Heating gypsum and driving off part of the water of crystalization
Casts
Diagnostic casts
Working casts
Dies
Diagnosticcast
Workingcasts
Gypsum products
Plaster
Die stone
Calcination
Less water
Higher density
The control of setting expansion is critical for accurate models and dies
Setting expansion occurs only during the hardening of the gypsum product
Gypsum products
Type I Impression plaster
Type II Model plaster
Type III Dental stone
Type IV Dental stone high strength
Type V Dental stone high strength & expansion
Model plaster requires 45 ml of H2O
Dental stone requires 30 ml of H2O
High strength stone requires 23 ml of H2O
High strength/expansion stone requires 20 ml of H2O
Gypsum setting is an exothermic reaction that generates heat
For regular-set products, the initial set occurs within 8 to 10 minutes from the beginning of the mix
Separation of impression from cast should occur 45-60 minutes after pouring
Before separating the impression from the cast, ensure that no part of the impression tray is connected to the gypsum. Do not pry or rock in one direction too far or the cast will likely break
If an alginate impression has dried out before the cast has been separated, soak the impression and cast in water for 15 minutes. The alginate will soften which allows removal of the cast without breaking anatomic structures
Pouring casts methods
Double-pour method
Single-step method
Boxing method
The anatomic portion and art portion (base) should be poured separately
The anatomic portion forms a point at the midline for maxillary casts, while mandibular casts are rounded from cuspid to cuspid
How are dental waxes used in clinical dental procedures?
1.
2.
3.
4.
Dental waxes
Natural
Synthetic
Dental waxes
Melting range
Flow
Excess residue
Dimensional change
Dental waxes
Pattern waxes
Processing waxes
Impression waxes
Pattern waxes
Inlay
Casting
Baseplate
Processing waxes
Boxing
Utility
Sticky
Impression waxes
Corrective
Impression
Bite registration
Diagnostic cast
Poured in plaster
Working Cast
Poured in dental stone
When the gypsum looses its glossy ?
Initial set has reached
Plaster
More water + air voids
Die Stone
Less water + less air voids
What is the strength of the gypsum depends on ?
The amount of water used in producing the “model”
A product with less water
Has a higher density of crystals (denser and stronger)
2 Factores that contributes to the strength and abrasion resistance of the final product