Classification Of Dental Casting Investment Materials

Classification Of Investment Materials

 

An investment is described as a ceramic material that is suitable for forming a mold into which molten metal or alloy is cast. The procedure for forming the mold is known as investing.

Classification of Dental Casting Investment Materials:

• Based on the Type of Binder Used: There are three types of dental casting investment materials based on the binder used.
  • Gypsum-bonded investments: They are used for casting gold alloys. They can withstand temperatures to 700°C.
  • Phosphate-bonded investments: They are used for metal ceramic and cobalt-chromium alloys. They can withstand higher temperatures.
  • Ethyl silica bonded investments: It is an alternative to phosphate-bonded investments for high-temperature casting. They are used in the casting of base metal alloy partial dentures.

2. Based on application:

• Type 1: For the construction of inlays, crowns, and other fields
  restorations.
• Type 2: For the construction of complete or partial dentures or other
  removable partial appliance.
• Type 3: For the construction of casts using brazing procedures.
• Type 4: For the construction of refractory dies

Composition of Phosphate – Bonded Investment material:

Setting mechanism of phosphate-bonded investment material:

Ammonium phosphate and magnesium oxide react with water to form hydrated magnesium ammonium phosphate and water.

The chemical reaction is:

NH₄H₂PO₄ +  MgO  +  5H₂O  →  NH₄MgPO₄.6H₂O  + MgO + NH₄H₂PO₄  +  H₂O
(Room temperature)

Excess of magnesia and water are present. Some of the magnesia is never fully reacted. Settling reaction continues to occur as there is an increase in temperature at the time of burnout. Water produced during this reaction decreases the viscosity of the mix as speculation continues.

The formation of magnesium ammonium phosphate plays a role as the binding medium in which filer particles get embedded in the matrix. This reaction is the hydration reaction which is followed by a crystallization reaction. The set matrix consists of multimolecular NH₄H₂PO₄ which aggregates around an excess of magnesium oxide and filers.

Setting Reaction at Higher Temperatures:

• Now as the temperature of the investment increases (burnout temperature) setting reaction continues. A sequence of both chemical and thermal reactions with various phase changes occurs at the time of settling and burnout, providing room-temperature strength, and high-temperature strength which enables the investment to withstand high casting temperatures of metal alloy.
• At a temperature of about 300°C, ammonia and water are liberated leading to the degradation of small amounts of magnesium ammonium phosphate which results in shrinkage of the investment, now this is compensated by the outward thrust of the expanding silica forms.
• At higher temperatures, phosphate reacts with silica and forms complex silicophosphates. The strength of the investment increases by the formation of silicophosphates on heating.