Gypsum & Investment material- Dr.Vaishnavi Ganesh

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GYPSUM PRODUCTS AND INVESTMENT MATERIAL

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Introduction

Gypsum - (Greek) - Gypsos - ChalkPlaster - (French) - Plastre - to mouldChemically, the gypsum used for dental

materials is pure calcium sulfate hemihydrate, which is achieved by boiling calcium sulfate dihydrate under various temperatures.

This procedure is called calciningGypsum CaSO4. 2H2O usually white to

yellowish white, found as compact mass.

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History

Different forms of gypsum has been used for centuries for construction of various artifacts.

Believed alabaster used in King Solomon’s Temple of biblical fame is a form of gypsum.

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Dental plaster & Dental Stone Production

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110o-130oC 130o-200oC 200o-1000oC

CaSO4.2H2O CaSO4.1/2H2O CaSO4

CaSO4Gypsum (calcium Plaster or Stone Hexagonal Orthorhombic sulfate dihydrate) (calcium sulphate anhydrite anhydrite hemihydrates)

CALCINING

Phillips, Science of dental materials 11th edi ,Anusavice

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The various forms are alpha – hemihydrate and beta-hemihydrate.

Gum arabic plus calcium carbonate added to reduce the water requirements

Grinding of hemihydrate after preparation can eliminate needle like crystals which decreases water requirement during mixing.

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Desirable Properties

These include:I. AccuracyII. Dimensional stabilityIII. Strength and resistance to abrasionIV. Compatibility with the impression materialV. ColorVI. BiocompatibilityVII.Easy to useVIII.Economical

Clinical Aspects of Dental Materials - Marcia Gladwin,Michael Bagby

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Dental Plaster Dental Stone

Beta – HemihydrateObtained by heating

gypsum in Open kettle at 110 to 120 degrees C.

Small irregularly shaped, and loosely packed crystals.

Alpha – Hemihydrate Also called hydrocalObtained by heating

gypsum under pressure in the presence of water vapor at 125 degree C

Larger ,denser, more regularly shaped crystals

Difference

Dental materials properties &manipulation 7thedi ,Craig,Powers,Wataha-

9

Dental plaster Dental stone

Porous in natureW/P ratio: 0.45 to

0.5Compressive

strength- 12MPaTensile strength-

2MPaSetting expansion –

0.20 to 0.30%

Relatively non-porous.W/P ratio: 0.28 to 0.30Compressive strength-

30MPaTensile strength- 4MPaSetting expansion –

0.08 to 0.10%

Difference

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Dental plaster Dental stone

Differences

Dental materials properties &manipulation 7thedi ,Craig,Powers,Wataha

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Densite

Also called improved stoneTypes 4 and 5 high-strength dental stones are

manufactured with a high-density raw material called densite. This variety is made by boiling gypsum rock in a 30% calcium chloride and magnesium chloride solution,after which the chloride is washed away with hot water (100" C) and the material is ground to the desired fineness

These act as deflocculants seperating the particles thereby smoother than stone.

Craig+ Introduction To Dental Materials 3rd Edition Richard Van Noort

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Setting Reaction

Exothermic reaction, magnitude of temperature can be upto 30° C at the center of the set mass

Calcium sulfate hemihydrate is mixed with water to produce calcium sulfate dihydrate.

(CaSO4)2.H2O + 3H2O 2CaSO4.2H2O+ unreacted(CaSO4)2.1/2H2O+ Heat


13

NUCLEI FORMATION

SPHERULITEFORMATION

GROWTH OF NUCLEI

Apllied dental materials 8th edition John F.McCabe & Angus W.G.Walls

Setting Reaction

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Setting Reaction

Hemihydrate is mixed with water and suspension is formed

Saturation of hemihydrate

Conversion of hemihydrate (solubility rate 0.8%) to form dihydrate

Solubility limit(0.2%) of dihydrate is exceeded

Crystallization of dihydrate

Apllied dental materials 8th edition John F.McCabe & Angus W.G.Walls

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THEORIES OF SETTING REACTION

COLLOIDAL THEORY:

Also called GEL theory. It states that when mixed with water, plaster enters a colloidal state through Sol-Gel Mechanism. In sol state hemihydrates particles are hydrated to form dihydrate, thereby entering into an active state. As measured amount of water is consumed, the mass converts into Solid Gel.

HYDRATION THEORY: This theory suggests that rehydrated

plaster particles join together through hydrogen bonding to the sulfate groups to form the set material.


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DISSOLUTION-PRECIPITATION THEORY:

Proposed in 1887 by Henry Louis Le Chatelier.

