casting procedures and casting defects

7/29/2019 Casting Procedures and Casting Defects

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CASTING PROCEDURE AND

CASTING DEFECTS


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CONTENTS

Master die

Wax pattern

Spruing

Liner

Investing procedure

Casting machines

Cleaning the casting

Causes of defective castings

Porosity

Conclusion

References


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Definitions

According to glossary of Prosthodontics terms (GPT-8)

Casting(1) something that has been cast in a mold.

(2) An object formed by the solidification of fluid that hasbeen poured or injected in to a mold.

Sprue former- A wax, plastic, or metal pattern used toform the channel allowing molten metal to flow in the moldto make a casting.

Investing The process of covering or enveloping wholly

or in part , an object such as denture , tooth, wax form,crown etc. with a suitable investment material beforeprocessing or casting.


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Master die

After making impression first step is die making. The mostcommonly used die materials are Type IV and Type Vimproved stones.

Setting expansion of Type IV stone is 0.1% and Type V is0.3%.

To reduce setting expansion accelerator( potassium sulfate )and retarder( borax) can be added.

Different types and methods of die making

1 Die stoneinvestment combination2 Other die materials( acrylic, polyester and epoxy resin)

3 Electroformed die


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Wax pattern

According to American dental association specificationnumber 4, wax used for making wax pattern are divided into

three types:

Type A - Hard or low flow wax used in indirect technique .

Type B Used for direct technique .

Type C Also used for direct technique.

To allow for a clean separation of the wax pattern from a

gypsum die, a die lubricant must be applied to gypsumsurface prior to placement of molten wax .

Lubricant is oil based . And excess of lubricant should not beused as it may obscure detail of die, or may get adsorbed to

the wax pattern.


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Spruing the wax pattern

Investing the wax pattern

Burning out the wax pattern

Casting the dental alloy

Sandblasting the framework

Finishing the framework

Steps involved in a casting procedure


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SpruingPurpose

To form a mount for the wax pattern and fix the pattern inspace so a mold can be made.

To create a channel for elimination of wax during burnout.

To form a channel for the ingress of molten alloy.

compensate for alloy shrinkage during solidification.

Sprue Size And Design

Large and small inlays require sprues that are 14 gauge (4 to 5

mm long) and 16 gauge (3 to 4 mm long), respectively.

Large and small crowns require 10- and 12gauge sprues,

respectively, with an average sprue length of 4 to 5 mm.


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Point Of Attachment

Sprue attachment must always be made at the bulkiest

portion of the pattern.

a- crucible former, b-sprue, c-cavity formed

by wax pattern after burnout, d-investment,

e- liner, f- casting ring, g- recommended

maximum thickness of approx. 6 mm. b/w

the end of invested ring to provide

pathway for gas escaping.

The wax sprue is the most commonly used.

A hollow-metal sprue pin is preferable to a solid metal pin.

Plastic sprues are not recommended.

Sprue Selection


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Liner The most commonly used technique to provide

investment expansions is to line the walls of the ring with

a ring liner.

Traditionally, asbestos was the material of choice.

Three types of non-asbestos ring liner materials havebeen produced:

1. an aluminosilicate ceramic liner

2. cellulose (paper) liner

3. Ceramic cellulose combination


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Functions of ring liner1. A liner is placed inside the ring to allow lateral expansion of

the investment.

2. When the ring is transferred from the furnace to the castingmachine it reduces heat loss as it is thermal insulator

3. Permits easy removal of the investment after casting.

Three millimeters of clearance is allowed at each end of thering so the mold is sealed and anchored.

The dry liner is tacked in position with sticky wax, and it isthen used either dry or wet.

With a wet liner technique, the lined ring is immersed inwater for a time, and the excess water is shaken away.

Squeezing the liner should be avoided, because this lead tovariable amounts of water removal and nonuniformexpansion.


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INVESTING PROCEDURE

The wax pattern should be cleaned of any debris.

Appropriate amount of distilled water (gypsuminvestments) or colloidal silica special liquid (phosphate

investments) is dispensed.

Mechanical mixing under vacuum removes air bubblescreated during mixing and evacuates any potentiallyharmful gases produced by the chemical reaction of thehigh-heat investments.


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Compensation for Shrinkage

Two liners allow a greater setting and thermalexpansion than does a single liner.

