this contains setting expansion of gypsum in detail. linear and hygroscopic expansion. control of expansion.

3. Setting Expansion Of Gypsum
Guide: -
• Dr. SUNITHA N SHAMANUR
Professor
• Dr. PAVAN KUMAR K. R.
Reader
[Dept of Prosthodontics,
Bapuji dental college & Hospital]
Presented by:
Parikshit Harnoor.
II B.D.S. 2016 batch
Reg No:14D0959

4. Hi !!! Friends
,
I’ll be guiding
you through
out this session

5. What will we learn?
 Intro – Setting expansion
 Mechanism
 Types (Linear and
Hygroscopic)
 Controlling the
expansion.

6. Fact to know.
 (CaSO4)2.H2O + 3H2O 2 CaSO4.2H2O
 Comparison. V(CaSO4.½H2O) = V(H2O) =
V(CaSO4.H2O),
Molecularmass (g) 290.28 54.05 344.33
Density(g/cm3) 2.75 1.00 2.32
Equivalent volume
(cm3)
105.56 54.21 148.41
Volume formed = 7.11% less than equivalent volumes.

7. Definition by GPT 8
Setting expansion :
The dimensional increase that occurs
concurrent with the hardening of various
materials, such as plaster of paris, dental
stone, die stone, and dental casting
investment

8. Setting expansion –
A Base to explain.
CRYSTALLIZATION.

9. A pre-requisiteknowledge
Nucleiofcrystallization(spherulites)

10. Mechanism of expansion
 Crystallization – growth of crystals
 Growth of nuclei
 Each crystal stops the growth of the other
 Intermesh
 Repetition of the process with 1000s of
nuclei
 An outward stress or thrust develops.
 After setting – interlocking crystals and
micro pores containing water

11. SEM of spherulites.

12. •Dimensional changes that occur during the setting of
a gypsum..
•Arrow indicates the initial set.

13. How much is
the expansion ?
As low as 0.06% or as high as
0.5%

14. Typesof
expansion(norma
l and
hygroscopic)

16. STAGE 1
ROUND
PARTICLES OF
HEMIHYDRATE
WATER

17. STAGE 2
Crystals begin to
form on nuclei

18. STAGE 3
Contact each other and water
around them decreases

19. STAGE 4
Should draw more water, but
outward thrust opposes this

20. STAGE 5
Crystals become intermeshed
and engaged

22. 1.What does
hygroscopic
mean ?
2.Does it apply
here ?
3.Technically
correct?

23. STAGE 1
Identical
mixture of
hemihydrate
WATER

24. STAGE 2
Distance b/w particles remain same as water
consumed by hydration will be replenished by
immersion water

25. STAGE 3
Crystals grow and contact each other,
no reduction in the distance b/w the
crystals is seen

26. STAGE 4
Crystal grow much freely

27. STAGE 5
Finally the crystal intermesh which
prevents the further expansion

28. Is the expansion
same for all the
five gypsum
products?
Type 1 and 2 (impression & model
plaster)
0.2% - 0.3%
Type 3 (Dental stone) 0.15% - 0.25%
Type 4 (dental stone with high strength) 0.08% - 0.10%
Type 5 (dental stone with high strength
& high expansion)
0.10% - 0.20%

29. • Desirable or not ?
• What is its
importance?

30. • Desirable or not ? Depends on the use.
• -ve’s and +ve’s both.
• -ve pt. : reduces accuracy in
impression plaster, model plaster &
stone.
• +ve pt. : casting shrinkages in
investments and divestment materials.
compensation for the shrinkage is
expansion

31. • Can this be
manually
controlled?
• How to decrease /
increase?

32. Reducing the expansion.
1. Chemicals like K2SO4 & borax – short, thick and
flat crystals
2. Increased W/P ratio. – less no. of nuclei, thin,
great space, less thrust.
3. Use of accelerators – attain rigid structure
quickly

33. Increasing the expansion.
1. Keeping the specimen in water – follows
hygroscopic expansion
2. Thermal expansion – (dental stone +
silica) = GBI. Heating GBI = expansion.
3. Decrease W/P ratio.

34. Review Of Literature.

35. Bibliography
1.Philips science of DM – Anusavice pg.193
2.Dental materials and their selection –
William.J.O’Brien. Pg no. 44
3.Materials science for dentistry– B. W. Darvell. Pg 45
4.Craig’s restorative dental materials –
Ronald L.Sakaguchi, John M. Power pg 308
5.Science of dental materials n clinical applications -
V.Sharma Bhat, B. T. Nandish pg 148
6.Essentials of dental materials– S. H. Soratur pg 136

36. Doubts are welcomed.