Techniques for compensation of solidification shrinkage

short topic of Techniques for compensation of solidification shrinkage

2. Presented By:- Dr Nishant Khurana
PG Student
Guided By:- Dr M.P. Singh
9/26/20132

3. •Solidification
•Shrinkage
•Problems Related
•Compensation For Solidification Shrinkage
Hygroscopic Seting Expansion {low heat
method}
Thermal Expansion {high heat method}
INTRODUCTION

4. Solidification is a phenomena that occurs
during the freezing of a metal.
- By phillips
SOLIDIFICATION

5. Graph Showing
Solidification…
A time temperature cooling curve for pure metal

6. All metals and alloys of practical
interest shrink when they change from
the liquid to solid state.
-By Phillips
SHRINKAGE

7. 1. The thermal contraction of a liquid metal between
the temperature to which it is heated and the liquidus
temperature.
2. The contraction of the metal inherent in its change
form the liquid to the solid state.
3. The thermal contraction of the solid metal occurs
on further cooling to room temperature.
THREE STAGES OF SHRINKAGE

8. •Dimensional errors
•Patient discomfort
PROBLEMS RELATED…

9. TableTable
INVESTMENT USES SHRINKAGE
(as per ADA sp.
No. 2)
TYPE I Employed for casting
of Inlays.
Maximum Setting
Expansion in air is
only 0.6 %
TYPE II Inlays , On lays ;
Major mode for
compensation for
alloy shrinkage
In water Maximum
Expansion permitted
is 2.2 %
TYPE III Construction of
partial denture
Its thermal expansion
is 0 – 0.6 % at 500 * C

10. The compensation for the shrinkages inherent in the dental
casting procedure may be obtained by any of the three
following methods:
Setting or hygroscopic expansion of the investment
Thermal expansion of the investment
Normal Setting expansion
COMPENSATION TECHNIQUES

11. A mixture of silica and calcinated gypsum (calcium
sulfate hemihydrate) results in setting expansion
greater than that of the gypsum product used alone.
The silica particles probably interfere with the
intermeshing and interlocking of the crystals as they
form. Thus the thrust of the crystals is outward
during growth, and they increase expansion.
NORMAL SETTING EXPANSION

12. The purpose of the setting expansion is to
aid in enlarging the mold to compensate
partially for the casting shrinkage.
It eventually gains sufficient strength to
produce a dimensional in the wax pattern and
mold cavity as setting expansion occurs.

13. Hygroscopic Setting Expansion, which is
greater in magnitude than normal setting
expansion differs from normal setting
expansion in that it occurs when the gypsum
product is allowed to set when placed in
contact with water.
It is one of the methods for expanding the casting
mold to compensate for the casting shrinkage of
the gold alloy.
HYGROSCOPIC SETTING EXPANSION

14. •The magnitude of the hygroscopic expansion is in
direct proportion to the amount of water added
during the setting period until a maximum
expansion occurs.
•The phenomenon is purely physical. The
hemihydrate binder is not necessary for the
hygroscopic expansion, because investments with
other binders exhibit a similar expansion when they
are allowed to set under water.

15. Effect of Composition
Effect of Water/Powder Ration
Shelf Life of the Investment
Effect of Time of Immersion
Effect of Confinement
Effect of Added Water
EFFECT OF HYGROSCOPIC EXPANSION

16. Fig:
CURVE A : Normal Setting
Expansion.
CURVE B : Hygroscopic Setting
Expansion.

17. DIAGRAMATIC COMPARISION BETWEEN
NORMAL & HYGROSCOPIC SETTING
CONDITIONS

18. Thermal Expansion is the tendency of matter to
change in volume in response to a change in
temperature. When a substance is heated, its
constituent particles move around more vigorously
and by doing so generally maintain a greater
average separation.
The desirable magnitude of the thermal expansion
of a dental investment depends on its use.
THERMAL EXPANSION

19. Effect of Water/Powder Ratio
Effect of Chemical Modifier
Strength
Thermal contraction
EFFECT OF THERMAL EXPANSION

20. THERMAL CONTRACTION

21. A number of factors influence the mould size. Two
liners allow a greater setting and thrmal expansion
than does a single liner. Also, the setting hygroscopic
and hermal expansions of investments can be
controlled to a certain extent by varying L/P ratio of
the investment. The lower the L/P ratio, the greater
the potential for investment expansion. Thinner
mixes reduces the expansion.
COMPENSATION FOR SHRINKAGE

22. 1.The mixture is too thin, a rough surface may
result o the casting.
2.Another possible problem is too much expansion
of mould when using the thermal expansion
technique with a cristobalite investment.
PROBLEMS ENCOUNTERS

23. The linear hygroscopic expansion increases
directly with the amount of water added until
maximum expansion is attained.
Eg. Flexible rubber ring
CONTROLLED WATER-ADDED TECHNIQUE

24. This technique obtains its compensation expantion from three
sources :
1.The 37* C water bath expands the water bath.
2.The warm water entering the investment mould from the top
adds some hygroscopic expansion ; and
3.The thermal expansion at 500*C provides the needed thermal
expansion.
ADVANTAGES:
1.Less investment degradation.
2.A cooler surface for smoother casting and
3.The convenience of placing the moulds direstly in the 500*C
furnace.
4.So its possible to keep one or more furnaces at the burnout
temperature so that moulds may be put in as they are ready.

HYGROSCOPIC LOW-HEAT TECHNIQUE

25. This approach depends almost entirely on using
of high heat burnout to obtain the required
expansion, while at the same time eliminating the
wax pattern. Additional expansion results from
the slight heating of gypsum investment on
setting, thus expanding the wax pattern and the
water entering the investment from the wet liner
adds a small amount of hygroscopic expansion to
the normal setting expansion.
HIGH-HEAT THERMAL EXPANSION

26. Thus we conclude that-
by the various techniques applied for compensation of
solidification shrinkage we can
•reduce the defects
• can maintain the dimensional stability thus reducing dimensional errors
Thereby producing a precise restoration in various dental procedures
CONCLUSION

27. PHILIPS’
Science of Dental Materials
Anusavice
Essentials of Dental Materials
SH Soratur
Basic Dental Materials
John J. Manappallil
Internet
 Google
 Wikipedia etc.
BIBLIOGRAPHY

28. Presented By :-
Dr Nishant KhuranaDr Nishant Khurana
PG1st Yr PG1st Yr
Dept. ConservativeDept. Conservative
&&
EndodonticsEndodontics
THANK YOU