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DENTAL INVESTMENT DENTAL INVESTMENT MATERIALS AND MATERIALS AND
PROCEDURESPROCEDURES
Dr.Dilip jainDEPT OF
CONSERVATIVE & ENDODONTICS
CONTENTS Introduction History Definition Requirements Material science Types of investments -composition -setting reaction -setting expansion -properties
Investing procedureDefects in investing procedureConclusion
3
Introduction
Lost wax casting procedure is adopted for fabrication of metal alloy appliances like inlays, crowns etc outside the mouth.
An accurate wax pattern prepared ,is invested in refractory mould materials.
After wax burn out the mould is filled with alloy liquid, which solidifies & cools down. During this procedure the possible dimensional change taking place are to be suitably compensated.
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Total shrinkage = Wax Shrinkage + Alloy Shrinkage
Methods of compensation:
Compensation is done by carefully controlling the mould expansions, by availing large setting expansion & adjusting the thermal expansions of the investment materials.
Setting Expansion + Thermal Expansion
History
Until the dawn of 20th century the investing procedure was largely ignored by modern industries,but in ancient days this procedure was used to make statues, jewels, and artifacts.
And it is the dental professionals who rediscovered it for preparing crowns and inlays.
The first authenticated record of use of investing casting in dentistry was given by Dr.Philbrook in 1897.
But the true significance of this process was given after research by Dr.Williams.H.Taggart in 1907.
Dr.Taggart not only developed and described the technique, he also reformulated a wax pattern component of excellent properties, he also developed an investment material and even invented an pressure casting machine.
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1933-BRODSKY –first refractory mold
70%-mullite,aluminium oxide and silicon dioxide and 30% plaster of paris (GERMAN BRILLAT NO-2)
MC CABE-GYPSUM CONTRACTS ON HEATING
Phillips-GBI should not be heated above 700 degrees
8
1949-MOORE AND WATTS-phosphate bonded investment
1959-Ethyl silicate bonded investment
1964-HOBO-Evaluated a range of refractory and ceramic combinations
1985-CALAMIA-Suggested –PBI-fabrication of all ceramic laminate veneers
2000- has an ISO 11245 for phosphate bonded refractory die materials
DEFINITIONInvestment
A material consisting primarily of an allotrope of silica and a bonding agent. The bonding substance may be gypsum (for use in lower casting temperatures) or phosphates and silica (for use in higher casting temperatures).
Investment(craig)
An investment can be described as a ceramic
material that is suitable for forming a mold
into which a metal or alloy is cast.
REQUIREMENTS OF INVESTMENT MATERIALSEasily manipulated.
Provide sufficient strength.
Capable of reproducing the shape, size and detail in the wax pattern.
Stability at higher temperatures:
On being heated to higher temperatures the investment should not decompose to give off gases that would damage the surface of the alloy.
Provide sufficient expansion.Provide sufficient expansion.
Investment should be porous enough to permit the air or other gases in the mold cavity to escape easily during the casting
Investment should produce a smooth surface and fine detail and margins on the casting.
The material should be inexpensive.
MATERIAL SCIENCE
In general an investment is a mixture of three distinct types of materials:
Refractory material
Binder material
Modifiers to improve the properties
REFRACTORY MATERIAL
Material capable of sustaining exposure to high temperature without significant degradation.
SILICA (silicon dioxide) is used as refractory material & to regulate the thermal expansion.
It is available in four allotropic forms such as Quartz Tridymite Cristobalite Fused quartz Quartz and Cristobalite are used extensively in
dental investments
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 rapid and readily reversible on cooling .this change is known as high – low inversion .
αα quartz quartz ββ quartz quartz
(low form)(low form) ▲ 575° C (high form)▲ 575° C (high form)
αα cristobalite cristobalite ββ
cristobalitecristobalite
▲ ▲ 200°- 270° C200°- 270° C
The beta allotropic forms are stable above the transition temperature and an inversion to the lower or alpha form occurs on cooling.
Thermal expansion of three forms of silicaThermal expansion of three forms of silica
The density decreases when the alpha form The density decreases when the alpha form changes to beta form with a resulting changes to beta form with a resulting increase in the new volumeincrease in the new volume
BINDER MATERIAL
Functions:- Hold the ingredients together- Provide rigidity
The common binders used are:
1) Gypsum-for low temperature casting investments.
2) Phosphateðyl silicate-for high temperature casting investments.
CHEMICAL MODIFIERSOther chemicals such as sodium chloride, boric acid,
graphite, copper powder, are often added in small quantities to modify physical properties
-BORIC ACID&SODIUM CHLORIDERegulate setting expansion and setting time and thus
prevent most of the shrinkage of gypsum when heated above 300 ºC .
-COPPER POWDER& GRAPHITEAct as reducing agents.
CLASSIFICATIONInvestments are classified :
According to type of binder used
According to the type of silica used
According to to use and melting range of alloy.
According to the type of binder used
-Gypsum bonded investments
-Phosphate bonded investments
-Ethyl silicate bonded investments
According to type of silica used
-Quartz investments
-Cristobalite investments
-
According to use
- Gypsum bonded investmentsUsed for conventional casting of gold alloy
inlays,onlays ,crowns&FPD’s.
ADA sp-2: divided them further into:
TYPE I- for casting inlays/crowns. mode of expansion: Thermal TYPE II- for casting inlays/onlays/crowns mode of expansion: Hygroscopic TYPE III- for partial dentures with gold
alloys.
- Phosphate bonded investmentsFor alloys used to produce copings or frameworks
for metal-ceramic prosthesis,press able ceramics.
ADA sp-42 divided them further into:
- TYPE 1: for inlays,crowns and other fixed restorations.
- TYPE 2 : for partial dentures and other cast,removable restorations.
- Ethyl silicate bonded investments
Used principally in casting of RPD with base metal alloys.
- Brazing investment or soldering investment
Used for brazing parts of a restoration such as clasps on RPD.
Acc to ADA sp-93 - TYPE I: gypsum bonded dental brazing
investment - TYPE II: Phosphate bonded brazing
investments.