In 1907, it received the full support of Jacobus

Hendricus vant’s Hoff.This is the most widely accepted mechanism. The theory is based on dissolution of plaster and instant recrystallization of gypsum, followed by interlocking of the crystals to form the set solid.


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TESTS FOR WORKING, SETTING, AND

FINAL SETTING TIMES

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Mixing Time (MT)

Mixing time is defined as the time from the addition of the powder to the water until the mixing is completed.

Mixing time for the below are as follows: Mechanical mixing – 20 to 30 sec Hand spatulation – 1 minute


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Working Time (WT)

It is the time available to use a workable mix, one that maintains a uniform consistency to perform one or more tasks.

In case of gypsum it is time that might be needed to pour an impression.

3 minutes is adequate.


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Setting Time (ST)

The powder is mixed with water, and the time that elapses from the beginning of the mixing until the material hardens is known as the setting time.

This is measured by PENETRATION TEST.The initial setting time is defined as the

time taken for the material to develop sufficient strength such that it will be able to support the lighter needles.

This is around 8 to 16 min (ANSI/ADA Sp.No:25)

The time at which the setting reaction is completed.

Phillips, Science of dental materials 11th edi ,Anusavice; & John.F.McCabe et al : Applied Dental Materials

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Loss Of Gloss For Initial Set

As the reaction proceeds, excess water is taken up in forming the dihydrate so that the mix loses its gloss.

Occurs at approximately 9 minutes.Material has no measurable compressive

strength, therefore the needle could be safely removed.


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Initial Gillmore Test For Initial Set

Smaller needle is used.The mixture is spread

out.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”.

Event is marked by definite increase in strength.

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Gillmore Test for Final Setting Time

It is done with an heavier needle.

½ lb needle and 1lb needle are used to check the initial and final setting time respectively.

The Elapsed time at which this needle leaves only a barely perceptible mark on the surface is called the final setting time.

½ lb Needle 1 lb Needle

Clinical aspects of dental materials – Marcia Gladwin,Michael Bagby

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Vicat test for setting time

The next stage is tested with Vicat Penetrometer.

The needle of 1mm diameter with the weighed plunger rod of 300g is supported and held just in contact with the mix.

Soon after the gloss is lost, the plunger is released.

The time elapsed until the plunger no longer penetrates to the bottom of the mix is known as the setting time

Dental materials properties &manipulation 7thedi ,Craig,Powers,Wataha-

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FACTORS AFFECTING SETTING TIME

I. Spatulation II. W/P ratioIII. Temperature IV. Modifiers : Accelerators & Retarders


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* Craig’s - Restorative Dental Materials - 11th Edi.

2.. Spatulation Time -

Within limits longer and faster the plaster mixed, the more no. of nuclei.

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If more water is used, the fewer the nuclei per unit volume.


3. W / P Ratio -

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3. Temperature –

If the room temperature(20 to 25°c) is raised to body temperature(37.5°C) , setting time decreases.

This is because at this temperature dihdrate becomes more soluble in water.

The ratio of solubilities of hemihydrate & dihydrate at 20°C is 4.5, as the temperature increases, the solubility ratio decreases until 100° C reached and the ratio becomes 1.

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4. Modifiers -

The rate of gypsum setting reaction of gypsum can be altered by adding certain chemicals.

These are called A. AcceleratorB. Retarder The act by changing the solubility of

hemihydrate and dihydrate forms .

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A. ACCELERATORS

Decreases the setting time.Accelerators used are:I. Terra alba 0.5 t0 1.0% - finely ground set

gypsumII. Sodium chloride - upto 2 %III. Sodium sulphate - up to 3 - 4 %IV. Potassium sulphate - upto 2 - 3 %

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B. RETARDERS -

It acts by reducing the solubility of calcium sulfate hemihydrate.

It increases the setting time.Commonly used retarders are:I. Borax (Na2B4O7.10H2O) - 2%

II. Potassium citrate III. Sodium chloride - >20%

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Manipulation

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MIXING OF GYPSUM PRODUCTS

• The strength is the inversely proportional to the W/P ratio, so it is important to keep w / p ratio optimum.

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Measurement & Combination Of Powder And Water

I. Clean starch free rubber / plastic bowl with a top diameter of 130mm is recommended.

II. Stiff spatula with round edged blade of 20-25mm width and 100mm length is used.

III. Required amount of water is added to the moist bowl.

IV. Sprinkle powder over the water slowly over 30 sec. This will allow entrapped air in the powder to escape.

V. Mix allowed to soak for 20min.


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Spatulation

The act of mixing powder and water together is called Spatulation.

