dental investment materials and investing techniques

8/13/2019 Dental Investment Materials and Investing Techniques

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first describe by Philbrook D, & Taggart WH

Mould/ die for pattern


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Sprue attached to patterns


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Patterns invested

Burnout in furnace

Flow of molten metal


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Fundamentals of Spruing, Investing &Casting


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After casting Finished & polished


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Investing: the process of covering or enveloping,wholly or in part, an object such as denture, tooth,wax form, crown, etc. with a suitable investmentmaterial before processing, soldering or casting.

(GPT 8)Dental casting investment: A material consisting

primarily of an allotrope of silica and a bondingagent. The bonding substance may be gypsum (foruse in lower casting temperatures) or phosphatesand silica (for use in higher casting temperatures).

(GPT 8)


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1. should be capable of reproducing the shape, size anddetail in the wax pattern.

2. easily manipulated.

3. setting time should be less .4. should maintain the integrity at higher temperatures andshould not decompose to give off gases.

5. Possess sufficiently high value of compressive strength at the

casting temperature to withstand the stresses set up when themolten metal enters the mould.6. should expand to compensate for the casting shrinkage


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7. Investment should be porous enough to permit the

air or other gases in the mold cavity to escape easilyduring the casting

8. Investment should produce a smooth

surface and fine detail and margins onthe casting.

9. should be inexpensive .


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Wax shrinkage- 0.3-0.4%.

Alloy shrinkage- 1.3- 1.6% ( noblealloys), 2.0- 2.3%( Predominantlybase metal alloys)


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Investment materials arebasically consist of1. A refractory material

2. A binder material

3. Additives used to improve thehandling character.


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SILICA (silicon dioxide) is used as refractory material & toregulate the thermal expansion.

It is available in four allotropic forms such as

QuartzTridymiteCristobaliteFused quartz

Quartz and Cristobalite are used extensively in dental investment


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Cristobalite occurs naturally as a rare mineral but isnormally manufactured by prolonged heating of

the quartz at high temperatures to induce theappropriate slow inversion.

Each form of silica exists in two phases.1. Low temperature phase or alpha phase

2. High temperature phase or Beta phase

On heating the change between the two phases is rapidand readily reversible on cooling .this change isknown as high low inversion .


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Depending on type of silica used theinvestment materials are classified as

1. Quartz investments

2. Cristobalite investments


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It binds the refractory materials together.According to the binder used investment materials areof three groups .

1. GYPSUM-BONDED INVESTMENTS

2. PHOSPHATE BONDED INVESTMENTS

3. SILICA BONDED INVESTMENTS


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Other chemicals such as sodium chloride, boric acid,graphite, copper powder, are often added in smallquantities to modify physical properties.

boric acid and sodium chloride, not only regulate

the setting expansion and the setting time, but also prevent most of the shrinkage of gypsum when it isheated above 300 C (572 F).


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They are the mold materials used in the castingof dental gold alloys with liquidustemperatures no more than 1080 c.Used for conventional casting of gold alloysinlays, onlays, crowns and Fpd.


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RefractoryCrystalline polymorphsof silica (quartz orcristobalite) 55-75%Silica is added to

provide a refractory

component during theheating of the investmentand to regulate thethermal expansion.


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BINDER Alpha hemi hydrate form ofgypsum(25-45%)

Strength of investment depends on amount of binder present.


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contracts


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MODIFIER - (4-7%)Used are

Reducing agentsModifying chemicalsColoring matter

Reducing agents : they reduce any metal oxides formedon the metal by providing a non oxidizing atmospherein the mold when the alloy enters mold.Ex Copper


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Modifying chemicals: They regulate setting expansionand thermal expansion and also prevent shrinkage ofgypsum when heated above 300 c .They act by reducing the two large contractions ofgypsum binder on heating to temperatures above 300

c .Ex Boric acidSoluble salts of alkali or alkaline earth metals


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SETTING TIME According to ANSI/ADA Specification No. 2 for dentalinlay casting investment, the setting time should not be

shorter than 5 min or longer than 25 min. Usually, themodern inlay investments set initially in 9 to 18 min.Should allow sufficient time for mixing and investingthe pattern.


