construction of dental restoration Dental waxes Casting investment dental material

1. Construction of
Dental
Restoration
1. Dental waxes
2. Casting investment

2. Dental restoration is a replacement for the lost
natural hard or soft dental tissues.

3. 1. Resin (Polymer) - based restorations
Composite fillingsDenture acrylics

4. Metal-ceramic All-ceramic
2. Ceramic - based restorations

5. Gold alloysAmalgam alloys
3. Metal - based restorations

6. Construction of metallic restorations
or .. metal copings
A. Direct Technique
e.g. direct amalgam filling
B. Indirect Technique
(Laboratory-made)
1. Swaging
(cold working)
Hammering the metal
over a die
2. Casting
(lost wax technique)
Pouring of molten
metal into a refractory
mold
3. Electroforming
Precipitation of metal
ions over a master die

7. Casting metallic restoration
using lost wax technique

8. Materials necessary for
constructing indirect metallic
restorations
using the lost wax technique
1. Dental waxes .. (to construct wax pattern)
2. Casting investment .. (to construct the mold)
3. Metallic alloys .. (restorative material)

9. Dental waxes
Dental waxes are hydrocarbons in nature (contain O2 ,
H, and C) and have many applications in Dentistry

10. Ideal requirements of pattern waxes
 Should be colored to be easy distinguished either from the die
material or the tooth structure.
 Should be cohesive and not adhesive
 Should soften without flaking.
 Should soften at reasonable temperature and harden at mouth
temperature
 Should have adequate flow to record the details of the
prepared tooth
 Should burn-out completely with no residue.

11. 1. Blue inlay pattern wax
Uses
To construct wax patterns for cast inlays, onlays ,
crowns and bridges

12. 1. Blue inlay pattern wax
Presentation forms
 Usually presents as Blue or green sticks or rods
 Recently, presents in different colors and formats
(blocks or in Tins)

13. 2. Casting wax
Uses
Constructing wax patterns of metallic frameworks either
for removable partial or complete dentures.

14. 2. Casting wax
Available in many shapes and colors to fit different applications
Presentation forms
 Preformed meshes and
clasps
 Sheets with different
thickness
 Bars with round, half
round and half bear
cross section.
 wires.

15. 3. Base plate wax
Uses
1. Temporary base plates and wax rims (bite blocks)
2. waxing the dentures up.

16. 3. Base plate wax
Presentation forms
 Pink or red sheets
 Preformed shapes are currently presented

17. Other types of Dental waxes
Processing waxes: Used for laboratory issues
1. Sticky wax.. to stick 2 portions of broken restoration or casts before repair.
2. Utility wax .. to position artificial teeth to their sheets.
3. Boxing wax .. to box an impression to facilitate its pouring in stone.
Impression waxes: No longer be used
1. Bite registration wax .. to check high points of dental restorations.
2. Corrective wax .. to make functional 2ry impressions.

18. Stop, give me a
break !!!!

19. Casting Investment
Casting investment is composed of silica particles
(SiO2) bonded together with different binding agents

20. Requirements of ideal investment
material
 Should be stable (not decompose) on heating
(Refractory)
 Should show a kind of expansion to compensate for
metal shrinkage
 Withstand the impact of molten metal
 Able to wet the wax surfaces registering its details
 Should be porous to permit escape of gases
 Not adhere to the surface of cast metal
 Easy breakdown after casting
 Easily manipulated, not expensive , not irritant and no
release of toxic products

21. Types of investment materials
Relative to the type of silica particles used
 Quartz-based investment
 Cristobalite- based investment
Cristobalite-based investment provide higher rates of thermal
expansion (displasive transformation) than quartz-based investment
does (when heated from 0- 600oC)
Relative to the type of binder used
 Gypsum-bonded investment .. Uses CaSo4. 2H2O binder
 Phosphate-bonded investment .. Uses magnesium ammonium
phosphate
 Silica-bonded investment .. Uses silica gel binder
In both types Tridymite
could present as impurity

23. 1. Gypsum-bonded Investment
Uses:
For casting metal alloys that melt below1000oC (i.e.
gold alloys only) because the investment decompose if
heated above 700oc.
Presentation:
Presented as powder to be mixed with regular tap water
After mixing, the calcium sulfate hemihydrates dissolves
in water and changes into calcium sulfate dihydrates in
the form of inter-meshed crystals having the silica
particles in between..