Stirred at a rate of 2 revolutions per second over 1 minute.

Either done in hand or mechanically. Spatulation should be done in a single direction

to prevent the breakage of dihydrate crystals While mixing keep the spatula flat against the

wall. This will help in wetting of all the particles and also removes the entrapped air bubbles.


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Fig 1: Powder and water are added to mixing bowl

Fig 2 Low speed is used for preliminary mixing.

Fig 3 High speed is used for energetic, smooth, final-phase mixing.

Fig 4 Final product is homogeneous and bubble-free.

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EQUIPMENTS FOR HAND MIXING

SCALE, RUBBER BOWL, SPATULAGRADUATED SCALE & CYLINDER

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Vacuum Mixer

For mixing investments, plaster, die material and duplicating silicone

100 mbar vacuum output evacuates air bubbles trapped in the mixing material and also prevents development of boiling bubbles

Vacuum mixing bowls are fixed to the agitator coupling by means of a vacuum, which eliminates time-consuming coupling to the mechanical holding device

Transparent vacuum mixing bowls in sizes 250 ml to 1200 ml

An electronic timer accurately controls the mixing time to the second and the manometer integrated into the front panel of the unit always shows the current vacuum in the mixing bowl

Spatula and mixing bowl geometry have been optimized for the investment and duplicating silicone processing

An attractively designed benchtop stand is available for use as bench unit


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PROPERTIES

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Setting Expansion

All gypsum products expand upon settingPlaster : 0.2 to 0.3 %Stone: 0.08 to 0.10 %High strength stone: 0.05 to 0.07%The growing crystals of gypsum cause an outward

thrust, this causes an external expansion and internal porosity in the set mass.

This can be manipulated by :1. Thicker mix & increased spatulation – increased SE2. Thinner mix and decreased spatuation –decreased

SE


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Setting Expansion

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Hygroscopic expansion :

Hygroscopic expansion is the linear expansion that takes place when the investment is placed under water.

Suggested Theories for the mechanism are I. Addition of water during setting increases the

surface film thickness on the inert particles and gypsum, thereby forcing them apart.

II. Water added during setting causes further hydration of calcium sulfate

III. Added water force gypsum to swell.IV. Due to crystal growth.

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REPRODUCTION OF DETAIL:

ADA specification no. 25 requires that: Type I and II reproduce a groove 75mm in width.

Type III, IV and V reproduce a groove 50mm in width.

Gypsum dies do not reproduce surface details very well because the surface of the set gypsum is porous at microscopic level.

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STRENGTH

Wet strength / Green Strength The strength obtained when the water in excess of

that required for hydration of the hemihydrate is left Dry strength

The strength obtained when the water in excess is driven off by drying 2-3 times more than wet strength

46SURFACE HARDNESS AND ABRASION RESISTANCE:

In general, hardness is defined as the resistance to penetration.

For dental purposes, the surface hardness of a material is generally measured in terms of its resistance to indentation.

The surface hardness of unmodified gypsum products is related in a general way to their compressive strength. High compressive strength of the hardened mass corresponds to high surface hardness.

After the final setting occurs, the surface hardness remains practically constant until most excess water is evaporated from the surface, after which its increase is similar to increase in compressive strength.

The surface hardness increases at a faster rate than the compressive strength, because the surface of the hardened mass reaches a dry state earlier than the inner portion of the mass.

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Effect of Hemihydrate on Set Gypsum

X-Ray suggests the presence of various forms of hemihydrate on set gypsum

1. Type 1: 90%2. Type 2: 60%3. Type4 & 5: less than 50%Increased concentration of hemihydrate

results in a weaker set material


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TYPE I : IMPRESSION PLASTERTYPE II : DENTAL PLASTERTYPE III: DENTAL STONETYPE IV : DENTAL STONE,HIGH STRENGTHTYPE V – DENTAL STONE, HIGH STRENGTH, HIGH EXPANSION

Classification of Gypsum Products

ADA specification no 25

Materials In Dentistry – Principles and application 2nd edition Jack L.Ferrancane

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TYPE I – Impression Plaster

It is used as an secondary impression material for completely edentulous patients.

Rigid and mucostatic in natureObtained by adding modifiers to dental plaster.It is hard and brittle and breaks when removed

from undercut regions.Non-toxic but may be unpleasant due to

dryness and heat evolved during settingExcellent at recording fine details - low viscosityDimensionally stable, although extremes of

temperatures should be avoided.

Jack L.Ferrance., Materials in Dentistry.

