CASTING PROCEDURE Casting procedure o The lost wax

CASTING PROCEDURE

Casting procedure o The lost wax casting technique was introduced by TAGGART in 1907

Steps in casting o o o o 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 pattern o o o Prior 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 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 o 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 o o o 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 o o o Wax plastic Metal

sprue o

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, o may block the escape of wax, o hollow plastics are available Metal: non rusting metal should be used, o removed before casting

diameter o o should be larger than the thickest portion of the wax pattern 2. 6 mm can be used for most patterns 2. 0 mm for premolar partial veneer restoration narrow sprue are sufficient for casting to be done on centrifugal machine

Positioning of the sprue o o o 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 o o Small auxiliary sprues or vent should be placed to improve the casting By allowing the gases to escape

Attaching the sprue to crucible former o o 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 ring Ideal properties of investment material o controllable expansion to compensate for shrinkage of cast alloy during cooling o Produce smooth castings without nodules o Chemical stability at high casting temp o Adequate strength to resist casting forces o Sufficient porosity to allow gas escape o 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) o satisfy most of the requirement for an ideal investment material o Not suitable for casting metal ceramic alloys as it is unstable at high temperature Three types of expansion o Setting expansion o Hygroscopic expansion o Thermal expansion o o o Used for alloys that fuses below 1, 975’F Mainly used with type I, III, gold alloys

Factors increasing the expansion o o o 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 o o o o Composition: binder megnesium oxide mono ammonium phosphate filler 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

o o High noble alloy -gypsum bonded investment (shows hygroscopic expansion) Base metal alloy –phosphate bonded investment (shows thermal expansion)

Ring and ring liners o Ring liners Asbestos (due to health hazard ) o Cellulose Used to aid in uniform expansion of the mold Wetting the liner increases the hygroscopic expansion It is coated on the casting ring o Bench set : min for one hour

Wax burnout o o o 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 1 st Batch

Casting technique Setting up the casting machine Pre heating the crucible Weighing the alloy Casting the alloy

Casting machine

Setting up the casting machine Three clock wise turns and locked in position using a pin Oxy-acetylene gas is used ordinary alloys Oxygen gas is used for metal ceramic alloys

Pre heating the crucible Freezing of the alloy – incomplete casting

Weighing the alloy 6 grams for pre-molar retainer castings n 9 grams for molar retainer castings n 12 grams pontic castings n

Casting the alloy n n n 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

recovery Removal of residual investment n Quenching with water gypsum bonded disintegrates n Using tooth brushes or ultrasonic cleaners n Phosphate bonded should be removed using sand blasting (aluminium oxide) n

Casting deffects • • • Distortion Surface roughness and irregularities Discoloration Porosity Incomplete casting

Distortion n n Distortion of wax pattern during fabrication Increased with increase in ambient temperature and time lag between fabrication and investing

Prevention Proper manipulation of wax Avoid stress on pattern during removal Immediate investment after removal from die

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

Solidification deffects § Localised shrinkage porosity –molten alloy prematurely solidifies in sprue before solidifying in mold space § Cause: diameter of sprue is small , lack of reservoir , improper sprue attachment-suck back porosity § Microporosity: too rapid solidification seen as small irregular voids § Avoided by increasing casting temperature, melting temperature

TRAPPED GASES o Pin hole porosity-tiny spherical voids Cause: release of entrapped gases while solidification o Gas inclusion porosity-larger spherical voids Cause : gas inclusion during casting procedure

Subsurface porosity –due to entrapped gases in molten alloy Prevention : controlling the rate of molten alloy entering the mold n

Residual air • Back pressure porosity Cause: inability of air to escape through pores in the investment , improper wax elimination , low casting temperature &pressure Prevention : use of porous investment, proper wax burn out , enough casting pressure& temperature

Surface roughness and irregularites Surface roughness – Cause : silica particle in investment Inaccurate powder liquid ratio , too rapid heating Surface irregularities –nodules or fins Cause : air bubble attached to pattern , water film on pattern , careless removal of pattern Prevention: correct powder liquid ratio, use of mechanical mixer , use of wetting agent

Discoloration of casting Appears dark due to coating of oxides Cause : under heating , prolonged heating carbon inclusion, high sulfur content of torch flame Removed by pickling l

Incomplete casting Ø Due to prevention of molten alloy from filling the mold space completely Cause : improper vending of air due to reduced casting pressure , Prevention: adequate casting pressure , adequate alloy

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