DENTAL COATINGS
Introduction :
The success of a dental rehabilitation depends largely on the dentist being familiar with laboratory procedures, and the technician being familiar with clinical procedures,
Obtaining a cast metal element from a wax model (lost wax casting) involves 3 steps:
Coating the model: which consists of covering it with a material allowing precise reproduction of its shape and morphological characteristics;
heating the cylinder which, by causing the elimination of the wax, leaves a mold which will occupy the alloy introduced during casting.
However; the metal shrinks when cooling. So to obtain a casting of the same dimension as the wax model, a so-called compensating coating must be used.
Definition
These are special refractory (heat-resistant) mixtures intended for the production of molds used in precision casting and to ensure compensatory expansion capable of compensating for variations in the volume of metal parts produced using the lost-wax casting method.
These coatings include:
A refractory substance: quartz or cristobalite, both derived from silica, responsible for thermal expansion; constitutes the major part of the coating, and which will ensure thermal expansion
A binder that allows the coating to harden, ensuring the mechanical resistance of the mold
Modifiers:
Classification
1. Low melting temperature alloy coating:
Plaster-bonded coatings: reserved for low-melting alloys below 1100 traditionally used for casting gold;
powder (refractory substance + binder (plaster) + modifiers)
Liquid distilled water
2. High melting temperature alloy coating
a. Ethyl silicate coatings:
Frequently used for casting removable partial dentures were “alcohol coatings”.
Powder : silica + glass + magnesium oxide
Liquid: ethyl silicate + water + colloidal silica
Their accuracy is questionable, they are not very durable and the operating procedures are complex. They are losing popularity.
b. Phosphate-bonded coatings
Phosphate coatings are the most popular because of the tolerance to high temperatures required for casting non-precious metals; titanium.
Phosphate coatings are divided into two categories:
-Type I: for inlays, onlays, overlays, crowns and bridges.
– Type II: for metal frames.
Powder : Refractory filler compounds (quartz and cristobalite). Silica glass, magnesium oxide, ammonium phosphate additives such as dyes
Most phosphate coatings contain the same components, however the granulation and quality of raw material can vary from product to product.
Liquid : The binder is usually composed of magnesium oxide, di-ammonium hydrogen phosphate, mono-ammonium phosphate and colloidal silica.
Due to the presence of ammonium phosphate in the liquid, these coatings are called phosphate coatings.
The Properties
The quality of a coating is determined by its various properties:
– Its ability to reproduce microscopic details,
– Its extremely smooth surface.
– Its controllable expansion.
– Its comfortable handling time.
– Its easy sandblasting.
– Be sufficiently porous to evacuate gases from the boiling of the wax
– Be a refractory, non-flammable material.
– Be solid and resistant: the cylinder must not crack either during the temperature rise or during casting (under the pressure of the metal)
Non-corrosive: it must not attack metal.
Coating:
-The wax model to be reproduced is attached to a casting rod and mounted on a base and placed in a casting ring (cylinder);
-Mix the powder and the liquid following the manufacturer’s instructions (to avoid air bubbles, the powder will be poured into the liquid gradually)
-Spatulate the mixture and tap lightly;
– Pour the coating inside the cylinder until it is filled starting from the same point in order to avoid trapping air bubbles;
-Wait for the coating to set
-Remove the base and the casting cone;
_remove the wax model by heat; there remains a cavity in the coating into which the molten metal is introduced
-Perform cylinder preheating,
At the beginning the temperature rise of the cylinder must be slow in order to avoid the appearance of cracks or fractures in the coating due to the evaporation of the liquid contained in it requires:
_A first stabilization of 30 minutes at approximately 270°C,
_A second stabilization of 30 minutes at approximately 570°C is necessary for expansion.
The temperature can be increased until the temperature corresponding to the desired expansion is reached; this temperature depends on the type of coating used and is indicated by the manufacturer;
The Expansion:
Investments used for casting dental prostheses are called compensating investments. This comes from having the ability to expand and thus compensate for the shrinkage of the metal during its crystallization.
There are two types of expansions:
– the expansion of the socket
– and thermal expansion.
1. Socket expansion
Setting expansion is the expansion of the coating during its hardening.
Between approximately 1.2% and 1.4%, may vary from one coating to another but varies depending on:
- Ambient temperature: The higher the ambient temperature, the greater the setting expansion will be.
- Material temperature: It is recommended to store these materials between 10°C and 12°C.
- Amount of liquid in the mixture: the proportions indicated by the manufacturer must be respected,
- Percentage of coating liquid and distilled water: Using only the coating liquid will give maximum setting expansion. As this liquid is diluted with distilled water, the setting expansion of the coating will decrease.
- Mixing intensity and duration
2. Thermal Expansion The thermal expansion of the coating is controlled by the heating rate of the furnace and the stabilization times at the different levels. The programming of the heating furnace must strictly follow the manufacturer’s recommendations.
Total expansion
The sum of the two expansions: setting expansion + thermal expansion represents the total expansion of the coating. This total expansion ends at around 600°C and reaches 2.5%.
Controlling expansion is difficult and not very accurate due to the number of factors involved. To achieve consistent results, it is important to follow a strict coating protocol.
Conclusion
Understanding the behavior of the different constituents of the coatings , their setting reaction and the factors on which compensatory expansion depends is a guarantee of final prosthetic success.
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DENTAL COATINGS
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