1. Models & Die
Materials
Applied Dental Materials
3rd year

2. Gypsum Products
Used extensively in dentistry to make dental models.
Obtained from natural gypsum rock (dihydrate form
of Calcium sulfate CaSO4 . 2H2O)
CaSO4 . 1/2H2O + 1 1/2 H2O
CaSO4 . 2H2O +
3900 cal/g mol

3. Dental Models
Three-dimensional reproductions of the teeth
and the surrounding soft tissue of a
patient’s
maxillary and mandibular arches.
Also referred to as study casts.
http://practicon.com/images/products/1_40132_FS.jpg

4. Use of Dental Models
Diagnosis for a fixed or removable
prosthetic.
http://www.summitdentalarts.com/media/7796/plaster-cast.gif
Diagnosis of orthodontic treatment.
Visual presentation of dental
treatment.
http://upload.wikimedia.org/wikipedia/commons/f/f1/Steg_ok_2a.jpg
Making of custom trays.
Making of orthodontic appliances.
http://roguedentalsolutions.com/web/wp-content/gallery/
photo-gallery/pauls-work-007.jpg

5. Ideal Requirements
Should have:
- Sufficient strength to prevent accidental
breakage.
- Sufficient hardness to avoid surface damage
during use.
Should produce fine details and sharp margins
of the impression.

6. Should show little dimensional change on
setting and remain dimensionally stable.
Should be compatible with the impression
materials. (No interaction between the
impression surface and the model surface)
Should have good color contrast with the other
materials being used.
Should be cheap and easily used.

7. Types
CaSO4 . 2H2O
Plaster
fluffy, porous
&least dense
Hydrocal
higher density &
more crystalline
Dansite
desist of
raw material

9. Manufactures
Plaster
Hydrocal
plaster
II
High-strength
low-expansion DS IV.
Densite
Low- to moderate
strength
III
High-strength
high- expansion DS V.

10. Model plaster: commonly called plaster of Paris, is
used primarily for pouring preliminary impressions
and the making of diagnostic models.
Dental stone: for use as a working model when a
more durable diagnostic cast is required. Examples
are in the making of custom trays and orthodontic
appliances.

11. High‑strength stone: also known as densite or
improved dental stone. Its strength, hardness,
and dimensional accuracy make it ideal to
create the dies used in the production of
crowns, bridges, and indirect restorations.
http://heraeus-dental.com/media/webmedia_local/image/650/Internet_Gipse_650_g_res_650.jpg

12. Plaster
Mineral
Dehydration
gypsum
Hydrocal
Densite
open kettle
110 - 120°C
under pressure
water vapor 125 °C
boiled
in 30% CaCl Sol.
Model Plaster
Dental stone
High strength
dental stone
Chemicals can be added to modify
their handling properties.

13. Chemical Reaction
Gypsum
Plaster
D Stone
high-strength DS
Mix water
37-50
28-32
19-24
Required water
18.6
18.6
18.6
The chemical reaction takes place
the amount of water needed.
Excess water
18-31
9-13
0-5
determine
Water thick mix, difficult to handle, trap air
bubbles but the set gypsum usually strong.
Water Thinner mix, porous easily handle, but
the set gypsum is inferior.

14. W/P Ratio for DS and
High Strength DS
Crystal shapes and concentration play a major
role in W/P ratio needed to obtain a workable
consistency.

15. Mechanism of Setting
Different solubility between CaSO4 . 2H2O &
CaSO4 . 1/2H2O.
CaSO4 . 1/2H2O + H2O
Supersaturation
Nucleation process ( needle-like crystals)
Bonding between contacting crystals
Final
cohesion.

16. Dissolution
Water
Ca++
SO --
CaSO4. 2 H2O
Nuclei of
crystalization
Precipitation

17. Effect of Spatulation
Definite effect on the setting time and setting
expansion.
amount of spatulation
setting time
http://www.whipmix.com/img/Product_img/vacuum_mixing.jpg

18. Effect of Temperature
Temp. of the mixing water as well as the temp.
of the environment affect the setting time in
one of two ways:
1. Change in the solubility of CaSO4 . 2H2O &
CaSO4 . 1/2H2O.
- solubility ratio
setting time.
slow the reaction

19. Temperature (°C)
CaSO4 . 1/2H2O
CaSO4 . 2H2O
20
0.90
0.200
25
0.80
0.202
30
0.71
0.209
40
0.61
0.210
50
0.50
0.205
100
0.17
0.170

20. 2. Change in the Ion mobility:
- Temp.
sol. & mobility of Ca++ & SO4-reaction rate
setting time.
- at 100°C
rate of reaction decreased and
the the setting time is lengthened.