GYPSUM BONDED INVESTMENTS
They are used for casting gold alloys They can withstand temp up to 700oCClassification:- According A.D.A specification no.2 for casting
investments for dental gold alloys, there are 3 types.
Type – IAre those employed for the casting of inlays or crowns when the allow casting shrinkage compensation is accomplished principally by thermal expansion of the investment.
Type – IIcompensation for alloy shrinkage is by the hygroscopic expansion
Type IIIInvestments are used in the construction of partial denture with gold alloys
composition
Refractory Crystalline polymorphs
of silica (quartz or cristobalite)
65-75% Silica is added to
provide a refractory component during the heating of the investment and to regulate the thermal expansion.
When these silica forms are heated change in crystalline form occurs at transition temperature…inversion from a low form known as alpha to a high form beta occurs.
- quartz: transition temperature is 575ºc - cristoballite: 200-270 ºcDensity decreases as alpha form changes to beta
form with resulting increase in volume that is exhibited by a rapid increase in linear expansion.
Binder
Alpha hemi hydrate form of gypsum(30-35%)
It is used as binder for investments used in casting gold containing alloys with melting ranges below 1000 ºc
Dimensional changes of three forms Dimensional changes of three forms of gypsum when heated..of gypsum when heated..
MODIFIER - (4-7%)
Used are :
Reducing agents
Modifying chemicals
Coloring matter
Reducing agents : they reduce any metal oxides formed on the metal by providing a non oxidizing atmosphere in the mold when the alloy enters mold.
Ex– Copper
Modifying chemicals: They regulate setting expansion and thermal expansion and also prevent shrinkage of gypsum when heated above 300 ºc .
They act by reducing the two large contractions of gypsum binder on heating to temperatures above 300 ºc .
Ex– Boric acid Soluble salts of alkali or alkaline earth
metals
Setting reaction- same as gypsumSetting reaction- same as gypsum
Dissolution of calcium sulfate hemihydrateDissolution of calcium sulfate hemihydrate
Formation of saturated solution of calcium sulfateFormation of saturated solution of calcium sulfate
Aggregation of calcium sulfate dihydrateAggregation of calcium sulfate dihydrate
Precipitation of dihydrate crystalsPrecipitation of dihydrate crystals
The crystallization of calcium sulfate dihydrate occurs while The crystallization of calcium sulfate dihydrate occurs while most of the remaining hemihydrate particles dissolve.most of the remaining hemihydrate particles dissolve.
Setting time
According to ANSI/ADA Specification No. 2 for dental inlay casting investment, the setting time should not be shorter than 5 min or longer than 25 min. Usually, the modern inlay investments set initially in 9 to 18 min.
SETTING EXPANSION
Setting expansion are of three type:
Normal setting expansionNormal setting expansion
Hygroscopic setting expansionHygroscopic setting expansion
Thermal expansionThermal expansion
PurposePurpose: To enlarge the mold to compensate for the casting shrinkage of the gold alloy
1).Normal setting expansion:1).Normal setting expansion:linear linear dimensional change as the investment dimensional change as the investment
setssetsSilica particles result in greater setting expansion
HOW??
silica particles interfere with the intermeshing silica particles interfere with the intermeshing and interlocking of crystalsand interlocking of crystals
resulting in outward thrust of crystalsresulting in outward thrust of crystals
resulting in expansionresulting in expansion.
According to According to ADA sp no 2: ADA sp no 2:
FOR TYPE I investments:FOR TYPE I investments:0.6%
Value of setting expansion for modern investments is 0.4%,regulated by retarders and accelerators.
Purpose :Purpose :
To aid in enlarging the mold to compensate partially for the casting shrinkage of gold.
2).HYGROSCOPIC SETTING 2).HYGROSCOPIC SETTING EXPANSIONEXPANSION-expansion that occurs as the
investment hardens while immersed in water
This is one of the methods for expanding the casting mold to compensate for casting shrinkage.
The hygroscopic setting expansion may be 6 or more times greater than the normal setting expansion of a dental investment
The water is drawn between the refractory particles by the capillary action and thus causes the particles to separate creating an expansion
The investment should be immersed in water before the initial set is complete.
ADA sp no 2 : type 2 investments require minimum setting expansion in water of 1.2% and maximum 2.2%.
HYGROSCOPIC SETTING EXPANSION & HYGROSCOPIC SETTING EXPANSION & NORMAL SETTING EXPANSIONNORMAL SETTING EXPANSION
FACTORS AFFECTING HYGROSCOPIC SETTING EXPANSION
1. Effect of compositionEffect of composition The finer the particle size of the silica, the
greater is the hygroscopic expansion. Higher the silica content greater is the
expansion.
2. Effect of w/p ratioEffect of w/p ratio The higher the W/p ratio of the original
investment water mixture, the less is the hygroscopic setting expansion.
.
.3.Effect of temperatureEffect of temperature Higher the temperature of immersion water,
less is the surface tension and hence greater is the expansion
4. Effect of time of immersionEffect of time of immersion Immersion before the initial set causes
greater expansion.
5. Effect of spatulationEffect of spatulation The shorter the mixing time, the less is the
hygroscopic expansion.
6. Effect of shelf life of the investmentEffect of shelf life of the investment The older the investment, the less is the
hygroscopic expansion.
7. Effect of confinementEffect of confinement Both the normal and the hygroscopic setting
expansions are confined by opposing forces, such as the walls of the container in which the investment is poured or the walls of a wax pattern.
3).Thermal expansion
The thermal expansion of the gypsum-bonded investment is directly related to the amount of silica present and the type of silica employed.
When the silica is subjected to high temperature a phase transformation occurs in the silica, producing a slightly different atomic configuration, which is accompanied by an expansion.
The amount of thermal expansion depends on the allotropic form of silica used, for example, the thermal expansion of Cristobalite is greater than that of quartz at most temperatures.
Type 1 investments should have thermal expansion of not less than1% and not greater than 1.6%.
Type II investments should have a thermal expansion ranging from 0-0.6%
Maximum thermal expansion should be attained at temperature not higher than 700degree centigrade.
