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• The dihydrate form of calcium sulfate, called• gypsum, usually appears white to milky

yellowish• and is found in a compact mass in nature.

GYPSUM

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• As the temperature is raised the remaining water is removed , and products are formed as indicated .

1100 –1300 1300 –2000 2000 –10000 CaSO4+2H2O [CaSO4]1/2 H2O CaSO4 CaSO4

GYPSUMCALCIUM SULFATE HEMIHYDRATE

HEXAGONALANHYDRATE

ORTHOROMBIC ANHYDRATE

GYPSUM

Densite

Mineralgypsum

Dehydration by heat or other means

Plastersformulation

Model plaster Lab plaster

Dental stone

High-strength dental stone

Hydrocal

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IT IS OF THE FOLLOWING TYPES :

Impression plaster TYPE IModel plaster. TYPE IIDental stone. TYPE IIIDental Stone , high strength . TYPE IVDental stone, high strength & TYPE VHigh expansion.

4

GYPSUM

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• Most gypsum products are obtained from natural gypsum rock. The reaction is exothermic, CaSO4 · ½ H2O + 1 ½ H2O →CaSO4 · 2H2O + 3900 cal/g mol Plaster of Paris Water Gypsum

Three types of base raw materials are derived from partial dehydration of gypsum rock, depending on the nature of the dehydration process.:

Plasters:are fluffy, porous, and least dense, whereas hydrocal variety :has a higher density and is more crystalline.

Densite :• is the densest of the raw materials

GYPSUM

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ISO Classification of dental gypsum products

Type 1 : Impression plaster. Type 2 : Dental plaster – model. Type 3 : Dental stone – die. Type 4 : Dental stone – die, high strength, low expansion.• Type 5 : Dental stone – die, high strength, high expansion.

• Although these types have identical chemical formulas of calcium sulfate hemihydrate, CaSO4 · ½H2O,they possess different physical properties,

Type 4 contains extra salts to reduce its setting expansion

DR .R.koohkan 7

• the main difference being the manner of driving off part of the water of the calcium sulfate dihydrate.

Plasters :• gypsum mineral is heated in an open kettle at a temperature of about 110° to 120° C β-calcium sulfate hemihydrate.• (irregular shape and porous)Hydrocal :• If gypsum is dehydrated under pressure and in the presence of water vapor at

about 125° C α-calcium sulfate hemihydrate. more uniform in shape and denser than the particles of plaster low- to moderate-strength dental stones..

Densite:• Boiling gypsum rock in a 30% calcium chloride solution, is washed away with

hot water(100° C) is ground fineness. • Types 4 and 5 high-strength dental stones are• A high-density

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• α – hemimhydrate

1. Type III , IV &V 2. Produced by wet calcination.3. Requires less water for

mixing.4. Better packing ability.5. Low surface free energy.6. Crystal habit of hexagonal

calcium sulfate .7. High apparent density.

β – hemihydrate

1. Type I & II2. Produced by dry calcination.3. Requires more water form

mixing.4. Less packing ability.5. High surface free energy.6. Crystal habit that of

hemihydrate.7. Low apparent density.

8

DR .R.koohkan 9

Gypsum Mixing Water(mL/100 g of powder)

Required Water(mL/100 g of powder)

Excess Water( mL/100 g of powder)

Model plaster 37-50 18.6 18-31

Dental stone 28-32 18.6 9-13

High-strength dental stone 19-24 18.6 0-5 *Water-powder ratio varies with each product.

When the set material is dried, the excess water evaporates and leaves porosity in the structure, weakening it.

This difference in the physical shape and nature of the crystals makes it possible to obtain the same consistency with less excess water with dental stone than with model plaster.

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SETTING REACTION • PROPOSED THEORIES :1- Colloidal theory – hemihydrate + water --> colloidal state through sol -gel

mechanism. In sol state hemihydrate particles

are converted to dihydrate

and as the measured amount of water is consumed the mass

converts to a solid gel.

