Gypsum-Bonded Investments: Composition, Properties, and Uses in Dental Casting

Gypsum Bonded Investments

Gypsum-bonded investments are used for casting gold alloys. They can withstand temperatures up to 700°C.

Classification of gypsum-bonded investment:

As per ADA specification No. 2

• Type I- For casting inlays or crowns
• Type II- For casting partial and complete denture bases.

Composition of gypsum-bonded investment:

The function of Each Constituent:

• Alpha hemihydrate:
  • It binds and holds the silica particles together.
  • Permits pouring of the mix into the mold.
  • It imparts strength to the mold.
  • Contributes to mold expansion (by setting expansion).
•  Silica: Quart or cristobalite
  • Acts as a refractory during heating.
  • Regulates thermal expansion.
  • Increases setting expansion of stone.
  • Silica in the investment eliminates the contraction of gypsum and changes it to an expansion during heating.
• Modifiers:
  • Coloring matter
  • Reducing agents: They reduce any oxides formed on the metal by providing a non-oxidizing atmosphere in the mold when the mold alloy enters, for example, Carbon
    or copper powder.
  • Modifying chemicals: They regulate setting expansion and setting time and also prevent shrinkage of gypsum when heated above 300°C, for example, Boric acid and sodium chloride.

Manipulation of gypsum-bonded investment: The measured quantity of powder and water is mixed manually using a flexible rubber bowl and spatula or in a vacuum investment mixing machine.

Setting Reaction of gypsum-bonded investment: When water is mixed the hemihydrates react to form dihydrate which sets to form a solid mass that binds the silica particles together.

Uses of gypsum-bonded investment: Casting of inlays, bridges, removable partial denture framework using gold alloys and another low-fusing alloy.