casting procedure

CASTING PROCEDURE

Dr. Deepak K. Gupta

Definition Casting is defined as something that has

been cast in a mold, an object formed by the solidification of a fluid that has been

poured or injected into a mold.(GPT)

“Casting is the process by which a wax pattern of a prepared tooth is fabricated and converted to its metallic replica”(Rosenteil)

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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


Steps involved in the casting


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 distortionwww.facebook.com/notesdental

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


SPRUE MATERIALS


SPRUE MATERIALSWax: 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


CRUCIBLE FORMER

The sprue is attached to a crucible former, usually made of rubber, which constitutes the base of the casting ring during investing.

May be metal , plastic or rubber The exact shape of the crucible

former depends on the type of casting machine used.

With most modern machines, the crucible former is tall to allow use of a short sprue and allow the pattern to be positioned near the end of the casting ring.

also referred to as a sprue formerwww.facebook.com/notesdental

Diagrammatic representation of a dental

casting moldA, Crucible former.B, Sprue. C, Cavity formed by wax pattern after burnout. D, Investment. E, Liner. F, Casting ring. G, Recommendedmaximum investment thickness of approximately 6 mm between the end of the mold cavity and the end of the invested ring to providepathways for sufficient gas escape during casting.


Casting rings Casting rings are used to confine the

fluid investment around the wax pattern while the investment sets. And …….

Considerations in selection of casting rings:


They are available as: 1) Shapes - Round

- Oval 2) Complete rings –

Rigid - Metal - Plastic Flexible - Rubber

3) Split rings - Metal - Plastic

Ring less casting system:- plastic rings which is conical in shape

with tapering walls are used.- Used for traditional gold-base alloys.

Flexible rings Split casting ringswww.facebook.com/notesdental

Casting ring liner

Materials used are:- Asbestos liner- Cellulose liner- Ceramic liner- Combination of

ceramic and cellulose liner


Function of casting ring liner:

Allow uniform expantion. In case of wet liner

technique- hygroscopic expansion.

Thickness of the liner should be less than 1mm.


RINGLESS CASTING TECHNIQUE With the use of higher-

strength, phosphate-bonded investments, the ringless technique has become quite popular.

The method uses a paper or plastic casting ring and is designed to allow unrestricted expansion.


INVESTING The process of covering or

enveloping an object such as a denture, tooth, wax form, crown, with a suitable investment material before processing, or casting

Investment materials are: Gypsum bonded investment Phosphate bonded

investment Ethyl silicate bonded

investmentwww.facebook.com/notesdental

-Thin film of cleaner on pattern reduces surface tension of wax better “wetting” of wax pattern

by the investment.

-Some of the commercially available debubblizing agents can be used.

• The wax pattern should not stand for more than 20-30 min

before being invested. So, it is best to invest the wax pattern as soon as possible


Investment mixing:1. Hand mixing 2. Vacuum mixing After mixing the

investment is poured in to the casting ring up to its rim


Wax elimination or burn out It is advisable to begin the burnout procedure while

the mould is still wet. Water trapped in the pores of the investment reduces

the absorption of wax. As the water vaporizes it flushes wax from the mold.

Purpose of burn out: to flush out wax pattern and create a hollow cavity

Heat application: Heating of ring should be done slowly. Hygroscopic low-heat technique High-heat thermal expansion technique


BURNOUT FURNACE


Hygroscopic low-heat technique The temperature used in this technique is 500°c for 60-90

mins. Obtain compensation expansion from three sources:

Immersion of investment in 37°c water bath. The warm water entering the investment mold from the top

adds some of the expansion. The thermal expansion at 500°c

This technique causes 0.55% of expansion.


HIGH-HEAT THERMAL EXPANSION TECHNIQUE

The investment is slowly heated to 650°c - 700°c in 60mins. Then maintained for 15-30 mins at this temperature.

Above 700°c sulfur dioxide

- Contaminates gold castings and makes them extremely brittle


ACCELERATED CASTING METHOD

To reduce the total time, Alternative Accelerated casting technique is proposed

Uses phosphate bonded investment which sets in 15 mins and then 15 min burn out is done at 815°c.

This method is used for preparing post and core restorations


HEAT SOURCES Torch flame

Gas air torch: melt conventional noble metal alloys (used for inlays, crown and bridge) whose melting points less than 1000°c

Gas oxygen torch: Used to melt metal ceramic alloys of higher temperature up to 1200°c.

Oxy acetylene torch: One volume of acetylene and two and half volume of oxygen are needed.

Hydrogen oxygen generator Electricity


Two type of torch tips:1.Multi-orifice2.Single-orifice

Zones of the blow torch flame:Zone 1 - colorless zone /Non combustion zoneZone 2 – Combustion zoneZone 3 – Reducing zone Zone 4 - oxidizing zonewww.facebook.com/notesdental

CHANGES SEEN IN METAL DURING FUSING

Initially appear spongy later small globules of fused metal appear molten metal flows assuming a spherical shape At proper casting temperature the molten alloy is light orange and

tend to spin or follow. At this stage the temperature of molten alloy is 38°c above its liquidus

temperature. During melting of the gold alloys flux may be added

Minimizing porosity To increase fusing of metal Prevent oxidation

Commonly used fluxes are fused borax powder ground with boric acid power or Charcoal www.facebook.com/notesdental

Casting machines1. Air pressure casting

machines:Alloy is melted in situ in crucible hollow of the ring, followed by applied air pressure.

