dental investments

2. 
first describe by Philbrook D, & Taggart WH
Mould/ die for pattern

3. Sprue attached to patterns

4. Patterns invested
Burnout in furnace
Flow of molten metal

5. Fundamentals of Spruing, Investing &
Casting

7. After casting
Finished & polished

8. Investing: the process of covering or enveloping,
wholly or in part, an object such as denture, tooth,
wax form, crown, etc. with a suitable investment
material before processing, soldering or casting.
(GPT 8)
 Dental casting investment: A material consisting
primarily of an allotrope of silica and a bonding
agent. The bonding substance may be gypsum (for
use in lower casting temperatures) or phosphates
and silica (for use in higher casting temperatures).
(GPT 8)


9. 1. should be capable of reproducing the shape, size and
detail in the wax pattern.
2. easily manipulated.
3. setting time should be less.
4. should maintain the integrity at higher temperatures and
should not decompose to give off gases.
5. Possess sufficiently high value of compressive strength at the
casting temperature to withstand the stresses set up when the
molten metal enters the mould.
6. should expand to compensate for the casting shrinkage

10. 7. Investment should be porous enough to permit the
air or other gases in the mold cavity to escape easily
during the casting
8. Investment should produce a smooth
surface and fine detail and margins on
the casting.
9. should be inexpensive.

11. 
Wax shrinkage- 0.3-0.4%.

Alloy shrinkage- 1.3- 1.6% ( noble
alloys), 2.0- 2.3%( Predominantly base
metal alloys)

12. 
Investment materials are basically
consist of
1. A refractory material
2. A binder material
3. Additives used to improve the
handling character.

13. SILICA (silicon dioxide) is used as refractory material & to
regulate the thermal expansion.
It is available in four allotropic forms such as
Quartz
Tridymite
Cristobalite
Fused quartz
Quartz and Cristobalite are used extensively in dental
investments

14. Cristobalite occurs naturally as a rare mineral but is
normally manufactured by prolonged heating of
the quartz at high temperatures to induce the
appropriate slow inversion.
Each form of silica exists in two phases.
1.
Low temperature phase or alpha phase
2.
High temperature phase or Beta phase
On heating the change between the two phases is rapid
and readily reversible on cooling .this change is
known as high – low inversion .

15. Depending on type of silica used the
investment materials are classified as
1.
Quartz investments
2.
Cristobalite investments

16. It binds the refractory materials together.
 According to the binder used investment materials are
of three groups.

1. GYPSUM-BONDED INVESTMENTS
2. PHOSPHATE BONDED INVESTMENTS
3. SILICA BONDED INVESTMENTS

17. Other chemicals such as sodium chloride, boric acid,
graphite, copper powder, are often added in small
quantities to modify physical properties.
 boric acid and sodium chloride, not only regulate
the setting expansion and the setting time, but also
prevent most of the shrinkage of gypsum when it is
heated above 300 ºC (572° F).


18.  They
are the mold materials used in the
casting of dental gold alloys with liquidus
temperatures no more than 1080 ºc.
 Used for conventional casting of gold alloys
inlays, onlays, crowns and Fpd.

19. Refractory
Crystalline polymorphs
of silica (quartz or
cristobalite)
 55-75%
 Silica is added to
provide a refractory
component during the
heating of the investment
and to regulate the
thermal expansion.


20.  BINDER
– Alpha hemi hydrate form of
gypsum(25-45%)
Strength of investment depends on amount of
binder present.

21. contracts

22. MODIFIER - (4-7%)
Used are
Reducing agents
Modifying chemicals
Coloring matter
Reducing agents : they reduce any metal oxides formed
on the metal by providing a non oxidizing atmosphere
in the mold when the alloy enters mold.
 Ex– Copper

23. Modifying chemicals: They regulate setting expansion
and thermal expansion and also prevent shrinkage of
gypsum when heated above 300 ºc .
 They act by reducing the two large contractions of
gypsum binder on heating to temperatures above 300
ºc .
 Ex– Boric acid
Soluble salts of alkali or alkaline earth metals

24. SETTING TIME
According to ANSI/ADA Specification No. 2 for
dental inlay casting investment, the setting time should
not be shorter than 5 min or longer than 25 min.
Usually, the modern inlay investments set initially in 9
to 18 min.
 Should allow sufficient time for mixing and investing
the pattern.


