ein 3390 chap 12 expendable-mold cast b spring_2012.ppt
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Chapter 12Expendable-Mold Casting
Processes(II)
EIN 3390 Manufacturing ProcessesSpring, 2012
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Sodium Silicate-CO2 Molding
Molds and cores can receive strength from theaddition of3-6% sodium silicate (WaterGlass)
Remains soft and moldable until it is exposed
to CO2 Na2SiO3+CO2 ->Na2CO3+SiO2 (Colloidal)
Hardened sands have poor collapsibility Difficult for shakeout and core removal
Heating from pour makes the mold stronger
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No-Bake, Air-Set, or ChemicallyBonded Sands Involves room-temperature chemical reactions
Organic and inorganic resin binders can be mixedwith the sand before the molding operation
Curing reactions begin immediately
No-bake sand can be compacted by lightvibrations Wood, plastic, fiberglass, or Styrofoam can be used
as patterns
System selections are based on the metalbeing poured, cure time desired, complexityand thickness of the casting, and thepossibility ofsand reclamation
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No-Bake Sands Air-Set, orChemically Bonded Sands
High dimensional precision and good surfacefinish
For almost all engineering metals
Good hot strength High resistance to mold-related casting
defects Molds decompose readily after the metal has
been poured, providing good shakeout
Cost of no-bake molding is about 20-30%morethan green-sand molding
Limited to low-medium production quantities
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Shell MoldingBasic steps
1) Individual grains of fine silica sand areprecoated with a thin layer ofthermosetting resin and heat-sensitiveliquid catalyst.
A metal pattern (usually some form of cast iron) is
preheated to 230 3150c Heat from the pattern partially cures a layer of
material
A strong, solid-bonded region adjacent to the patternis formed in 10-20 mm in thickness.
2) Pattern and sand mixture are inverted andonly the layer of partially cured materialremains
3) The pattern with the shell is placed in anoven and the curing process is completed
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Shell MoldingBasic steps (- continue)
4) Hardened shell is stripped from thepattern
5) Shells are clamped or glued togetherwith a thermoset adhesive
6) Shell molds are placed in a pouringjacket and surrounded with sand,gravel, etc. for extra support
Casting Materials:Casting irons, alloys of aluminum, andcopper
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Shell Molding
Advantages: Excellent dimensional accuracy withtolerance of 0.08 0.13 mm
Very smooth surfaces
Excellent Collapsibility and permeability
Less cost of cleaning, and machining
Less amount of required mold material
High productivity, low labor costs.
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Shell MoldingDisadvantages: Cost of a metal pattern is often high
Design must include the gate and therunner
Expensive binder Limited Part size
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Dump-Box Shell Molding
Figure 12-18 Schematic of the dump-box version of shell molding. a) A heated pattern isplaced over a dump box containing granules of resin-coated sand. b) The box is inverted, and
the heat forms a partially cured shell around the pattern. c) The box is righted, the top is
removed, and the pattern and partially cured sand is placed in an oven to further cure the
shell. d) The shell is stripped from the pattern. e) Matched shells are then joined and
supported in a flask ready for pouring.
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Shell-Mold Pattern
Figure 12-19 (Top) Two
halves of a shell-mold
pattern. (Bottom) The two
shells before clamping,
and the final shell-moldcasting with attached
pouring basin, runner, and
riser. (Courtesy of Shalco
Systems, Lansing, MI.)
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Shell-Mold Casting
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Other Sand-Based MoldingMethods V-process or vacuum molding
Vacuum serves as the sand binder
Applied within a specific vented pattern,drawing the sheet tight to its surface
Flask is filled with vibrated dry, unbondedsand
Compacts the sand and gives the sand its
necessary strength and hardness When the vacuum is released, the patternis withdrawn
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V-Process
Figure 12-20 Schematic of the V-process or vacuum molding. A) A vacuum is pulled on a pattern,
drawing a heated shrink-wrap plastic sheet tightly against it. b) A vacuum flask is placed over the
pattern and filled with dry unbonded sand, a pouring basin and sprue are formed; the remaining sand
is leveled; a second heated plastic sheet is placed on top; and a mold vacuum is drawn to compact the
sand and hold the shape. c) With the mold vacuum being maintained, the pattern vacuum is then
broken and the pattern is withdrawn. The cope and drag segments are assembled, and the molten
metal is poured.
