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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