dental casting investment material

Dental CastingInvestment Materials

Dr. Deepak K. Gupta

Introduction

• A material which is suitable for forming a mould into which molten metal or alloy is cast.

• These materials can withstand high temperatures.

• Also known as refractory materials.• In general, an investment is a mixture of three

distinct types of materials– Refractory material, – Binder material– Other chemicals

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Components of Investment

• Refractory Material– a form of silicon dioxide, such as quartz, tridymite, or

cristobalite, or a mixture of these– They serve two functions:

• Acts as a material that can withstand high temperatures.• Regulates the thermal expansion.

• Binder Material– refractory materials alone do not form a coherent solid mass, – Some kind of binder is needed. Ex: Gypsum, phosphate,

ethyl silicate

• Other Chemicals– sodium chloride, boric acid, potassium sulfate, graphite,

copper powder, or magnesium oxide,– Small quantities - modify various physical properties.


Properties Required of an Investment

• Easily manipulated• Sufficient strength at room temperature• Stability at higher temperatures• Sufficient expansion: compensate for shrinkage

of the wax pattern and metal.• Porosity: porous enough to permit the air or

other gases in the mold cavity to escape.• Smooth surface: Fine detail and margins on the

casting.• Ease of divestment• Inexpensive.


Types of Investment

• Based on the nature of Binder, its classified on 3 types– Gypsum bonded investments: used for casting gold

alloys, withstand temperature up to 700°C.

– Phosphate bonded investments: For metal ceramic and cobalt-chromium alloys, withstand higher temperatures.

– Ethyl silica bonded investments• alternative to the phosphate bonded investments, for

high temperature casting.

• Principally used in the casting of base metal alloy partial dentures


Gypsum Bonded Investments

• ADA Sp. No. 2 for gold alloy casting investments again classify it 3 types.

• Constituents– 65% to 75%: quartz or cristobalite, or a blend of the two,

– 25% to 45%: α-calcium sulfate hemihydrate

– 2% to 3% chemical modifiers

Investment Use compensation for alloy casting shrinkage

Type I casting of inlays or crowns

Thermal expansionof the investment

Type II casting inlays, onlays, or crowns

hygroscopic expansion achieved by immersingthe invested ring in a warm water bath

Type III casting gold alloys Thermalfacebook.com/notesdental

Gypsum: α-calcium sulfate hemihydrate

• Casting gold-containing alloys with melting ranges below 1000 °C

• Material is heated at temperatures sufficiently high – 200-400 °C: shrink considerably

– 400 °C - 700 °C : slight expansion takes place between

– Above : decomposition and the release of sulfur dioxidecausing contamination of alloy


Silica• It exists in 4 allotropic forms: quartz, tridymite,

cristobalite and fused quartz.– Quartz and cristobalite are of dental importance

• Investment should expand thermally -compensate partially or totally for the casting shrinkage of the solidifying alloy.

• Gypsum - shrinks considerably when it is heated,– so it should be silica which should expand to cause

overall expansion of investment


Silica

• When heated allotropes of silica changes from α(low room temperature form) to β (high temperature form)

– Phase transformation is called an inversion.

– Causes linear expansion – overall causing volume expansion


MODIFIERS• Certain modifying agents

– Coloring matter, – Reducing agents, such as

carbon and powderedcopper,

• Alkali-earth and transition-metal chlorides, boric acid, and sodium chloride– regulate the setting

expansion and the setting time


Properties

• Setting time: set initially in 9 to 18 mins– It can be manipulated in the

same way as of gypsum product

• NORMAL SETTING EXPANSION– silica particles probably

interfere with the intermeshing and interlocking of the crystals

– thrust of the crystals is outward during growth,

– Soft wax and thin pattern may get distorted,


Properties: HYGROSCOPIC SETTING EXPANSION

• Expansion when contact with heated water.

• Greater in magnitude than normal setting expansion

• Type II investments : 1.2% - 2.2%

• Directly proportional– Silica content of the

investment– Water mixed during

setting reaction

• Indirectly proportional : size of silica particles

A, Normal setting expansionB, Hygroscopic setting expansion


Properties: THERMAL EXPANSION

• Thermal expansion of silica must be increased to counterbalance the contraction of the gypsum

• Contraction of the gypsum is entirely balanced when the quartz content is increased to 75%

• Type I: 1% - 1.6%, Type II: - 0% and 0.6% at 500 °C• It depends on

– Particle size of the quartz, – Type of gypsum binder: α or β– Resultant W/P ratio – Allotropes of quartz : Cristobalite > quartz


Properties

• THERMAL CONTRACTION– Inversion of the β form to its stable α form at

room temperature.

– Contracts to less than its original dimension

• EFFECT OF CHEMICAL MODIFIERS– Increasing the silica content – reduces the

strength of investment

– Small amounts of sodium, potassium, lithium chlorides or boric acid – eliminates the need for adding silica to cause expansion


Properties

• STRENGTH

– Adequate to prevent cracking, bulk fracture, or chipping of the mold

– affected by the W/P ratio same way as gypsum.

