Investment materials and procedures

DENTAL INVESTMENTDENTAL INVESTMENT
MATERIALS ANDMATERIALS AND
PROCEDURESPROCEDURES
Dr.Dilip jain
DEPT OF
CONSERVATIVE &
ENDODONTICS

CONTENTS
 Introduction
 History
 Definition
 Requirements
 Material science
 Types of investments
-composition
-setting reaction
-setting expansion
-properties
Investing procedure
Defects in investing procedure
Conclusion

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.

4
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.

7
 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

DEFINITION
Investment
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 MATERIALS
Easily 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 quartzquartz ββ quartzquartz
(low form)(low form) ▲ 575° C (high form)▲ 575° C (high form)
 αα cristobalitecristobalite 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

Thermal expansion of three forms of silicaThermal expansion of three forms of silica

The density decreases when the alpha formThe density decreases when the alpha form
changes to beta form with a resultingchanges 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) Phosphate&ethyl silicate-for high
temperature casting investments.

CHEMICAL MODIFIERS
Other chemicals such as sodium chloride, boric acid,
graphite, copper powder, are often added in small
quantities to modify physical properties
-BORIC ACID&SODIUM CHLORIDE
Regulate setting expansion and setting time and thus
prevent most of the shrinkage of gypsum when heated
above 300 ºC .
-COPPER POWDER& GRAPHITE
Act as reducing agents.

CLASSIFICATION
Investments 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 investments
Used 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 investments
For 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 700o
C
Classification:- According A.D.A specification no.2 for casting
investments for dental gold alloys, there are 3 types.
Type – I
Are 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 – II
compensation for alloy shrinkage is by the hygroscopic
expansion
Type III
Investments 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 ºc
Density 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 formsDimensional 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 whileThe 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:linearlinear
dimensional change as the investmentdimensional change as the investment
setssets
Silica particles result in greater setting expansion
HOW??
silica particles interfere with the intermeshingsilica 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 toAccording 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 SETTING2).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 and
hardens the investment,it disintegrates during
heating of the investment and may result in
roughened 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 enters
the mold, the air must be forced out ahead of it…
if
the air is not completely eliminated a backpressure
builds to prevent gold alloy from completely filling
the mold resulting in void formation.
Thus the common method of venting the mold is
through 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 nowOriginally phosphoric acid was used but now
ammonium phosphate has replaced it as it canammonium 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 the
divesting of the castingdivesting of the casting from the mold.Used only
when 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 investmentThermal expansion of a phosphate bonded investment
mixed with water as compared with the specialmixed with water as compared with the special
liquid….liquid….

concentration of liquidconcentration of liquid
The influence of the liquid concentration on the settingThe 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.
composition
1).Refractory material – Silica
2). Binder –Silica gel or ethyl silicate
Several methods may be used to produce the silica or
salicylic acid gel binders.
- FORMATION OF SILICA GEL
When 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

80
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

84
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.

85
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 .

87
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

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.

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 castPreparation 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 fabrication
Sprue the wax patternSprue the wax pattern
Invest the wax patternInvest the wax pattern
Eliminate the wax pattern by burning the waxEliminate the wax pattern by burning the wax
out of the investment in a furnace,thus makingout 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 andClean 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 thea 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 completerelief 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-MADEMADE
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 sprues
Wax 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’s
Disadvantage
-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:def
ormation 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 furnaceb. When the ring is transferred from the furnace
to the casting machines, it reduces loss ofto 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 fromc. 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 wereAsbestos fiber bundles were
found to produce hazardous-found to produce hazardous-
respirable particles capable ofrespirable particles capable of
causing lung diseasecausing lung disease
NEW ALTERNATIVE RINGNEW 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 SPRUETHREE 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 wax
pattern should be cleaned of any, debris
grease 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 ofThere are two different methods of
investing the wax pattern:investing the wax pattern:
1). Hand investing
2). Vacuum investing

1). HAND MIXING1). HAND MIXING
The 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 untilMixing 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

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 incorporate1). Hand spatulate the mix to incorporate
powder quicklypowder quickly

2). Vacuum mix the investment according2). 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 the4). Fill the ring holding it on the
vibrator.tilt the ring from side to side tovibrator.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- 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 theStep-6) remove the filled ring and the
crucible former from the bowl.crucible former from the bowl.

Step-5Step-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 meansCasting machines provide the means
for transferring the molten alloyfor transferring the molten alloy
from the melting crucible to thefrom 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 isalloy is
melted in a separate crucible by a torch flamemelted 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 turnscasting 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

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 areaThe 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 inThe alloy heated in
the reducing part ofthe reducing part of
the flame andthe flame and
casting machinecasting machine
arm released toarm released to
make the castingmake the casting

The machine is allowed to spin until it hasThe machine is allowed to spin until it has
slowed enough that it can be stopped by hand, andslowed 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

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

 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

--------
--------
--------
--------
--------
----
--------
--------
--------
--------
--------
----
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 4000o
C 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 oxidesprocess of removing the oxides
from the surface of the casting by heatingfrom 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)
DISADVANTAGESDISADVANTAGES
- fumes of acid corrode laboratory metal
furnishings
- fumes are health hazard.

Casting defects
Casting defects can be classified as
1. Distortion
2. Surface roughness, irregularities and
discolouration
3. Porosity
4. Incomplete or missing details
Casting defects can also be classified as
1. Dimensionally inaccurate castings
2. 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 defects
Localised shrinkage porosity
Microporosity
 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 importantInvesting a wax pattern is a very important
procedure as it will determine the seatingprocedure as it will determine the seating
accuracy of the final casting .accuracy of the final casting .
knowledge of material science & propertiesknowledge of material science & properties
of the investment material is necessary forof the investment material is necessary for
proper handling & manipulation of theproper handling & manipulation of the
same.same.

ReferencesReferences
• Phillips sccience of dental materialsPhillips sccience of dental materials
Kenneth J.Annusavice -11Kenneth J.Annusavice -11thth
editionedition
• Introduction to dental materialsIntroduction to dental materials
Richard van noort -1Richard van noort -1stst
editionedition
• Applied dental materialsApplied dental materials
John F.Mc.Cabe -8John F.Mc.Cabe -8THTH
editionedition
• Dental materials and their selectionDental materials and their selection
Willian J .O’BrienWillian J .O’Brien -2-2ndnd
editionedition
• Restorative dental materialsRestorative dental materials
Robert .G.CraigRobert .G.Craig -11-11th EDITIONth EDITION

• Contemporary fixed prosthodontics-Contemporary fixed prosthodontics-
Rosenstiel - 3Rosenstiel - 3rdrd
editionedition
• Fundamentals of fixed prosthdonticsFundamentals of fixed prosthdontics
Shillingburg - 4Shillingburg - 4thth
editionedition
• Dental laboratory proceduresDental laboratory procedures
Rudd & Morrow - 2Rudd & Morrow - 2ndnd
editionedition
• Operative dentistry-modern theory & practiceOperative dentistry-modern theory & practice
M.A.MarzoukM.A.Marzouk
• Internet sourcesInternet sources

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