25. SETTING EXPANSION
 1. normal setting expansion: the expansion takes
place when the material sets in air
 2. hygroscopic setting expansion: when setting occurs
under water.
 3. thermal expansion: when it is affected by heat
Purpose: to enlarge the mold to compensate for the
casting shrinkage of the gold alloy

27. ADA sp no 2 for type 1 investment permits a maximum
setting expansion in air of 0.6% setting expansion of
modern investments is 0.4%which can be regulated by
accelerators and retarders .
 It is enlarging the mould containing the wax pattern
and it depends on the thermal expansion of pattern
caused by heat of reaction.
 High gypsum content- high expansion.


28. Other variables – lower w/p ratio, drier mix of
investment- high NSE.
 Thin wall pattern- more expansion than thick
walls.
 Soft wax- more NSE.
 Wax softer than type II inlay wax- NSE causes
serious distortion of the pattern.


29. Greater in magnitude than NSE.
 the gypsum product is allowed to set under or in
contact with water for mould expansion.


30. The hygroscopic setting expansion may be 6 or more times
greater than the normal setting expansion of a dental
investment
 The increased amount of expansion is because the water
helps the outward growth of crystals
 The investment should be immersed in water before the
initial set is complete.
 ADA sp no 2 for such type 2 investments require minimum
setting expansion in water of 1.2% and maximum 2.2%.


31. Composition: more finer silica particles -more HSE.
α hemihydrate greater HSE than β hemihydrate.
 W:P ratio: less water, more powder in mix.
 Spatulation: more mixing time- more HSE.
 Time of immersion: immerse in water before initial
set.
 Confinement: less opposing force from walls of
casting ring (wet cellulose), immersion in water bath at
37.7°C- expansion of wax pattern.
 Water: more immersion water.
 Shelf life: fresher investment.


32. The thermal expansion is directly related to the amount
and type of silica present.
 Contraction of gypsum is balanced- quartz 75%
 More Expansion in the presence of cristobalite and
initial contraction of gypsum is eliminated.
 Type 1 investments should have thermal expansion of
not less than1% and not greater than 1.6%.
 Type 1I investments should have thermal expansion of
between 0% to 0.6% at 500°C.


33.  SETTING
CONTRACTION: When an
investment is cooled from 700°C, it contracts.
On reheating it expand again but not
recommended as investment may crack on
heating.

34.  COMPRESSIVE
STRENGTH:
According to ADA sp no 2 the compressive strength
should not be less than 2.5MPA to withstand force of
molten metal.
 FINENESS: Surface roughness of the casting and
setting time depend upon the fineness of the material.
Fine silica- more hygroscopic expansion.

35.  POROSITY: The material should be porous to
allow escape of air from mold space while casting.
Lower content of hemihydrate- more water gaugingmore porous.

36. It is a gypsum bonded material mixed with colloidal
silica
 Setting expansion is 0.9%
 Thermal expansion is 0.6% when it is heated to 677 ºc
 As it is a gypsum bonded material it is not
recommended for high fusing alloys.
 Divestment phosphate is a phosphate bonded
investment used as a divestment for high fusing alloys.


37. Advantages





Easier to manipulate.
Adequate strength.
Adequate porosity.
Less expensive.
Amount of dimensional
change easier to control.
Disadvantages



Thermally unstable at
high temp.
Powder is hygroscopic.
Improper wax burnout &
high casting force may
produce cracks in
investment leading to
fins.

38. Most palladium and base metal alloys used for partial
dentures and porcelain fused to metal restorations have
high melting temperatures. They should be cast
a mold temperature higher than 700 ºc.
To withstand these high temperatures ,molds require
different types of binders such as phosphate
compounds.
at

39. 
They are used in construction of high melting
.
temperature dental alloys
 Soldering and porcelain veneering

40. Type 1
 For casting of inlays crowns and other restorations
especially for alloys like gold, platinum ,palladium
cobalt chromium and nickel chromium
Type 2
 For casting of removable partial dentures

41. Refractory materials – (concentration of
approximately 80%)silica in quartz , cristobalite or a
mixture of two .
Purpose
 To provide high temperature thermal shock resistance
 To provide high thermal expansion.

42. Binder (<20%)
 Magnesium oxide (acid) and a phosphate (base)
 Originally phosphoric acid was used but mono
ammonium phosphate has replaced it as it can be
incorporated in powder form

43. Modifiers :
 Carbon is often added -clean casting.
 Facilitates easy divesting of casting and mold.
 Generally added when casting alloy is gold.
 When silver palladium or base metal alloys are
invested with the investment containing carbon, it
embrittles the alloys even though the investment is
heated to the temperature that burn out the carbon.
 Palladium reacts with carbon even at above temp
1500°C, so carbon free phosphate bonded invst used
for higher temp.