21. Effect of Humidity
When humidity reaches 70% and above:
the moist convert CaSO4 . 1/2H2O into CaSO4 .
2H2O which will act as extra centers of nucleation,
thus accelerating the reaction.
Further contamination
CaSO4 . 1/2H2O
amount of available of
retarding the reaction.

22. Effect of Colloidal
Systems & PH
Agar and alginate retard the reaction
(potassium sulfate).
High pH accelerate the reaction, while low pH
retard the reaction.

23. Properties
Setting time: the time required for the reaction to
be completed (final setting time).
Final setting time: when the material can be
removed from the impression without distortion.
Initial setting time: the time required to reach a
certain arbitrary stage of firmness in their setting
process.
working time.

24. Control of Setting
Time
Addition of accelerators or retarders.
Temp. of water.
Degree of spatulation.
W/P ratio.

25. Viscosity
more voids are observed in cast made of stones
with higher viscosity.
Material
Viscosity (cp)
high strength DS
A
21,000
B
29,000
C
50,000
D
54,000
E
101,000
Impression Plaster
23,000

26. Compressive
Strength
Water
Compressive strength.
Wet strength: with some or all the excess
water present in the specimen.
Dry strength: with all the excess water drive
out.
Dry C.S is almost twice the wet C.S.
8.8% excess water is in the hardened stone.

27. 0.50
11
9
31
0.30
20.5
0.50
10.5
0.24
High-strength
Dental Stone
12.5
0.27
Dental Stone
Compressive Strength
(MPa)
0.55
Model Plaster
W/P Ratio (ml/g)
0.45
Material
38
0.30
21.5
0.50
10.5

28. Surface Hardness and
Abrasion Resistance
After setting occur the C.S remain constant tell
all the excess water evaporate from the surface,
after which it increase.
Hardness increase at higher rate than C.S.
Attempts have been made to increase the
hardness of gypsum products.
Impregnating with epoxy or MMA, oils,
colloidal silica.

29. Production of Details
ADA specification requires type I & II
reproduce a groove 75 µm in width, type III, IV
& V reproduce a groove 50 µm in width.
Gypsum Vs Epoxy.
Elastomeric impressions.
Contaminations.

30. Setting Expansion
The % of setting expansion varies from one type
to another.
Plaster 0.2% to 0.3%.
Low to moderate-strength DS 0.15% to 0.25%.
High-strength DS 0.08% to 0.10%.
High-strength high expansion DS 0.10% to 0.20%.

31. almost 75% of the expansion observed during
the first 24 hrs occurs at the first hour of
setting.
Machine mixing.
W/P ratio.
chemicals.
hygroscopic expansion.
http://www.whipmix.com/img/Product_img/vacuum_mixing.jpg

32. Manipulation
The powder is added to the water and allowed
to settle for 30 sec.
Mixing can be done by hand spatulation or by
mechanical spatulation.
Pouring requires care to avoid trapping air.
Once poured it has to be allowed to set for 45
to 60 min.

34. Other Die Materials
Silico Phosphate Cement: Hard but shrink on setting.
Amalgam: Amalgam reproduces fine details but has
some disadvantages:
Should be packed against rigid impression materials.
Needs a long time to reach maximum strength.
Needs a separating medium.
Has high thermal conductivity so it cools the wax
pattern rapidly, which may lead to pattern
distortion.

35. Acrylic resin and other polymeric materials:
They have high abrasion resistance and they
are harder than gypsum but their shrinkage
during polymerization leads to inaccuracy.
Epoxy die: Epoxy die is very toxic and it
shrinks during polymerization.

36. Metal sprayed die:
Bismuth tin metal sprayed
directly on impression to form metal shell then
the gypsum is directly poured in it.
Electroplated die: Silver plated die is used
with rubber impression materials but is not
used with impression compound.
Copper plated die is used with impression
compounds.

37. QUESTIONS?