Dental investments and refractory materials (part 1)

A refractory is a non metallic material with can with stand high temperature without degrading , softening or losing its strength.

A investment is a ceramic material which is suitable for forming a mold into which a molten metal or alloy is cast. The process is called investing

REQUIREMENTS OF AN INVESTMENT MATERIAL

  1. The investment mold must expand to compensate for the alloy shrinkage, which occurs during the cooling of the molten alloy.
  1. The powder should have a fine particle size to give a smooth surface to the casting.
  2. The manipulation should be easy. It should have a suitable setting time.
  3. The material should have a smooth consistency when mixed.
  4. The set material should be porous enough to permit air in the mold cavity to escape easily during casting.
  5. At higher temperatures, the investment must not decompose to give off gases that may corrode the surface of the alloy.
  1. It must have adequate strength at room temperature to permit handling, and enough strength at higher temperatures to withstand the impact force of the molten metal.

GENERAL COMPOSITION of INVESTMENTS

All investment materials contain a refractory, a binder and modifiers.

REFRACTORY

A refractory is a material that will withstand high temperatures without decomposing or disintegrating, e.g. silica.

Allotropic forms Silica exists in at least four allotropic forms.

Quartz

Tridymite

Cristobalite

Fused quartz

They serve two functions

  1. Act as a material that can withstand high temperatures.
  2. Regulate the thermal expansion.

BINDER

A material which will set and bind together the particles of refractory substance,

e.g. gypsum, phosphate and silicate. The common binder used for gold alloys is dental stone (alpha hemihydrate).

The investments for casting cobalt chromium alloys use ethyl silicate, ammonium sulphate or sodium phosphate.

CHEMICAL MODIFIERS

Chemicals such as sodium chloride, boric acid, potassium sulfate, graphite, copper powder or magnesium oxide are added in small quantities to modify properties.

Source – basic science of dental materials – manappallil

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