Casting Methodes Ref

7/25/2019 Casting Methodes Ref

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The casting method consists of forming a wax pattern, surrounding it with investment

material, and later heating the investment mold to remove the wax before casting the

molten metal into the mold.

Defects in Wax Pattern

1. Distortion2. Surface roughness and irregularities

3. Air bubbles

. !ater film

Defects in Burn out Procedure

1. "apid #eating "ates

2. $nder heating3. %rolonged heating

Rough surface on casting

Due to prolonged overheating of g&psum bonded investment.%revention' use correct heating c&cle for burnout procedure. Do not heat above 12()*

+))-

%roblems D$"/0 -AST/0 %"-D$"

1. /ncomplete castings Allo& not hot enough

#ave casting temperature above indicated fusion temperature of allo&

4old too cold

ven too cold' chec5 p&rometer6 mould should heat soa5 1 hour at burnout

temperature.

/nsufficient casting force *orce allo& into the mold rapidl&6use centrifugal machine with good springs

!inds.

/nsufficient gold

2. "ough Surface on casting Due to 'Too much casting pressure causing investment brea5down from force of

allo&.

limination'

A gauge pressure of .1) to.14%a in an air pressure casting machine

Three to four turns of the spring in an average t&pe of centrifugal casting machines is

sufficient for small castings

-lassification of casting defects +A0$SA7/-

Defects in castings can be classified under four headings8+1 Distortion6

+2 Surface roughness and irregularities6

+3 %orosit&6

+ /ncomplete or missing detail


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Distortion

An& mar5ed distortion of the casting is probabl& related to a distortion of the waxpattern.This t&pe of distortion can be minimi9ed or prevented b& proper manipulation of

the wax and handling of the pattern.Some distortion of the wax pattern occurs as the

investment hardens around it.The setting and h&groscopic expansions of the investmentma& produce a nonuniform expansion of the walls of the pattern.

This t&pe of distortion occurs in part from the nonuniform outward movement of the

proximal walls, the gingival margins are forced apart b& the mold expansion, whereas thesolid occlusal bar of wax resists expansion during the earl& stages of setting

The configuration of the pattern, the t&pe of wax, and the thic5ness influence the

distortion that occurs.

Distortion increases as the thic5ness of the pattern decreases.The less the setting expansion of the investment, the less the distortion.

Surface Roughness, Irregularities, and Discoloration

The surface of a dental casting should be an accurate reproduction of the wax pattern

from which it is made.xcessive roughness or irregularities on outer surface of the casting necessitate additional

finishing and polishing, whereas irregularities on the cavit& surface prevent a proper

seating of an otherwise accurate casting.Surface roughness is defined as relativel& finel& spaced 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. The surface roughness of the dental casting is invariabl& somewhatgreater than that of the wax pattern from which it is made. The difference is probabl&

related to the particle si9e of the investment and its abilit& to reproduce the wax pattern in

microscopic detail.

Surface irregularities on an experimental casting caused b& air bubbles +A,water film

+:, and inclusion of foreign bodies+-.


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AirBubbles

Small

nodules ona casting

are caused

b& air

bubblesthat

become

attached to

the patternduring or

subse;uentto the

investing

procedure.The best

method to avoid air bubbles is to use the vacuum investing techni;ue. /f a manual method

is used, various precautions can be observed from the investment mix before the

investing. The use of a mechanical mixer with vibration both before and after mixingshould be practiced routinel&.

!etting agent6 A wetting agent ma& be helpful in preventing the collection of air bubbles on the

surface of the pattern. The wetting agent be applied in a thin la&er.

Air'dr& the wetting agent, because an& excess li;uid dilutes the investment, possibl&

producing surface irregularities on the casting.

Water Films

!ax is repellent to water, and if the investment becomes separated from the wax pat'

tern in some manner, a water film ma& form irregularl& over the surface.

ccasionall&, this t&pe of surface irregularit& appears as minute ridges or veins on the

surface . /f the pattern is slightl& moved, % ratio

ma& also produce these surface irregularities.

Rapid Heating


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4a& result in fins or spines on the casting.

Also, a characteristic surface roughness ma& be evident because of fla5ing of the

investment when the water or steam pours into the mold. *urthermore, such a surge ofsteam or water ma& carr& some of the salts used as modifiers into the mold, and these

salts are left as deposits on the walls after the water evaporates.

The mold should be heated graduall&6 at least ?) min should elapse during the heatingof the investment'filled ring from room temperature to ))@ -.

The greater the bul5 of the investment, the more slowl& it should be heated.

nderheating

/ncomplete elimination of wax residues ma& occur if the heating time is too short or

if insufficient air is available in the furnace. These factors are particularl& important with

the low'temperature investment techni;ues. 7oids or porosit& ma& occur in the casting from the gases formed when the hot allo&

comes in contact with carbon residues. ccasionall&, the casting ma& be covered with a

tenacious carbon, coating that is virtuall& impossible to remove b& pic5ling.

