Investment materials

1. PRESENTED BY Dr EKTA GARG
MDS 1st YEAR
DEPARTMENT OF
CONSERVATIVE DENTISTRY &
ENDODONTICS

2.  An investment can be
described as a
material which is
suitable for forming a
mold into which a
metal or alloy is
appropriately cast.
 The procedure for
forming the mold is
described as

3.  Easily Manipulated – not only should it be possible to mix & manipulate the
mass readily & to paint the wax pattern easily, but the investment should also
harden within a short time.
 Sufficient strength at room temperature – To permit ease in handling &
provide enough strength at higher temperature to withstand the impact force of
the molten metal.
 Stability at higher temperature – Investment must not decompose to give off
gases that could damage the surface of the alloy.
 Sufficient Expansion – Enough to compensate for shrinkage of the wax
pattern & metal that takes place during the casting procedure.

4.  Porosity – Porous enough to permit the air & other gases in the mold cavity to escape
easily during the casting procedure.
 Smooth surface – Fine details & margins on the casting.
Ease of Divestment – The investment should break away readily from the surface
of the metal & should not react chemically with it.
 Inexpensive
 Not be bio hazardous
 Reasonable setting time
 Long shelf life

5. REFRACTORY
MATERIAL
BINDER
OTHER
CHEMICALS

6.  Refractory material: Usually a form of silicon dioxide, such as
quartz, tridymite, cristobalite or a mixture of these.
 Binder material: Common binder used for dental casting gold
alloy is α calcium sulphate hemihydrate, phosphates and ethyl
silicate.
 Other Chemicals: Such as sodium chloride, boric acid
potassium sulphate, graphite, copper powder or magnesium
oxide.

7. • Phosphate
bonded
• Silicate bonded
High
temperature
casting
investment
• Gypsum bonded
Low
temperature
casting
investment
CLASSIFICATION
1. Based on processing temperature :-

8. 2. Based on type of
Binder Used
Gypsum bonded
investment
 Type I
 Type II
 Type III
Phosphate bonded
investments
Ethyl silicate
Bonded
investments

9. 3. BASED ON TYPE OF REFRACTORY USED
a.) Silica -
quartz
cristobalite
b.) Magnesium oxide
c.) Zirconia based investments

10. The Gypsum based materials represent
the type traditionally used for
conventional casting of gold alloy inlays,
onlays, crowns, & fixed partial dentures
(FPDs).

11. ADA specification No. 2 for casting investments for dental gold
alloys encompasses three types of investments.
 Type I: Employed for the casting of inlays or crowns when the
alloy casting shrinkage compensation is accomplished principally
by thermal expansion of the investment.
 Type II: Investments are also used for the casting of inlays or
crowns, but the major mode of compensation is by the hygroscopic
expansion of the investment.
 Type III: Used in the fabrication of partial dentures with gold
alloys.

12. Quartz or Cristobalite – Allotropic
forms of silica [ 55 – 75 % ]
alpha– hemihydrate – provides strength and
rigidity serves as a binder [ 25 – 35 %]
Chemical modifiers – 5 %

13.  The α-hemihydrate form of gypsum is generally the
binder for investments used in casting gold
containing alloys with melting ranges below
1000°C.
 When this material is heated to the temperature
required for complete dehydration and sufficiently
high to ensure complete castings, it shrinks
considerably and frequently fractures.

14. Dimensional Change
of three forms of
gypsum when heated

15.  Silica (SiO2) is added to provide a refractory component
during the heating of the investment and to regulate the
thermal expansion.
 During the heating, the investment is expected to
expand thermally to compensate partially or totally for
the casting shrinkage of the gold alloy.
 If proper forms of silica are employed in the investment,
the contraction of gypsum during heating can be
eliminated and changed to an expansion.

16. Thermal expansion of
four forms of silica
Quartz 575°c
Cristobalite b/w
200°c & 270°c
Tridymite two
inversions occur at
117°c & 163°c

17. Cristobalite
Tridymite
Fused Quartz

18.  Modifying agents, coloring matter, & reducing agents,
such as carbon & powdered copper.
 Reducing agents provide a nonoxidizing atmosphere in
the mold when gold alloy is cast.
 Some of the added modifiers, such as boric acid, and Na
chloride, not only regulate the setting expansion and the
setting time, but they also prevent most of the shrinkage
of gypsum when it is heated above 300°C.

19.  According to ADA specification No. 2 for dental inlay
casting investment, the setting time should not be
shorter than 5 minutes nor longer than 25 minutes.
 Usually, the modern inlay investments set initially in 9
to 18 minutes.
 Sufficient time should be allowed for mixing and
investing the pattern before the investment sets.

