EIN 3390 Chap 12 Expendable-Mold Cast B Spring_2012.ppt

7/28/2019 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



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Investment Casting

Figure 12-29 Investment-casting steps for the shell-casting procedure. (Courtesy of Investment



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Investment Casting

Figure 12-29 Investment-casting steps for the shell-casting procedure. (Courtesy of Investment



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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)

Documents

EIN 3390 Chap 12 Expendable-Mold Cast B Spring_2012.ppt