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Advantages and Disadvantagesof the V-Process
Advantages Absence of moisture-related defects Binder cost is eliminated Sand is completely reusable Finer sands can be used Better surface finish No fumes generated during the pouringoperation
Exceptional shakeout characteristics
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Advantages and Disadvantagesof the V-Process
Disadvantages Relatively slow processUsed primarily for production ofprototypes
Low to medium volume partsMore than 10 but less than 50,000
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12.3 Cores and Core Making
Complex internal cavities can beproduced with cores
Cores can be used to improve castingdesign
Cores may have relatively low strength Iflong cores are used, machining may
need to be done afterwards
Green sand cores are not an option formore complex shapes
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Dry-Sand Cores
Produced separate from the remainder ofthe mold Inserted into core prints that hold the
cores in position
Dump-core box Sand is packed into the mold cavity Scrap level with top surface (like paring line) Invert box and leave molded sand on a plate Sand is baked or hardened
Single-piece cores in a split-core box Two-halves of a core box are clamped together
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Dry-Sand Cores
Figure 12-21 V-8 engine block
(bottom center) and the five dry-
sand cores that are used in the
construction of its mold.(Courtesy of General Motors
Corporation, Detroit, MI.)
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Additional Core Methods
Core-oil process (1% vegetable oil) Sand is blended with oil to develop strength
Wet sand is blown or rammed into a simplecore box
In convection ovens at 200 2600c for curing
Hot-box method Sand is blended with a thermosetting binder
Heat to 230 0c for curing
Cold-box process Binder coated sand is packed and then sealed
Gas or vaporized catalyst polymerizes theresin
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Additional Core Methods
Figure 12-23 (Right) Upper Right; A
dump-type core box; (bottom) corehalves for baking; and (upper left) a
completed core made by gluing two
opposing halves together.
Figure 12-22 (Left) Four methods of making ahole in a cast pulley. Three involve the use of
a core.
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Additional Core Considerations
Air-set or no-bake sands may be used Eliminate gassing operations
Reactive organic resin and a curing catalyst
Shell-molding
Core making alternative Produces hollow cores with excellent strength
Selecting the proper core method isbased on the following considerations
Production quantity, production rate, requiredprecision, required surface finish, metal beingpoured
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Casting Core Characteristics Sufficient strength before hardening
Sufficient hardness and strength afterhardening
Smooth surface
Minimum generation ofgases Adequate permeability
Adequate refractoriness
Good collapsibility
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Techniques to Enhance CoreProperties
Addition of internal wires or rods
Vent holes formed by small wire into core
Cores can be connected to the outer
surfaces of the mold cavity Core prints
Chaplets- small metal supports that areplaced between the cores and the moldcavity surfaces and become integral to thefinal casting
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Chaplets
Figure 12-24 (Left) Typical chaplets. (Right) Method of supporting a core by use of
chaplets (relative size of the chaplets is exaggerated).
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Mold Modifications Cheeks are second parting lines that allow
parts to be cast in a mold with withdrawablepatterns
Inset cores can be used to improveproductivity
Figure 12-25 (Left) Method of making a reentrant angle or
inset section by using a three-piece flask.
Figure 12-26 (Right) Molding an
inset section using a dry-sand
core.
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12.4 Other Expendable-MoldProcesses with Multiple-Use
Patterns Plaster mold casting Mold material is made out of plaster withadditives to improve green strength, drystrength, permeability, and castability
Slurry is poured over a metal pattern
Hydration of plaster produces a hard mold
Bake plaster mold to remove excess water
Improved surface finish and dimensionalaccuracy
Limited to the lower-melting-temperaturenonferrous alloys
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12.4 Other Expendable-MoldProcesses with Multiple-UsePatterns
Antioch process
Variation of plaster mold casting
50% plaster, 50% sand mixed with water Improvement of permeability and reducesolidification time
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Plaster Molding
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Ceramic Mold Casting
Mold is made from ceramic material Ceramics can withstand higher
temperatures
Greater cost and not reusable for mold
Shaw process Reusable pattern inside a slightly tapered flask
Mixture sets to a rubbery state that allows thepart and flask to be removed
Mold surface is then ignited with a torch
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Ceramic Mold Casting
Figure 12-27 Group of intricate
cutters produced by ceramic mold
casting. (Courtesy of Avnet Shaw
Division of Avnet, Inc., Phoenix, AZ)
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Other Casting Methods Expendable graphite molds
Some metals are difficult to cast
Titanium
Reacts with many common mold materials
Powdered graphite can be combined with additives
and compacted around a pattern Mold is broken to remove the product
Rubber-mold casting Artificial elastomers can be compounded in liquid
form and poured over the pattern to produce asemirigid mold
Limited to small castings and low-melting-pointmaterials
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12.5 Expendable-Mold ProcessesUsing Single-Use Patterns
Investmentcasting One of the oldest
casting methods Products such as
rocket components,and jet engine turbine
blades Complex shapes
Most materials canbe casted
Figure 12-30 Typical parts produced by investment
casting. (Courtesy of Haynes International, Kokomo, IN.)