10

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2- Hydration theory: suggests that rehydrated plaster particles join together through

hydrogen bonding to the sulfate groups to form the set material.

3-Dissolution – precipitation theory:• (most widely accepted theory) the setting reaction of water with calcium sulfate - hemihydrate to from calcium sulfate dihydrate is caused by the difference in solubility between these two components. based on dissolution of plaster and instant recrystallization of gypsum, followed by interlocking of the crystals to form the set solid.

11

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Hemihydrate is 4 times more soluble in water than is the dihydrate near room temp (20°C). Thus the setting reaction can be understood as follows:--

hemihydrate + water

Suspension is formed that is fluid and workable.

Hemihydrate dissolves until it forms a saturated solution.

This saturated solution supersaturated in dihydrate, precipitates out dihydrate.

12

DR .R.koohkan

. . Thus solution is no longer saturated with hemihydrate,

so it continues to dissolve.

Dissolution of the hemihydrate and precipitation of dihydrate proceeds as new crystals form or further

growth occurs on the present crystals.

• In practice about 0.2% to 0.4% linear expansion is • obtained.

13

DR .R.koohkan

W/P RATIO The ratio of the water to the hemihydrate powder is usually

expressed as the W/P ratio W/P ratio setting time , strength , setting

expansion . example : if 100g is mixed with 60 ml of water, the W/P ration is 0.6.

14

DR .R.koohkan 15

Volumetric Contraction

The fact that the contraction of gypsum is not visible does not invalidate its existence, and when the volumetric contraction is measured by a dilatometer, it is determined to be about 7%.

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Effect of Spatulation

The mixing process, called spatulation, has a definite effect on the setting time and setting expansion

spatulation (either speed of spatulation or time or both)

setting time

Because an increased amount of spatulation causes more nuclei centers (dihydrate) to be formed, the conversion of calcium sulfate hemihydrate to dihydrate is accelerated.

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I. The first effect of increasing temperature is achange in the relative solubilities of calcium sulfatehemihydrate and calcium sulfate dihydrate, which alters the rate of the reaction.

II. In general, as the temperature increases, the mobility of the calcium and sulfate ions the rate of the reaction ,the setting time

the temperature is raised over37° C, the rate of the reaction decreases, and the setting time is lengthened.

17

20° C 30° C Over 37° C 100° C

The ratio of the solubilities of calcium sulfate dihydrate and calcium sulfate hemihydrate

4.5 3.4 1

the mobility of the calcium and sulfate ions increase increase increase increase

the rate of the reaction increase decreases no reaction

Effect of Temperature

DR .R.koohkan 18

EFFECT OF HUMIDITY

The initial result is acceleration of setting. Produce dihydrate, providing more nuclei for crystallization . The final result is further contamination by moisture can reduce the amount of hemihydrate remaining to form gypsum retardation of setting will occur setting time

DR .R.koohkan 19

EFFECT OF COLLOIDAL SYSTEMS

Colloidal systems (agar , alginate) retard the setting of gypsum

by being adsorbed on the hemihydrate and dihydrate nucleation sites retards the setting it is more effective on dihydrate nucleation

potassium sulfate are added to improve the surface quality of the set CaSO4 · 2H2O against agar or alginate.

DR .R.koohkan 20

Liquids with low pH, such as saliva, retard the setting reaction

Liquids with high pH accelerate setting.

EFFECT OF PH

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The important properties of gypsum products include qualityfluidity at pouring time Setting time linear setting expansion compressive strengthhardness abrasion resistanceReproduction of detail

PROPERTIES

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The time required for the reaction to be completed is called the final setting time.

The initial setting time is defined by: The loss of gloss from the surface Increase temperature of the mass INITIAL GILLMORE TEST FOR INITIAL SET

The final setting time is defined as the time at which the material can be separated from the impression

SETTING TIME

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• The smaller needle is most frequently used for cements but it is sometimes used for gypsum products .

The mixture is spread out, and the needle is lowered onto the surface. The time at which it no longer leaves an impression is called the initial set , noted as “Initial Gillmore”.