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COMPOSITION

1. β Hemihydrate2. Accelerator : potassium sulfate, potassium chloride &

potassium nitrate.3. Retarder: sodium carbonate, sodium bicarbonate,

potassium carbonate, borax4. Gums: tragacanth added to improve handling

characteristics. They help in cohesion & facilitate border boulding.

5. Potato starch6. Flavoring agents: used to offset the bland taste.7. Anti-expansion solution: It is added to plaster. Potassium sulfate: 4% Borax: 0.4-1% Alzarin red: 0.04%

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TYPE II - Dental Plaster

It is composed of calcium sulphate β hemihydrate with modifiers.

Laboratory or model plaster.It should flow evenly in all parts of the

impression.It should neither contract nor expand while

setting.After setting it should not wrap or change

shape.Uses - study casts, to mount casts on

articulator, moulds for dentures.

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TYPE III - Dental Stone

Laboratory stone.Chemical composition is alpha hemihydrate.Harder and stronger than type II.Uses – secondary casts for fabrication of

dentures

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Composition

CaSO4. 1/2 H2O - α - hemihydrate Coloring agents - 2 - 3 %Potassium sulphate - acts as

acceleratorBorax - retarder

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Type IV – Dental Stone, high strength

Has high strength of 79 Mpa / 11,500 psiHigh hardess.

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Type V – Dental Stone, High Strength, High Expansion

Higher expansion material which compensates for greater shrinkage that occurs in many newer, high melting alloys for dental castings

Stronger than all other gypsum products.

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Properties

TYPE SETTING TIME

(min )

SETTING EXPANSION

RANGE ( %)

COMPRESSIVE STRENGTH

( MPa )Min. Max.

REPRODUCTION OF DETAIL

( mm )

Type I 2.5-5.0

0.0-0.15 4.0 8.0 75 + 8

Type II +20 % 0.0-0.30 9.0 - 75 + 8

Type III +20 % 0.0-0.20 20.0 - 50 + 8

Type IV +20 % 0.0-0.15 35.0 - 50 + 8

Type V +20 % 0.16-0.30 35.0 - 50 + 8

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Uses

It is used for impression making procedures.In making study casts for orthodontic purposes.For making diagnostic casts for diagnosis and

treatment planning. Master cast / working cast preparationDie preparation.Used for making face masks.Used as interocclusal record material.Used for mounting casts on articulator.Used in flasking proceduresUsed as dental investment material.

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Dental Investment

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Introduction

The technique used for casting of alloys is Lost Wax Technique.

Also called investment casting. DEFINITION: The casting of a metal into a

mold produced by surrounding (investing) an expendable (wax) pattern with a refractory slurry that sets at room temperature, after which the pattern is removed through the use of heat.

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Into (contd)

Dental Casting investment is a material consisting principally of an allotrope of silica and a bonding agent. The bonding substance may be gypsum (for use in low casting temperature) or phosphates and silica (for high casting temperature).

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

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History :

Its history can be traced back around 3000 B.C. This

meticulous procedure of casting was used by various craftsmen to produce jewellary and ornaments.

11th Century Theophilus Described lost wax

technique, which was a common practice prevailed in 11th century.

1558 Benvenuto Cellini claimed to have attempted, use of wax and clay for preparation of castings

1884 Aguilhon de saran used 24K gold to form Inlay

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History (contd):

1887 J. R. Knapp invented Blowpipe. 1897 Phillibrook Described a method of casting

metal filling.1907 Taggart Dr.Taggart discribed this technique

and further devolped on this technology. Deviced a practically useful casting machine.

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Requirements of an Investment

1. Easily manipulated 2. Sufficient strength at room temperature.3. Stability at higher temperatures.4. Sufficient expansion.5. Beneficial casting temperature.6. Porosity enough to let the air and gases

escape.7. Smooth surface.8. Ease of divestment.9. Inexpensive.


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Composition

1. REFRACTORY MATERIAL:Investment material capable of sustaining exposure

to high temperature without significant degradation.

Quartz, tridymite, or cristobalite or a mixture of these.

Function:1. Resist the heat and forces of casting2. To expand and compensate for casting

shrinkage.


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2 . BINDER MATERIAL: It is used to bind the refractory mass that

is not coherent Binders used are alpha-calcium

hemihydrate, phosphate, ethyl silicate.

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3. OTHER CHEMICALS: These are added to achieve desirable properties.1. Reducing agents – Ex : Charcoal2. Sodium chloride, boric acid – to shrinkage,

during dehydration of calcium sulfate dihydrate.3. Potassium sulfate (accelerator)4. Copper powder(reducing agent)/magnesium oxide

.