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Investment Casting Sequential steps for investment
casting
1) Produce a master pattern2) Produce a master die
3) Produce wax patterns4) Assemble the wax patterns onto acommon wax sprue
5) Coat the tree with a thin layer ofinvestment material
6) Form additional investment aroundthe coated cluster
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Investment Casting Sequential steps for investment
casting (- continue)
7) Allow the investment to harden8) Remove the wax pattern from the
mold by melting or dissolving9) Heat the mold10) Pour the molten metal11) Remove the solidified casting
from the mold
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Advantages and Disadvantagesof Investment Casting
Advantage Complex shapes can be cast
Thin sections, down to 0.4 mm can be made
Excellent dimensional precision
Very smooth surface Machining can be eliminated or reduced
Easy for process steps automation
Disadvantage Complex process Costly for die
Quantity of investment casting 100 10,000/year
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Investment Casting
Figure 12-28 Investment-casting steps for the flask-cast method. (Courtesy of Investment
Casting Institute, Dallas, TX.)
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Investment Casting
Figure 12-28 Investment-casting steps for the flask-cast method. (Courtesy of Investment
Casting Institute, Dallas, TX.)
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Investment Casting
Figure 12-29 Investment-casting steps for the shell-casting procedure. (Courtesy of Investment
Casting Institute, Dallas, TX.)
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Investment Casting
Figure 12-29 Investment-casting steps for the shell-casting procedure. (Courtesy of Investment
Casting Institute, Dallas, TX.)
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Investment Casting
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Counter-Gravity InvestmentCasting
Pouring process is upside down Vacuum is used within the chamber
Draws metal up through the central sprue and intothe mold
Free of slag and dross
Low level ofinclusions
Little turbulence
Improved machinability
Mechanical properties approach those of wrought
material Simpler gating systems
Lower pouring temperatures
Improved grain structure and better surface finish
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Evaporative Patterns
Metal mold or die is used to mass-produce the evaporative patterns
Pattern: 2.5% polymer, 97.5% air
For multiple and complex shapes, patterns
can be divided into segments or slices Assembled by hot-melt gluing
Full-mold process Green sand is compacted around the patternand gating system
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Evaporative Patterns
Lost-foam: (see Figure 12-32) Make polystyrene pattern assembly
Make a thin refractory coating for Polystyrenepattern
Place dried pattern into a flask surrounded by fineunbounded sand
Compact sand by vibration
Pour molten metal onto pattern
Dump sand and remove casting from flask Backup sand can be reused
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Lost Foam Process
Figure 12-32 Schematic of the lost-foam casting process. In this process, the
polystyrene pattern is dipped in a ceramic slurry, and the coated pattern is then
surrounded with loose, unbonded sand.
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Advantages of the Full-Mold andLost-Foam Process Sand can be reused Castings of almost any size
Both ferrous and nonferrous metals
No draft is required
Complex patterns
Smooth surface finish
Cores are not required
Absence ofparting lines Higher metal yield
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Lost-Foam Casting
Figure 12-33 The
stages of lost-foam
casting, proceeding
counterclockwise from
the lower left:
polystyrene beads
expanded polystyrenepellets three foam
pattern segments an
assembled and dipped
polystyrene pattern
a finished metal casting
that is a metal duplicate
of the polystyrene
pattern. (Courtesy of
Saturn Corporation,
Spring Hill, TN.)
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Lost-Foam Casting
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12.6 Shakeout, Cleaning, andFinishing
Final step of casting involves separatingthe molds and mold material
Shakeout operations Separate the molds and sand from the flasks
Punchout machines Force entire contents of a flask from a contaner
Vibratory machines
Rotary separators Remove sand from casting (iron, steel, brass)
Blast cleaning Remove sand, oxide scale, parting line burrs.
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12.7 Summary
Different expendable-mold castingprocesses are developed to create shapedcontainers, and then utilize liquid fluidityand subsequent solidification to produce
desired shapes of casting products. Each process has unique advantages
and disadvantages
Best method is chosen based on the
product shape, material and desiredproperties
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Homework for Chapter 12:
Review questions: 6, 11, 34, 42, 48, 49(on page 311 312)
Problems: 1-b, 1-d (on page 122)