By varying the L/P ratio of the investment.

Casting procedure

Invested ring is placed in a room temp. furnace and

heated up to

For gypsum bonded ( hygroscopic ) ------ 468 oC

For gypsum bonded ( thermal expansion ) ---- 650 oC

For phosphate bonded investment -------700 - 870 oC


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Hygroscopic low heat technique

In this technique compensation for shrinkage is done with :

1. 37oc water bath expands the wax pattern.

2. Warm water enters the mold and hygroscopic expansion

is obtained .

3. thermal expansion

During this technique care must be taken to allow sufficient

burnout time because wax is slowly eliminated at low

temperature.

Porosity is more in low temp. technique then high temp.

technique


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The hygroscopic technique was developed for alloys with

high gold content but there was a need for slightly moreexpansion for newer noble alloys and this addedexpansion may be obtained from :

1. increasing the water bath temp. to 40 0c

2. using two layer of liner.

3. increasing the burnout temp. to a range of 600oc -650oc


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High heat thermal expansion technique

This technique mainly depends on high heat burnout toobtain the required expansion.

Gypsum investment isnt heated more than 700 0c,because above this temperature carbon react withcalcium sulfate binder & forms sulfur dioxide whichcontaminates gold casting and make them extremelybrittle.


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In phosphate investment expansion is obtained from :

1. Expansion of wax pattern

2. Setting expansion which is higher than gypsum

investment because in phosphate investment colloidal

silica is used as a liquid

3. Thermal expansion which is also greater.

A total expansion of 2% or more is required for porcelain

bonded alloys.


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WAX ELIMINATION AND HEATING

It is also advisable to begin the burnout procedurewhile the mold is still wet. Water trapped in thepores of the investment reduces the absorption ofwax, and as the water vaporizes, it flushes wax from

the mold.

If the burnout procedure does not immediately

follow the investing procedure, the invested ring isplaced in a humidor at 100% humidity.


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Gypsum Investments

Rapid heating can generate steam, which can causeflaking or spalling of the mold walls.

Too many patterns in the same plane within the

investment often cause separation of a whole section

of investment, because the expanding wax creates

excessive pressure over a large area.

Too rapid a heating rate may also cause cracking of the

investment


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The outside layer starts to expand thermally, resulting incompressive stress in the outside layer, whichcounteracts tensile stresses in the middle regions of themold.

Such a stress distribution causes the brittle investment tocrack from the interior outwardly in the form of radialcracks. These cracks, in turn, produce a casting with fins

or spine.

Fins on the surface of a casting that formedas a result of cracks in the investment

before casting of the metal.


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Phosphate Investments

Phosphate investments obtain their expansion from thefollowing sources:

Expansion of the wax pattern this is considerable becausethe setting reaction raises the mold temperaturesubstantially.

Setting expansion this is usually higher than in gypsuminvestments, especially because special liquids are used toenhance such expansion.

Thermal expansion

this is greater when taken totemperatures higher than those used for gypsum-bondedinvestments.


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The usual burnout temperatures for phosphate-bondedinvestments range from 750 to 1030 C.

Highest temperatures are required for base metal alloys.

The heating rate is usually slow to 315 C and is quite rapidthereafter, reaching completion after a hold at the uppertemperature for 30 minutes.

To save time, the metal ring is being replaced with a plastic

ring that is tapered so that once the investment has set, itcan be pushed out of the ring, held for a specified time toensure complete setting, and then placed directly into thehot furnace.


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Time gap before doing Casting

The investment contracts thermally as it cool.

Under average conditions of casting, approximately 1minute can pass without a noticeable loss in dimension.

In the low-heat casting technique, the temperaturegradient between the investment mold and the room isnot as great as that employed with the high-heattechnique.

Alloy should be cast soon after removal of the ring fromthe oven; otherwise a significant variation from thedesired casting dimensions may occur.


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CASTING MACHINES

Alloys are melted in one of the four following ways,depending on various types of casting machines:

The alloy is melted in a separate crucible by a torchflame and is cast into the mold by centrifugal force.

The alloy is melted electrically by a resistance heating orinduction furnace, then cast into the mold centrifugallyby motor or spring action.

The alloy is melted by induction heating, then cast into

the mold centrifugally by motor or spring action.