– It decreases after heating to 700 0C – microcracks

• FINENESS

– Fine silica results in a higher hygroscopic expansion

– Also it will give finer details accurately and minimal surface roughness.


PHOSPHATE-BONDED INVESTMENT

• Increased use of metal-ceramic, hot-pressed ceramic prostheses and base metal alloy –requires investment which can bear higher temperature

• Phosphate-bonded or silicate-bonded investments can be used in such condition

• But they are difficult to disinvest – however these problem has been sorted out recently.


COMPOSITION• Refractory filler: silica (80% by weight) in the form of

cristobalite, quartz, or a mixture of the two forms.– particle size varies from a submicron level to that of a fine sand

• Binder: magnesium oxide (basic) and a phosphate (acid)– Originally phosphoric acid was used, but monoammonium

phosphate has replaced

• Other: Carbon– Produce clean castings and facilitate the divesting of the casting

from the investment mold– Appropriate when the casting alloy is gold– Others: palladium reacts with carbon at temperatures above

1504 °C to make it brittle

• Colloidal silica suspensions in place of water– Greater expansion: newer gold-containing alloys and other

alloys - higher melting temperature ranges than traditional gold alloys,


SETTING REACTION• Ammonium diacid phosphate reacts with

magnesium oxide - green strength, or room temperature stregth.

• ammonium diacid phosphate is used in a greater amount.

• additional amount can react with silica at an elevated temperature– P2O5 and SiO2 forms silicophosphate

– increases the strength of investment


SETTING AND THERMAL EXPANSION

• slight expansion occurs compared to gypsum bonded

• increased considerably by using a colloidal silica solution in place of water

• early thermal shrinkage -decomposition of the binder, accompanied by evolution ofammonia.

• Gypsum investments the shrinkage is caused by the transformation of calcium sulfate from the hexagonal to the rhombic form.


WORKING AND SETTING TIME

• markedly affected by temperature– warmer the mix, the faster it sets

– setting reaction itself gives off heat, and thisfurther accelerates the rate of setting

• Increased mixing time and mixing efficiency, – result in a faster set

– Better smoothness and accuracy of the casting.

– Mechanical mixing under vacuum is preferred

• L/P ratio– increase in the L/P ratio increases the working time


SURFACE QUALITY OF CAST METALS

• In the past, detail reproduction and surface smoothness were inferior to gypsum bonded

• increasing the ratio of special liquid to water used for the mix – gave a better details

• lead to oversized extracoronalcastings

• Improvement in the technique and also inthe investment composition -few surface imperfections


ETHYL SILICATE–BONDED INVESTMENT

• Its use has declined - more complex and time-consuming procedures

• But still used in the construction of high-fusing base metal partial denture alloys.

• Here the binder is silica gel.

• Its made either by 2 method from

– Sodium Silicate

– Ethyl silicate


Types• Sodium silicate

– pH lowered by addition of an acid or anacid salt,

– a bonding silicic acid gel forms.

– Accelerator: ammonium chloride

• Ethyl silicate• It is hydrolyzed in the presence of hydrochloric acid, ethyl

alcohol, and water.

• Sol is then mixed with quartz or cristobalite,

• Finely powdered magnesium oxide is added - keep the mixture alkaline.

• A coherent gel of polysilicic acid then forms, accompanied by a setting shrinkage


Ethyl silicate

• When this soft gel is dried at a temperaturebelow 168 °C– loses alcohol and water to form a concentrated, hard

gel

– Green shrinkage, which is additive to the settingshrinkage

– As well it takes longer time to hydrolyze and gelation –amines can be added to faster the reaction

• Thus in this type of investment, mold enlargement should compensate not only casting shrinkage, but green shrinkage and setting shrinkage


Manipulation

• The powder is added to the hydrolyzed ethyl silicate liquid, mixed quickly, and vibrated into a mold that has an extra collar to increase the height

• The mold is placed on the platform of a special type of vibrator that provides a so-called tamping action.

• This allows the heavier particles to settle quickly while the excess liquid and some of the fine particles rise to the top.

• In about 30 minutes, the accelerator in the powder hardens the settled part and the excess at the top is poured off.

• Thus, the L/P ratio in the settled part is greatly reduced and the setting shrinkage is reduced to 0.1%.

• The remaining cast is somewhat fragile because the amount of binder is quite low and it is essentially composed of silica.


Various Investment Material


References

• Phillips' Science of Dental Materials- Phillip Anusavice_12th

• Basic Dental Materials -2nd.ed Mannapalli

• Clinical Aspects of Dental Materials Theory, Practice, and Cases, 4th Edition

• Craig's Restorative Dental Material 13th edition


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dental casting investment material

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