Investment material

1. Investment materials

2. • 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 used in higher casting
temperatures).
(GPT 8)

3. DEFINITION
• 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)

4. • Acc to Craig ,
-- An investment can be described as a ceramic
material that is suitable for forming a mold into
which a metal or alloy is cast.
-- The operation of forming a mold is described as
investing.

5. Introduction
• The adoption of the casting practice in dentistry
for making crowns, bridges, gold alloy inlays and
other restorations represent one of the major
advancement in restorative dentistry.

6. • Following the production of a wax pattern either
the direct or indirect method, the next stage in
many dental procedures involves the investment
of the pattern to form a mold.
• Lost wax casting procedure is adopted for
fabrication of metal alloy appliances like inlays,
crowns etc outside the mouth.

7. • An accurate wax pattern is prepared ,is invested
in refractory mould materials.
• After wax burn out the mould is filled with alloy
liquid, which solidifies & cools down. During
this procedure the possible dimensional change
taking place are to be suitably compensated.

8. Methods of compensation:
Compensation is done by carefully controlling the
mould expansions, by availing large setting
expansion & adjusting the thermal expansions of
the investment materials.
Setting Expansion + Thermal Expansion

9. Properties required of an investment
1. Easily manipulated.
2. Sufficient strength at room temperature: the
inner surface of mold should not break down at
higher temperatures due to force of molten
metal.

10. 3. Stability at higher temperatures: it should not
decompose to give off gases that could damage
the surface of the alloy.
4. Sufficient expansion: it must expand enough to
compensate for shrinkage of the wax pattern and
metal that takes place during the casting
procedure.

11. 5. Beneficial casting temperatures: preferably the
thermal expansion versus temperature curve
should have a plateau of the thermal expansion
over a range of casting temperature.
6. Porosity: it should be porous enough to permit
the air or other gases in the mold cavity to
escape easily.

12. 7. Smooth surface: fine detail and margins on the
casting should be preserved.
8. Ease of divestment: The investment should
break away readily from the surface of the metal
and should not have reacted chemically with it.

13. 9. Inexpensive.

14. Composition
• An investment is a mixture of three distinct
types of materials: refractory material, binder
material, and other chemicals.

15. • Refractory material
-- It is usually a form of silicon dioxide, such as
quartz, tridymite, or cristobalite, or a mixture of
these.
• Binder material
-- As the refractory material alone do not form a
coherent solid mass, some kind of binder is
needed.

16. -- Common binder used are: α-calcium sulfate
hemihydrate.
-- Phosphate, ethyl silicate, and other similar
materials also serve as binder for high
temperature casting investments.

17. • Other chemicals
-- Alone refractory material and binder material
cannot produce the desirable properties
required.
-- Other chemicals , such as sodium chloride, boric
acid, potassium sulfate, graphite, copper
powder, or magnesium oxide are added in small
quantities.

18. -- Small amounts of boric acid or chlorides
enhance thermal expansion of investments
bonded by calcium sulfate.

19. There are in general three types of investments:
• Gypsum bonded investment
• Phosphate bonded investment
• Silica bonded investment

20. • Gypsum bonded investments are the oldest
materials and are used for casting conventional
gold alloys.
• The phosphate bonded investments are used for
base metal alloys in fixed partial prosthesis.
• Silica bonded investments are principally used
for the casting of base metal alloy partial
dentures.

21. Calcium Sulphate-Bonded Investments
• They are used for casting metal inlays, onlays,
crowns and bridges.
• They are used for casting gold alloys.
• They can withstand temp up to 700oC.

22. • Acc to ADA specification no-2 there are three types
of gypsum bonded investment materials:
-- Type 1: thermal expansion type; for casting inlays
and crowns.
-- Type 2 : hygroscopic expansion type; for casting
inlays and crowns
-- Type 3 : for casting complete and partial dentures.

23. • These materials are supplied as powders which
are mixed with water and are composed of a
mixture of silica (SiO2) and calcium sulphate
hemihydrate.
• Other minor components including graphite, or
powdered copper.