It takes place at approx 13 mins. 23

INITIAL GILLMORE TEST FOR INITIAL SET

DR .R.koohkan 24

The Vicat apparatus: is commonly used to

measure the initial setting time of gypsum products. It consists of a rod weighing 300g with a needle of 1-mm diameter.

MEASUREMENT SETTING TIME

DR .R.koohkan 25

The setting time can control by: changing the temperature of the mix water a changing the degree of spatulation. Water-powder(W/P) ratio The easiest and most reliable way to change the setting time is to add

different chemicals.The setting time is usually shortened for power mixing compared with hand mixing.

CONTROL OF SETTING TIME

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ACCELERATORS:

Sodium Sulphate - 3-4 % Potassium Sulphate - 2-3 % Sodium Chloride - 2 % Gypsum - < 20% Potassium sulfate “ROCHELLE potassium sodium tartrate SALT” The acceleration caused by an additive depends on

the amount and rate of solubility of the hemihydrate versus the same effect on the dihydrate.

26

DR .R.koohkan 27

RETARDERS: Act by forming an adsorbed layer on the hemihydrate and on gypsum

crystals to reduce its solubility.(organic materials) forms a layer of calcium salt that is less soluble than is the sulfate salt.

(salts) ORGANIC MATERIALS: Glue, gelatin and some gums. SALTS: Borax 1-2% (sodium tetraborate decahydrate) sodium chloride ~ 20%

DR .R.koohkan 28

Manipulative Variable Setting Time Consistency Setting Expansion

Compressive Strength

Increase water/powder ratio

Increase Increase Decrease Decrease

Increase rate of spatulation Decrease Decrease Increase No effect

Increase temperature of mixing water from 23° to 30° C

Decrease Decrease Increase No effect

SUMMARY OF EFFECT OF MANIPULATIVE VARIABLES ON PROPERTIES OF GYPSUM PRODUCTS

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Setting time Expansion

K2SO4 ↓ ↓ ↓ ?↓ ↓ ↓

Borax ↑ ↑ ↓

K2SO4 + Borax ↓ ↓ ↓ ↓

Sodium chloride in small amounts

Terra alba effective accelerators.(set calcium sulfate dihydrate)

Gypsum products may be formulated with chemicals that modify their handling characteristics and properties.

29

sodium citrate is a dependable retarder.

2% aqueous solution of borax prolong the setting time (a few hours)

DR .R.koohkan 30

• More voids were observed in casts made from the stones with the higher viscosities.

VISCOSITY

DR .R.koohkan 31

The compressive strength is inversely related to the W/P ratio of the mix. The more water , the lower compressive strength.

The excess water is uniformly distributed in the mix and contributes to the volume but not the strength of the material

the most porous the weakest stone

Porous Model plaster>Dental stone > High-strength stone strength Model plaster<Dental stone <. High-strength stone

COMPRESSIVE STRENGTH

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The dry compressive strength is usually about twice that of the wet strength.

32

Com

pres

sive

stre

ngth

(MPa

) 40

30

50

60

20 0 2 4 6 8 10 Weight loss (%)

Effect of loss of excess water on Compressive strength of dental stone.

DR .R.koohkan 33

The surface hardness of un modified gypsum materials is related in a general way to their compressive strength.

Increased surface hardness does not necessarily mean improved abrasion resistance because hardness is only one of many factors that can affect wear resistance

SURFACE HARDNESS AND ABRASION

impregnating the set gypsum by surfaceHardness

compressive strength

scratchresistance

epoxy or methylmethacrylate monomer plaster - -

epoxy or methylmethacrylate monomer dental stone

colloidal silica (about 30%) dental stone - -

DR .R.koohkan 34

ANSI/ADA specification No. 25 requires that: types 1 and 2 reproduce a groove 75 mm in width types 3, 4, and 5 reproduce a groove 50 mm in width

Gypsum dies do not reproduce surface detail as well as electroformed or epoxy dies

gypsum does not wet some impression material (silicon)

use of vibration during the pouring of a cast Use surfactants in silicone impression materials Rinsing the impression

REPRODUCTION OF DETAIL

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Gypsum product shows linear expansion during the setting due to outward thrust of crystals that is change from hemihydrate to dihydrate.