The alloy is vacuum arc melted and cast by pressure inan argon atmosphere.


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Torch Melting/Centrifugal Casting Machine The alloy is melted by a torch flame

in a glazed ceramic crucibleattached to the "broken arm" of thecasting machine.

The broken arm feature accelerates

the initial rotational speed of thecrucible and casting ring, thusincreasing the linear speed of theliquid casting alloy as it moves intoand through the mold.

Once the metal has reached thecasting temperature and the heatedcasting ring is in position, themachine is released and the spring

triggers the rotational motion.


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METAL FLOWS IN TO THE MOLD


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As metal fills hydrostatic pressure gradient developsalong the length of the casting. The pressure gradientfrom the tip of the casting to the bottom surface is

quite sharp and parabolic in form, reaching zero atthe button surface.

Pressure gradient at the moment beforesolidification reaches is about 0.21 to 0.28 MPa (30

to 40 psi) at the tip of the casting.There is also a gradient in the heat transfer rate suchthat the greatest rate of heat transfer to the mold isat the high pressure end of the gradient (i.e., the tipof the casting).

Because this end also is frequently the sharp edge ofthe margin of a crown, there is further assurancethat the solidification progresses from the thinmargin edge to the button surface.


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Electrical Resistance-Heated Casting Machine Current is passed through a resistance heating

conductor, and automatic melting of the alloy occurs in

a graphite or ceramic crucible.

Advantages:

Used for metal-ceramic prostheses, which are alloyedwith base metals in trace amounts that tend to oxidizeon overheating.

The crucible in the furnace is located flush against thecasting ring.

Therefore the alloy button remains molten slightlylonger, again ensuring that solidification progressescompletely from the tip of the casting to the buttonsurface.


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Induction MeltingMachineAlloy is melted by an induction field that

develops within a crucible surrounded bywater cooled metal tubing.

The electric induction furnace is atransformer in which an alternating currentflows through the primary winding coil andgenerates a variable magnetic field in thelocation of the alloy to be melted in acrucible.

Once the alloy reaches the casting

temperature in air or in vacuum, it is forcedinto the mold by centrifugal force, by airpressure, or by vacuum.

Used for melting base metal alloys.


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Direct Current Arc Melting Machine The direct current arc is produced between two

electrodes: the alloy and the water cooled tungstenelectrode.

The temperature within the arc exceeds 4000C, andthe alloy melts very quickly.

This method has a high risk for overheating the alloy,and damage may result after only a few seconds ofprolonged heating.


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Vacuum or Pressure Assisted Casting Machine The molten alloy is heated to the casting temperature,

drawn into the evacuated mold by gravity or vacuum,and subjected to additional pressure to force the alloy

into the mold.

For Titanium and Titanium alloys, vacuum arc heated

argon pressure casting machines are required.


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Casting Crucibles

Four types of casting crucibles are:

Clay, Carbon,

Quartz, and

Zirconium-alumina

Clay crucibles are appropriate for many of the crownand bridge alloys, such as the high noble and noble

types.

Carbon crucibles can be used not only for high noblecrown and bridge alloys but also-for the higher-fusing,

gold-based metal-ceramic alloys.


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Torch Melting of Noble Metal Alloy

Fuel used is a mixture of natural or artificial gas and air,

oxygen-air and acetylene can also be used. When the reducing zone is in contact, the surface of the

gold alloy is bright and mirror like.


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FLAME


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Mirror like surface of the metal indicates proper fusion. B, Cloudy surface

indicates surface oxidation by improper positioning of the torch flame.

When the oxidizing portion of the flame is in contact with

the alloy, there is a dull film of "dross" developed over the

surface.

The alloy first appears to be spongy, and then smallglobules of fused alloy appear.


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The molten alloy soon assumes a spheroidal shape.

At the proper casting temperature, the molten alloy is

light orange and tends to spin or follow the flame whenthe latter is moved slightly.

At this point, the alloy should be approximately 38 to66 C above its liquidus temperature.

The casting should be made immediately when the

proper temperature is reached.


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HEAT TREATMENT

Heat treatment is helpful in finishing margins.

To adequately resist excessive wear, however, therestoration must be returned to a hardened state prior toits clinical use.

It is done for all gold alloys.

Softening heat treatment: consist of heating the alloy to

700C and maintaining that temperature for

approximately 15 minutes, followed by quenching in room

temperature water.