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Classification

Based on casting temprature :1. Low heat investment: a) Gypsum bonded investment upto (700°C)2. High heat investment : a) Phosphate bonded investment b) Silica bonded investment3. Brazing investment4. Investment for all ceramic restorations

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Classification –Based on composition

•Conventional casting of gold alloy inlays, onlays, crowns, and FPD’s

Gypsum-Bonded

Investment

•Copings or frameworks for metal ceramics & for some base metal alloys

Phosphate-Bonded

Investment

•In casting RPD with base metal alloy

Silicate-Bonded

InvestmentPhillips, Science of dental materials 11th edi ,Anusavice

Gypsum Bonded Investments

These are any casting investment with gypsum as the binder, also containing cristobalite or quartz as a refractory material. Cristobalite and quartz are responsible for the thermal expansion of the mold during wax elimination. Because gypsum is chemically stable at temperatures exceeding 650°C,these investments are typically restricted to casting type I, II, III, and IV gold alloys

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Classification (ANSI/ADA Sp.No.2 [ISO7490])

Type 1 • Thermal

Expansion type.

• For casting inlays and crowns

Type 2• Hygroscopic

expansion type.

• Inlays, onlays, or crowns

Type 3• For casting

partial dentures & complete dentures for gold alloys


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Requirements of Gypsum Bonded Investments

72

Components

I. Binder II.Refractory

component III.Modifiers

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I Binder –

Gypsum (α hemihydrate) – 25% to 35%.All forms shrink after dehydration at around

200°C & 400°C.A slight expansion takes place at 400°C &

700°C Then contraction takes place, this is caused by

decomposition and release of sulfur gases which also contaminates the casting.

Therefore it should not be heated above 700°C Ideal temperature for heating is 650°C

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II Refractory component-

Silica(SiO2) – 65 % to 75% Quartz, Cristobalite, Tridymite and fused silica are

the 4 forms. Quartz, Crystobalite are used in dentistry. Fused quartz & Tridymite has low linear coefficient

of thermal expansion. Fused quartz doesn’t undergo phase

transformations. These at high temperature convert from “low”-

form α-quartz to “high”-form known as β-quartz. β-quartz is stable only above the transition

temperature with decrease in temperature.

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Effect of temperature on silica

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III Modifiers :

2 to 3%Reducing agents – carbon & powdered

copper. It is used to produce non-oxidising

atmosphere in the mold.Boric acid & sodium chloride – Regulate

setting time & setting expansionColoring agents.

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Expansion

Three types1. Setting expansion2. Hygroscopic expansion 3. Thermal expansion

Setting expansion is the linear expansion that takes place during normal setting .


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Setting Expansion

Inclusion of silica results in interlocking of silica in between the crystals, causing outward thrust and thus resulting in expansion.

The growing crystals of gypsum cause an outward thrust, this causes an external expansion and internal porosity in the set mass.

ADA Specification No.2 for Type I investment permits a maximum setting expansion in air - 0.6%.

This can be manipulated by :1. Thicker mix & increased spatulation – increased SE2. Thinner mix and decreased spatulation –decreased

SE

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Setting Expansion

80



It is a continuation of normal setting expansion, because immersion water replaces water of hydration.

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Hygroscopic expansion

Suggested Theories for the mechanism are I. Addition of water during setting increases

the surface film thickness on the inert particles and gypsum, thereby forcing them apart.

II. Water added during setting causes further hydration of calcium sulfate

III. Added water force gypsum to swell.IV. Due to crystal growth.

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Hygroscopic expansion

Water is added immediately to the setting gypsum investment, after the ring is filled.

Usually done by submerging the ring in water bath at 37˚C for 1 hour.

Expansion is 1.2% to 2.2% for Type II.Significant additional setting expansion

results.A wet liner can also be used .

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Setting Expansion (Setting Vs. Hygroscopic)

STAGE I : Initial Mix

STAGE II : Crystals Of Dihydrate Are Formed

STAGE III : Crystals Grow

STAGE IV & V : More marked intermeshing of crystals

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86FACTORS AFFECTING HYGROSCOPIC EXPANSION

EFFECT OF COMPOSITION :Proportional to silica content of the

investment.Finer particle size of silica – ↑ HSEα Hemihydrate - ↑ HSE with silica,

particularly when expansion is unrestricted.EFFECT OF W/P RATIO :Higher W/P ratio - HSE

87FACTORS AFFECTING HYGROSCOPIC EXPANSION

EFFECT OF SPATULATION

Mixing time is reduced – HSE

SHELF LIFE OF GYPSUM

Older investment – HSE

EFFECT OF TIME OF IMMERSION

Before initial setting ↑ HSE After initial setting HSE

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FACTORS AFFECTING HYGROSCOPIC EXPANSION

EFFECT OF CONFINEMENT:The confining effect on hygroscopic expansion is

more pronounced.When placed in 37.7°c (100°F) water it causes

heating, expanding & softening of wax pattern. EFFECT OF ADDED WATER:Magnitude HSE α Amt of H2O added during settingWater is drawn between by capillary action & thus

causes separation of quartz particles - Expansion

89

90

Factors Affecting Expansion

91

Thermal expansion

Occurs when mould is heated to eliminate wax.