Low – 0.06 % High – 0.5 %

[CaSO4]₂ •H₂O + 3H₂O 2 CaSO4 •2H₂OMolecular mass 290.284 54.048 344.322Density(g/cm³) 2.75 0.997 2.32Equivalent volume 105.556 54.211 148.405

Total volume 159.767 148.405

35

SETTING EXPANSION

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SETTING EXPANSION CONTD . .

Net change in volume is : (148.405 – 159.767) * 100 = -7.11% 159.767 CRYSTALLIZATION MECHANISM: crystals grow outward thrust or stress

develops expansion of the entire mass .• Practically the product is greater in external

volume but less in crystalline volume.

36

DR .R.koohkan 37

) Hygroscopic expansion: In one technique the investment is immersed in water

after setting has begun. A greatly increased setting expansion occurs. So less thermal expansion is required.

Increased hygroscopic expansion is obtained in the following cases:

1) When a lower water/powder ratio is used2) For an investment material of greater silica content.3) If water of higher temperature is used.4) For longer immersion in water

HYGROSCOPIC SETTING EXPANSION

DR .R.koohkan 38

HYGROSCOPIC SETTING EXPANSION

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If during the setting process, the gypsum materials are immersed in water, the setting expansion increases.

Mechanical mixing decreases setting expansion.

39

HYGROSCOPIC EXPANSION

Properties of a High-Strength Dental Stone Mixed by Hand and by a Power-Driven Mixerwith Vacuum

DR .R.koohkan 40

Flexible rubber mixing bowl and metal spatula with a stiff blade. (Courtesy of Whip Mix Corporation,Louisville, KY.)

A Vibrator is designed to promote the release of bubbles in the gypsum mix and to facilitate pouring of the impression. (Courtesy of Whip Mix Corporation,Louisville, KY.)

Power-driven mechanical spatulator with a vacuum attachment. (Courtesy of Whip Mix Corporation,Louisville, KY.)

DR .R.koohkan 41

Properties Required of an Investment :1. Easily manipulated:2. Sufficient strength at room temperature:3. Stability at higher temperatures:4. Sufficient expansion:5. Beneficial casting temperatures:6. Porosity:7. Smooth surface:8. Ease of divestment:

9. Inexpensive.

CASTING INVESTEMENTS

DR .R.koohkan 42

Composition Investment is a mixture of three distinct types

of materials: 1. Refractory material2. Binder material3. Other chemicals

CASTING INVESTEMENTS

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REFRACTORY MATERIAL

During the heating, the investment is expected to expand thermally to compensate partially or totally for the casting shrinkage of the gold alloy. Such as:

Quartz Tridymite Cristobalite or a mixture of these.

43

Microstructure of the surface of a set cristobalite investment. The large, irregular particles are ,silica, and the rodlike particles are cristobalite (~3000) Courtesy oR Earnshdw.)

DR .R.koohkan 44

BINDER MATERIAL

The common binder used for dental casting:

α-calcium sulfate hemihydrate (gold alloy) Phosphate, (for high-temperature casting ) ethyl silicate, (for high-temperature casting)

DR .R.koohkan 45

To produce the desirable properties required of an investment. such as : Sodium chloride Boric acid Potassium sulfate Graphite Copper powder Magnesium oxide

small amounts of chlorides or boric acid enhance the thermal expansion of investments bonded by calcium sulfate.

OTHER CHEMICAL

DR .R.koohkan 46

The final product’s properties are influenced by both the ingredients present in the investment and the manner in which the mass is manipulated and used in making the mold.

The investment may contain 25% to 45% of the calcium sulfate hemihydrate. The remainder consists of silica allotropes and controlling chemicals.

