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Casting Processes
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Casting Processes
Module 2
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Casting Processes
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2.1
Casting Processes
2.1.1 Exploration: Metal Casting
Definition: Casting is a process where an industrial material is changed into its liquid
state. This liquid is poured into a mold cavity and allowed to solidify. The solid
material is then removed from the cavity before other processes are performed.
Casting is the most cost-effective method of producing shaped metal products.
http://ibase093.eunet.be/en/textfoundry.htm
Activity:
Develop a list of advantages and disadvantages of
using casting to manufacture materials.
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2.1.2 Dialog: Metal Casting
Advantages of casting
Products that have intricate shapes or complex form may be cast at a relatively low
cost.
One casting can produce a product the would be made up of multiple parts using a
different process.
Castings can vary from very small to very large, over 100 tons.
You can produce a high volume of cast products at a relatively inexpensive cost.
Casting can be used to produce any number of shapes with a variety of tolerances
using many different types of materials.
Most times the casting process can be easily automated.
Go to the web site http://www.lovson.com/castings.html
Disadvantages of casting
Cast products do not have the internal strength of forged products.
Impurities can become trapped easily and cause deformations.
Product can not be produced that have extremely smooth surfaces or high
tolerances.
Metal shrinkage can cause problems with final product.
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Go to the web site http://www.backyardmetalcasting.com/defects.html
2.2.1 Dialog: Expendable-Mold Casting
Definition: The mold is expendable and only used one time. The mold must be
destroyed to remove the casting. Each new casting requires a new mold.
2.2.2 Sand Casting
Description: In sand casting the mold is made of specially treated sand. The
composition is usually mad up of silica sand, clay, water, and other additives.
2.2.3 Equipment:
2.2.3.a Pattern
Material that is the shape of the product being cast used to create the cavity.
Patterns are a replica of the part with proper shrinkage allowances added.
Types: Loose patterns
Split patterns
Match-plate patterns
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Go to the web site www.williamtatham.ltd.uk/ foundry/
2.2.3.b Foundry Flask
A container that is divided into two parts: the top (cope) and the bottom (drag).
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Go to the web site http://budgetcastingsupply.com/Flasks.htm
2.2.3.c Riddle
Screen used to sift sand and remove impurities and large stones.
2.2.3.d Hand tools
Rammer
Used to compact sand in flask and form mold cavity.
Trowels
Used for finish work.
Gate cutters
Used to cut gates from sprue and riser into mold cavity.
Go to the web site http://www2.spsu.edu/cteacad/bseaboltx/tools/foundry.html
2.2.4 Application: Design and construct a foundry flask and tool set from materials
you find around your house. This set could be used for casting a small object. Use
the pictures in the text for ideas!
Design Ideas:
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2.3.1 Dialog: Materials
Materials: Almost any solid material that can be melted can be cast
Pouring Temperature Chart
Alloy
°F
°C
Solder
~450
~230
Tin
~600
~300
Lead
~650
~345
Zinc Alloys
650-850
345- 455
Aluminum Alloys
1150-1350 620- 735
Magnesium Alloys
1150-1350 620- 735
Copper-base Alloys
1650-2150 900-1180
Cast Irons; Gray, Ductile
2450-2700 1340-1480
Monel (70 Ni, 30 Cu)
2500-2800 1370-1540
Nickel-based Superalloys
2600-2800 1430-1540
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High Alloy Steels
2700-2900 1480-1600
High Alloy Irons
2800-3000 1540-1650
Carbon & Low Alloy Steels 2850-3100 1565-1700
Titanium Alloys
3100-3300 1700-1820
Zirconium Alloys
3350-3450 1845-1900
2.4.1 Exploration: Material Selection
Fluidity (flowing characteristics)
Go to the web site http://inaba.nims.go.jp/movie/CAST/cast.html
Chemical stability when in contact with air and mold material
Resistance to bonding to mold material
Meet the requirements of end product
Ferrous
Cast iron: Gray iron is the most common form of cast iron used in industry today.
Named for the fracture gray appearance it has excellent machineability, dampening
characteristics and self-lubricating properties.
Steel
Nonferrous
Aluminum alloys
Brass-bronze
Zinc alloys
Titanium
2.5.1 Exploration: Green Sand Casting
Go to the web site:
(http://www.castmetalsfederation.com/process.asp?procid=2&name=Green%20San
d)
List seven requirements of green sand:
1.
2.
3.
4.
