DENTAL CASTING INVESTMENT MATERIAL

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

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COMPONENTS OF INVESTMENT

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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

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TYPES OF INVESTMENT

• Based on the nature of Binder
• • Gypsum bonded investments
  • Phosphate bonded investments
  • Ethyl silica bonded investments

GYPSUM BONDED INVESTMENTS

ISO (1990) lists 3 types Gypsum Bonded of Investments:

• Type I – for inlays and crowns utilizes thermal expansion of the mould.
• Type II – for inlays and crowns utilizes hygroscopic setting expansion.
• Type III – for gold partial denture utilizes thermal expansion.

COMPOSITION

1. Binder : 25% to 35% of α-calcium sulfate hemihydrate is used

-To react with water and on hydration binds the silica together.

-To impart sufficient strength to the mould and

-To contribute to the mould expansion by its setting expansion.

• • 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

2. Refractory materials:65% to 75% quartz or cristobalite, or a blend of the two(form of silica)

- It withstands high temperatures and

- Provides mould expansion by thermal expansion

• • 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

3. A reducing agent: such as powdered charcoal, reduces any oxide formed on the metal. If present should not be heated more than 650ºC

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4. Modifying chemicals: 2% to 3%: eg. boric acid or sodium chloride to shrinkage on heating.

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PROPERTIES

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SETTING TIME OF GYPSUM BONDED INVESTMENT

• ADA specification no.2 recommends minimum setting time of 5 mins. and a maximum of 25 minutes.
• The modern investments usually have a initial setting time between 9 to 18 minutes.

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NORMAL SETTING EXPANSION

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– 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,

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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

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

THERMAL CONTRACTION

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– Inversion of the β form to its stable α form at room temperature.
– Contracts to less than its original dimension

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EFFECT OF CHEMICAL MODIFIERS

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– 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

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STRENGTH

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– 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

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 FINENESS

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– Fine silica results in a higher hygroscopic expansion
– Also it will give finer details accurately and minimal surface roughness.

MCQs

Gypsum in the investment acts as;

A, Filler

B. Binder

C. Refractory

D. Reactor

Answer B