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
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
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
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%
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.
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