DR .R.koohkan 47

• Dimensional change of Three form of gypsum When heated

• The calciumsulfate portion of the investment decomposes into

embrittle the casting metal.Temperature ("C)

(Courtesy of K. Neiman,Whip-Mix (‘corporation, Louisville, I<Y.)

at temperatures over 700° Csulfur dioxide sulfur trioxide

Tend

ing to

DR .R.koohkan 48

EFFECT OF TEMPERATURE ON INVESTMENT

Effect of Temperature on Silicon Dioxide Refractories Effect of Temperature on Calcium Sulfate Binders

DR .R.koohkan 49

EFFECT OF TEMPERATURE ON SILICON DIOXIDE REFRACTORIES

The percentage of expansion varies from one type to another

cristobalite and quartz each exist in two polymorphic forms β-Form .Stable at a higher temperature α-Form Stable at a Lower temperature. stable at room

Tridymite has three stable polymorphic forms.

DR .R.koohkan 50

573°c C

Quartz:At 573° C also shows a break inthe expansion curve, .

cristobalite, the expansion is uniform up toabout 200° C.

0.5% to 1.2%

, above 250° C it again becomes more uniform.

Expansion increases

Tridymite :Shows a similar break at a much lower temperature

105° and 160° C

DR .R.koohkan 51

• Displacive transition temperatures. • A displacive change involves expansion or contraction in the

volume of the mass without breaking any bonds. ( cristobalite 220° C quartz573° C , tridymite105° and 160° C )

• changing α-form β-form at all three forms of silica expand. • The amount of expansion is highest for cristobalite and lowest for

tridymite.

to

DR .R.koohkan 52

• Reconstructive transition during which bonds are broken and a new crystal structure is formed.

• The quartz can be converted to cristobalite and tridymite by being heated through a

B-quartz B-tridymite B-cristobalite Fused silica870° C

160° C

Middle tridymite

105° C

α-tridymite

1475° C

220° C

1700° C

α-cristobalite

Fused silica

α-quartz

573° C

Displacive transition temperatures

DR .R.koohkan 53

EFFECT OF TEMPERATURE ON CALCIUM SULFATE BINDERS

Dehydration of the dihydrate and a phase change of the calcium sulfate anhydrite cause a contraction.

up to about 200° C between 200° -700° C. Remains unchanged

Registers varying degrees of expansion,

anhydrous calcium sulfateThermal expansionInvestment contracts slightly 105° C then

Depending on the silica and composition of the investment

DR .R.koohkan

COOLING OF THE INVESTMENT

When the investment is allowed to cool, the refractory and binder contract according to a thermal contraction curve that is different from the thermal expansion curve of the investment

54

Thermal expansion and contraction curves for calcium sulfate–bonded investment (thermal expansion type). Curve 1 is first heating, curve 2 is cooling curve 3 is reheating.

DR .R.koohkan 55

) Hygroscopic expansion: In one technique the investment is immersed in water

after setting has begun. A greatly increased setting expansion occurs. So less thermal expansion is required.

Increased hygroscopic expansion is obtained in the following cases:

1) When a lower water/powder ratio is used2) For an investment material of greater silica content.3) If water of higher temperature is used.4) For longer immersion in water

HYGROSCOPIC SETTING EXPANSION

DR .R.koohkan 56

Thermal expansion curves for calciumsulfate–bonded investments. A, Hygroscopic type; B,thermal expansion type.

For hygroscopic expansion,:the additional water provided must be presented to the investment during setting.

The additional water be presented before the observed loss of gloss, which is when the setting reaction is not complete. This allows the additional water to join the remaining mix water and extend the water surface so that the action of surfacetension is either delayed or inactive.

DR .R.koohkan 57

ALLOY AND MELTING TEMPERATURES

Base metal alloys are usually cast into molds at 850° to 110° C. To with stand these high temperatures, the molds require different types of binders, such as silicate and phosphate compounds.