44. It is available as two component
systems
 1- It is a Powder which contains
refractory materials and binders
and modifiers
 2- Aqueous solution stabilized
with colloidal silica
Colloidal silica suspension
facilitate greater expansion of the
investment which can compensate
the greater casting shrinkage of
alloys used in MC & newer gold
alloys.

45. Expansion can be varied by the proportions of silica
and water. More silica and less water – more
expansion.
 When mixed with water- initial shrinkage is seen same
as that of GBI. Completely eliminated with colloidal
silica.
Liquid can be used as full strength or diluted with water
to provide some degree of control over setting or
thermal expansion. But if more liquid is used it caused
dense nonporous investment which can effect casting.


46. Early thermal expansion is associated withdecomposition of binder, magnesium ammonium
phosphate and evolution of ammonia gas.
 Some shrinkage is masked by expansion of
refractory filler ( crystobalite)


47. 1 Compressive strength
 Type 1- 2.5 mpa
 Type 2- 3 mpa
2 Thermal expansion
 0.8% when 50:50 mixture of liquid and water

48. Temperature
Warmer the mix faster it sets
The setting reaction liberate the heat and
accelerates rate of setting

Mixing time
increased mixing time and mixing efficiency
result in faster set.


49. 1.They have high fired strength. This make them
handle without breaking before they are placed in a
furnace for the wax burn out process and strong
enough to with stand the impact and the pressure of
centrifugally cast molten alloy
2.They also provide high setting and thermal
expansion enough to compensate cast metal
prosthesis or porcelain veneers during cooling
3. They can withstand temp more than 900°C

50. 
These are used for high fusing base metal alloy

51. Refractory material – Silica
Binder –Silica gel or ethyl silicate
Modifier –
Magnesium oxide (strengthen the gel)
Ammonium chloride - accelerator

52. Silica gel: pH of sodium silicate lowered by
addition of an acid or acid salt.
 Colloidal silica converted to gel by adding
ammonium chloride.


53. Ethyl silicate: colloidal polysilicic acid is formed by
hydrolizing ethyl silicate in presence of Hcl, ethyl
alcohol & water. It is then mixed with quartz or
cristobalite and magnesium oxide ( alkaline). Coherent
gel of polysilicic acid formed, dried at temp 168°C,
loses alcohol & water to form concentrated hard gel .
Volumeric contraction during drying is green
shrinkage.
 Amines added to ethyl silicate- hydrolysis and gelation
occurs simultaneously.


54. It is supplied as a powder and liquid or two liquids
If supplied as a powder and liquid
 Powder consists of refractory particles of silicas
and glasses along with the calcined magnesium
oxide and some other refractory oxides in minor
amounts
 Liquid contains stabilized alcohol solution of silica
gel

55. If supplied as 2 liquids
 One is ethyl silicate
Certain types of amines are added for hydrolysis and
gelation to occur simultaneously
 Other is acidified solution of denatured alcohol

56. The powder is added to hydrolyzed ethyl silicate
liquid, mixed quickly and vibrated into a mold , that
has an extra collar to increase the height.
The mold is placed on a vibrator that has a tamping
action
This allows the heavier particles to settle while the
excess liquid and some of the finer particles rise to
the top .
The top of the mold is prone to cracking due to greater
drying shrinkage from evaporation of the ethyl
alcohol

57. In about 30 minutes the accelerator in the powder
hardens the settled part, and the excess is poured
off (to avoid crack formation).
The liquid powder ratio in settled part is greatly
reduced and the setting shrinkage is reduced to
0.1%

58. Can be used for higher temp castings ( 1090°C1180°C) and compactible with higher fusing
alloys.
 Care should be taken while handling and burnout
as inflammable alcohol is given off.


61. Investment can be mixed in 2 ways
1. Hand mix: The liquid is added to a clean, dry mixing
bowl, and the powder is gradually added to the
liquid, using the care and caution to minimize air
entrapment. Mixing is formed gently until all the
powder has been wet.
2. Vacuum mixing: this is a type of mechanical mixing
done under vacuum created by a vacuum mix
machine.

66. Patterns invested
Burnout in furnace
Flow of molten metal