Dental coatings
1)-Introduction:
The transfer of alloys to dental prostheses requires the creation of molds and the melting of the alloy at a high temperature. To ensure good precision, we need a material that is resistant to the increase in temperature and capable of compensating for the volumetric variations of the wax and the alloy.
This material is the compensating coating.
2)-Definition:
Compensating coatings are special refractive materials or mixtures used in the manufacture of molds in precision casting, and they are capable of compensating for volumetric variations in wax models and metals for their reproductions.
3)-Classification:
Is a function of alloy type:
*precious alloys: low melting temperature
(exp. 850°C) =>coating for low melting range = gypsum-bonded coating.
*non-precious alloys: high melting temperature
(exp.1400°C)=> Coating for high melting range = coating with phosphate or silicate binder.
So there are two types of coating:
&-Gypsum binder coating;
&-Phosphate or silicate binder coating.
These two types of materials are incompatible with each other. Plaster bowls must be clearly distinct. The manufacturer’s instructions must be followed scrupulously.
Plaster binder coating:
Used for casting gold alloys of type I, II and III,
Silica, whether quartz or cristobalite, accounts for 60 to 65% of the overall composition, and is responsible for thermal expansion.
The plaster, in the form of calcium α hemihydrates, acts as a binder and must constitute 30 to 35% of the coating.
Phosphate-bonded coatings
Due to the decomposition of gypsum above 750 °C, alloys that require a high melting temperature “precious alloys for ceramics, non-precious alloys” are cast in molds made of phosphate-bonded investment.
The composition of these coating masses is similar to gypsum-bound masses. They are also made up of a refractory material, silica, and a binder. This is a mixture of magnesium oxide and ammonium phosphate.
- Phosphate coatings are divided into two categories:
Type I : for inlays, onlays, overlays, crowns and bridges.
Type II : for metal frames.
4)-Composition:
*Refractory mass: a variety of silica.
*Binder: low fusion alpha or beta plaster, or phosphate for high fusion.
*Chemical modifiers: e.g. carbon as a reducing agent => non-oxidizing atmosphere at the time of casting.
*Expansion reducer: e.g. boric acid and sodium chloride.
5)-Presentation:
*Powder: refractory mixture (coating);
*Liquid: Distilled water.
6)-Fundamental qualities required for coatings:
*Easy storage and handling;
*Non-toxic;
*Tp related to manipulation;
*Fine grains to be able to reproduce the details of the wax model;
*Have an expansion related to the contraction of the metal;
*Be porous for gas escape during casting;
*Should not decompose during handling;
*Resist temperature increase;
*Easily detach from the casting.
The fact that molten alloys used for dental restorations contract during solidification is of great importance in the investment of wax models. It is therefore necessary to compensate for the solidification contraction of the alloy used by causing the mold to expand by an amount equal to the contraction.
There are four mechanisms that cause the expansion of the refractory mold: setting expansion of the investment; hygroscopic expansion of the investment; expansion of the wax model; and thermal expansion.
7)-Physicochemical properties:
*Tp: between 05 to 25mn;
*Normal setting expansion: greater /plaster between 4 to 5%. It increases with the concentration of plaster.
*Hygroscopic expansion:
Can increase 6 times or more / socket expansion
-the smaller the particles => hygro expansion increases
-E/P increases => hygro expansion decreases;
-Kneading time increases => hygro expansion increases;
-Storage duration: the older the coating, the less hygro expansion;
-Confinement effect: cylinder walls resist expansion.
Note: If more than 30 seconds elapse between the cylinder being removed from the furnace and the casting, the loss of investment heat causes the alloy to contract more as it solidifies, therefore all the material required for the casting must be ready before the cylinder is taken from the furnace to the sling.
8)-Conclusion:
*Deformation: deformation of the wax model;
*Rough surface: => coating qualities: particle size;
*Air bubble;
*Burrs or thorns : => cracks in the coating => heating too quickly
Dental coatings
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