Casting Shrinkage Of Certain Dental Alloys

Casting Shrinkage Of Certain Dental Alloys

Compensation of Casting Shrinkage:

For Gypsum-bonded Investments

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

1. Normal Setting Expansion:

As the water gets mixed with investment powder, water surrounds the calcium sulfate crystals and convert them into calcium α–dihydrate. Setting expansion enlarges the mold partially for casting shrinkage of gold alloys. The effectiveness of setting expansion depends on the thermal expansion of the wax pattern.

Expansion in the wax pattern is due to exothermic heat which is released from the investment while setting. 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 permits maximum setting expansion in air of 0.5%. Modern investments show a 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 the 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.

The water here is provided to gypsum crystals from the 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 N o. 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 than 700°C (If the temperature is more than 700°C, the investment is
a 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 is present in the investment.
• This contraction can be eliminated and changed in expansion.
• Because when heated, quart or crystabolite changes its crystalline form this occurs at a transition temperature.
• Quart when heated, inverts from “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 crystabolite 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.

For Phosphate-bonded Investments:

The expansion of mold is desirable to compensate for casting shrinkage. Expansion of phosphate investment occurs from three sources, i.e.

• Wax pattern expansion: Heat during setting allows significant impression of the wax pattern.
• Setting expansion: It is 0.7 to 1%.
• Thermal expansion: It is 1 to 1.5%.