Investments for High Melting Alloys in Dentistry

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Questions and Answers

What is the primary use of phosphate-bonded investment?

For alloys with very high melting temperatures (up to 1400o C)

For casting of high-melting temperature alloys used for porcelain fused to metal restorations (correct)

For crowns and bridges with low melting temperatures

For removable dentures frameworks

What is the composition of the powder in phosphate-bonded investment?

Ammonium dihydrogen phosphate, magnesium oxide, and colloidal silica (correct)

Colloidal silica, water, and ammonium dihydrogen phosphate

Magnesium oxide, colloidal silica, and carbon

Ammonium dihydrogen phosphate, magnesium oxide, and sodium chloride

What is the primary reaction that occurs in phosphate-bonded investment at room temperature?

A polymerization reaction occurs, producing a polymer

An acid-base reaction occurs, producing magnesium ammonium phosphate matrix and water (correct)

Dehydration occurs, producing a crystalline polymer

A exothermic reaction occurs, producing heat

What is the maximum temperature at which phosphate-bonded investment can be used?

1200o C Signup and view all the answers

What is the primary advantage of phosphate-bonded investment?

High strength at room temperature Signup and view all the answers

What is a disadvantage of phosphate-bonded investment?

Difficult divesting Signup and view all the answers

What is the primary use of silicate-bonded investment?

For alloys with very high melting temperatures (up to 1400o C) Signup and view all the answers

What is the primary difference between phosphate-bonded and silicate-bonded investment?

Temperature range of use Signup and view all the answers

What is the purpose of using a vibrator when filling the ring with investment material?

To aid the flow of the investment material Signup and view all the answers

What is the main difference between low heat technique and high heat technique in investing?

The type of expansion achieved Signup and view all the answers

What is the purpose of burnout in investing?

To remove wax pattern and sprue Signup and view all the answers

What is the minimum time recommended for the investment material to set after filling the ring?

45 minutes Signup and view all the answers

What is the purpose of mechanical mixer in investing?

To mix the investment material Signup and view all the answers

What is the temperature range used for burnout in hygroscopic technique?

450-475oC Signup and view all the answers

What is the purpose of vacuum mixing in investing?

To give a smoother mix with less air bubbles Signup and view all the answers

What is the purpose of hand mixing in investing?

To mix the investment material Signup and view all the answers

What is the main purpose of washing the wax pattern with a surface tension reducing agent?

To reduce the surface tension of the investment material Signup and view all the answers

What is the reaction that occurs when equal volumes of the two liquids are mixed in silicate-bonded investment?

Hydrolysis Signup and view all the answers

What is the advantage of silicate-bonded investment in terms of alloy casting?

Ability to cast high melting alloys Signup and view all the answers

What is the composition of the powder in silicate-bonded investment?

Silica, glass, and MgO Signup and view all the answers

What is the purpose of the glass fiber liner in the metallic ring?

To prevent the investment from sticking to the ring Signup and view all the answers

What is the disadvantage of silicate-bonded investment in terms of its strength?

Low strength fired molds Signup and view all the answers

What is the third stage of the reaction in silicate-bonded investment?

Drying Signup and view all the answers

What is the purpose of selecting the type of investment in the investing procedure?

To choose the correct investment material for the specific application Signup and view all the answers

Study Notes

Phosphate-bonded Investment

Used for alloys with high melting temperatures up to 1200°C
Used for casting of high-melting temperature alloys used for porcelain fused to metal (ceramo-metal) restorations
Composition:
- Powder: Ammonium dihydrogen phosphate, magnesium oxide (binding system), and colloidal silica (refractory material)
- Liquid: Colloidal silica of SiO2 in water or water
- Modifiers: Carbon as reducing agent
Reaction:
- At room temperature: acid-base reaction between ammonium dihydrogen phosphate and magnesium oxide, producing magnesium ammonium phosphate matrix and water
- At higher temperature: dehydration occurs, and a crystalline polymer is formed
Types:
- Type I: For inlays, crowns, and bridges
- Type II: For removable dentures frameworks
Advantages:
- High strength at room temperature
- High strength at high temperature
- High setting and thermal expansion
Disadvantages:
- Show breakdown above 1400°C
- Difficult divesting
- More dense and less porous

Silicate-bonded Investment

Used for alloys with very high melting temperatures (up to 1400°C)
Mainly for casting alloys for partial dentures bases
Composition:
- Powder: Silica, glass, and MgO
- Liquid: Usually supplied as two liquids (ethyl silicate and hydrochloric acid)
Reaction:
- Hydrolysis: ethyl silicate hydrolyzed to silicic acid
- Gelation: silicic acid and alcohol react with silica and MgO powder leading to gel formation
- Drying: loss of alcohol and water, and shrinkage of the gel
Advantages:
- Ability to cast high melting alloys
- High thermal expansion
- Can produce castings with fine details
- Much easier in divesting
Disadvantages:
- Gives flammable component during heating (for ethyl silicate binder)
- Low strength fired molds
- High density and less porous (so need a vent)
- More complicated technique

Investing Procedures

Prepare the casting ring (metallic or rubber) and the sprue base
Wash the pattern with a surface tension reducing agent (surfactant)
Select the type of investment
Mix the investment (hand mixing or vacuum mixing)
Fill the ring with the investment material
Set the investment (at least 45 minutes)
Burnout: place the casting ring in the oven to burn out the wax pattern and sprue (wax or plastic)
The temperature of the burnout depends on the type of expansion

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Description

Learn about phosphate-bonded investments used for casting high-melting temperature alloys in porcelain fused to metal restorations in dentistry. Understand the composition and uses of these investments.