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5.
6.
7.
2.5.2 Dialog: Green Sand
Green sand is comprised of clean silica sand sifted to between 53 µm to 595 µm.
It contains from 4 to 15% clay, usually bentonite, and is mixed into the sand.
There is from 0.5 to 2% cereal flours mixed in to serve as a sticky cushion.
The water content is usually added to a 3 or 4% level of total weight
2.6.1 Exploration: Green Sand Casting Process
Got to web site http://inaba.nims.go.jp/movie/CAST/cast.html
List five steps of green sand casting
1.
2.
3.
4.
5.
2. 6.2 Dialog: Green Sand Casting Process
1. Production of mold: The first step in sand casting is making a pattern. Patterns
are the shape of final product with dimensional allocation for shrinkage and final
finishing. Patterns can be made of wood, plastic, metal, or plaster. The material
used is determined by the number of castings to be made and final finish
requirements.
2. Core-making: Cores are similar to molds but are used to create openings or
cavities within the final casting. They are made of a sand mixture which is baked to
form the core. This allows the cores to be strong yet they can be easily removed
from the finished casting.
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3. Molding: Molding is process in which the appropriate cavities are created. The
mold is contained in a flask made up of a two piece frame. The top portion of the
flask is called a cope and the lower portion is a drag. First, molding sand is packed
into a flask around the pattern. The pattern is removed and gates and runners are
positioned in the drag the sprue is placed the cope or top portion of the flask. The
sprue and gates are necessary to allow the molten metal to flow into the cavity. Then
the cope and drag are put together and the mould is ready to receive the molten
metal.
4. Liquid is poured into mold cavity and allowed to solidify: The casting material is
changed from a solid to a liquid state in a furnace. It is then poured down the sprue
and through the gates into the mould cavity. After the metal has solidified the final
casting is removed from the flask.
5. Additional processes to the casting: The casting is cleaned to remove of all
materials that are not part of the finished casting. The gating systems and any mold
or core material are removed from the casting. The surface of the casting is cleaned
and any unwanted parts of the casting are removed. The part could now should be
inspected and can either be a finished product or one that will need additional
processing to be complete.
Go to web site http://www.class.et.byu.edu/.../consolidation/greensandcasting.htm
2.6.3
Dialog: Quality
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Casting defects
1. Miss-runs: material solidifies before filling the mold cavity
2. Cold shunt: two or more portions of the metal flow together but do not fuse before
solidifying.
3. Sand blow: cavity caused by gas released during the pouring process.
4. Sand wash: An irregular surface on the casting caused by the erosion of the sand
during the pouring process.
5. Net shape: no additional processing is necessary to obtain desired geometry or
dimensions.
6. Near net shape: additional processing is required to achieve accurate geometry
or dimensions.
2.6.4 Alternative Process Selection
Examples: http://www.aluminumfoundry.cc/
http://www.sawyerandsmith.com/main/
http://www.azom.com/details.asp?ArticleID=1392#_Sand_Casting
http://www.richards-engineering.co.uk/Video/video.html
http://www.castingtrade.com/html/departments/sand.html
2.7.1
Dialog: Lost-foam Casting
Description: Patterns are usually made of a expanded polystyrene material that
includes the product geometry as well as all gates, sprue, risers and vent holes. The
pattern is placed in a flask and usually green sand is used and compacted around
the pattern. When the molten metal is poured into the form the polystyrene
vaporizes and the molten metal fills the void.
Equipment: The equipment used in the lost foam casting process is similar except
for the pattern making equipment. Different processes are required to make the
polystyrene patterns. One of a kind patterns are usually made by hand and complex
shapes can be made by gluing together polystyrene sections. In multiple production
applications automated equipment is used to pre-make the polystyrene patterns.
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Go to web site
http://www.vulcangroup.com/equipment.htm#Foam%20Processing%20Equipment
2.7.2 Process
Materials: The materials used in the lost-foam casting process are similar to those
used in green sand casting except different sands can be used the pattern is made
of an expandable polystyrene material.
Production of mold: The first step in lost-foam casting is making a pattern. Patterns
are the shape of final product with the addition of all gates, runners, sprues, risers
and vents. Patterns are made of an expandable polystyrene material. Cores are not
usually needed because and openings or cavities are built right into the pattern. The
process used is determined by the number of castings to be made. The patterns for
one of a kind item are usually produced by hand while multiple casting patterns are
created in an automated manufacturing process.
