Dental Investments and Refractory Materials

Dental Investments And Refractory Materials

Casting shrinkage:  At the time of casting, the molten metal is forced in an investment mold to occupy the space occupied by the wax pattern before. As the molten metal cools at room temperature, it shrinks dimensionally this is known as casting shrinkage or solidification shrinkage.

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Shrinkage Occurs in Three Stages:

• Thermal contraction of the liquid material.
• Contraction of metal while changing from liquid to solid mass.
• Thermal contraction of solid metal as it reaches room temperature.

Compensation of Casting Shrinkage:

Shrinkage affects the fi of the restoration, therefore it must be controlled and compensated by the expansion of investment material. During heating, the investment expands thermally and this expansion compensates for the casting shrinkage.

Expansions of Investment by Gypsum Bonded Investment Materials

Gypsum-bonded investment materials are used for the conventional casting of inlays, onlays, crowns, and fixed partial dentures made of gold alloys. Expansion of casting shrinkage by gypsum bonded investment is of 3 types:

1. Normal settng expansion.
2. Hygroscopic setting expansion.
3. Thermal expansion.

1. Normal Setting Expansion:

• As water gets mixed with investment powder, water surrounds the calcium sulfate crystals and converts them into calciumα–dihydrate. Setting expansion enlarges the mold partially for casting shrinkage of gold alloys. The effectiveness of setting expansion depends on the thermal expansion of wax pattern.
• Expansion in the wax pattern is due to exothermic heat which is released from the investment while settling. As the soft is the wax and thin is the wax pattern, more is the expansion since the investment can move soft and thin wax more easily. ADA specification N o. 2 for type I investment permit maximum setting expansion in air of 0.5%. Modern investments show setting expansion of 0.4%.

2. Hygroscopic Setting Expansion

• Hygroscopic setting expansion is a continuation of normal setting expansion when it is allowed to set in the presence of water. Water has to be incorporated before the setting of investment takes place. This water replaces the water of hydration and prevents the confinement of crystal growth. So the crystals continue to grow outward in the presence of water which causes more expansion.
• Water here is provided to gypsum crystals from a wet liner which is placed over the inner surface of the casting ring or from the exposed surface of the investment which is immersed in water. Hygroscopic setting expansion is 5 to 6 times that of normal setting expansion. ADA specification no. 2 for type II investments requires a minimum of 1.2% and a maximum of 2.2% expansion.

3. Thermal Expansion: In the case of gypsum investment thermal expansion is achieved by placing the mold in the furnace at a temperature below 700°C (If the temperature is more than 700°C, the investment is breakdown and gases are released which contaminate the gold alloy).

• The thermal expansion of gypsum-bonded investment is directly related to the amount and type of silica.
• A considerable amount of quart or allotropic form of silica is necessary to counterbalance the contraction in the gypsum during heating.
• When investment is heated:
  • The gypsum contracts between 200 to 400°C and a slight expansion takes place between 400°C to 700°C and a large contraction occurs after this.
  • If a considerable amount of allotropes of silica are present in the investment this contraction can be eliminated and change in expansion. Because when heated, quart or crystabolite changes its crystalline form this occurs at a transition temperature.
• Quart when heated, inverts from a “low” form known as alpha quart to a “high” form known as beta quartz at a temperature of 370°C.
• Crystabolite similarly when heated inverts from ‘low’ or alpha crystalline to high or beta crystalline form at temperatures between 200°C and 270°C.
• The density changes (decreases) as alpha form changes to beta form with a resulting increase in volume and a rapid increase in linear expansion.

The permissible minimum and maximum thermal expansion for gypsum-bonded investments are 0 to 0.5% at 500°C and 1 to 1.6% at 700°C temperatures.

Reservoir

A reservoir is that part of a spread network that solidifies last and hence can act as a source of supply of molten material to the mold cavity. It helps to compensate for solidification shrinkage and prevent localized shrinkage porosity.

 Ring Liner

A material that allows uniform expansion of the investment which compensates for casting shrinkage is called as ring liner.

 

Casting Investments And Casting Procedures

An investment material is defined as a ceramic material that is suitable for forming a mold into which a metal or alloy is cast.

Ideal Requirements of an Investment Material:

• The investment mold must expand to compensate for the alloy shrinkage that occurs during the cooling of the molten alloy.
• The powder should have a fine particle size to give a smooth surface to the casting.
• The manipulation should be easy. It should have a suitable setting time.
• The material should have a smooth consistency when mixed.
• The set material should be porous enough to permit air in the mold cavity to escape easily during casting.
• At higher temperatures, the investment must not decompose to give of gases that may corrode the surface of the alloy.
• It must have adequate strength at room temperature to permit handling, and enough strength at higher temperatures to withstand the impact force of the molten metal.
• Casting temperature should not be critical.
• After casting, it should break readily from the surface of the metal and should not react chemically with it.
• The material should be economical