24. Composition
• 25% to 35% - α-calcium sulfate hemihydrate.
• 65% to 75% - quartz or cristobalite.
• 2% to 3% - chemical modifiers.

25. Gypsum
• α- hemihydrate form of gypsum act as a binder
to hold the other ingredients together and to
provide rigidity.
• When this material is heated to the temperatures
required for complete deydration and
sufficiently high to ensure complete castings, it
shrinks considerably and occassionally fractures.

26. • All forms shrink considerably after dehydration
between 200oC and 400oC.
• A slight expansion takes place between 400oC
and 700oC.
• And a large contraction then occurs.

27. • The shrinkage on heating is due to the
dehydration of the set gypsum in two stages.
2CaSO4 · 2H2O  (CaSO4)2 H2O + 3H2O
(CaSO4)2 · H2O  2CaSO4 + H2O
• Shrinkage is due to the transformation of
calcium sulphate from the hexagonal to the
orthorhombic configuration.

28. • This later shrinkage is most likely caused by
decomposition and the release of sulfur gases,
such as sulfur dioxide.
• This decomposition not only causes shrinkage
but also contaminates the castings with the
sulfides.

29. • Thus it is imperative the gypsum products not be
heated above 700oC and these effects can be
minimized by ‘heat soaking’ the mould at 700°C
for at least an hour to allow the reactions to be
completed before casting commences.

30. • The gypsum products containing carbon should
not be heated above 650oC.
• In this way , proper fit and uncontaminated
alloys are obtained.

31. Silica (SiO2)
• It is added to provide a refractory component during the
heating of the investment and to regulate the thermal
epansion.
• Usually, the wax pattern is eliminated from the mold by
heat.
• During the heating, the investment is expected to expand
thermally to compensate partially or totally for the
casting shrinkage of the gold alloy.
• Gypsum shrinks considerably when it is heated,
regardless of whether it is set plaster or stone.

32. • If the proper forms of silica are employed in the
investment, this contraction during heating can
be eliminated and changed to an expansion.
• Silica exists in at least four allotropic forms:
quartz, tridymite, cristobalite, and fused quartz.

33. • When quartz, tridymite, or cristobalite is heated,
a change in crystalline form occurs at a
transition temperature characteristic of the
particular form of silica.
• For example, when quartz is heated, it inverts
from a "low" form, known as α-quartz, to a
"high" form, called β-quartz, at a temperature of
575o C (1067o F).

34. • In a similar manner, cristobalite undergoes an
analogous transition between 200o C (392o F)
and 270o C (518o F) from "low" (α-cristobalite) to
"high" (β-cristobalite).
• Two inversions of tridymite occur at 117o C (243o
F) and 163o C (325o F), respectively.

35. • The β-allotropic forms are stable only above the
transition temperature noted, and an inversion
to the lower α form occurs on cooling in each
case.
• In powdered form, the inversions occur over a
range of temperature rather than
instantaneously at a specific temperature.
• The density decreases as the α form changes to
the β form, with a resulting increase in volume
that is exhibited by a rapid increase in the linear
expansion.

36. The density decreases when the alpha
form changes to beta form with a
resulting increase in the new volume

37. • Fused quartz is amorphous and glasslike in
character, and it exhibits no inversion at any
temperature below its fusion point.
• It has an extremely low linear coefficient of
thermal expansion and is of little use in dental
investments.

38. • Thermal expansion of three forms of
silica

39. • Quartz, cristobalite, or a combination of the two
forms may be used in a dental investment.
• Both are now available in pure form.

40. • Tridymite is no longer an expected impurity in
cristobalite.
• On the basis of the type of silica principally
employed, dental investments are often
classified as quartz or cristobalite investments.

41. Modifiers
• The reducing agents are used in some
investments to provide a nonoxidizing
atmosphere in the mold when the gold alloy is
cast.
• Some of the added modifiers, such as 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 100oC (572o F)

42. • They are limited to the usual balancing agents to
regulate the setting time and setting expansion.

