CASTING PROCEDURE

Casting procedure The lost wax

casting technique was introduced by TAGGART in 1907

Steps in casting

Preparing the wax pattern Spruing the wax pattern Attaching the sprue to crucible former Investing the pattern in the casting ring Burnout of the wax Casting Recovery Finishing and polishing

Preparing the wax patternPrior to casting

margin of the wax pattern should be readapted

pattern checked for smoothness, finish and contour

Sprue should be attached to the thickest portion of the wax pattern

wax pattern can be removed from the die using sprue

surfactant should be applied on the wax to obtain wetting of the investment

invested immediately to prevent distortion

spruing of wax pattern

Sprue is defined as “the channel or hole through which plastic or metal poured or cast into gate or reservoir and then into mold”

sprue design

it must allow the molten wax to escape from the mold

enable molten metal to flow into the mold with minimal turbulence

metal within the sprue must remain molten slightly longer than the alloy that has filled the mold this will act as a reservoir to compensate the shrinkage

material used for sprue

Wax plastic Metal

sprue

Wax: preferred for most casting because they melt at the same rate as the pattern and allow for easy escape for molten wax

Plastic: resist distortion rigid, may block the escape of wax, hollow plastics are availableMetal: non rusting metal should be used, removed before casting

diameter

should be larger than the thickest portion of the wax pattern

2.6 mm can be used for most patterns 2.0mm for premolar partial veneer

restoration narrow sprue are sufficient for casting to be

done on centrifugal machine

Positioning of the sprue Sprue should be attached to the point of

greatest bulk 45 angulation near the bulk of the

pattern axial wall should have obtuse angle 135 This prevents air entrapment during

investing and suck back porosity after casting

attachment should be flared to prevent turbulence during metal flow

venting

Small auxiliary sprues or vent should be placed to improve the casting

By allowing the gases to escape

Attaching the sprue to crucible former

Crucible former is conical structure with a base placed at one end of the ring ,free end of the sprue attached to tip of crucible former

The sprue should be adjusted such that terminal end of wax pattern is six mm away from free end of the investment

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Investing the pattern in the casting ringIdeal properties of investment material controllable expansion to compensate for shrinkage

of cast alloy during cooling Produce smooth castings without nodules Chemical stability at high casting temp Adequate strength to resist casting forces Sufficient porosity to allow gas escape Easy recovery of casting Investment material Gypsm bonded and phosphate bonded

Gypsum bonded Alfa hemi hydrate of Gypsum used as binder with crystobalite or

quartz as refracting material and modifiers (coloring matter and reducing agents)

satisfy most of the requirement for an ideal investment material Not suitable for casting metal ceramic alloys as it is unstable at

high temperature Three types of expansion Setting expansion Hygroscopic expansion Thermal expansion

Used for alloys that fuses below 1,975’F Mainly used with type I, II, III, gold alloys

Factors increasing the expansion

Use of full width ring liner Prolonged spatulation Storage at 100% humidity Lower water powder ratio use of dry liner Use of two ring liners

Phosphate bonded investment Composition: binder megnesium oxide

mono ammonium phosphatefiller silica ( quartz or crystabolite form)

They are stable at high temp Material of choice for metal ceramic They expand rapidly Due to heat from the setting reaction softens the wax and allow

freer setting expansion The increase strength of the material at high temp restrict

the shrinkage of the alloy as it cools The powder mixed with colloidal silica reduces the surface

roughness of casting and it also increases expansion thus expansion can be conveniently controlled by diluting the colloidal silica slightly with distilled water

High noble alloy -gypsum bonded investment (shows hygroscopic expansion)

Base metal alloy –phosphate bonded investment (shows thermal expansion)

Ring and ring liners Ring liners

Asbestos (due to health hazard ) Cellulose Used to aid in uniform expansion

of the moldWetting the liner increases the hygroscopic

expansionIt is coated on the casting ring

Bench set : min for one hour

Wax burnout

The removal of wax from the mould usually by heat Melted 200’c for 30 min It does not melt but sublimes directly to gases and

escapes through the pores of investment To burnout remaining traces of wax final burnout

650’c for 45 min Gypsum beyond 650’c disintegrate investment and

form sulfur dioxide and dis colour the casting

CASTING TECHNIQUE

FAZNA M. ALI 1st Batch

Casting techniqueCasting technique

Setting up the casting Setting up the casting machinemachine

Pre heating the Pre heating the cruciblecrucible

Weighing the alloyWeighing the alloy

Casting the alloyCasting the alloy

Casting machineCasting machine

Setting up the casting machineSetting up the casting machine

Three clock wise turns and locked in Three clock wise turns and locked in position using a pinposition using a pin