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May be conducted in either of two ways :

Heat soaking at a constant temperature of

approximately 450C for 15 minutes.

Slow cooling from 450C 250C over a period of 15

minutes, followed by quenching in water.

CLEANING THE CASTING


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CLEANING THE CASTING

Two advantages are gained in quenching:

(1) The noble metal alloy is left in an annealed condition for

burnishing, polishing, and similar procedures,(2) When the water contacts the hot investment, a violent reaction

ensues, resulting in a soft, granular investment that is easilyremoved.

The surface of the casting appears dark with oxides and tarnish.

Such a surface film can be removed by a process known aspickling,which consists of heating the discolored casting in anacid.

One of the best pickling solutions for gypsum-bonded

investments is a 50% hydrochloric acidsolution.


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The disadvantage of hydrochloric acid is that the fumes from the acid are likely

to corrode laboratory metal furnishings.

The pickling process can be performed ultrasonically while the prosthesis is

sealed in a Teflon container.

A solution of 50% sulfuric acid may also be more advantageous in this respect.

Cold hydrofluoric acid dissolves the silica refractory quite well without damageto a gold-based or a palladium-silver alloy.

Base metal alloys require a light Sandblasting,

usually with fine alumina.


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Selecast System Titanium casting

system for dentistry

Designed as full-automatic operation

It includes LCD operation panel

It includes Inverter-motor for more stability of turn-table.

There more large size vacuum pump

The electric devices have wider margin to its limit value, it

means to be more safety.


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Characteristics

In 'SPIN CAST

METHOD' pores are

scarcely formed, since

mold rotates (with

3,000 r.p.m.) before

casting.

Structure
http://www.selec-inc.com/eng/Selecast%20from%20selec%20co.,%20Ltd.


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Structure

It has a vacuum chamber in its upper body, which consists of argonarc melting system and centrifugal casting system.

The body contains vaccum pump, high-voltage electrode formelting, turn-table rotary motor, bulbs and control system whichcovers all of the device.

When 'POWER' button is pushed and touch to auto start, the

chamber is evacuated and argon gas flows in it.

Arc melting with tungsten electrode, titanium ingot in cruciblemelts down in argon atmosphere.

After the rotary speed of the mold (which is horizontally spun) getshigher than fixed, molten titanium is poured into the gate from thetilting crucible

Casting method Horizontal centrifugal


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Casting method Horizontal-centrifugal

casting with tilting - type

crucible

Melting method After vacuumed, electric are

melting in argon atmosphere

Melting ability Max.41g

(pure titanium and titanium

alloy)

Electric control Automationexhaustion - feeding of

argon - spinning of mold -

melting - casting

Chamber volume 14.6 liters

Revolution of

mold

Max. 3,000 R.P.M

Size (outward) W510 * D725 * H 1,130

(mm)

Weight 220kg


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CAUSES OF DEFECTIVE CASTINGS

Defects in castings can be classified under four headings:

(1) Distortion

(2) Surface roughness and irregularities

(3) Porosity

(4) Incomplete or missing details

Di i


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Distortion Is probably related to distortion of the wax pattern.

This type of distortion can be minimized by proper

manipulation of the wax and handling of the pattern.

Distortion of the wax pattern occurs as the investment

hardens around it.

The setting and hygroscopic expansions of the investmentmay produce a non-uniform expansion of the walls of thepattern.

The gingival margins are forced apart by the moldexpansion, whereas the solid occlusal bar of wax resists

expansion during the early stages of setting. The lesser the setting expansion of the investment, the less

the distortion.

S f R h I l iti d Di l ti


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Surface Roughness, Irregularities, and Discoloration

Surface roughness is defined as relatively finely spared

surface imperfections whose height, width, and direction

establish the predominant surface pattern.

Surface irregularities are isolated imperfections, such as

nodules, that are not characteristic of the entire surface

area.

Surface irregularities on anexperimental casting caused byair bubbles (A), water film (B)and inclusion of foreign body (C)

bbl


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Air Bubbles Small nodules on a casting are caused by air bubbles that

become attached to the pattern during or subsequent to

the investing procedure. For nodules on margins or on internal surfaces removal of

these irregularities might alter the fit of the casting.