43. Setting Time
• The setting time should not be shorter than 5
mins or longer than 25 mins.
• Usually, the modern inlay investments set
initially in 9 to 18 mins.

44. Normal setting expansion:linear dimensional
change as the investment sets
Silica particles result in greater setting
expansion
HOW??
silica particles interfere with the
intermeshing and interlocking of
crystals
resulting in outward thrust of
crystals
resulting in
expansion.

45. According to ADA sp no 2:
FOR TYPE I investments:0.6%
Value of setting expansion for modern
investments is 0.4%,regulated by
retarders and accelerators.
Purpose :
To aid in enlarging the mold to
compensate partially for the casting
shrinkage of gold.

46. Factors determining the effective setting expansion
• Greater the gypsum content of the
investment, greater the exothermic heat
transmitted to the wax pattern and greater
the mould expansion.
• Lower the W/P ratio for the investment,
greater the exothermic heat and greater
the setting expansion.

47. • Thinner the walls of the wax pattern, greater the
setting expasion of the investment.
• Softer the wax, greater the setting expansion. If a
wax softer than Type B inlay wax is used, the
setting expansion may cause a serious distortion
of the pattern.

48. 2).HYGROSCOPIC SETTING EXPANSION-
expansion that occurs as the investment
hardens while immersed in water
• This is one of the methods for
expanding the casting mold to
compensate for casting shrinkage.
• The hygroscopic setting
expansion may be 6 or more times
greater than the normal setting
expansion of a dental investment

49. • The water is drawn between the
refractory particles by the
capillary action and thus causes
the particles to separate creating
an expansion
• The investment should be
immersed in water before the
initial set is complete.
• ADA sp no 2 : type 2 investments
require minimum setting
expansion in water of 1.2% and
maximum 2.2%.

50. • In one, method known as the ‘water immersion’
technique, the investment mould is placed into
water.
• Another method is the ‘water added’ technique.
• Here a measured volume of water is placed on the
upper surface of the investment material within the
casting ring.
• This produces a more readily
controlled expansion.

51. FACTORS AFFECTING HYGROSCOPIC SETTING
EXPANSION
1. Effect of composition
v The finer the particle size of the
silica, the greater is the hygroscopic
expansion.
v Higher the silica content greater is
the expansion.
vAlpha hemihydrate produces more
expansion in the presence of silica,
than beta hemihydrate.

52. HYGROSCOPIC
SETTING
EXPANSION &
NORMAL SETTING
EXPANSION

53. • The hygroscopic setting expansion of stone or
plaster alone is very slight.
• The investment should have at least 15% binder
to provide strength after hygroscopic setting
expansion, and to prevent drying shrinkage

54. FACTORS AFFECTING HYGROSCOPIC
SETTING EXPANSION
1. Effect of composition
v The finer the particle size of the silica,
the greater is the hygroscopic expansion.
v Higher the silica content greater is the
expansion.

55. 2. Effect of w/p ratio
v The higher the W/p ratio of the
original investment water mixture, the
less is the hygroscopic setting expansion.

56. .3.Effect of temperature
v Higher the temperature of immersion
water, less is the surface tension and hence
greater is the expansion
4. Effect of time of immersion
v Immersion before the initial set causes
greater expansion.
5. Effect of spatulation
v The shorter the mixing time, the less is the
hygroscopic expansion.

57. 6. Effect of shelf life of the investment
v The older the investment, the less is the
hygroscopic expansion.
v The material should be stored in air tight
containers and should not be exposed to humidity.
v It is better to purchase small amounts of the
investment at a time.
7. Effect of confinement
v Both the normal and the hygroscopic setting
expansions are confined by opposing forces,
such as the walls of the container in which the
investment is poured or the walls of a wax
pattern.

