dental casting investment material

Dental CastingInvestment Materials

Dr. Deepak K. Gupta

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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


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 immersing the invested ring in a warm water bath

Type III casting gold alloys Thermal


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 dioxide causing 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 this

further 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 extracoronal castings

• 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 an

acid 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 temperature

below 168 °C– loses alcohol and water to form a concentrated, hard

gel– Green shrinkage, which is additive to the setting

shrinkage– 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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