Casting Investments and Casting Procedures

Casting Investments And Casting Procedures

Following are the different types of expansions to compensate for the casting shrinkage:

Normal Setting Expansion:

• As the water gets mixed with investment powder, water surrounds the calcium sulfate crystals and converts 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 no. 2 for type I investment permits a maximum setting expansion in air of 0.5%. Modern investments show a setting expansion of 0.4%.

Hygroscopic Setting Expansion or Low Heat Technique:

• 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 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.

Thermal Expansion or High Heat Technique:

For Gypsum-bonded Investment

• In the case of gypsum investment thermal expansion is achieved by placing the mold in the furnace at a temp 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.

For Phosphate-bonded Investment:

• The expansion of phosphate-bonded investments depends on the thermal expansion of the material. At higher temperatures, melting alloys have high contraction at the time of solidification which necessitates greater expansion of investment.
• This is done by thermal expansion of investment and inversion of silica. Replacement of liquid components from water to colloidal silica increases hygroscopic expansion.
• So the expansion of investment varies as per the ratio of colloidal silica to water. As the ratio of colloidal silica to water increases, the expansion increases. So when the ratio of colloidal silica to water is 1:3 expansion of phosphate investment material is 1.3% to 1.5%; the ratio of 1:1 leads to an expansion of 1.5 to 1.7% and a ratio of 3:1 leads to an expansion of 1.7 to 1.9%