2. Centrifugal casting machine:Alloy is melted in a crucible, and forced in to mold by centrifugal force.


ELECTRICAL RESISTANCE - HEATED CASTING MACHINE

It is used to melt ceramic alloys. Here the alloy is automatically

melted in graphite crucible. Direct-current arc melting machine Produce between two electrodes:

the alloys and the water cooled tungsten electrode. > 4000°C – alloy melts very quickly.

High risk of over heating of the alloy.


Induction melting machine

Metal is melted by an induction field that developed with in the crucible surrounded by water- cooled metal tubing.

It is more commonly used for melting base metal alloys

not been used for noble alloy casting as much as other machines


CASTING FORCE Force required to overcome the surface tension of alloy +

Resistance offered by gas in the mold. This can be done by use of following different type of force

Vacuum force Air or Gas Pressure Centrifugal force

Sufficient mass of alloy must be present to sustain adequate casting pressure 6g is typically adequate for premolar and anterior casting 10g is adequate for molar casting 12 g is adequate for pontic


Casting CrucibleThey are of 3 types:

• Clay Crucibles• Carbon Crucibles• Quartz Crucibles

(zircon-alumina)


Cleaning of the casting

Consider the gold crown & bridge alloys.

After casting has been completed, ring is removed & quenched in water.Advantages:

1. Noble metal is left in an annealed condition for burnishing & Polishing.

2. When water contacts hot investment, violent reaction ensues. Investment becomes soft, granular & casting is more easily cleaned.


PICKLING:

Surface of the casting appears dark with oxides and tarnish. Such a surface film can be removed by a process called Pickling.

Best method for pickling is to place a casting in a dish & pour acid over it.

Heat the acid but don't boil it.


Hydrochloric acid Sulfuric acid Ultrasonic devices Gold and palladium based metal

ceramic alloys and base metals, these alloys are not generally pickled.


Casting Defect


Introduction An unsuccessful casting results in

considerable trouble and loss of time. With present techniques, casting

failures should be the exception, not the rule.

It can be classified under four headings distortion; surface roughness and irregularities; Porosity Incomplete or missing detail


DISTORTION It is usually due to distortion of wax

pattern minimized or prevented by proper

manipulation of the wax and handling of the pattern

Some distortion of the wax pattern occurs as the investment hardens around it. Setting and hygroscopic expansions of the

investment. Produce a non-uniform expansion of the

walls of the patternwww.facebook.com/notesdental

SURFACE ROUGHNESS, IRREGULARITIES, AND DISCOLORATION

Accurate reproduction of the surface of the wax pattern from which it is made.

Surface roughness and irregularity should not confused, both are different terms

These depends on size of investment particles. Various other causes

air voids water films liquid/powder ratio composition of the investment foreign bodies impact of molten alloy on the mold wall pattern position carbon inclusions


Porosity Porosity or void fraction is a measure of

the void (i.e., "empty") spaces in a material, and is a fraction of the volume of voids over the total volume.

It can be either internal or external What it does to the casting

Weakens Discoloration Surface roughness Plaque accumlation


Classification Solidification defects

Localized shrinkage porosity Microporosity Suck-back porosity

Trapped gases Pinhole porosity Gas inclusions Subsurface porosity Back pressure porosity


Localized shrinkage porosity

Caused by premature termination of the flow of molten metal during solidification.

Generally occurs near the sprue-casting junction, but it may occur anywhere


Microporosity It also occurs due to solidification shrinkage. generally present in fine-grain alloy castings. solidification is too rapid for the microvoids to

segregate to the liquid pool. causes the formation of small, irregular voids. mold or casting temperature is too low. Not detectable unless the casting is sectioned

and is not a serious defect


Suck-back porosity localized shrinkage porosity, interior of a crown near the area of the sprue Occlusoaxial line angle or incisoaxial line angle

that is not well rounded. entering metal impinges onto the mold surface at

this point – hot spot This spot may retain a localized pool of molten

metal after other areas of the casting have solidified.

Creates a shrinkage void or suck-back porosity.


Suck-back porosity


Pinhole and gas inclusion Entrapment of gas during

solidification gas inclusion porosities are usually

much larger than pinhole porosity, Many metals dissolve or occlude

gases while they are molten Copper and silver: oxygen Platinum and palladium: hydrogen as

well as oxygen.www.facebook.com/notesdental

Subsurface porosity simultaneous nucleation of solid

grains and gas bubbles. diminished by controlling the rate at

which the molten metal enters the mold


Back-pressure Porosity Entrapped air bubbles on the inner

surface of the casting. large concave depressions. caused by the inability of the air in

the mold to escape through the pores in the investment


INCOMPLETE CASTING Occasionally only a partially complete

casting or perhaps no casting at all is found.

The obvious cause is that the moltenalloy has been prevented in some manner from completely filling the mold. insufficient venting of the mold and high

viscosity of the fused metal. incomplete elimination of wax residues from

the moldwww.facebook.com/notesdental

INCOMPLETE CASTING Incomplete venting

Magnitude of the casting pressure

pressure should be applied for at least4 seconds.

lower L/P ratio is associated with lessporosity of the investment.

Incomplete elimination of wax Pores in the investment

may become filledwww.facebook.com/notesdental

facebook.com/notesdental

References Phillips' Science of Dental Materials-

Phillip Anusavice_12th

Basic Dental Materials -2nd.ed Mannapalli

Clinical Aspects of Dental Materials Theory, Practice, and Cases, 4th Edition

Craig's Restorative Dental Material 13th edition


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casting procedure

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