Prevention

The use of a mechanical mixer with vibration both beforeand after mixing should be practiced routinely.

The wetting agent be applied in a thin layer. It is best to air dry the wetting agent, because any excess

liquid dilutes the investment, possibly producing surfaceirregularities on the casting.

W Fil


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Water FilmsWax is repellent to water and if the investmentbecomes separated from the wax pattern in somemanner, a water film may form irregularly over thesurface.

This type of surface irregularity appears as minute

ridges or veins on the surface.

Too high an L/P ratio may also produce these surfaceirregularities.

A wetting agent is of aid in the prevention of suchirregularities.

Under Heating


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Under Heating

Incomplete elimination of wax residues may occurparticularly with the low-temperature investment

techniques.

Voids or porosity may occur in the casting from thegases formed when the hot alloy comes in contact with

the carbon residues.

The casting may also be covered with a tenaciouscarbon coating that is virtually impossible to removeby pickling.

P l d H ti


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Prolonged Heating

If the high-heat casting technique is used, a prolongedheating of the mold at the casting temperature is likely tocause a disintegration of the gypsum-bonded investment,and the walls of the mold are roughened as a result.

The products of decomposition are sulfur compounds thatmay contaminate the alloy to the extent that the surface

texture is affected.

Temperature of the Alloy

If an alloy is heated to too high a temperature beforecasting, the surface of the investment is likely to beattacked, and a surface roughness may result.

Casting Pressure


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Casting Pressure Too high a pressure during casting can produce a rough

surface on the casting.

A gauge pressure of 0.10 to 0.14 MPa in an air pressurecasting machine orthree tofour turns of the spring in anaverage type of centrifugal casting machine is sufficient forsmall castings.

Composition of the Investment

The ratio of the binder to the quartz influences the surface

texture of the casting.

A coarse silica causes a surface roughness.

Foreign Bodies


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Foreign Bodies

A rough crucible former with investment clinging to

it may roughen the investment on its removal sothat bits of investment are carried into the moldwith the molten alloy.

Carelessness in the removal of the sprue former canalso be a cause for it.

Casting that shows sharp, well defined deficienciesindicates the presence of some foreign particles inthe mold, such as pieces of investment and bits ofcarbon from a flux.

Bright appearing concavities may be the result of


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Bright appearing concavities may be the result offlux being carried into the mold with the metal.

Surface discoloration and roughness can resultfrom sulfur contamination, either frominvestment breakdown at elevated temperatures

or from a high sulfur content of the torch flame.

The interaction of the molten alloy with sulfur

produces a black or grey layer on the surface ofgold alloys that is brittle and does not cleanreadily during pickling.

I t f M lt All


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Impact of Molten Alloy The molten alloy may fracture or abrade the mold

surface on impact, regardless of its bulk.

The direction of the sprue former should be such thatthe molten gold alloy does not strike a weak portion ofthe mold surface.

It is unfortunate that sometimes the abraded area issmooth so that it cannot be detected on the surface ofthe casting.

Such a depression in the mold is reflected as a raisedarea on the casting.

Pattern Position


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Pattern Position

If several patterns are invested in the same ring, theyshould not be placed too close together.

Positioning too many patterns in the same plane in themold should be avoided.

The expansion of wax is much greater than that of theinvestment, causing breakdown or cracking of theinvestment if the spacing between patterns is less than 3

mm.

Carbon Inclusions


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Carbon Inclusions

Carbon, as from a crucible, an improperly adjustedtorch, or a carbon containing investment, can beabsorbed by the alloy during casting.

These particles may lead to the formation of carbidesor even create visible carbon inclusions.

Porosity


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Porosity Porosities in noble metal alloy castings may be classified as

follows:

I. Solidification defects

A. Localized shrinkage porosity

B. Microporosity

II. Trapped gases

A. Pinhole porosity

B. Gas inclusionsC. Subsurface porosity

III. Residual air


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Porosities

Solidification

DefectsTrapped gases Residual Air

Localized

ShrinkageMicroporosity

Pinhole

Porosity

Gas

Inclusions

Subsurface

Porosity


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Localized shrinkage is generally caused by premature

termination of the flow of molten metal during solidification.

The linear contraction of noble metal alloys in changing from

a liquid to solid is at least 1.25%.

Continual feeding of molten metal through the sprue must

occur to make up for the shrinkage of metal volume duringsolidification.