Oxy-acetylene gas is used ordinary alloysOxy-acetylene gas is used ordinary alloys

Oxygen gas is used for metal ceramic Oxygen gas is used for metal ceramic alloysalloys

Pre heating the cruciblePre heating the crucible

Freezing of the alloy – incomplete castingFreezing of the alloy – incomplete casting

Weighing the alloy

6 grams for pre-molar retainer castings 9 grams for molar retainer castings 12 grams pontic castings

Casting the alloy

Heated using open flame from torch Reducing part of flame is used to prevent

oxidation Flux should be added Melts If the alloy is ready to cast locking pin is

released and casting is completed

recoveryrecovery

Removal of residual investment Removal of residual investment Quenching with water gypsum Quenching with water gypsum

bonded disintegratesbonded disintegrates Using tooth brushes or Using tooth brushes or

ultrasonic cleanersultrasonic cleaners Phosphate bonded should be Phosphate bonded should be

removed using sand blasting removed using sand blasting (aluminium oxide)(aluminium oxide)

Casting deffects• Distortion• Surface roughness and

irregularities• Discoloration • Porosity • Incomplete casting

DistortionDistortion

Distortion of wax pattern during Distortion of wax pattern during fabrication fabrication

Increased with increase in ambient Increased with increase in ambient temperature and time lag between temperature and time lag between fabrication and investingfabrication and investing

Prevention Prevention

Proper manipulation of waxProper manipulation of wax

Avoid stress on pattern during removalAvoid stress on pattern during removal

Immediate investment after removal from Immediate investment after removal from diedie

Porosity

Can occur on internal or external surface Weakens the casting Types :A Solidification defects B Trapped gases C Residual air

Solidification deffects Solidification deffects

Localised shrinkage porosity –molten alloy Localised shrinkage porosity –molten alloy prematurely solidifies in sprue before solidifying in prematurely solidifies in sprue before solidifying in mold spacemold space

Cause:diameter of sprue is small ,lack of Cause:diameter of sprue is small ,lack of reservoir ,improper sprue attachment-suck back reservoir ,improper sprue attachment-suck back porosity porosity

Microporosity:too rapid solidification seen as small Microporosity:too rapid solidification seen as small irregular voids irregular voids

Avoided by increasing casting temperature,melting Avoided by increasing casting temperature,melting temperaturetemperature

TRAPPED GASES

Pin hole porosity-tiny spherical voids

Cause: release of entrapped gases while solidification

Gas inclusion porosity-larger spherical voids

Cause :gas inclusion during casting procedure

Subsurface porosity –due to entrapped gases in Subsurface porosity –due to entrapped gases in molten alloymolten alloy

Prevention : controlling the rate of molten alloy Prevention : controlling the rate of molten alloy entering the moldentering the mold

Residual airResidual air

• Back pressure porosity Back pressure porosity Cause:inability of air to escape through Cause:inability of air to escape through

pores in the investment ,improper pores in the investment ,improper wax elimination ,low casting wax elimination ,low casting temperature &pressuretemperature &pressure

Prevention :use of porous Prevention :use of porous investment,proper wax burn investment,proper wax burn out ,enough casting pressure& out ,enough casting pressure& temperature temperature

Surface roughness and irregularitesSurface roughness and irregularites

Surface roughness – Surface roughness – Cause :silica particle in investment Cause :silica particle in investment Inaccurate powder liquid ratio ,too rapid Inaccurate powder liquid ratio ,too rapid

heating heating Surface irregularities –nodules or fins Surface irregularities –nodules or fins

Cause :air bubble attached to pattern ,water Cause :air bubble attached to pattern ,water film on pattern ,careless removal of patternfilm on pattern ,careless removal of pattern

Prevention: correct powder liquid ratio,use of Prevention: correct powder liquid ratio,use of mechanical mixer , use of wetting agentmechanical mixer , use of wetting agent

Discoloration of castingDiscoloration of casting

Appears dark due to coating of oxides Appears dark due to coating of oxides

Cause : under heating ,prolonged heating Cause : under heating ,prolonged heating carbon inclusion, high sulfur content of carbon inclusion, high sulfur content of torch flame torch flame

Removed by pickling Removed by pickling

Incomplete casting Incomplete casting

Due to prevention of molten alloy from Due to prevention of molten alloy from filling the mold space completely filling the mold space completely

Cause :improper vending of air due to Cause :improper vending of air due to reduced casting pressure ,reduced casting pressure ,

Prevention:adequate casting Prevention:adequate casting pressure ,adequate alloy pressure ,adequate alloy

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