( less than 20% binder, remainder of investment is quartz or another form of silica.)

melting temperatures

Over 700° C

under700° C calcium sulfate–bonded investments

Investment

Phosphate- bonded investments

Silica-bonded investments

cobalt-

chromium alloys

Gold alloys

DR .R.koohkan

PARTICLE SIZE OF SILICA

The particle size hygroscopic expansion

calcium sulfate hemihydrate little effect

silica significant effect

58

Finer silica produces higher Setting and hygroscopic expansions.

Silica/Binder Ratio:Investments usually contain 65% to 75% silica,25% to 35% calcium sulfate hemihydrate, 2% to 3% of some additive chemicals to control the different physical properties

If the silica/stone ratio is increased, The hygroscopic expansion of the investment

The strength of the investmentin

crea

ses,

decr

ease

s.

DR .R.koohkan 59

Manipulative Variable Setting Time Consistency Setting Expansion

Compressive Strength

Increase water/powder ratio

Increase Increase Decrease Decrease

Increase rate of spatulation Decrease Decrease Increase No effect

Increase temperature of mixing water from 23° to 30° C

Decrease Decrease Increase No effect

SUMMARY OF EFFECT OF MANIPULATIVE VARIABLES ON PROPERTIES OF GYPSUM PRODUCTS

DR .R.koohkan 60

WATER-BATH TEMPERATURE

The water bath has a measurable effect on the wax pattern.

At higher water-bath temperatures 1-the wax pattern expands, requiring less expansion of the investment to compensate for the total casting shrinkage.

2- soften the wax provides less resistance to the expansion of the investment

making the setting expansion more effective.

DR .R.koohkan

THERMAL AND HYGROSCOPIC CASTING INVESTMENT

Casting techniques involving gypsum bonded investments are often classified as

1- Thermal techniques

61

the invested ring placing into the burnout oven(649° C),

directs

after setting

the invested ring before setting immersing in a water bath after setting(482° C).

the burnout oven

Although all gypsum-bonded investments exhibit both thermal and hygroscopic setting expansion,

2- Hygroscopic techniques.

DR .R.koohkan 62

Investments used in the thermal technique usually contain cristobalite has a high thermal expansion.

Investments used in the hygroscopic technique usually contain

lower thermal expansions higher hygroscopic setting expansions.

quartz or tridymite

Temperature ( C)

Expa

nsio

n (%

)

Thermal expansion of mixed hygroscopic-thermal gold casting investment.

DR .R.koohkan 63

Temperature ( C

Expa

nsio

n (%

)

Setting and hygroscopic expansion ofmixed hygroscopic-thermal gold casting investment.

DR .R.koohkan 64

PHOSPHATE-BONDED INVESTMENTS

1. A water-soluble phosphate ion.

2. The second component reacts with phosphate ions at room temperature.

3. The third component is a refractory, such as silica.

This type of investment consists of three different components.

ANSI/ADA specification No. 126 (ISO 9694) fordental phosphate-bonded casting investments Specifies two types of investments for alloys having a solidus temperature above 1080° C:Type 1: For inlays, crowns, and other fixed restorationsType 2: For removable dental prostheses

DR .R.koohkan 65

an acid-base reaction between acid

monoammonium phosphate+ Mgo

forming a binding medium with fillerparticles embedded in the matrix.

Spatulation continues

The water produced by this reaction at room temperature

Phases formed at high temperatures

DR .R.koohkan 66

• The special liquid is a form of silica sol in water

• phosphate-bonded investments possess higher setting expansion when they are mixed with the silica sol than when mixed with water. (increases its strength)

Effect of silica sol concentration onthermal expansion (solid lines) at 800° C and setting expansion (dotted lines) of two phosphate-bonded investments.A, Thermal expansion type; B, hygroscopic expansion type.

Thermal expansion type

DR .R.koohkan 67

• This type of investment may derive its silica bond from ethyl silicate, an aqueous dispersion of colloidal silica, or from sodium silicate.