Porosity in pontic area is caused by its ability to retain heatbecause of its bulk.

This problem can be solved by attaching one or more smallgauge sprues called as chill setsprues.

These act as cooling pins to carry away heat from pontic.

Void may also occur externally, usually in the interior of a


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Void may also occur externally, usually in the interior of acrown near the area of the sprue, if a hot spot has beencreated by the hot metal impinging from the sprue channelon a point of the mold wall.

This hot spot causes the local region to freeze last andresults in what is called suck-back porosity.

Suck-back porosity often occurs at an occlusoaxial line angle

or incisoaxial line angle that is not well rounded.

Suck-back porosity can be eliminated by flaring the point of

sprue attachment and reducing the mold melt temperature

differential, that is, lowering the casting temperature by

about 30 C.


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Microporosityalso occurs from solidification shrinkage but isgenerally present in fine grain alloy castings when thesolidification is too rapid for the micro voids to segregate to

the liquid pool.

Pinholeand the Gas inclusionporositiesare related to theentrapment of gas during solidification.

The gas inclusion porosities are usually much larger than

pinhole porosity.

On solidification, the absorbed gases are expelled and pinhole

porosity results.


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Larger spherical porosities can be caused by gas occludedfrom a poorly adjusted torch flame, or by use of the mixing oroxidizing zones of the flame rather than the reducing zone.

These types of porosity can be minimized by pre- melting thegold alloy on a graphite crucible or a graphite block, if thealloy has been used before, and by correctly adjusting andpositioning the torch flame during melting.

Subsurface porosityoccurs on occasion, the reasons for suchvoids have not been completely established.

They may be caused by the simultaneous nucleation of solidgrains and gas bubbles at the first moment that the alloyfreezes at the mold walls.

This type of porosity can be diminished by controlling the

rate at which the molten metal enters the mold.

Entrapped air porosity on the inner surface of the casting,


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pp p y g,sometimes referred to as back-pressure porosity, canproduce large concave depressions.

This is caused by the inability of the air in the mold to escapethrough the pores in the investment or by the pressuregradient that displaces the air pocket toward the end of theinvestment via the molten sprue and button.

The entrapment is frequently found in a "pocket" at thecavity surface of a crown or mesio-occlusal-distal casting.


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Incomplete Casting


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Incomplete Casting Occasionally, only a partially complete casting, or perhaps no

casting at all, is found.

Cause is that the molten alloy has been prevented in somemanner, from completely filling the mold.

CAUSES


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The first consideration- insufficient venting, is directly related tothe back pressure exerted by the air in the mold. If the air cannotbe vented quickly, the molten alloy does not fill the mold before itsolidifies.

In such a case, the magnitude of the casting pressure should be

suspected. If insufficient casting pressure is used, the back pressure cannot

be overcome.

Furthermore, the pressure should be applied for at least 4seconds.

The mold is filled and the alloy is solidified in 1 second or less; yetit is quite soft during the early stages. The pressure should bemaintained for a few seconds beyond this point.

CAUSES

A second common cause for an incomplete casting is


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incomplete elimination of wax residues from the mold.

If too many products of combustion remain in the mold, thepores in the investment may become filled so that the aircannot be vented completely.

If moisture or particles of wax remain, the contact of the

molten ; alloy with these foreign substances produces anexplosion that may produce sufficient back pressure toprevent the mold from being filled.

CONCLUSION


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CONCLUSION

Production of an accurate casting requires development

of a precise casting procedure.

Many problems can be encountered during castingprocedure resulting defects in final castings.

Adherence to the casting procedures described will aid inthe production of clean, well fitting castings.

REFERENCES


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Science of dental materials- Anusavice, 11th Edn

Restorative dental materials - Craig,10th Edn.

Applied dental Materials -John F. Mc Cabe,7th Edn. Dental materials, Properties & Manipulation -Robert G. Craig et.al,

5th Edn.

Journal of Prosthet Dent. 2002; 87:94-98.

Journal of Prosthet Dent. 1988; 80: 691-698.




Dental materials. 1987; 3: 125-130.

Dental materials. 1993; 9: 177-181.

Intl Journal Prosthodont. 1991;4:152-158.

J Dent Research. 1990; 69: 67-68.

casting procedures and casting defects

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