Core-making: Cores are similar to molds but are used to create openings or cavities
within the final casting. They are made of a sand mixture which is baked to form the
core. This allows the cores to be strong yet they can be easily removed from the
finished casting.
Molding: Molding is process in which the appropriate cavities are created. The
pattern in lost-foam casting is placed inside a flask and the sand is compacted
around the pattern. A vibrator is usually used to compact the molding sand.
Liquid is poured into mold cavity. The polystyrene vaporizes and allows the liquid to
fill the void. The casting material is changed from a solid to a liquid state in a
furnace. It is then poured down the sprue and through the gates into the mould
cavity. After the metal has solidified the final casting is removed from the flask.
Additional processes to the casting: The casting is cleaned to remove of all materials
that are not part of the finished casting. The gating systems and any mold or core
material are removed from the casting. The surface of the casting is cleaned and
any unwanted parts of the casting are removed. The part could now be inspected
and can either be a finished product or one that will need additional processing to be
complete.
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2.7.3 Quality
High dimensional accuracy can be achieved using the lost-foam process.
Complex shapes can be made that are not possible with other processes.
Alternative Process Selection
2.8.1
Exploration: Other Expendable Mold Casting Processes
Investment casting/Lost-wax process:
http://www.castmetalsfederation.com/process.asp?procid=7&name=Investment%20
Casting
1. Give one example of when one would use the Investment casting/Lost-wax
process.
Plaster-mold casting process:
http://www.efunda.com/processes/metal_processing/plaster_mold.cfm
1. Give one example of when one would use the Plaster-mold casting process.
Ceramic-mold casting:
http://www.efunda.com/processes/metal_processing/ceramic_mold.cfm
1. Give one example of when one would use the Ceramic-mold casting process.
2.8.2
Multiple-Use-Mold Casting
Definition: The mold is not expendable and can be used many times. The mold
must be strong enough to stand up to multiple use. Most permanent molds are
made of steel or cast iron.
2.8.3 Dialog: Permanent-mold casting
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Description: In this multi-use-mold process molten metal is poured into a mold
cavity. After the metal solidifies the mold is opened and the parts removed.
2.8.3.a
Equipment
Two part metal mold usually made of steel or cast iron that contains the basic part
pattern as well as all gating systems.
2.8.3.b
Materials
Permanent -mold casting is usually limited to metals that have a lower melting point,
such as aluminum, magnesium, and copper alloys.
Cores can be used but of they are metal they must allow for product extraction.
Sand cores can be used if necessary.
2.8.3.c
Process
The mold is preheated and coated
Cores are added if needed
Molten metal is poured into the mold
Mold is open and part removed
Go to web site www.castsolutions.com/archive/02_feature_article.html
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2.8.3.d
Quality
Good dimensional control
Good surface finish
More rapid cooling allows finer grain structure and a stronger casting
Alternative Process Selection
2.9.1 Exploration: Other Multi-use Mold Casting Processes
Slush Casting:
http://www.efunda.com/processes/metal_processing/permanent_mold.cfm
1. Give one example of when one would use the Investment casting/Lost-wax
process.
Low Pressure Mold Casting:
http://www.efunda.com/processes/metal_processing/permanent_mold.cfm
1. Give one example of when one would use the low pressure mold casting process.
Vacuum Permanent-mold Casting:
http://www.efunda.com/processes/metal_processing/permanent_mold.cfm
1. Give one example of when one would use the Ceramic-mold casting process.
2.9.2 Exploration: Examples
http://www.key-to-metals.com/ViewArticle.asp?ID=59
2.10.1
Exploration: Die casting
Description: In this mullti-use-mold process molten metal is injected into a mold
cavity under high pressure (up to 350 MPa). The pressure is maintained until the
metal solidifies then the mold is opened and the parts removed. The molds in this
process are called dies which gives the process it's name..
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Go to web site http://www.diecasting.org/faq/
Go to web site http://www.diecasting.org/environment/
List 5 advantages of die-casting.
1.
2.
3.
4.
5.
2.10.2
Dialog: Die Casting Equipment
Special die-casting machines are used in this process. The machines close the
multi-part molds and maintain alignment while liquid metal is injected into the cavity.
Go to Web page
http://www.thecannongroup.com/prodotti/tecnologie.asp?tecpassata=mistec
2.10.3
Materials
Permanent -mold casting is usually limited to metals that have a lower melting point,
such as aluminum, magnesium, and copper alloys. Cores can be used but of they
are metal they must allow for product extraction. Sand cores can be used if
necessary.