Factors affecting thermal expansion
Effect of water / powder ratio
The magnitude of thermal expansion is
related to the amount of solids
present.therefore more the water used in
mixing the investment the less the thermal
expansion achieved during subsequent
heating.
Effect of chemical modifiers
As the weakening effect of silica is too
great,therefore certain modifiers are added to
eliminate the contraction caused by gypsum
and increase the expansion without the
presence of excessive amount of silica.
SILICAS DO NOT PREVENT GYPSUMSILICAS DO NOT PREVENT GYPSUM
SHRINKAGE BUT COUNTERBALANCESHRINKAGE BUT COUNTERBALANCE
IT,WHEREAS MODIFIERS ACTUALLYIT,WHEREAS MODIFIERS ACTUALLY
REDUCE GYPSUM SHRINKAGE ………REDUCE GYPSUM SHRINKAGE ………
Modifiers added areModifiers added are:
- Sodium,potassium,lithium chloride
- Boric acid..
Although boric acid decreases contraction andhardens the investment,it disintegrates duringheating of the investment and may result inroughened surface on the casting.
STRENGTH-maximum stress required to fracture a structure.
According to ADA sp no.2 : the compressive
strength of gypsum bonded investment should not
be less then 2.4Mpa..
The strength of the investment is affected by the
water/powder ratio, more the water that is
employed in mixing ,the lower the compressive
strength.
FINENESS
Fineness affects the setting time,surface
roughness of the casting.
Although fine silica results in higher hygroscopic
expansion than coarser silica..A fine particle is
preferable because finer the investment smaller
the surface irregularities.
POROSITY
To a certain extend investment material should be porous.
During casting process as the molten metal entersthe mold, the air must be forced out ahead of it…
ifthe air is not completely eliminated a backpressurebuilds to prevent gold alloy from completely fillingthe mold resulting in void formation.
Thus the common method of venting the mold isthrough the pores of the investment….
CLINICAL IMPLICATIONS
Gypsum bonded investments cannot withstand temperatures greater than 700 ºc
Heating above 700 ºc results in shrinkage with the liberation of:
- CaSO4+SiO2 CaSiO3+SO3CaSO4+SiO2 CaSiO3+SO3
- - CaSO4+4C CaS+4COCaSO4+4C CaS+4CO
- 3CaSO4+CaS 4CaO +4SO2- 3CaSO4+CaS 4CaO +4SO2
This decomposition not only causes shrinkage but also contaminates the casting with the sulphides of the non noble alloying elements .
So gypsum should not be heated above 700 ºc
PHOSPHATE BONDED INVESTMENTS
Rapid growth in use of metal ceramic & hot
pressed ceramic prostheses has resulted in an
increased use of phosphate bonded investments.
Main use of this investment is for casting alloys
that need to be heated to casting temperatures in
excess of about 1200ºto 1300ºC.
APPLICATION
TYPE I: for casting inlays,crowns and other restorations.
TYPE II :for removable partial dentures.
Make soldering fixtures.
Refractory dies for fabrication of custom veneers from dental porcelains.
COMPOSITION
Phosphate bonded investments are available as two component system :
1) Powder 2). Liquid
1). Powder: consists of 2).Liquid: consists of- Refractory fillers - colloidal silica.
- Binders - water- Carbon
Refractory fillers:
Silica in the form of cristobalite,quartz-80%cristobalite,quartz-80%Purpose: to provide high temperature thermal
shock resistance and a high thermal expansion.
Binder: consists of-
- Magnesium oxide:3-5%Magnesium oxide:3-5% - Mono ammonium phosphate:7-10%- Mono ammonium phosphate:7-10%Originally phosphoric acid was used but now Originally phosphoric acid was used but now
ammonium phosphate has replaced it as it can ammonium phosphate has replaced it as it can be incorporated into the investment powder.be incorporated into the investment powder.
CARBON:
Added to produce clean castingsclean castings and facilitatefacilitate thedivesting of the castingdivesting of the casting from the mold.Used onlywhen the casting alloy is gold.
Not used with Ag-palladium alloys or base metal alloys as:
- carbon embrittles the alloys.- Palladium reacts with carbon at temperatures
above 1504ºC.
SETTING REACTION
Chemical reaction that causes the investment to set is:
NH4H2PO4 + MgO + 5H2O NH4MgPO4 6H2O
Thus predominantly colloidal multimolecular
magnesium ammonium phosphate aggregate
around excess of MgO and fillers is formed.
The final products are crystalline Mg2P2O7 and
some excess MgO Along with unchanged
quartz,cristoballite or both.
Setting and thermal expansion
Depends on:Depends on:
Type of liquid usedType of liquid used
Concentration of liquid used.Concentration of liquid used.
Type of liquid usedType of liquid used
Phosphate bonded investments show slight
expansion rather then shrinkage because of
the presence of colloidal silica as liquid
which results in increased conc. Of silica
leading to expansion.
Thermal expansion of a phosphate bonded investment Thermal expansion of a phosphate bonded investment mixed with water as compared with the special mixed with water as compared with the special liquid….liquid….
concentration of liquidconcentration of liquid
The influence of the liquid concentration on the setting The influence of the liquid concentration on the setting and thermal expansion o phosphate bonded investments.and thermal expansion o phosphate bonded investments.
Properties
1).Compressive strength Type 1: 2.5 mpa Type 2: 3 mpa
2). Thermal expansion 0.8% when 50:50 mixture of liquid and
water
working & setting time-influenced by:
Temperature
Warmer the mix faster it sets.
The setting reaction liberate the heat and further
accelerates rate of setting
Mixing time
Increased mixing time and mixing efficiency
result in faster set.
Advantages
1.Posses high fired strength.
2. Provide high setting and thermal expansion.
3. They can withstand the burnout process (1-1.5hrs)with temp more that reach 900°C
Disadvantages
When used with alloys having casting temperature greater than 1375ºC result in mold breakdown & rougher surfaces on castings.
Although high strength of these investments is an advantage during casting,but can make divesting a difficult and tedious task.
