gypsum is the commonly used product in dentistry for preparing study model, die and cast.
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2. INTRODUCTION
• Gypsum is a natural occurring , white powdery mineral with the
chemical name calcium sulphate dihydrate (CaS04.2H2O).
• Gypsum products used in dentistry are based on calcium sulphate
hemihydrate (CaSO42)2.H2O.
4. MODEL:
The term “MODEL” is a replica of several teeth and their associated
soft tissues or alternatively to an edentulous arch.
DIE:
The term die is normally used when referring to a replica of a single tooth.
6. Type 3
dental stone
die, model
USES
Making cast for
CD and RPD
construction
Type 4
Dental stone
Die, high
strength, low
expansion
USES
For cast and dies
for inlay and
crown fabrication.
Type 5
dental
stone,Die, high
strength, high
expansion
USES
STRONG
DIES
7. 1. dimensional accuracy
2. adequate mechanical properties
3. The material should be fluid at the time it is poured into the impression so that fine detail
can be recorded.
4. The set material should be strong to resist accidental fracture and hard enough to resist
abrasion during the carving of a wax pattern.
8. COMPOSITION
• Gypsum products use in dentistry are formed by driven off part of the
water of crystallization from gypsum to form calcium sulphate
hemihydrate.
•
10. DENTAL PLASTER
• It is also called plaster of paris.
• It is produce by process called calcination.
• Beta hemihydrate- Type I , II are fibrous aggregate of fine crystals with capillary pores. Fluffy
porous and least dense
12. DENTAL STONE
• Dental stone. a calcined gypsum derivative similar to but stronger than Plaster of
Paris, used for making dental casts and dies. Also called artificial stone.
• . Alpha hemihydrate- Type III, IV, V cleavage fragments and crystals in form of rods
or prisms Higher density and more crystalline
13. DENTAL STONE
METHOD 1
If gypsum is heated to about
125*C under steam pressure in an
autoclave more regular and less
porous hemihydrate is formed.
This is sometimes referred to as
alpha- hemihydrate.
METHOD 2
Gypsum may be boiled in a solution
of salt such as CaCl2. this give a
material similar to that produced by
autoclaving but with even less porosity.
They May be differentiated from dental
plaster which is white.
14. DIFFERENCE
BETWEEN
β– hemihydrateα - hemihydrate
When dehydrate is
heated under steam
pressure.
Dense & prismatic
crystals.
Stronger & harder
products on mixing
with water.
( type 3 ,4 & 5 )
Smaller w/p ratio
When dehydrate is
heated by a
process called
calcination.
Spongy &
irregularly shape
crystal
Less strong
products on mixing
with water. Type
1& 2
More w/p ratio
15.
16. MIXING
• Plaster and stone powder are mixed with water to produce a
workable mix.
• For hand mixing a clean, scratch free rubber or plastic bowl is
normally recommended.
• A stiff spatula with a round-edged blade of around 20-25 mm
width and 100 mm length is used.
• The amount of water is added to a moist bowl and the powder
added slowly to the water over about 10 seconds
• The mix is allowed to soak for about another 20 seconds and
then mixing/spatulation carried out doe around 60 seconds
using a circular stirring motion,.
17. SETTING PROCESS
• The setting process begins rapidly after mixing the powder and water.
• The first stage : the water becomes saturated with hemihydrate, which has a solubility
of around 0.8% at room temperature. The dissolved hemihydrate is then rapidly
converted to dihydrate which has a much lower solubility of around 0.2%.
• The material should be used as soon as possible after mixing since its viscosity
increases to the stage where the material is unworkable within a few minutes.
• Potassium sulphate is a commonly used accelerator which is thought to act by
increasing the solubility of the hemihydrate .
• Borax is the most widely used retarder .
18. W/P RATIO
Increasing the W/P ratio retards setting by decreasing the concentration of crystallization of nuclei
19. PROPERTIES OF SET MATERIALS
• Gypsum is a very brittle material.
• The strength of gypsum depends on the porosity of set material.
• Stone is always mixed at lower W/P ratio, it is less porous much stronger and
harder.
• Solubility increases with the temperature of the water.
• Stone is less fragile but must be treated with care if fracture is to be avoided.
• Dimensional stability of gypsum is good.
20. SETTING TIME
• Setting time of gypsum is controlled by 2 factors :
• MANUFACTURERS :
• It may add chemical accelerators (potassium sulphate) or retarders
(borax)
• Gypsum products required the use of VICAT needle for judging setting
time.
• OPERATOR :
• Temperature , W/P ratio , and mixing time
21. SETTING EXPANSION
• Increased spatulation increases setting expansion.
• Increase W/P ratio reduces the setting expansion.
• Acelerators and retarders.
Another physical change which accompanies setting is
a small expansion caused by growing crystals
22. APPLICATION:
Plaster is often used for mounting stone model onto articulator and sometimes for
preparing study models.
When strength, hardness and accuracy are required dental stone are normally used
in preference to dental plaster.
The stone material is less likely to be damaged.
23. ADVANTAGES
• Gypsum model and die materials have the advantages of being inexpensive and easy
to use.
• The accuracy and dimensional stability are good.
• they are able to reproduce fine detail from the impression.
24. DISADVANTAGES
• The mechanical properties are not ideal and the brittle nature of gypsum occasionally
leads to fracture- particularly through the teeth, which form the weakest part of any
model.
• Problems occasionally arise when gypsum model and die materials are used in
conjunction with alginate impression.