Silica refractory material is responsible.Cristobalite changes from α to β form

between 200˚C -270˚C.Expansion is 0.1% to 0.6% at 500˚C for type

II.Quartz changes form at 575˚C.

92

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Effect Of Cristobalite & Quartz On Gypsum Bonded Investments

Crystobalite to quartz ratio is very important for adequate thermal expansion.

When cristobalite is added in lesser amounts, it results in higher shrinkage at 250°C.

But when added from 42-47gm/pct it resulted in adequate expansion of 1.6%, where the required expansion rate is 1.5% - 2.0%.

J.Mater.Sci.Techno,vol24 No.1,2008

94

Factors Affecting Thermal Expansion

EFFECT OF W/P RATIO:Depends on the amount of solids present. W:P ratio Thermal expansion.EFFECT OF CHEMICAL MODIFIERS:Excessive silica - weakening of the material.Sodium, Potassium, or Lithium- Eliminates the contraction of gypsum Thermal expansionBoric Acid Thermal expansion Hardness of set investment

95

THERMAL CONTRACTION

After thermal expansion if the investment is allowed to coal. The cooling path will follow the expansion curve.

The investment contract to less than its original dimension due to shrinkage of gypsum when it is first heated.

The process of heating and cooling cause internal cracks

96

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STRENGTH:

Depends on the amount & type of gypsum binder used.

Adequate strength of investment is needed to prevent fracture/chipping of the mold and to resist contraction of the investment.

After burn-out of the mold, the strength should be no greater than that required to resist the impact of the metal entering the mold.

According to A.D.A. sp 2,the compressive strength of investment should not be less than 2.5 Mpa ( 2 hrs after setting

98

Factors Affecting Strength

1. Increased W:P ratio - compressive strength.

2. Heating of investment to 700 o C – may (or) strength by 65%.

3. Investments with NaCl - strength .4. Repeated heating & cooling cause internal

cracks - strength

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FINENESS:

More fine silica particles , higher hygroscopic expansion rate and less surface irregularities of the casting.

100

POROSITY:

This occurs due to the pressure in the mold.This helps in venting out the gasesTherefore wax pattern should not be covered

more than approximately 6mm of investment.This provides sufficient interconnectivity of

porous network.More hemihydrate less porosity,Uniform particle size, greater porosity.Therefore less hemihydrate with more

amount of gauging water is used.

101

STORAGE

High relative humidity :1. Setting time changed2. Setting expansion and hygroscopic

expansion changed. Different components have different specific

gravity, therefore the separate as they settle under normal vibration.

Therefore should be stored in Airtigth & moisture free containers and should be purchased at small quantities.

102

INVESTMENT FOR CASTING HIGH-MELTING POINT ALLOYS

Most base metal and palladium alloys have a melting temperature of 850 – 1100°C.

The type of investment used for such alloys are:

I. Phosphate-Bonded InvestmentII. Silicate-Bonded Investment In this the binder composition is less than

20% to withstand the high temperatures.

103

PHOSPHATE BONDED INVESTMENT

Any casting investment with a binder formed from magnesium oxide and mono ammonium phosphate. The resulting colloidal multi molecular (NH4MgPO4.6H2O)n coagulates around MgO and fillers. This is combined with silica in the form of quartz, cristobalite or both in a concentration of about 80%. Upon heating the binder undergoes several several changes believed to be responsible for increased strength at high temperature.

104

Types (ANSI/ADA Sp.No.42[ISO 9694])

Type I •Inlay, crown and restoration.

Type II •R. P. D frame work

105

Phosphate Bonded Investment

Most common type of investment for high melting temperatures

It has three different components:A. Binder – Magnesium oxide & phosphate (acidic

monoammonium phosphate)

B. Refractory Material – 80% Silica (Quartz/Crystobalite). 33% dilution of Colloidal silica is being used, it

gives greater expansion

106

Setting reaction

Acid- base reaction.Monoammonium phosphate reacts with basic

magnesia . Soluble phosphate reacts with water, forming

a binding media with filler particles embedded in it.