ETHYL SILICATE-BONDED INVESTMENT
These are used for high fusing base metal alloy.
composition1).Refractory material – Silica2). Binder –Silica gel or ethyl silicateSeveral methods may be used to produce the silica or
salicylic acid gel binders.
- FORMATION OF SILICA GELWhen the Ph of sodium silicate is lowered by the addition
of an acid or acid salt, a bonding silicylic acid gel forms.
An aqueous suspension of colloidal silica can also be converted to a gel by the addition of an accelerator ,such as AMMONIUM CHLORIDE.
- FORMATION OF ETHYL SILICATE
Another method of binder formation is based on ethyl silicate.a colloidal silicic acid is first formed by hydrolyzing ethyl silicate in the presence of hydrochloric acid, ethyl alcohol and water
The solution is then mixed with quartz or cristoballite, to which is added small amount of MgO.Thus polysilicic acid gel is formed.
3). Modifier – Magnesium oxide (strengthen the gel) Ammonium chloride - accelerator
It is supplied as a powder and liquid or two liquids
If supplied as a powder and liquid
Powder consists of refractory particles of silicas and glasses along with the calcined magnesium oxide and some other refractory oxides in minor amounts
Liquid contains stabilized alcohol solution of silica gel
If supplied as 2 liquids One is ethyl silicate
Certain types of amines are added for hydrolysis and gelation to occur simultaneously
Other is acidified solution of denatured alcohol
76
Advantages
Final set mass is cristobalite. Hence it can withstand temperature even beyond 1150 o C
High strength.
77
Disadvantages
Non porous material.
Complicated manipulation.
Cannot be used for titanium and its alloys as silica can oxidize titanium or its alloys easily
78
Other investment materials
Soldering investments
Divestment materials
Investment material for titanium alloys
Silica free investments
79
Soldering investments/Brazing investment
RPD /FPD frameworks are cast sometimes in separate parts and then soldered/brazed.
Parts are assembled on master casts , joined with sticky wax and invested in special investment material on a tile
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Dental brazing investments
ADA specification no 93 Two types;
1: Type 1: Gypsum bonded dental brazing investments
2: Type 2 : Phosphate bonded dental brazing investments
81
This method used to avoid distortion of the appliance during soldering
Investment material should not undergo setting or thermal expansion which may distort the appliances
Gypsum mixed with colloidal silica liquid
The die is made from this mix and the wax pattern is then constructed on it.
The whole complex is then invested in a mixture of Divestment & water thereby eliminating the possibility of distortion of the pattern on removal from the die or during the setting of the investment.
83
Properties
Setting expansion : 0.9 % Thermal expansion : 0.6 % at 650 o C Thermal expansion : 1.2% at 850 o C
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Divestment materials
Die stone & investment combination.
These mixed with colloidal silica liquid
The die is made from this mix and the wax pattern is then constructed on it.
The whole complex is then invested in a mixture of Divestment & water
This combination used to compensate distortion of wax pattern of long span bridges or RPD frameworks during removal from die.
Special GBI or PBI investment materials are used.
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Properties
Setting expansion : 0.9 % Thermal expansion : 0.6 % at 650 o C Thermal expansion : 1.2% at 850 o C
Investment Material For Ceramic/Silica free investment Casting of ceramic crowns using castable glasses
is done in refractory moulds. Used with castable moldable ceramic are hot
pressed into ceramic mould by pressure. where no compensation expansion is needed
Special GBI of low thermal expansion are used, made from pure gypsum ,calcium sulfate semihydrate,
Same investment additives as GBI , prevent their contraction or deterioration on heating .
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Investments For Titanium Alloys
Molten titanium is highly reactive with oxygen and is capable of reducing some of the oxides commonly found in those investments.
Titanium can also dissolve residual oxygen, nitrogen, and carbon from the investment; these elements can harden and embrittle titanium in the solid state.
As a result, either modifications of existing refractory formulations and binders or new refractory formulations and binder systems are required.
Dental Materials and Their Selection - 3rd Ed. (2002)by William J. O'Brien
88
Properties
Reduce breakdown of the investment and the contamination of titanium:
Reduction of refractory oxides of the investment.
Refractory materials that are less easily reduced by titanium should be used
Dental Materials and Their Selection - 3rd Ed. (2002)by William J. O'Brien
89
Modifications Of Investment Material
Investment consisting of a phosphate binder, magnesia, and quartz was developed under the hypothesis that quartz would not be as reactive as silica.
This investment was recommended for use as a room-temperature mold, to reduce reaction with titanium.
However, contamination of castings by reaction with the investment was still encountered.
Dental Materials and Their Selection - 3rd Ed. (2002)by William J. O'Brien
90
Modifications Of Investment Material
A phosphate investment that contains both magnesia and alumina as refractories was developed.
This investment can attain large expansion by the spinel reaction of alumina and magnesia (MgO + Al2O3 ® MgO - Al2O3) when it is burned out at 1,150°C to 1,200°C.
Another approach to obtaining the needed expansion is through the use of Li2O - Al2O3 - SiO2.
It expands irreversibly upon heating through the temperature range of 900°C to 1100°C.
Investing procedure
Investing:Investing:
The process of covering or envelopingThe process of covering or enveloping
wholly or in part,an object such aswholly or in part,an object such as
denture,wax form ,crown,tooth etc with adenture,wax form ,crown,tooth etc with a
suitable investment material beforesuitable investment material before
processing,soldering orprocessing,soldering or
casting.casting.
Various steps involved are:Various steps involved are:
Preparation of tooth or teeth to receive a cast Preparation of tooth or teeth to receive a cast restoration.restoration.Prepare the impression of the prepared tooth.Prepare the impression of the prepared tooth.Wax pattern fabricationWax pattern fabricationSprue the wax patternSprue the wax patternInvest the wax patternInvest the wax patternEliminate the wax pattern by burning the wax Eliminate the wax pattern by burning the wax out of the investment in a furnace,thus making out of the investment in a furnace,thus making the mold.the mold.
Force the molten metal into the mold-Force the molten metal into the mold-castingcasting
Clean the casted metal,finish,polish and Clean the casted metal,finish,polish and cement it on the tooth surface.cement it on the tooth surface.
