Classification Of Investment Materials

Explain the setting expansions of investment materials. Describe them in details.
Answer:

Setting Expansion of Investment Materials:

Normal Setting Expansion:

A mixture of silica and dental stone results in setting expansion which is more than when the gypsum product is used alone.
The silica particles probably interfere with the intermeshing of the crystals as they form. Thus, the thrust of the crystals is outward during growth.
ADA specification No. 2 for type I investments permits maximum setting expansion in air of 0.5%

Hygroscopic Setting Expansion:

When gypsum products are allowed to set in contact with water, the amount of expansion exhibit is much greater than the normal setting expansion.
The increased amount of expansion is because the water helps the outward growth of crystals.
This expansion is known as hygroscopic setting expansion.
Investment should be immersed in water before initial set is complete.
ADA specifications no. 2 for type II investments require minimum of 1.2% and maximum of 2.2% expansion.

Thermal Expansion:

In case of gypsum investment thermal expansion is achieved by placing the mould in furnace at a temperature below than 700°C, if temperature is more than 700°C, the investment is breakdown and gases are released which contaminate 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:

Gypsum contracts between 200 to 400°C and a slight expansion takes place between 400°C to 700°C and a large contraction occur after this.
If considerable amount of allotropes of silica 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 “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 crystabolite to high or beta crystabolite form at temperature 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.

Reader Interactions