Casting Technology Uploaded by MIUDENTIST

8/13/2019 Casting Technology Uploaded by MIUDENTIST

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Casting Technology

The purpose of the castingprocedure is

to provide exact metallic duplicateof

missing tooth (or teeth) structure with

maximum possible accuracy in all

dimensions,

For

Inlays

Onlays

Crowns & Bridges


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Direct restoration


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Indirect restoration

Inlay

Onlay


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Gold bridge


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Reducedtooth


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Impression

Cast / Die


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Gold

Crown


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Steps of casting

proceduresWax pattern construction

Spruing

Investing

Wax elimination

CastingFinishing

Polishing


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I- Wax pattern construction

Direct technique

Indirect technique

Indirect direct technique


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II- Spruing

Create a channel to allow the molten wax toescape from the mold.

Enable the molten alloy to flow into the mold

which was previously occupied by the wax

pattern.

Role of a sprue:

The wax pattern is attached toa sp rue

for proper handl ing wi thou t dis tor t ion


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Sprue materials

Waxesmelt at the same rate as the wax pattern thus

allowing easy escape of the molten wax

Plastics

Metals

Hollow

Or

solid


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Attachment and Direction:Sprue should be attached to thethickestpart of the wax pattern

with a 45C angleto the proximal wallThis or ientat ion avoids :

* Turbulent flow of the molten metal

air bubbles leading to

# porosity


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Diameter (Th ickness ):

Equal or slightly Thickerthan the thickest partof the wax pattern

*improves flow of alloy into the mold provides reservoir during solidification.

Reservoir : piece of wax attached 1 mm.

Away from the wax pattern it is the last part to solidify,

allows continuous feeding of molten alloy

to compensate solidification shrinkage

and to avoid shrinkage porosity


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Number of spruesmay be single, doubleor multiple

There may be a main sprue with accessory

sprues.

Single Double Multiple

L th f


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Length of sprueShould keep the wax pattern

about 6-8 mm from the

end of the casting ring

Short sprue prevent escape of air leading to

back p ressu re po rosi ty.

Long sprue f racture of the investment, asmoldwill not withstand the impact force of the entering

molten alloy.


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Preparation of the casting ring


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Crucible former:usually made of rubber, which

constitutes the base of thecasting ring during investing.


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Cast ing r ing and l iner:

Casting ringmay be metallic, or rubberring.Selection depends on:

choice of casting alloy and investment material.

Casting ring lined using asbestos,

then wet the liner.

Funct ions o f l iner :

1)Creates space to allow for investment expansion.

2)Allow setting of investment material under water to give

hygroscopic expansion.

3)Facilitate removal of investmentfrom the casting ring after

casting.


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Types of ring

Metallic Rubber Split


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III- Investing

Surfactant


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Invest ing the wax pattern:

Wax pattern must be invested immediatelyto avoiddistortion of wax pattern,

which may be due to release of internal stresses

Wax is a hydrophobic material

Improper wetting of wax pat.

irregular surface of the metallic cast.

Minimized by applying

surfactants (surface active agents)on the wax surface

*increases surface energy of the wax

*decrease contact angle between and

investment material.

Methods of invest ing the wax pat tern:


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Methods of invest ing the wax pat tern:

Handinvesting procedure and vacuum

***Vacuum mixing of investment

is highly recommendedto obtain casting withminimal surface defect.

Invest ing w ax pattern m ay be

done by regular mixinvestment technique OR

by double mixinvestment technique *improve adaptability of the investment material

to the surface of wax pattern

*avoid distortion of wax

#After the ring is filled to the rim,

#allow the investment to set for about one hour

#remove the crucible former and the metallic sprue


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Investing technique

Hand mixing Vaccum


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IV- Wax

elimination


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Wax elimination:Heating the investment must be done

to achieve the following:

1)All traces of the wax are vaporized: bring thefurnace to 200 Cand hold this temperature for 30 minutes.

Most of the wax will be eliminated by this time.

2) Increase the temperature gradually to the finalburnout temperature (about 650 C),this isaccompanied by thermal expansionof theinvestment.The mold temperature is now ready for casting.


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V- Casting

A) Melting the alloy

Using flame Using electric furnac


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B) Cast ing :A cast ing machine requires

* a heat source to melt the alloy.

* casting force, so thecasting will be completed.

Method s o f melt ing al loys:

* flame:

Melting gold alloysis done by a blow pipe flamewhile melting basemetalsis done by oxyacetylene flame.

* electr ic furnace.

Method s o f Cast ing: (driving the alloy into the mold)

* Air pressure casting machine Compressed air

* Centrifugal casting machine.

*Both centrifugal and gas pressure


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Casting machines

Air pressure casting machine Centrifugal casting machine


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Solidification of the alloy is done under pressure,

then

Allow for cooling and

when the color of the molten alloy has changed

from redto blackcolor,

the ring should be quenched in water.

Devest ing , Finish ing and pol ish ingof the casting

are subsequently performed.


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VI- Devesting and finishing


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VI-Polishing


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Casting Defects


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I) Disto rted cas t ing :

Due to distortion of the wax pattern.

**Improper handling of the wax pattern and/or its

** Delayed investing.

II) Dimens ional inaccurate cast ing :

Small or largecasting may be due to :a.Impropertype of investmentmaterial.

b.Impropertemperature.


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III) Surface roughness o f the cast ing:

due to:

*** Air bubbles on the surface of wax patternduring the investment procedure

* Improper use of wetting agent

* Improper vacuum investing


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VI) Porosity :

1)Shr inkage spot po rosi ty :

at the point of attachment to the sprue, it is due to using a

thin sprue,which leads to incomplete feedingof the molten alloy

during solidification.

2) Back p ressure po ros i ty:It is due to using

***short sprue. The distance between the waxpattern and the end of the ring is more than 6 mm;

***Non porous investment

air will not escape from the moldand creates back


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V) Incom plete cast ing and rounded

marg in :

Due to resistance of the flowof molten alloy intothe mold:

*Using short sprue.

*Insufficient venting of the Investment.*Insufficient casting pressure.

*Under heating of the alloy causing

premature solidification.

*Incomplete wax elimination.


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Casting Technology Uploaded by MIUDENTIST