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8/6/2019 Casting Procedure 1
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CASTING PROCEDURE
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Intracoronal
restorations±
Inlays,onlays
Extracoronalrestorations± crowns ,
bridges
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Phosphate bonded investments
Binder ± MgO and ammonium phos.
Refractory ± silica ±cristobalite , quartz
Liquid ± water or colloidal silica
Setting and thermal expansion
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Ethyl silica bonded investments Binder ± silica gel or silicic acid gel,MgO
methods to produce silicic gel binders : -sod. Silicate -> silicic acid gel
-colloidal silica ->silica gel
-Ethyl silicate ± hydrolysis-> colloidal silicicacid ->silica gel
-Ethyl silicate ± hydrolysis and gelation
(amines)->silica gel
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CROWN PREPARATION
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Impressions
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Model / Die
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Wax pattern
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Wax pattern
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Sprue attached to wax pattern
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Principles of optimal sprue design
.sprue diameter sprue position
Sprue attachment
Sprue length
Sprue direction
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Direct \ Indirect sprue
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Sprue and wax pattern and
crucible former
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Casting ring liner
Asbestos
Ceramic
cellulose
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Investing
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Investing
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Mould after investing
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Burn out / Wax elimination
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Burnout temperatures for diff
investments
Type-1 GBI- thermal expansion-650c
Type-2 GBI-hygroscopic expansion-500c
PBI-thermal expansion-750-1000c
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Crossection of mould after
burnout
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casting
Melting the metal/alloy
Different casting machines used
Cleaning the castings
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Melting the alloy
Torch flame
Induction melting
Electrical resistance
direct current arc
melting
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Zones in the flame
Mixing zone ± black color
Combustion zone- green color
Reducing zone ± blue color
Oxidizing zone ± red color
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Casting machines
Centrifugal ± spring
driven, motorised
Air pressure
Vaccum generated
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casting
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Crossectional view
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Cleaning the castings
QUENCHING for GBI
PICKLING for Gold alloys50% HCL
H2SO4
SAND BLASTING for PBI
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crown
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Causes of defective castings
Distortion
Surfaceroughness,irregularities,discoloration
Porosity
Incomplete castings
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1.Distortion
Delay in investing
Using larger diameter sprues for small
patterns
Using soft waxes
Careless handling
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2 a. Surface roughness
Increased L/P ratio
Prolonged heating of the mould
Temp. of the alloy
Greater casting pressures
comp. Of the investment
Foreign bodies
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2 b.surface irregularities
Air bubbles
Water films Rapid heating of the mould
Many patterns in the same mould
Direction of the sprue
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2 c.surface discoloration
Under heating
Prolonged heating
Carbon inclusions
Mixing diff. alloys
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3.Porosity
Solidification defects
Trapped gases
Residual air
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Trapped gases
Pin hole porosity
Gas inclusions
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Residual air
Back pressure porosity or entrapped air
porosity ± seen as large concave
depressions in the castings
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Incomplete castings
Low casting pressures
Insufficient venting of the mould
Incomplete elimination of the wax
Greater viscosity of the alloy