=i;uid>%owder "atio The amount of water and investment should be measured accuratel&.

The higher the =>% ratio, the rougher the casting. #owever, if too little water is used,the investment ma& be unmanageabl& thic5 and cannot be properl& applied to the pattern.

/n vacuum investing, the air ma& not be sufficientl& removed. /n either instance, a

rough surface on the casting ma& result.%rolonged #eating

!hen the high'heat casting techni;ue is used, a prolonged heating of the mold at the

casting temperature is li5el& to cause a disintegration of the g&psum'bonded investment,

and the walls of the mold are roughened as a result. *urthermore, products ofdecomposition are sulfur compounds that ma& contaminate the allo& to the extent that the

surface texture is affected

Such contamination ma& be the reason that the surface of the casting sometimes doesnot respond to pic5ling.

!hen the thermal expansion techni;ue is emplo&ed, the mold should be heated to the

casting temperaturenever higherand the casting should be made immediatel&.

!emperature of the Allo"

/f an allo& is heated to too high a temperature before casting, the surface investment is

li5el& to be attac5ed, and a surface roughness ma& result. /n all probabilit&, the allo& willnot be overheated with a gas'air torch when used with the gas supplied in most localities.

/f oil is used, special care should be observed that the color emitted b& the molten allo&,

for example, is no lighter than a light orange.

#asting Pressure

Too high a pressure during casting can produce a rough surface on the casting.

A gauge pressure of ).1) to ).1 4%a in an air pressure casting machine or three tofour turns of the spring in an average t&pe of centrifugal casting machine is sufficient for

-omposition of the /nvestment

The ratio of the binder to the ;uart9 influences the surface texture of the casting.

A coarse silica causes a surface roughness.


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/f the investment meets A0S/> ADA Specification 0o. 2, the composition is not a

factor in surface roughness small castings.

Foreign Bodies

!hen foreign substances get into the mold, a surface roughness ma& be produced.

*or example, a rough crucible former with investment clinging to it ma& roughen the

investment on its removal so that bits of investment are carried into the mold with themolten allo&. -arelessness in the removal of the sprue former ma& also be a cause.

$suall&, contamination results not onl& in surface roughness but also in incomplete

areas or surface voids. An& casting that shows sharp, well'defined deficiencies indicatesthe presence of some foreign particles in the mold, such as pieces of investment and bits

of carbon from a flux. :right'appearing concavities ma& be the result of flux being

carried into the mold with the metal.

Surface discoloration and roughness can result from sulfur contamination, either frominvestment brea5down at elevated temperatures or from a high sulfur content of the torch

flame. The interaction of the molten allo& with sulfur produces a blac5 or gre& la&er on

the surface of gold allo&s that is brittle and does not clean readil& during pic5ling.

/mpact of 4olten Allo&The direction of the sprue former should be such that the molten gold allo& does not

stri5e a wea5 portion of the mold surface.ccasionall&, the molten allo& ma& fracture or abrade the mold surface on impact,

regardless of its bul5.

/t is unfortunate that sometimes the abraded area is smooth so that it cannot bedetected on the surface of the casting

Such a depression in the mold is reflected as a raised area on the casting, often too

slight to be noticed &et sufficientl& large to prevent complete seating of the casting. This

t&pe of surface roughness or irregularit& can be avoided b& proper spruing so as toprevent the direct impact of the molten metal at an angle of () degrees to the investment

surface.

Pattern Position

/f several patterns are invested in the same ring, the& should not be placed too close

together. =i5ewise, positioning too man& patterns in the same plane in the mold should be

avoided. The expansion of wax is much greater than that of the investment, causingbrea5down or crac5ing of the investment if the spacing between patterns is less than 3

mm.

#arbon Inclusions

-arbon, as from a crucible, an improperl& ad


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Thus it is not a good practice to place a new amalgam restoration ad


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mechanicall& trapped b& the molten metal in the mold or b& gas that is incorporated

during the casting procedure.All castings probabl& contain a certain amount of porosit&.

#owever, the porosit& should be 5ept to a minimum because it ma& adversel& affect theph&sical properties of the casting.

-astings that are severel& contaminated with gases are usuall& blac5 when the& are

removed from the investment and do not clean easil& on pic5ling.The porosit& thatextends to the surface is usuall& in the form of small pinpoint holes. !hen the surface is

polished, other pinholes appear.