The procedure in briefThe procedure in brief
THE PROCEDURE INCLUDES THE FOLLOWING STEPS
STEP1 -PREPARE the tooth or teeth to receive a cast restoration(I.e-inlay,onlay,full crown preparation,dowel and core).
STEP2- Make the impression of prepared tooth..
Pour the impression(type4 or type5gypsum slurry) to make a positive cast,from which DIE(S) representing prepared tooth or teeth is sectioned.
WHAT S A DIE?
It is the positive replica /reproduction of the form of prepared tooth.
Preparation of master die
Commonly used die materials are:
TYPE4 dental stone-with setting expansion of 0.1%(ADAsp25)
TYPE5 dental stone -with setting expansion of 0.3% hence used for base metal alloys.
Disadvantage of type 4 gypsum die is:
Abrasion during carving of the wax pattern
Means to increase abrasion resistance:
-silver plating-coating the surface with cyanoacrylate.-adding a die hardener to gypsum.
Methods of altering die dimensions
1). Additional accelerator(potassium sulphate) and retarder (borax) can be added to the gauging water to reduce the setting expansion of the type4 DIE STONE TO LESS THAN 0.1%.
DIVESTMENTDIVESTMENT- - a die material and the a die material and the investing medium with a comparable compositioninvesting medium with a comparable composition
Die stone is mixed with investment to form Divestment .
Divestment mixed with colloidal silica Setting expansion is 0.9% Thermal expansion is 0.6% when it is heated to
677 ºc As it is a gypsum bonded material it is not
recommended for high fusing alloys. Divestment phosphate is a phosphate bonded
investment used as a divestment for high fusing alloys.
Other die materials
Amalgam,acrylic,polyester,epoxy resinsAmalgam,acrylic,polyester,epoxy resins
Not commonly used due to
- Great dimensional variationsGreat dimensional variations.:in cases of resins the curing contraction is excessive(0.6%)
- Compatibility to impression materials is lessCompatibility to impression materials is less. Electroformed dies-metal dies formed from
electroplated impression material.they have high strength,adequate thickness,and good abrasion resistance
2). DIE SPACER). DIE SPACER::To produce relief space for the cement
Commonly used spacers are resins.others include model paint,nail polish,thermoplastic polymers dissolved in volatile solvents
Applied in several coats to within 0.5 mm of the preparation finish line to provideto provide relief for the cement and ensure complete relief for the cement and ensure complete seating of castingseating of casting
Step 3- wax pattern fabrication
MAKE THE WAX PATTERN WHICH WILL BE REPRESENTATIVE OF THE LOST TOOTH STRUCTURE.IT CAN BE MADE BY DIRECT TECHNIQUE –PATTERN MADE ON TOOTHPATTERN MADE ON TOOTH ;INDIRECT TECHNIQUE-MADE MADE ON DIEON DIE
Various steps involved are:
Selection of the waxSelection of the wax
Selection of techniqueSelection of technique
Selection of the waxSelection of the wax
Inlay casting wax is used for forming the pattern for cast restorations.
Composed of:
- paraffin-40-60%- Dammar resin- reduce flaking- Carnauba,candelilla waxes- to raise the melting
temperature.- Dyes- to provide color.
Acc.to ADA sp-4:Acc.to ADA sp-4:TYPE I- TYPE I- medium wax-used with direct techniquemedium wax-used with direct technique
TYPE II- TYPE II- softer wax-used with indirect techniquesofter wax-used with indirect technique
Requirements of good inlay wax:Requirements of good inlay wax:
- It must flow readily when heated,without chipping,flasking or losing its smoothness.
- When cooled, it must be rigid.- It must be capable of being carved
precisely,without chipping,distorting or or smearing..
Step 4-spruing
A spruing system is intended to create a channel or series of channels in the set investment through which molten alloy flows to reach the pattern areas after the burnout process
Types of spruesTypes of spruesWax spruesWax sprues
-Preferred for most castings-Melt at the same time as the pattern and thus allow easy escape of molten wax.
Plastic spruesPlastic sprues
-Used when casting FPD’sDisadvantage-Soften at higher temperature then the wax pattern and thus may block the escape of wax,resulting in increased casting roughness.
Metal spruesMetal sprues-Should be made of non rusting metal to avoid possible contamination of the wax.-AdvantageAdvantage: provides rigidity during investment.-DisadvantageDisadvantage:deformation of the sprue walls or wax pattern during there removal.
The general principles of spruingThe general principles of spruing
The general principles of spruing, include ;-
spruing method (direct versus indirect), sprue diameter Sprue position Sprue attachment Sprue direction Sprue length reservoir location,
Spruing method – direct spruingSpruing method – direct spruing
The flow of molten metal is straight (direct) from the casting crucible to the pattern area in the ring.
Requires less time and effort.
A straight sprue former is luted (attached) to the thickest part of the wax pattern at one end and secured to the crucible former at the other.
The sprue former can be modified by placing a ball, or round reservoir, between the pattern and the button.
Direct spruing is used most frequently for single units and small, multiunit patterns
A basic weakness of direct spruing is the potential for suck-back porosity at the junction of restoration and the sprue.
Spruing method – Indirect spruingSpruing method – Indirect spruing
With indirect spruing, molten alloy does not flow directly from the casting crucible into the pattern area in the heated mold
Instead, the casting alloy takes an indirect route before it reaches the pattern areas, thus the name indirect spruing.
The connector (or runner) bar is often 6- or 8-gauge round wax to which the wax pattern sprue formers are attached on one side with two larger ingot sprue formers on the other side,
AdvantagesAdvantages
Indirect spruing offers : Greater predictability and reliability in
casting Enhanced control of solidification
shrinkage
Sprue diameterSprue diameter
Is an important factor in dictating the speed with which the melt enters and fills the mold.
Sprue diameter should be of the same size as the thickest area of wax pattern.
Sprue positionSprue position The sprue former attached to
the wax up should be luted to the thickest part of the pattern to allow the molten alloy to flow from regions of large volume (thick areas) to regions of lesser volume (thin sections).
Placing the sprue former elsewhere might result in an incomplete casting if a thin section undergoes solidification before the mold can fill completely.
