G
F
F–G
G
E
D
D
G
G
G–E
G
G
D
E
G
F
F
F
F
E
F
F
E
F
G
VP
VP
E
D
E
Casting Alloys
Ferrous
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Cast Irons
Largest quantity of metals
 Wear resistance, hardness and good machinability
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Cast Steels
High cast temperature (1650°C, 3000 °F)
 Applied to railroads, mining, chemical plants, oil
fields and heavy construction
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Stainless steels
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Casting similar to normal steels
Economics of Casting
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Cost depends on several factors
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Materials-materials with higher melting points require expensive dies and
machinery
Equipment-the more sophisticated the equipment the more the cost
Labor-single more costly factor in manufacturing
TABLE 12.6
Cost*
Process
Die
Sand
L
Shell-mold
L–M
Plaster
L–M
Investment
M–H
Permanent mold
M
Die
H
Centrifugal
M
* L, low; M, medium; H, high.
Equipment
L
M-H
M
L-M
M
H
H
Labor
L–M
L–M
M–H
H
L–M
L–M
L–M
Production
rate (Pc/hr)
<20
<50
<10
<1000
<60
<200
<50
References
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http://en.wikipedia.org/wiki/Image:Bronze_spearhead_mold.J
PG
http://www.emachineshop.com/machines-molding/images/diecast-machine.jpg
http://www.diecasting.org/faq/
http://www.diecasting.org/design/case2/images/VacuumMetal
Pour.jpg
http://www.offshoresolutions.com/products/metal/images/casti
ngs/centrifugalCastingDiagram.jpg
http://www.empirecastings.com/images/Proces7.gif
http://www.me.gatech.edu/jonathan.colton/me4210/ccdc3.gif
http://www.stahl-online.de/images/2-1-1_Bild5_30862.jpg
http://www.inductothermindia.com/photos/furnace.jpg
http://www.nbm-houston.com/images/jpg/holding-furnaceSMALL.jpg
http://www.dueker.de/ENG/bilder/kupol.jpg
References(cont)
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http://www.bhrgroup.co.uk/graphics/pwteeturbulent.jpg
http://i7.ebayimg.com/04/i/04/b4/60/35_1_b.JPG
http://www.me.gatech.edu/jonathan.colton/me4210/mfgvide
os.html
http://www.nd.edu/~manufact/index3.htm
http://www.norcanhydro.com/ images/nor_index2.jpg
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http://www.kitchenkitchen.com/.../ images/scanpan-fp.jpg
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http://www.greg-j.com/ files/pipes.jpg
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http://www.cs.berkeley.edu/.../ IMGS/Crankshaft.jpg
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http://www.mech.northwestern.edu
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http://www.thaimetalcasting.com/
Product/product.html
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http://www.eere.energy.gov/. ../profile.html
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References(cont)
http://www.ima-eu.org/ en/textfoundry.htm
http://www.tth.com/ Metal%20casting.htm
https://webmail.unm.edu/Redirect/ifcln1.ifc.org/ifcext/envi
ro.nsf/AttachmentsByTitle/gui_foundries_WB/$FILE/fo
undries_PPAH.pdf
https://webmail.unm.edu/Redirect/www.grede.com/foundr
y_terms/foundry_terms_frameset.shtml