=arger spherical porosities can be caused b& gas occluded from a poorl& ad


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"ounded margins

/ncomplete burnout of wax pattern


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/nsufficient heating of allo& before casting

4argins melted while attaching pattern to sprue or former

/mproper length>diam of sprue restricts flow of allo& into mold metal free9es beforemargins are complete

Porosit"

/mproper sprue former,si9e,length or placement


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"ough surface on casting

xcess moisture on pattern

!ater powder ratio too high in mixing investmentToo much casting pressure causing investment brea5down from force of allo&


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%rolonged overheating of g&psum bonded investment


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%its in casting

Debris in mould

Dirt& wax=oose debris in crucible

4old temperature too hot


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*ins on casting

vervibration during investment or disturbing ring during set

#eating ring too rapidl& causes moisture in mold to form steam and rupture outmold


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:ubbles or nodules on casting

/nvestment not mixed under enough vacuum to remove entrapped air bubbles


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Air lea5 between crucibles,sprue former,and casting allowed bubble to form andbecome trapped under pattern


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Incomplete #asting

ccasionall&, onl& a partiall& complete casting, or perhaps no casting at all, is

found. The obvious cause is that the molten allo& has been prevented, in some manner,

from completel& filling the mold. At least two factors that ma& inhibit the ingress of theli;uefied allo& are insufficient venting of the mold high viscosit& of the fused metal.

The first consideration, insufficient venting, is directl& related to the bac5 pressure

exerted b& the air in the mold./f the air cannot be vented ;uic5l&, the molten allo& does not fill the mold before it

solidifies. /n such a case, the magnitude of the casting pressure should be suspected

/f insufficient casting pressure is used, the bac5 pressure cannot be overcome.

*urthermore, the pressure should be applied for at least sec.The mold is filled and the allo& is solidified in 1 sec or less6 &et it is ;uite soft

during the earl& stages. Therefore the pressure should be maintained for a few secondsbe&ond this point. These failures are usuall& exemplified in rounded, incomplete margins.

"ounded,incomplete margins are evidence of insufficient casting pressure


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A second common cause for an incomplete casting is incomplete elimination of waxresidues from the mold. /f too man& products of combustion remain in the mold, the

pores in the investment ma& become filled so that the air cannot be vented completel&

/f moisture or particles of wax remain, the contact of the molten allo& with theseforeign substances produces an explosion that ma& produce sufficient bac5 pressure to

prevent the mold from being filled. This shin& condition of the metal is caused b& the

strong reducing atmosphere created b& carbon monoxide left b& the residual wax./ncomplete casting results from incomplete wax removal


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Different allo&s exibit var&ing viscosities in molten state depending on compositionand temperature.

#owever, both the surface tension and the viscosit& of a molten allo& are decreased

with an increase in temperature.An incomplete casting resulting from too great a viscosit& can be attributed to

insufficient heating. The temperature of the allo& should be raised higher than its li;uidus

temperature so that its viscosit& and surface tension are lowered and so that it does notsolidif& prematurel& as it enters the mold. Such premature solidification ma& account for

the greater susceptibilit& of the white gold allo&s to porosit& because their li;uidus

temperature are higher. Thus the& are more difficult to melt with a gas' air torch flame.


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Sprue former selection is based on the following five principles6

1' *or a small pattern a small si9e sprue is selected because a large sprue attached to

a thin delicate pattern could cause distortion. A sprue former diameter if too thin will leadto a shrin5age porosit& +Fsuc5' bac5 porosit&. "eservoir sprues are used to overcome this

problem.

2'The sprue former should be attached to the portion of the pattern with the largestcross sectional area. /t is best for the molten allo& to flow from a thic5 section to a

surrounding thin area +e.g margins and not the reverse. This design minimi9es the ris5

for turbulence.3'The length of the sprue former should be long enough to properl& position the

pattern in the casting ring within ? mm of the trailing end and &et short enough so that

molten allo& does not solidif& before it fills the mold.A reservoir should be added to a

spruing networ5 to prevent locali9ed shrin5age porosit&.

!hen the molten metal allo& fills the heated casting ring, the pattern area shouldsolidif& first and the reservoir last. :ecause of its large mass of allo& and position in the

heat centre of the ring, the reservoir remains molten to furnish li;uid allo& into the moldas it solidifies. The resulting solidification shrin5age occurs in the reservoir bar and not in

the restoration. *laring facilitates the entr& of fluid allo& into the pattern area.

Sprue *ormer Direction'The sprue former should be directed awa& from an& thin ordelicate parts of the pattern, because the molten metal ma& abrade or fracture investment

in this area and result in a casting failure. /t should not be attached at a right angle to a


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broad flat surface as this leads to turbulence within the mold cavit& and severe porosit& in

the region. The sprue former should alwa&s be attached to pattern at B degree angle.

Sprue *ormer =ength'The length of the sprue former depends on the length of thecasting ring. The sprue length should be ad


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CASTING METAL CROWNS

- ERROR-

Coordinating teacher: rc Tiberiu

Student: A! "aidhani A##ar

"adea Ioi$ "!cecu E!ena

"uciu %dtreanu A!e&andru

Mr u Mihai

M' III( GR) * A

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