Sprue directionSprue direction
Sprue should be directed away from any thin or delicate parts of the pattern.
Should not be attached at 90 degrees to broad flat surface.
Angulation of 45 degrees gives satisfactory castings.
Sprue lengthSprue length
The length of the sprue depends on the length of the casting ring and the spatial location of the pattern within the ring.
Short sprue as compared to the ring length may result in porosity in the casting.
Distance between the top of the pattern and the open end of the ring is:
- for gypsum bonded investments:6mm - for phosphate bonded investments:3-4mm
Diagrammatic representation of the dental casting mold.Diagrammatic representation of the dental casting mold.
Location of the reservoirLocation of the reservoir
The reservoir portion of a spruing system, should be positioned in the heat center of the ring
This permits the reservoir to remain molten longer and enables it to furnish alloy to the patterns until they complete the solidification process
Aside from being in the heat center, the reservoir should have the largest mass of any part of the sprue system.
Choice of the Ring
For the hydroscopic expansion technique, a rubber ring is used.
For the thermal expansion technique, a metal ring is used .
For Injection Molding technique pf moldable ceramic, a special two part flask of standard size is used .
CASTING RING LINERSCASTING RING LINERS
THE MOST COMMONLY USED TECHNIQUE TO PROVIDE INVESTMENT EXPANSION IS TO LINE THE WALLS OF RING WITH RING LINER
FUNCTIONSFUNCTIONS
a. Allows for mould expansiona. Allows for mould expansion
b. When the ring is transferred from the furnace b. When the ring is transferred from the furnace to the casting machines, it reduces loss of to the casting machines, it reduces loss of heat, as it is a thermal insulator.heat, as it is a thermal insulator.
c. Permits easy separation of the investment from c. Permits easy separation of the investment from the ring after the casting is over. the ring after the casting is over.
VARIOUS TYPES OF RING LINERSVARIOUS TYPES OF RING LINERS
TRADITIONALLYTRADITIONALLY
Asbestos ring liners were used.Asbestos ring liners were used.
DisadvantageDisadvantage
- High carcinogenic potentialHigh carcinogenic potential
- Asbestos fiber bundles were Asbestos fiber bundles were found to produce hazardous- found to produce hazardous- respirable particles capable of respirable particles capable of causing lung diseasecausing lung disease
NEW ALTERNATIVE RING NEW ALTERNATIVE RING LINERSLINERS
- Aluminosilicate ceramic linerAluminosilicate ceramic liner
- cellulose linercellulose liner
LINER APPLICATIONLINER APPLICATION
IT IS CUT TO FIT THE INSIDE DIAMETER OF THE RING WITH NO OVERLAP
IT IS TACKED INTO POSITION WITH STICKY WAX;AND USED DRY OR WET
Wet liner techniqueWet liner technique
The lined ring is immersed in water and the excess water is shaken away.
Squeezing the liner should be avoided as:
- it leads to variable amounts of water removal & non uniform expansion.
Liner provides greater normal setting expansion and the absorbed water also results in semi hygroscopic expansion.
AS EXPANSION OF THEAS EXPANSION OF THE
INVESTMENT IS ALWAYSINVESTMENT IS ALWAYS
GREATER IN THEGREATER IN THE
UNRESTRICTEDUNRESTRICTED
LONGITUDINAL DIRECTIONLONGITUDINAL DIRECTION
THAN IN RADICALTHAN IN RADICAL
DIRECTION,THATDIRECTION,THAT
IS,TOWARDS THE RINGIS,TOWARDS THE RING
HENCE LINER SHOULDHENCE LINER SHOULD
BE PLACED 3MM SHORTBE PLACED 3MM SHORT
ON BOTH THE ENDS TOON BOTH THE ENDS TO
PRODUCE UNIFORMPRODUCE UNIFORM
EXPANSIONEXPANSION
RINGLESS CASTING SYSTEMRINGLESS CASTING SYSTEM
A ring less system that provides maximum expansion of investment is available commercially
The system, called the power cast ring less system consists of 3 sizes of rings and formers,preformed wax sprues and investment powder, and a special investment liquid.
THREE SIZES OF RINGS AND SPRUE THREE SIZES OF RINGS AND SPRUE FORMERSFORMERS
- These rings are tapered to allow for removal of the investment mold after the material has set.
This system is suited for casting of alloys that require greater mold expansion than traditional gold-based alloys
.CRUCIBLE FORMER.CRUCIBLE FORMER
The sprue is attached to a crucible former. Usually made of rubber,wax which constitutes the base of the casting ring during investing.
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.
Step-5 Investing
Includes:Includes:
- preparation of the investment mixpreparation of the investment mix
- Liquid used for mixingLiquid used for mixing
- Methods of investingMethods of investing
Preparation of the investment mix
Before preparation of the mix,the waxpattern should be cleaned of any, debrisgrease or oils..Cleaner used:Cleaner used: wax pattern cleaner or diluted
synthetic detergent.Function:Function:-Reduces the surface tension of the wax & permits
better wetting of the investment to insure complete coverage of the intricate portions of the pattern.
LIQUID USE FOR MIXING:LIQUID USE FOR MIXING:
1).For gypsum bonded investments: distilled water
2).For phosphate bonded investments: colloidal silica.
Methods of investing
There are two different methods of There are two different methods of investing the wax pattern:investing the wax pattern:
1). Hand investing
2). Vacuum investing
1). HAND MIXING1). HAND MIXINGThe appropriate amount ofThe appropriate amount of
Liquid is dispensed. TheLiquid is dispensed. The
liquid is added to a clean, dryliquid is added to a clean, dry
mixing bowl, and the powder ismixing bowl, and the powder is
gradually added to the liquid,gradually added to the liquid,
using the care and caution tousing the care and caution to
minimize air entrapment.minimize air entrapment.
Mixing is formed gently until Mixing is formed gently until allall
the powder has been wet;the powder has been wet;
otherwise, the unmixed powderotherwise, the unmixed powder
may inadvertently be pushedmay inadvertently be pushed
out of the bowl. Although handout of the bowl. Although hand
mixing is an option, it is farmixing is an option, it is far
more common to mix all castingmore common to mix all casting
investments mechanically underinvestments mechanically under
vacuum. vacuum.