24. A. Dimensional changes:
1. Setting expansion: as a result of gypsum setting
2. Hygroscopic setting expansion: as a result of adding water after initial
setting (6 min from the start of mix) …. water could be gained through
the following approaches;
a. Water adding using hypodermic syringe
b. Immersion in warm water
c. Contact with wet ring liner
3. Thermal expansion: as a result of silica displasive transformation
4. Heating (evaporation of water) and cooling (after casting) contraction
Important characters of Gypsum-
bonded Investment

25. Setting Vs Hygroscopic expansion
Step 1. Setting expansion
Step 2. Hygroscopic expansion
Outward
thrusting
Adding water after 6 min
Extra expansion 3 times more than the normal setting expansion
Crystal growth

26. B. Interfacial properties:
• Gypsum-bonded investment usually gives castings with little surface
roughness.
Important characters of Gypsum-
bonded Investment (cont’d)
C. Chemical properties:
• The set investment material is porous permitting the escape of gasses
through out the mold…… This fact looks advantageous in preventing
the back-pressure porosity in the cast metal
• The material could not be heated above 700oC ……. to avoid the
disintegration of gypsum binder

28. Phosphate-bonded Investment
Uses:
For casting metal-ceramic alloys and some base metal alloys (Can be
heated above 1000oc).
Presentation:
Presented as powder (containing ammonium diacid phosphate and
magnesium oxide) to be mixed with either water or colloidal silica
solution
After mixing, the following reaction occurs to form crystals of magnesium
ammonium phosphate which bind the silica particles together giving the
investment its room temperature strength.
MgO + NH4 H2Po4 ---------------> Mg NH4 Po4 + H2O

29. Important characters of phosphate-
bonded Investment
A. Dimensional changes:
1. Setting expansion
This expansion is greater with the investment mixed with colloidal silica.
2. Thermal expansion
As a result of silica displasive transformation. It is greater in investments mixed
with colloidal silica.
3. Drying shrinkage
in water-mixed investment, a drying shrinkage will occur at 200-400oC and this is
completely eliminated if the colloidal silica is used.

30. B. Interfacial properties
 PBI gives rougher cast surface than the GBI.
 increasing the silica solution ratio enhances the surface smoothness.
C. Chemical properties
The set PBI shows the same degree of porosity as that of GBI.
PBI could be heated up to 1100oC
D. Strength properties
Set PBI shows a higher strength at room temperature than that of GBI.
Set PBI shows a higher strength at high temperature than that of GBI.
Important characters of phosphate-
bonded Investment (cont’d)

32. Silica-bonded Investment
Uses:
For casting high fusing base metal alloys that usually used for
constructing partial denture frameworks.
Presentation:
Presented as powder (containing silica and MgO) and two bottles of liquids (1.
diluted silicate solution, 2. diluted HCl).
Equal volume of each solution should be mixed at first to form silicic acid sol. Powder is
then mixed with the sol that will change into silicic acid gel (room temp binder) in
presence of MgO. That gel will dry and change into silica on heating

33. Important characters of Silica-
bonded Investment
A. Dimensional changes
1. Setting contraction
Due to the transformation of silicic acid sol into silicic acid gel.
2. Green contraction
On heating, the investment mass dries and loses alcohol and water.
3. Thermal expansion
Due to displasive transformation of silica particles.
Because of higher silica content, the amount of thermal expansion
is fair enough to compensate for the shrinkage of wax pattern,
solidification of metal as well as the setting and green contraction of
the investment.

34. B. Interfacial properties
SBI produces the roughest cast in comparison to other types of
investment.
C. Strength properties
SBI has higher strength than both GBI and PBI…
D. Chemical properties:
SBI has a complicated and time consuming manipulative procedure.
SBI could be heated up to 1300oC, and it is used to cast base metal
alloys with very high melting temperatures (1300-1500oC) accordingly.
Important characters of silica-bonded
Investment (cont’d)

35. Gases comes
out
Metal gets
in
Air vent
Investment
Mold cavity
Sprue
channel
Silica particles of the set material are closely packed together, producing
a negligible degree of porosity. Lack of porosity may lead to pack-
pressure porosity in the cast metal. Air spaces or vents must be left in the
investment to allow the escape of gases.

36. Thank you