SILICA-BONDED INVESTMENTS

DR .R.koohkan 68

SILICA-BONDED INVESTMENTS

Setting reaction:a) Stage 1 : hydrolysis. Ethyl silicate can be

hydrolysed to silica acid, with liberation of ethyl alcohol:

Si(OC2H5)4 + 4H2O Si(OH)4 + 4C2H5OHethyl silicate

In practical, a polymerised form of ethyl silicateis used, yielding a sol of polysilicate acid.

a colloidal solution of silicic acid and ethyl alcohol,

DR .R.koohkan 69

b) Stage 2: gelation. The sol is mixes with cristobalite or quartz, then gel formation is made to occur under alkaline conditions by adding magnesium oxide. There is a slight shrinkage at this stage.

c) Stage 3: drying. on heating, considerable shrinkage occurs and there is a loss of alcohol and water, leaving a mould made of silica particles tightly packed together.

As alternative of the above, simultaneous hydrolysis and gel formation can occur, when an amine such as piperidine is incorporated.

DR .R.koohkan 70

• ANSI/ADA specification No. 126 (ISO 11246)• The setting time must not differ by more than 30%

from the time stated by the manufacturer.

• The compressive strength at room temperature shall not be less than 1.5 MPa.

• The linear thermal expansion must not differ by more than 15% from the time stated by the manufacturer.

• Brazing Investment• When brazing (soldering) the parts of a

DR .R.koohkan 71

BRAZING INVESTMENT

ANSI/ADA specification No. 126 (ISO 11244) for dental brazing investments defines two types

of investment: Type 1: Gypsum-bonded dental brazing investments Type 2: Phosphate-bonded dental brazing investments

Soldering investments are designed to have lower setting and thermal expansions than casting investments , a feature that is desirable so the assembled

DR .R.koohkan 72

CASTING

INTRODUCTIONCasting is the process by which a wax pattern of a restoration is converted to a replicate in dental alloy.

It is used to make dental restorations such as inlays, onlays, crowns, bridges, and removable partial dentures

DR .R.koohkan 73

STEPS IN MAKING A CAST RESTORATION

1 . TOOTH PREPARATION .2 . IMPRESSION .3 . DIE PREPARATION .4 .WAX PATTERN FABRICATION .- There are 4 methods for making wax patterns

for a cast restoration .

5. SPRUING .

DR .R.koohkan 74

Gingival retraction Retraction cord

DR .R.koohkan 75

THE STEPS INVOLVED IN THE PROCESS OR THE LOST WAX CASTING ARE:

1 .Create a wax pattern of the missing tooth / rim2 .Sprue the wax pattern3 .Invest the wax pattern4. Eliminate the wax pattern by burning it (inside the furnace or in hot water). This will create a mould.5 . Force molten metal into the mould - casting.6 .Clean the cast.7 .Remove sprue from the cast8 . Finish and polish the casting on the die

DR .R.koohkan 76

DR .R.koohkan 77

DIMENSIONAL CHANGES

The management of these dimensional changes is complex, but can be summarized by the equation:

wax shrinkage + metal shrinkage = wax expansion + setting expansion + hygroscopic expansion + thermal expansion

DR .R.koohkan 78

the wax will shrink significantly because of the high coefficient of thermal expansion of waxes.

Metal shrinkage occurs when the molten metal solidifies, but this shrinkage is compensated by introducing more metal as the casting solidifies

Cooling shrinkage may reach 2.5% for an alloy that cools from a high solidus temperature (1300" to 1400' C), depending on the coefficient of thermal expansion of the alloy.

DR .R.koohkan 79

SPRUE

FUNCTIONS OF SPRUE1 . Forms a mount for the wax pattern .2 . Creates a channel for elimination of wax .3 .Forms a channel for entry of molten metal4 . Provides a reservoir of molten metal to compensate for the alloy shrinkage

DR .R.koohkan 80

SELECTION OF SPRUE

• 1 . DIAMETER :It should be approximately the same size of the thickest portion of the wax pattern .Too small sprue diameter suck back porosity results .

• 2 . SPRUE FORMER ATTACHMENT :Sprue should be attached to the thickest portion of the wax pattern .It should be Flared for high density alloys & Restricted for low density alloys .