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SETTING EXPANSION1. normal setting expansion: the expansion takes

place when the material sets in air2. hygroscopic setting expansion: when setting occursunder water.3. thermal expansion: when it is affected by heat

Purpose: to enlarge the mold to compensate for thecasting shrinkage of the gold alloy


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Gypsum bondedinvestment

Inlay & onlayinvestment

Thermalexpansion (

ISO Type 1)

Hygroscopicexpansion (

ISO Type 2)Dentureinvestment ( ISO

Type 3)


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ADA sp no 2 for type 1 investment permits a maximumsetting expansion in air of 0.6% setting expansion ofmodern investments is 0.4%which can be regulated byaccelerators and retarders .It is enlarging the mould containing the wax patternand it depends on the thermal expansion of patterncaused by heat of reaction.High gypsum content- high expansion.


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Other variables lower w/p ratio, drier mix ofinvestment- high NSE.Thin wall pattern- more expansion than thickwalls.Soft wax- more NSE.Wax softer than type II inlay wax- NSE causesserious distortion of the pattern.


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Greater in magnitude than NSE.the gypsum product is allowed to set under or incontact with water for mould expansion.


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The hygroscopic setting expansion may be 6 or more timesgreater than the normal setting expansion of a dental

investmentThe increased amount of expansion is because the waterhelps the outward growth of crystalsThe investment should be immersed in water before the

initial set is complete .ADA sp no 2 for such type 2 investments require minimumsetting expansion in water of 1.2% and maximum 2.2%.


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Composition : more finer silica particles -more HSE. hemihydrate greater HSE than hemihydrate.

W:P ratio : less water, more powder in mix. Spatulation : more mixing time- more HSE. Time of immersion: immerse in water before initialset. Confinement : less opposing force from walls ofcasting ring (wet cellulose), immersion in water bath at37.7C- expansion of wax pattern.

Water : more immersion water. Shelf life : fresher investment.


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The thermal expansion is directly related to the amountand type of silica present.Contraction of gypsum is balanced- quartz 75%More Expansion in the presence of cristobalite andinitial contraction of gypsum is eliminated.Type 1 investments should have thermal expansion ofnot less than1% and not greater than 1.6%.

Type 1I investments should have thermal expansion of between 0% to 0.6% at 500C.


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SETTING CONTRACTION: When aninvestment is cooled from 700 C, it contracts.On reheating it expand again but not

recommended as investment may crack onheating.


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COMPRESSIVE STRENGTH:According to ADA sp no 2 the compressive strengthshould not be less than 2.5MPA to withstand force ofmolten metal.

FINENESS: Surface roughness of the casting andsetting time depend upon the fineness of the material.Fine silica- more hygroscopic expansion.


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POROSITY: The material should be porous toallow escape of air from mold space while casting.Lower content of hemihydrate- more water gauging-

more porous.


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It is a gypsum bonded material mixed with colloidalsilicaSetting expansion is 0.9%Thermal expansion is 0.6% when it is heated to 677 cAs it is a gypsum bonded material it is notrecommended for high fusing alloys.Divestment phosphate is a phosphate bondedinvestment used as a divestment for high fusing alloys.


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Most palladium and base metal alloys used for partialdentures and porcelain fused to metal restorations havehigh melting temperatures. They should be castat a mold temperature higher than 700 c.

To withstand these high temperatures ,molds requiredifferent types of binders such as phosphatecompounds.


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Type 1 For casting of inlays crowns and other restorationsespecially for alloys like gold, platinum ,palladiumcobalt chromium and nickel chromium

Type 2 For casting of removable partial dentures


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Refractory materials (concentration ofapproximately 80%)silica in quartz , cristobalite or amixture of two .

Purpose To provide high temperature thermal shock resistanceTo provide high thermal expansion.