2). Vacuum mixing2). Vacuum mixing
- Mechanical mixing under vacuum removes air bubbles created during mixing and evacuates any potentially harmful gases produced by the chemical reaction of the high-heat investments.
Once mixing is completed, the pattern may be hand-invested or vacuum invested.
Advantages of vacuum mixing
1). Amount of porosity is reduced
2). The texture of the cast surface is smoother with better detail reproduction.
3). Tensile strength of vacuum mixed investment is also increased.
Hand-Investing Procedure-BRUSH TECHNIQUE
For investing by hand, the entire pattern is painted (inside and out) with a thin layer of investment. The casting ring is positioned on the crucible former, and the remainder of the investment is vibrated slowly into the ring.
Step by step procedureStep by step procedure
1). Hand spatulate the mix to incorporate 1). Hand spatulate the mix to incorporate powder quicklypowder quickly
2). Vacuum mix the investment according 2). Vacuum mix the investment according to manufacture’s recommendations.to manufacture’s recommendations.
3).Coat the wax pattern ,teasing the3).Coat the wax pattern ,teasing the
investment ahead of the brushinvestment ahead of the brush
4). Fill the ring holding it on the 4). Fill the ring holding it on the vibrator.tilt the ring from side to side to vibrator.tilt the ring from side to side to avoid trapping air under the pattern.avoid trapping air under the pattern.
vacuum-Investing Procedurevacuum-Investing Procedure
STEP-1.Hand spatulate the mix.
STEP-2) with the crucible former in place,attach the ring to the mixing bowl.
STEP-3 Attach the vacuum hose
STEP- 4) mix according to manufacture’s recommendations.
STEP-5) Invert the bowl and fill the ring under vibration and remove the hose before shutting of the mixer.
Step-6) remove the filled ring and the Step-6) remove the filled ring and the crucible former from the bowl.crucible former from the bowl.
Step-5 Step-5 wax-eliminationwax-elimination(BURN-OUT)(BURN-OUT)
BURN-OUT:BURN-OUT:Elimination of the wax pattern from the moldElimination of the wax pattern from the mold
of set investment material is referred to asof set investment material is referred to as
burn out.burn out.
- Consists of heating the investment in a thermostatically controlled furnace until all the traces of the wax are vaporized.
Step by step-Procedure
1. Allow the investment to set for the recommended time (usually 1 hour) and then remove the rubber crucible former
a. If a metal sprue is used, remove it as well
b. Any loose particles of investment should be blown off with compressed air.
2.Place the ring with the sprue facing down in the furnace on a ribbed tray. The tray allows the molten wax to flow out freely
.3.Bring the furnace to 200C (400F) and hold this temperature for 30 minutes most of the wax is eliminated by this time.
4.Increase the heat to the final burnout temperature [generally 650C (1200F) or 480C (900F) follow the manufacturers instruction] and hold for 45 min. because the heating rate affects the expansion.
STEP-6 CASTINGSTEP-6 CASTING
Casting machines provide the means Casting machines provide the means for transferring the molten alloy for transferring the molten alloy from the melting crucible to the from the melting crucible to the mold.mold.
Various casting machines used:Various casting machines used:
1). Torch melting/ centrifugal casting machine). Torch melting/ centrifugal casting machine
2). Electrical resistance-heated casting machine2). Electrical resistance-heated casting machine
3). Induction melting machine3). Induction melting machine
4). Direct current arc melting machine4). Direct current arc melting machine
5) Vacuum or pressure assisted casting machine.5) Vacuum or pressure assisted casting machine.
Centrifugal casting machineCentrifugal casting machine- - alloy is alloy is melted in a separate crucible by a torch flame melted in a separate crucible by a torch flame & is cast into the mold by centrifugal force.& is cast into the mold by centrifugal force.
PROCEDUREPROCEDURE
1). casting machine given three (or) four clock wise turns casting machine given three (or) four clock wise turns
2).Crucible for the alloy being cast is placed on the machine).Crucible for the alloy being cast is placed on the machine
ZONES OF A FLAME
Zone 1 – Directly from the nozzle
Air and gas are mixed before combustion .
No heat is present
Zone 2 – Combustion zone
Gas & Air are partially burned
Color – GREEN Oxidizing – KEEP AWAY 158
Zone 3 : Reducing zone
Hottest part of the flame.
Most effective zone for melting and should be kept constantly over the alloy .
Color – Blue
Zone 4 : Oxidizing zone
Combustion occurs with the oxygen in the air .
KEEP AWAY .
159
CRAIG’S METHOD FOR DETERMINING THE EFFECTIVE FLAME :
-Checking & interpreting the flame condition.
Apply the flame to a copper coin – on a soldering block.
Bright & Clean Dark , dull red colour
Oxidation and ineffective heating
Visual scenario practically : Morphology
Spongy Small globules of fused metal appear Spheroidal shape
Color :- The molten alloy is light orange and tends to spin or follow the flame when it is moved slightly.
160
161
Disadvantages :
Excessive heat may distill lower melting components .
Overheating – gases to dissolve in the casting – porosity
Highly technique sensitive
AIR ACETYLENE & OXYGEN ACETYLENE GAS
- These were designed mainly for Cobalt chromium base alloys higher fusion temperatures
Advantage : Hottest flame hence faster burnout .
162
The torch is lit and adjustedThe torch is lit and adjusted
The crucible is pre-heated in the area The crucible is pre-heated in the area that would be in contact with the alloythat would be in contact with the alloy
The alloy heated in The alloy heated in the reducing part of the reducing part of the flame and the flame and casting machine casting machine arm released to arm released to make the castingmake the casting
The machine is allowed to spin until it has The machine is allowed to spin until it has slowed enough that it can be stopped by hand, and slowed enough that it can be stopped by hand, and the ring is removed with casting tongsthe ring is removed with casting tongs
ELECTRICAL RESISTANCE -- HEATED CASTING MACHINE
PRINCIPLE :
- During Electrical melting of alloys heat energy is produced when
electric current is passed through a conductor depending upon the
voltage applied across it.