DR .R.koohkan 81

3 . SPRUE FORMER POSITION Shape & form of the wax pattern . Patterns may be sprued directly or indirectly .. Indirect method is commonly used

4.Reservoir prevents localised shrinkage porosity . Reservoir And Its Location Reservoir portion of a Spruing system is a round ball or a bar located 1mm away from the wax pattern. . Round ball reservoir & a bar reservoir also called connector Significance of Reservoirs:

DR .R.koohkan 82

Reservoir should be positioned in the heat centre of the ring . This permits the reservoir to remain molten for longer and enables it to furnish alloy to the pattern until they complete solidification process .

5. SPRUE FORMER DIRECTION Ideal angulation is 45 degrees

6 . SPRUE FORMER LENGTH Depends on the length of casting ring

7.TYPES OF SPRUES - Wax . II . Solid- Plastic . Hollow- Metal .

DR .R.koohkan 83

VENTING

Small auxilliary sprues or vents improve casting of thin patterns

Acts as a HEAT SINK .

DR .R.koohkan 84

WETTABILITY

To minimise the irregularities on the investment & the casting a wetting agent can be used .

FUNCTIONS OF A WETTING AGENT1 . Reduce contact angle between liquid & wax surface .2 .Remove any oily film left on wax pattern

DR .R.koohkan 85

CRUCIBLE FORMER

It serves as a base for the casting ring during investing .Usually convex in shape.May be metal , plastic or rubber

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CASTING RING LINERS

Most common way to provide investment expansion is by using a liner in the casting ring .Traditionally asbestose was used .Non asbestose ring liner used are :

1) Aluminosilicate ceramic liner .2) Cellulose paper liner .

a resilient liner is to allow different types of investment expansion

facilitate venting during casting procedure. facilitate the removal of the investment block after casting .

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RINGLESS INVESTMENT TECHNIQUE

Useful for high melting alloys used for phosphate bonded investments This method uses paper or plastic casting ring Solid rings do not permit the investment to expand

laterally during the setting and hygroscopic expansions of the mold

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INVESTMENT TECHNIQUES the correct water powder ratio of the investment

mix, A required number of spatulation turns, A proper investing technique are essential to obtain

acceptable casting results. hand investing and vacuum investing. requires 45 to60 minutes

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CASTING TORCH SELECTION

Two type of torch tips: • Multi-orifice is widely used for metal ceramic alloys. Main advantage is distribution of heat over wide area for uniform heating of the alloy.

• Single-orifice tip concentrate more heat in one area.

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Three fuel sources are used for Casting Torch; 1-Acetylene 2-Natural Gas 3-PropaneCASTING CRUCIBLESFour types are available ;1) Clay .2) Carbon .3) Quartz .4) Zirconia –Alumina .

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CASTING MACHINES

It is a device which uses heat source to melt the alloy casting force .

Heat sources can be :1) Reducing flame of a torch .( conventional alloys & metal ceramic alloys )2) Electricity .(Base metal alloys )

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Advantages of electric heating :-heating is evenly controlled .-minimal undesirable changes in the alloy composition - Appropriate for large labs .

Disdvantages :Expensive .

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CASTING MACHINES USE

1) Air pressure .2) Centrifugal force .3) Evacuation technique

Centrifugal casting machine,

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ALLOYS CAN BE MELTED BY

1) Alloy is melted in a separate crucible by a torch flame & is cast into the mold by centrifugal force .(centrifugal C M )

2) Alloy is melted electrically by a resistance heating or by induction furnace & then cast centrifugally by motor or spring action (spring wound CM electrical resistance )

3) Alloy is melted by induction heating cast into mold centrifugally by motor or spring action .(Induction CM )

4) Alloy is vacuum melted by an argon atmosphere

Torch melting / Centrifugal casting machine Electrical resistance /Heated casting machine

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Electrical resistance Spring-wound casting

Induction melting casting machine, water cooling induction coil.

Induction melting casting machine,vertical crucible positional within the induction coil

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