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Binder (


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Modifiers :Carbon is often added - clean casting .Facilitates easy divesting of casting and mold.Generally added when casting alloy is gold .When silver palladium or base metal alloys are invested

with the investment containing carbon, it embrittles thealloys even though the investment is heated to thetemperature that burn out the carbon.Palladium reacts with carbon even at above temp

1500 C, so carbon free phosphate bonded invst usedfor higher temp.


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It is available as two component systems 1- It is a Powder which containsrefractory materials and bindersand modifiers

2- Aqueous solution stabilizedwith colloidal silicaColloidal silica suspensionfacilitate greater expansion of the

investment which can compensatethe greater casting shrinkage ofalloys used in MC & newer goldalloys.


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Expansion can be varied by the proportions of silicaand water. More silica and less water moreexpansion.When mixed with water- initial shrinkage is seen sameas that of GBI. Completely eliminated with colloidalsilica.

Liquid can be used as full strength or diluted with water

to provide some degree of control over setting orthermal expansion. But if more liquid is used it causeddense nonporous investment which can effect casting.


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Early thermal expansion is associated with-decomposition of binder, magnesiumammonium phosphate and evolution ofammonia gas.Some shrinkage is masked by expansion ofrefractory filler ( crystobalite)


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TemperatureWarmer the mix faster it setsThe setting reaction liberate the heat andaccelerates rate of setting

Mixing timeincreased mixing time and mixing efficiencyresult in faster set.


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1.They have high fired strength . This make themhandle without breaking before they are placed in afurnace for the wax burn out process and strongenough to with stand the impact and the pressure ofcentrifugally cast molten alloy

2.They also provide high setting and thermalexpansion enough to compensate cast metal

prosthesis or porcelain veneers during cooling3. They can withstand temp more than 900 C


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These are used for high fusing base metalalloy


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Refractory material Silica

Binder Silica gel or ethyl silicate

Modifier Magnesium oxide (strengthen the gel)Ammonium chloride - accelerator


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Silica gel: pH of sodium silicate loweredby addition of an acid or acid salt.Colloidal silica converted to gel byadding ammonium chloride.


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Ethyl silicate: colloidal polysilicic acid is formed byhydrolizing ethyl silicate in presence of Hcl, ethylalcohol & water. It is then mixed with quartz orcristobalite and magnesium oxide ( alkaline). Coherentgel of polysilicic acid formed, dried at temp 168 C,loses alcohol & water to form concentrated hard gel .Volumeric contraction during drying is greenshrinkage.Amines added to ethyl silicate- hydrolysis and gelationoccurs simultaneously.


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It is supplied as a powder and liquid or two liquidsIf supplied as a powder and liquid

Powder consists of refractory particles of silicasand glasses along with the calcined magnesiumoxide and some other refractory oxides in minoramounts

Liquid contains stabilized alcohol solution of silicagel


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If supplied as 2 liquidsOne is ethyl silicate

Certain types of amines are added for hydrolysis andgelation to occur simultaneouslyOther is acidified solution of denatured alcohol


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The powder is added to hydrolyzed ethyl silicateliquid, mixed quickly and vibrated into a mold , thathas an extra collar to increase the height.

The mold is placed on a vibrator that has a tamping

actionThis allows the heavier particles to settle while theexcess liquid and some of the finer particles rise tothe top .

The top of the mold is prone to cracking due to greaterdrying shrinkage from evaporation of the ethylalcohol


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In about 30 minutes the accelerator in the powder

hardens the settled part, and the excess is pouredoff (to avoid crack formation).

The liquid powder ratio in settled part is greatlyreduced and the setting shrinkage is reduced to0.1%


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Can be used for higher temp castings (1090C-1180C) and compactible with higherfusing alloys.Care should be taken while handling andburnout as inflammable alcohol is given off.


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Investment can be mixed in 2 ways

1. Hand mix: The liquid is added to a clean, dry mixing bowl, and the powder is gradually added to theliquid, using the care and caution to minimize airentrapment. Mixing is formed gently until all the

powder has been wet.2. Vacuum mixing: this is a type of mechanical mixingdone under vacuum created by a vacuum mixmachine.


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Patterns invested

Burnout in furnace

Flow of molten metal


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dental investment materials and investing techniques

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