- The alloy is melted electrically by a resistance heating .
- Current is passed through a resistance heating conductor, and
automatic melting of the alloy occurs in a graphite or ceramic crucible.
165
- Resistance heat develops when flow of current was opposed by a opposite power
e p
e
pe
p
e
p
RESISTANCE HEATING
166
Advantages:
– For metal ceramic prosthesis.– Base metals in trace amounts that tend to oxidize on overheating.– Crucible located flush against casting ring.
Carbon crucibles should not be used in melting of:
– High Pd– Pd-Ag– Ni-Cr– Co-Cr
167
168
INDUCTION MELTING MACHINE
The alloy is melted by an induction field that develops within a crucible surrounded by water-cooled metal tubing.
169
VERTICAL CRUCIBLE POSITOINED WITHIN INDUCTION COIL
WATER COOLED INDUCTON COIL
170
171
The electric induction furnace is a transformer in which an alternating current flows through the primary winding coil and generates a variable magnetic field in the location of the alloy to be melted in a crucible
172
Alloy reaches melting temp. forced into mold by air pressure, or by vacuum.
It is more commonly used for melting base metal alloys, more in jewelry .
Not been used for noble alloy casting as much as other machines.
173
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DIRECT CURRENT ARC MELTING MACHINE
Arc is produced between two electrodes: The alloy and the water-cooled tungsten electrode generates heat.
anode
cathode
AB
C
D
174
The temperature within the arc exceeds 4000oC and the alloy melts very quickly.
This method has a high risk for over heating the alloy.
175
- Casting machines provide the means for transferring the molten alloy from the crucible to the mold
CASTING MACHINES
176
Recovery of the casting-devestingRecovery of the casting-devesting
After the casting has solidified, the ring is removed and quenched in water as soon as the button exhibits a dull red glow…
AdvantagesAdvantages
1). Noble metal alloy is left in an annealed condition for burnishing and polishing.
2). Soft granular investment is easily removed when water contacts the hot investment.
PicklingPickling--process of removing the oxides process of removing the oxides from the surface of the casting by heating from the surface of the casting by heating
the discolored casting in an acid.the discolored casting in an acid.
Pickling solution used:Pickling solution used:- 50% HCl- sulphuric acid(H2SO4)
DISADVANTAGES DISADVANTAGES
- fumes of acid corrode laboratory metal furnishings
- fumes are health hazard.
Casting defects
Casting defects can be classified as
1. Distortion2. Surface roughness, irregularities and
discolouration3. Porosity4. Incomplete or missing details
Casting defects can also be classified as
1. Dimensionally inaccurate castings2. Having a rough surface and/or fins 3. Porous, contaminated or incomplete
castings
Distortion
• Wax pattern
• Investment material
Surface roughness
Improper finishing of wax pattern
Excess surfactant
Too high casting pressure
Improper w/p ratio
Composition of the investment
Foreign bodies
Impact of molten alloy
Nodules Large nodules – air trapped during
investing
Multiple nodules- inadequate vaccum
-improper brush tech.
-lack of surfactant
Nodules on occlusal surface- excessive vibration
Water films
Appear as MINUTE RIDGES OR VEINS on the surface
If pattern is moved during investing
Lack of intimate contact of investment with wax pattern
Remedy – Surfactant Increased W:P ratio
Fins / spines Increased w/p ratio – ↓ Strength
Premature heating
Too rapid heating
Discolouration
Contamination with gases when the molten alloy comes in contact with the carbonaceous residue
Porosity
Solidification defectsLocalised shrinkage porosityMicroporosity
Trapped gases Gas inclusions Subsurface porosity Pinhole porosity Residual air
Localised shrinkage porosity
Premature termination of flow of metal
Occurs near sprue – casting junction
Microporosity
Occurs – rapid solidification
-low casting or mold temp.
Pinhole/Gas inclusion porosity
Entrapment of gas during solidification Many metals like Copper and Silver dissolve oxygen Palladium has affinity to hydrogen
Subsurface porosity
Not established
Occurs- simultaneous nucleation of solid grains and gas bubbles along the mold walls
Suckback porosity
Occurs-occlusoaxial,incisoaxial line angle
Hot spot created
Back pressure porosity
Large concave depressions Inability of the air to escape
Incomplete casting
Insufficient venting of the mold
Incomplete elimination of wax residue
High viscosity of fused metal
ConclusionConclusion
Investing a wax pattern is a very important Investing a wax pattern is a very important procedure as it will determine the seating procedure as it will determine the seating accuracy of the final casting .accuracy of the final casting .
knowledge of material science & properties knowledge of material science & properties of the investment material is necessary for of the investment material is necessary for proper handling & manipulation of the proper handling & manipulation of the same.same.
ReferencesReferences
• Phillips sccience of dental materialsPhillips sccience of dental materials Kenneth J.Annusavice -11Kenneth J.Annusavice -11 thth edition edition• Introduction to dental materialsIntroduction to dental materials Richard van noort -1Richard van noort -1stst edition edition• Applied dental materials Applied dental materials John F.Mc.Cabe -8John F.Mc.Cabe -8THTH edition edition• Dental materials and their selectionDental materials and their selection
Willian J .O’BrienWillian J .O’Brien -2 -2ndnd edition edition
• Restorative dental materialsRestorative dental materials
Robert .G.CraigRobert .G.Craig -11 -11 th EDITIONth EDITION
• Contemporary fixed prosthodontics- Contemporary fixed prosthodontics- Rosenstiel - 3Rosenstiel - 3 rdrdeditionedition
• Fundamentals of fixed prosthdonticsFundamentals of fixed prosthdontics Shillingburg - 4Shillingburg - 4 thth edition edition
• Dental laboratory proceduresDental laboratory procedures Rudd & Morrow - 2Rudd & Morrow - 2ndnd edition edition
• Operative dentistry-modern theory & practiceOperative dentistry-modern theory & practice M.A.Marzouk M.A.Marzouk
• Internet sources Internet sources