58. vThis confinement can be avoided largely by
placing damp asbestos as a liner on the inner
wall of the ring.
vThe water in the asbestos also is utilized for
hygroscopic expansion.

59. 8. Amount of added water
vAn increase in the amount of water added,
increases the hygroscopic setting expansion upto
a certain point, after which further addition of
water does not create any expansion.

60. • This degree of maximum expansion is called the
“critical point”.
• This critical point can be raised or lowered easily
by changing the manipulative conditions like
W/P ratio, time of spatulation, age of investment
etc.

61. Particle size of silica
• Finer particles of silica produce greater
hygroscopic expansion.
• The hemihydrate particles have little effect on
this expansion.

62. Silica/binder ratio
• If this ratio increases, greater will be the
hygroscopic expansion and lesser the strength.
• This is because the added water can easily
diffuse through the silica particles.

63. Thermal Expansion
• Thermal expansion is directly related to the
amount and type of silica employed.
• The contraction of gypsum is entirely balanced
when the quartz content is about 75%.

64. • Type I investments – thermal expansion of not
less than 1.0% nor greater than 1.6%.

65. • maximal thermal expansion be attained at a
temperature not greater than 700°C.
• Gold alloys are apt to be contaminated above
this temperature.

66. Factors affecting thermal expansion
• W/P ratio
More the water used for mixing – less will be the
thermal expansion.
• Effect of chemical modifiers- The addition
of small amounts of chlorides of sodium,
potassium or lithium to the investment
eliminates the contraction and increases
expansion.

67. Strength
• ADA specification no. 2 requires a minimum
compressive strength of 2.5 Mpa, 2 hours after
setting of the investment

68. • Modifiers aid in increasing the strength as more
of the binder can be used without much
reduction in thermal expansion.
• Use of alpha hemihydrate increases compressive
strength( than beta hemihydrates).

69. Fineness
• Fineness of the investment affects the setting
time and surface roughness of the casting. Fine
silica particles increase the hygroscopic setting
expansion and gives smoothness to the casting.

70. Porosity
• More gypsum crystals present in the set
investment – less is the porosity.
• Less the hemihydrate content – greater the
amount of gauging water – more is the porosity.
• Mixture of coarse and fine particles exhibits less
porosity than an investment composed of
uniform particle size.

71. Storage
• Air tight & moisture proof containers.
• Purchase in smaller quantities.

72. Phosphate bonded investment
material
• The use of phosphate or silica boned investment
is increased as rapid growth in use of metal
ceramic restorations and increased use of higher
melting alloy.
• Base metal alloys and higher melting range gold
alloys require phosphate bonded investment
material.

73. Composition
 This type of investment consists of three types of
components, each of which is responsible for certain
characteristic properties:
 The components are:
1. Ammonium Diacid phosphate
• It gives strength, soluble in water, provide
phosphate ions, reacts with silica at high temp to
increase strength.
2. Refractory filler : silica in the form of cristobalite,
quartz or a mixture of the two and in concentration
of approx 80%.

74. • Purpose of the filler: to provide high
temperature thermal shock resistance and a high
thermal expansion.

75. • 3. Binder - Magnesium oxide and phosphate
that is acid in nature.
• Initially phosphoric acid was used but
monoammonium phosphate has replaced it,
because it can be incorporated into the
investment powder.

76. • Colloidal silica suspensions are available for use
with phosphate investments instead of water.
• These suspensions can freeze and become
unusable – so should be stored in frost free
environment.

77. • Phosphate bonded investments are mixed with a
special liquid .
• The liquid is a form of silica sol in water, which
gives higher thermal expansion.

78. • Carbon is added to the powder to produce clean
casting and facilitate the divesting of the casting
from the mold.
• This addition is appropriate when casting alloy is
gold but not with base metal alloys.
• Carbon embrittles the alloy even though the
investment is heated to temp that burn out the
carbon.

