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chapter 47 notes

part 2

QuestionAnswer
Dental Plaster Also commonly referred to as plaster of Paris,
model plaster is white and used primarily for pouring up preliminary impressions to make diagnostic models for a case presentation, or in the use of orthodontics.
The crystals in plaster are irregularly shaped and very porous, similar in appearance to a sponge.
Dental stone is a form of gypsum
gypsum that is typically what in color yellow
gypsum is used when a more durable diagnostic cast is required, or when a working model is needed for the fabrication of a fixed or removal prosthetics, retainer, and custom tray
The crystals in dental stone are are more uniform in shape and less porous than those in plaster.
Using dental stone results in a .... much stronger and denser than one made from plaster.
High-Strength Stone Also known as densite or improved dental ston
high-strength stone has a strength, hardness, and dimensional accuracy that make it ideal for use by the dental laboratory technician
The crystals in high-strength stone are smooth and very dense and require the least amount of water for mixing.
Dental plaster (100 g) 45–50 ml water
Dental stone (100 g) 30–32 ml water
High-strength stone (100 g) 19–24 ml water
When too little water is used, the mix will be dry, which will provide a shorter working time.
When too much water is used, the mix will be thin and runny and will take longer to set; this creates a model that is considerably weaker.
The water is measured by volume with a measuring device such as a large syringe or a milliliter-graduated cylinder.
If a scale is not available, the powder may be measured by volume.
Mixing Dental Plaster Add the powder to the water in steady increments. Allow the powder to settle into the water for about 30 seconds.
Mixing Dental Plaster A smooth and creamy mix should be achieved in about 20 seconds.
Mixing Dental Plaster Complete mixing and vibration of the plaster for no longer than 2 minutes.
A model consists of two parts: the anatomic portion and the art portion,
the anatomic portion, which is created from the alginate impression,
and the art portion, which forms the base of the cast
Three different pouring methods can be used to create the base or art portion of the cast Double-Pour Method, Box-and-Pour Method and Inverted-Pour Method
Double-Pour Method the anatomic portion of the cast is poured first
IN double pour the 2nd mix of plaster is used to prepare the art portion.
Box-and-Pour Method The impression is surrounded with a “box” made from boxing wax.
The inverted-pour approach consists of mixing one large mixture of plaster or stone and pouring up the impression first,
This is the most common way of “pouring up” an impression. Inverted-Pour Method
How long should you wait before you separate the model from the impression? Wait 45 to 60 minutes after the base has been poured before separating the impression from the model.
Created by: alejac0360
 

 



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