17938858
Beschreibung
Mindmap von Alice Dolman, aktualisiert more than 1 year ago
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Erstellt von Alice Dolman
vor etwa 5 Jahre
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What would be the physical effects onthe
completed restoration if the
firingtemperatures and sequences
usedwere incorrect?
- The precise temperatures and times that are required
when firing dental porcelains will vary from
manufacturer to manufacturer and furnace to furnace.
- After each individual firing, the temperature may be slightly
reduced (5-10 deg C) so each firing will not affect the previous
firing. However, it important to know that this is not necessarily
always the case, as in some modern porcelains, each consecutive
dentine cycle after the first retains the same temperature.
- Thermal Shock
- All porcelains require slow, controlled heating and cooling
to prevent thermal shock (causing cracking). This will vary,
but generally, a 6 minute preheat is required when firing
margin or dentine cycles. This is controlled by the speed of
the muffle closing, combined with the rate of temperature
increase within the muffle during the cycle.
- In order to provide a slow cool, most furnaces reduce
power to the heating element and begin a gradual
opening of the muffle. Preventing thermal shock is
particularly important when firing bonded porcelains.
- All porcelains require slow, controlled heating and cooling
to prevent thermal shock (causing cracking). This will vary,
but generally, a 6 minute preheat is required when firing
margin or dentine cycles. This is controlled by the speed of
the muffle closing, combined with the rate of temperature
increase within the muffle during the cycle.
- Porosity
- All modern dental porcelains are designed to be fired in a partial
vacuum. Exactly when the vacuum activates in the cycle will vary,
depending on the stage, the porcelain type and the furnace itself.
The same is said for when the partial vacuum is released.
- The purpose of creating a vacuum during the
firing process is to reduce the risk of porosity
within the porcelain, by removing trapped gases
from the material as the particles sinter. This
increases the strength of the porcelain, and also
increases the aesthetic properties.
- The porosity volume has a detrimental effect on the
aesthetics of the material by increasing opacity.
- The porosity volume has a detrimental effect on the
aesthetics of the material by increasing opacity.
- All modern dental porcelains are designed to be fired in a partial
vacuum. Exactly when the vacuum activates in the cycle will vary,
depending on the stage, the porcelain type and the furnace itself.
The same is said for when the partial vacuum is released.
- Grain size/Strength
- The finer the grain size the
greater the strength but this is
accompanied by increased
opacity.
- The final grain size is determined by conditions
present during the sintering phase, such as firing
duration and firing temperature. Higher
temperatures and longer cycles lead to larger grain
size.
- Lower temperatures will cause the porcelain to be weaker as the alumina crystals will not fully fuse together
- Lower temperatures will cause the porcelain to be weaker as the alumina crystals will not fully fuse together
- The strength and opacity of aluminous porcelain is
related to the grain size of the alumina crystals; the
grain size of the fused alumina crystals is selected to
provide optimum strength without producing
excessive opacity.
- The finer the grain size the
greater the strength but this is
accompanied by increased
opacity.
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