79. • The chemical reaction is as follows that causes
the investment to set and harden:
NH4H2PO4 + MgO + 5H2O  NH4MgPO4 6H2O

81. Working and Setting time
• Unlike gypsum investments, phosphate investments are
markedly affected by temperature.
• Warmer the mix, faster the set.
• The reaction gives off heat, which further accelerates the
setting. Increased mixing time and mixing efficiency
result in a faster set; these two factors give smoothness
and accuracy to the casting.
• Mechanical mixing under vacuum is preferred.
• An increase in liquid/powder ratio increases the working
time.

82. Setting and thermal expansion
• As powder(gypsum) and liquid is mixed there is
slight expansion. This can be increased by using
colloidal silica solution instead of water.
• When phosphate bonded investment material is
mixed with liquid containing silica first there is
early thermal shrinkage.

83. • This is associated with the decomposition of the
binder, magnesium ammonium phosphate and
is accompanied by evolution of ammonia.
• Shrinkage is masked because of the expansion of
the refractory filler, like cristobalite.

84. Setting and thermal expansion
• The combined setting and thermal expansion for
phosphate investments is around 2% if the
special silica liquid is used

85. Ethyl Silicate bonded investments
• Less popular because of more complicated and
time consuming procedures.
• Used in construction of high fusing base metal
partial denture alloys.
• Binder is silica gel that reverts to silica on
heating.

86. • Several methods are used to produce silica or
silicic acid gel binders.
• When a pH of sodium silicate is lowered by the
addition of an acid or an acid salt such as
monoammonium phosphate, a bonding silicic
acid gel forms. The addition of Magnesium oxide
will strengthen the gel.
• An aqueous solution of colloidal silica can be
made to gel by the addition of an accelerator,
such as ammonium chloride

87. • A colloidal silicic acid is first formed by
hydrolyzing ethyl silicate in the presence of
hydrochloric acid, ethyl alcohol and water, as
follows:
Si(OC2H5) + 4H2O  Si (OH)4 + 4C2H5OH
• Because a polymerized form of ethyl silicate is
actually used, a colloidal sol of polysilicic acids is
formed instead of the simpler silicic acid shown
in the above reaction.

88. • The formation of poly silicic acid constitutes the
1st stage of the setting reaction, called
“hydrolysis”.
• Stage 2 is called “gelation”. Here the sol is
mixed with quartz or cristobalite to which is
added a small amount of finely powdered MgO to
render the mixture alkaline. A coherent gel of
polysilicic acid then forms accompanied by a
slight ‘setting shrinkage’.

89. • Stage 3 is called “drying”.
• Here the soft gel is dried to a temperature below
168°C. During drying, the gel loses alcohol and
water to form a hard, concentrated gel of silica
particles tightly packed together.
• A considerable volumetric contraction
accompanies the drying. Which reduces the size
of the mould. This contraction is known as “green
shrinkage” and it occurs in addition to the setting
shrinkage.

90. • A faster method to obtain silica gel is by the
addition of amines such as piperidine to the
solution of ethyl silicate. Here hydrolysis and
gelation occurs simultaneously. But an
unacceptable shrinkage may occur, mainly in the
stage of hydrolysis.

91. • Stock solutions of hydrolysed ethyl silicate
binder may be prepared and stored in dark
bottles.
• The solution gels slowly on standing and its
viscosity may increase noticeably after 3-4 weeks
when it has to be discarded.

92. • Silica-bonded investments being more refractory
than phosphate-bonded investments, can
tolerate higher burn-out or mould-casting
temperatures.
• Temperatures between 1090 and 1190°C are
employed when the higher fusing chromium
containing alloys are cast.

93. Investments for small casting
OBJECTIVE
-casting
To provide metallic duplication of missing
tooth structure with accuracy.

94. • Steps in making metal inlays, onlays crowns
and bridges
case selection
tooth preparation
gingival retraction
making impression
die preparation

95. • Wax pattern preparation
• Investment of wax pattern
• Burnout procedure
• Casting
• finishing and polishing
• Cementation
• Recall.

96. Wax pattern with sprue attached