107

NH4H2PO4 + MgO +5H2O [room temperature]

↓

(NH4 Mg PO4 6 H2O)n

MgO [colloid –like particles ] NH4H2PO4

H2O [dehydration at up to 50C

↓ (NH4 Mg PO4 6 H2O)n dehydration at 160C(NH4 Mg PO4 H2O)n [300C-650C] ↓ [non-crystalline polymer] (Mg2P2O7)n

↓ crystalline Mg2P2O7 [+ MgO + quartz + cristobalite] overheating above 1040C↓

Mg3 (P2O4)2

108

SETTING & THERMAL EXPANSION

Thermal expansion when with water have a shrinkage at 200-4000c, due to decomposition of binder magnesium ammonium phosphate with evolution of ammonia

This can be eliminated by adding colloidal silica esp cristobalite

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110

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WORKING TIME & SETTING TIME

working time – 2 min.setting time – 1 hour Factors affecting and setting time temperature fast set : (as the setting

reaction itself is exothermic) mixing time fast set (generally mechanical

mixing under vacuum is preferred L : p ratio working time.

112

Requirements of phosphate bonded investments

113

The influence of handling technique on the strength of phosphate bonded investments

Mechanical spatulation under vacuum and pressure suggested by Jhonson resulted in the least amount of nodules.

Recommended spatulations per second : 45 – 60Total handling time :1. HA,HP,MVA,MVP,MA -180 -195 seconds2. HV – 225 seconds Powder / liquid ratio (g/ml): 100/12 – 100/16 Chilled liquid would increase the setting time.

A.S.Juszczyk,Dental Materials J 16;2000:26-32

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Surfactant containing phosphate bonded investment

Addition of surfactants to phosphate bonded investment can increase the hygroscopic setting expansion .

The surfactant also makes the unset investment more viscous and reduces the compressive strength

Mueller, et al 1986,JPD-54: 367: 1985.

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RECENT ADVANCES

Titanium casting with phosphate bonded investment with zirconia:

1. Thickness of the reaction layer of titanium casting was reduced, resulting in a smooth casting without cracks and the investment was easily removed from the casting but the micro hardness was reduced.

X.P.Lou,T.W.Guo et al;Dent Mater J:18,2002:512-515

SILICATE BONDED INVESTMENT

Definition :A casting investment with ethyl silicate or a

silica gel as the binder, the later reverting tosilica upon heating. This is combined with cristobalite or quartz as the refractory material. These investments exihibit considerable thermal expansion and can be used when casting higher fusing chromium alloys.

GPT-8

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SILICATE BONDED INVESTMENT

It is rarely used because its time consuming and complicated.

The silica refractory material, which is bonded by the hydrolysis of ethyl silicate in the presence of hydrochloric acid.

The product of the hydrolysis is a colloidal solution of silicic acid and ethyl alcohol

Si(OC2H5 ) 4 + 4H2 0 _ hcl Si(OH)4 + 4C2H5OH

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Composition

Binder silica acid gel Refractory silica (cristobalite) Additive magnesium oxide - Make it

alkaline, Strengthen then the gel

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This investment is usually supplied with two bottles of special liquid.

In one of the bottles the manufacturer usually supplies a properly diluted water-soluble silicate solution.

The other bottle usually contains a properly diluted acid solution, such as a solution of hydrochloric acid.

Before use, mix an equal volume from each bottle and allow the mixed liquids to stand for a prescribed time, according to the manufacturer's instructions, so hydrolysis can take place and freshly prepared silicic acid forms.

120

Setting Reaction

One such investment consists of a silica refractory, which is bonded by the hydrolysis of ethyl silicate in the presence of hydrochloric acid.

The product of the hydrolysis is a colloidal solution of silicic acid and ethyl alcohol

HCl

Si(OC2H5 ) 4 + 4H2 0 Si(OH)4 + 4C2H5OH

SiO2.2H2O is formed in practice.Has higher silica content and has better refractory properties.

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Green shrinkage

The Soft Gel is dried at a temperature below 168°C.

During this process, Gel loses alcohol & water to form a concentrated, Hard Gel.

Volumetric contraction accompanies the drying, reducing the size of the mold.

This contraction is called GREEN SHRINKAGE.

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DISADVANTAGES :Gives flammable content during processingExpensive.ANSI/ADA SP.NO:91 FOR THE FOLLOWING:Recommended setting time- 30% from wat

stated by manufacturerRecommended compressive strength- 1.5

MpaRecommended linear expansion- 15% from

wat stated by manufacturer

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Manipulation :

Powder + hydrolyzed ethyl silicate liquid mixed quickly and vibrated into a mold allow heavier particles to settle quickly while the excess liquid and some of the fine particles rise to top.