2.10.4
Process
The mold is preheated and coated with a non-stick substance. Cores are added if
needed. Liquid metal is then poured into the mold. The mold is open and the part
removed
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Go to web page http://www.dynacast.com/die-casting/images/dg.hot1.jpg
2.10.5
Quality: http://www.gibbsdc.com/html/quality.html
Good dimensional control
Good surface finish
More rapid cooling allows finer grain structure and a stronger casting
2.10.6
Examples
Go to web page http://www.diecasting.com/2who.htm
Go to web page http://www.azom.com/details.asp?ArticleID=1392#_Die_Casting
2.11.1
Application: Casting Lab: Rubber Mold Casting
Objective: Experience a casting process in order to become more familiar with the
terms and procedures used in casting. Also, to identify important factors which must
be considered when casting objects.
References: "Manufacturing Processes MET212 Casting Project" from Community
College of Allegheny County-Engineering Technology Department.
Introduction: In order to make this lab safer and less messy, we will use a low
melting point metal alloy (tin/bismuth) and a mold made of RTV silicone rubber. The
metal has a melting point of 280oF and the rubber can withstand a temperature of
500oF.
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Materials:
1. Macro-Mark, One to One/Rapid RTV Silicone Rubber Mold material
2. 2-part flask. Wood, plastic or metal containers
3. Pattern of part to be produced
4. Mold Release
5. Plaster of Paris
6. Small Paint Brushes
7. Sprue pin, riser pin, sprue base material, gate material
8. Leather Welding Spats
9. Molder's Leggings
10. Tongs
11. Ring Stand and clamp
12. Propane burner
13. Face shield
14. Heat resistant gloves
Procedure
Building the Flask
1. Obtain the pattern
2. Obtain flask material ( Ex. empty soup can). Remove top and bottom of can and
smooth edges.
3. Measure the height of the casting and determine the parting line. The can will be
cut into 2 pieces, one part for the cope and one part for the drag. The drag must
have ½ -3/4 inch of mold material between the lowest part of the casting and the
bottom of the drag.
4. Scribe a line around the can at the required height for the drag.
5. Using a hacksaw, carefully saw the can into 2 pieces along the scribe line.
6. The cope should be 2 to 3 inches above the top of the casting. If there is not
enough can left, obtain a second can and cut it in the same manor.
7. Flip the cop and drag so that the ends of the cans meet. This will give a smooth
flat joint between the cope and drag.
Making the Mold
1. Using a can that is smaller in diameter than the flask but large enough in
diameter to fit around pattern, cut a ring that is a little taller than the dimension from
the top of the pattern to the parting line.
2. Place the ring on a flat board and place the pattern topside down and a little of
center.
3. Use plaster of Paris to fill around the pattern up to the parting line. Allow the
plaster to cure.
4. Remove the metal ring and place pattern and plaster on a plywood surface. Coat
the pattern, plywood and plaster with mold release and allow to dry. Place drag
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around pattern/plaster so that one side of the plaster/pattern is about ¼ inch form
the edge of the drag.
5. Mix an adequate amount of rubber mold material to cover pattern and plaster,
and fill the drag half of the flask. Allow to rubber to set.
6. Turn over mold and remove the board and plaster. Drill 2, 3/8” diameter holes
1/8” deep into the drag. These will be used to align drag and cope.
7. Align the cope of the flask with drag using the original seam of the can. Locate
the sprue pin, gate and riser pin.
8. Coat the parting line, pattern, gate, sprue and riser pin with mold release and
allow to dry.
9. Mix adequate amount of rubber and fill the cope. Allow to cure.
10. Open the flask and remove the sprue pin, gate material and pattern. Remove
any loose rubber.
Making the Casting
1. Obtain an adequate amount of tin/bismuth.
2. Brush the inside of the mold with industrial talcum powder.
3. Assemble flask and tape or clamp together.
4. Melt the metal using a propane torch and stainless steel saucepan.
5. Measure the temperature with an optical pyrometer. Casting temperature should
be about 290oF.
6. Using a skimmer, clear any dross from the top of the metal.
7. Start pouring the casting by pouring in the basin next to the sprue hole and then
the sprue hole itself. Continue pouring until the sprue hole is filled to the pouring
basin.
8. Allow to cool and then remove form mold.
9. Cut the sprue and riser off the part.
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