Within 1½ hrs the accelerator (NH4cl) hardens the settled part. The excess on top is poured off and there by decreasing the L:P ratio at the bottom.

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Divestment :

Combination of die stone with investment material in a particular composition. The die is made with divestment and colloidal silica liquid mix, Then the entire assembly with the wax pattern is invested in a mixture of divestment and water, and casting is completed.

Properties Setting expansion – 0.9% Thermal expansion 0.6% (at 9770c)Divesting : process of removing an investment

from a casting or hot pressed ceramic.

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BRAZING INVESTMENT :

ADA Sp No 93 Type I - Gypsum bonded dental brazing

investment Type II - Phosphate bonded. Steps 1) Broken parts are Stabilize by sticky wax

2) The broken parts are then embedded in investment with portion to be solder is left exposed and free of investment.

They should have low setting and thermal expansion. Particle size is usually not fine. They possess usually a compressive strength of 2-10 Mpa.

Dental Materials: Properties And Manipulation 8th Ed: Robert G.Craig et al

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REQUIREMENTS

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INVESTING

Wax pattern cleaned of debris, greases & oils The ring is lined with a ceramic liner, this aids in expansion. This thin film of cleanser left on wax pattern reduces surface

tension of the wax & permits better wetting of the investment.

Appropriate amount of distilled water / colloidal silica is dispensed.

Powder is then dispensed gradually to prevent entrapment of air bubbles & All the powder particles should be wet thoroughly to prevent pushing out of the unmixed particles to the surface.

The lid is applied to the bowl, and the vacuum is attached to remove air.

The entire apparatus is applied to the power mixer. Once mixed the investment is poured slowly into the casting

ring in a vibrator, until the casting ring is full.Dental Materials: Properties And Manipulation 8th Ed: Robert G.Craig et al

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VACUUM MIXING

Mechanical mixing prevents entrapment of air bubbles reducing porosity & evacuates potential harmful gases released during mixing.

After mixing pattern is either hand invested / vacuum invested.

In hand mixing the pattern is painted with the material and then vibrated slowly into the ring.

In vacuum mixing, the investment is directly invested from the mixing equipment.

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Wax Heating & Elimination:

In Gypsum bonded investment – temperature is critical, because it causes release of carbon which gets incorporated in the pores.

In Phosphate bonded investment – at high temperature carbon is converted into carbon di oxide and evaporated.

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EXPANSION OF THE INVESTMENT

Once set the investment is allowed to set for at least 45 min.

Investment should expand sufficiently to compensate for the casting shrinkage.

All metals shrink when they cool from the solid state from 1% to 2.5%.

Three modes of expansion include:1. Setting Expansion2. Hygroscopic Expansion3. Thermal Expansion Total expansion will be equal to the sum of all the

three expansions.

Dental Materials: Properties And Manipulation 8th Ed: Robert G.Craig et al

131

CASTING TECHNIQUES

Two techniques :1. Hygroscopic Low-Heat Technique2. High-Heat Thermal Expansion Technique

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Vacuum Mixer

For mixing investments, plaster, die material and duplicating silicone

100 mbar vacuum output evacuates air bubbles trapped in the mixing material and also prevents development of boiling bubbles

Vacuum mixing bowls are fixed to the agitator coupling by means of a vacuum, which eliminates time-consuming coupling to the mechanical holding device

Transparent vacuum mixing bowls in sizes 250 ml to 1200 ml

An electronic timer accurately controls the mixing time to the second and the manometer integrated into the front panel of the unit always shows the current vacuum in the mixing bowl

Spatula and mixing bowl geometry have been optimized for the investment and duplicating silicone processing

An attractively designed bench top stand is available for use as bench unitDental materials properties &manipulation 7thedi ,Craig,Powers,Wataha

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References

1. Phillips - Science of dental materials - 11th edition - Anusavice

2. Restorative dental materials - 11th edition - Robert G Craig & John M. Powers3. Skinner’s Science of dental materials - 9th edition4. Dental materials & their selection - 3rd edition - William J O’Brien5. Clinical aspects of dental materials - 2nd edition - Marcia Gladwin, Michael bagby6. Introduction to dental materials – Richard Van Noort.7. Materials in Dentistry principles and application- 2nd edition Jack.L.Ferrance

8. J Prosthet Dent- 2008;99:282–286

9. J Prosthet Dent-2000,84,32-37

10 J Prosthet Dent-1998:26:59

11. J Prosthet Dent-1999:81:129-134

Gypsum & Investment material- Dr.Vaishnavi Ganesh

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