20.  The purpose of setting expansion is to aid in enlarging the mold
to compensate partially for the casting shrinkage of the gold.
 ADA specification No. 2 for Type I investment permits a
maximum setting expansion ‘in air’ of only 0.6%.
 The setting expansion of such modern investment is
approximately 0.4%. It can be regulated by retarders and
accelerators.
 A mixture of silica and gypsum hemihydrate results in setting
expansion greater than that of the gypsum products when it is
used alone.

21.  Setting expansion is influenced by the exothermic heat
transferred to the pattern.
 Variables other than the exothermic heat of reaction also
influence the effective setting expansion.
• As the investment sets, it essentially gains sufficient
strength to produce a dimensional change in the wax
pattern & mold cavity as setting expansion occurs.
• Also, the softer the wax, the greater the effective setting
expansion, because the softer wax is more readily moved
by the expanding investment.

22.  One of the methods for expanding the casting mold to compensate
for the casting shrinkage of the gold alloy.
 Occurs when the gypsum product is allowed to set when placed in
contact with water.
 Greater in magnitude than normal setting expansion
 ADA specification No. 2 for Type II investments requires a
minimum setting expansion in water of 1.2% while the maximum
allowed is 2.2%.

24.  Factors influencing hygroscopic expansion-
a. Effect of composition-
The magnitude of setting expansion of a dental
investment is generally proportional to the silica content
of the investment.
• Finer the particle size of silica greater the
expansion.
• α-hemihydrate will produce a greater expansion
than β-hemihydrate.

25. b. Effect of water:powder ratio:
The highest the W:P ratio of the original investment
water mixture, the less the hygroscopic setting
expansion.
c. Effect of spatulation:
With most investments, as the mixing time is reduced,
the hygroscopic expansion is decreased.

26. d. Shelf life of the investment:
Older the investment, the lower its hygroscopic
expansion.
e. Effect of time of immersion:
The greatest amount of hygroscopic setting expansion is
observed if the immersion takes place before the initial
set.

27. f. Effect of the amount of water added:
 The magnitude of hygroscopic expansion is in direct proportion to
the amount of water added during the setting period until a
maximum expansion occurs, no further expansion is evident
regardless of any amount of water added.
 Expansion can be detected when water is poured into a vessel
containing only small, smooth quartz particles. The water is drawn
between the particles by capillary action and thus causes the particle
to separate, creating an expansion.

28.  The effect is not
permanent after the water is
evaporated, unless a binder is
present.
 The greater the amount of
the silica or the inert filler,
the more easily the added
water can diffuse through the
setting material and the
greater is the expansion.

29.  The thermal expansion of a gypsum bonded investment is
directly related to the amount of silica present and to the type of
silica employed. A considerable amount of quartz is necessary to
counterbalance the contraction of gypsum during heating.
 The contraction of the gypsum is entirely balanced when the
quartz content is increased to 75%.
 The investments containing cristobalite expand earlier and to a
greater extent than those containing quartz.

30. The desirable magnitude of the thermal expansion of a
dental investment depends on its use.
If hygroscopic expansion is to be used to compensate
for the contraction of the gold alloy, as for the Type II
investment. ADA specification No. 2 requires that the
thermal expansion be between 0% and 0.6% at 500°C.

31. However, for Type I investment, which rely principally
on thermal expansion for compensation, the thermal
expansion must be not less than 1% nor greater than 1.6%.
Another desirable feature of an inlay investment is that
its maximum thermal expansion be attained at a
temperature not higher than 700°C. Thus when a thermal
expansion technique is employed, the maximum mold
temperature for casting of gold alloy should be less than
700°C.

33.  Factors affecting thermal expansion-
a. Effect of Water:Powder ratio
The magnitude of thermal expansion
is related to the amount of solids
present. Therefore it is apparent that
the more water that is used in mixing
the investments, the less is the
thermal expansion that is achieved
during subsequent heating.

34. b. Effect of chemical modifiers:
The addition of small amounts of sodium, potassium, or
lithium chlorides to the investment eliminates the
contraction caused by the gypsum and increases the
expansion without the presence of an excessive amount
of silica.

35. c. Thermal contraction/cooling
of investment:
When the investment is cooled from
700˚c, its contraction follows the expansion
curve during the inversion of the beta-
quartz or beta crytobalite to its stable alpha
form at room temperature.
On cooling – investment shows overall
contraction as compared to its dimensions
before heating .
On reheating – does not expand to its
previous level – also it can cause cracks &
affect quality of casting .

36. d. Strength:
According to ADA specification No. 2, the compressive
strength for an inlay investment should not be less than
2.4Mpa tested 2 hours after setting.
Heating the investment to 700°C may increase or
decrease the strength as much as 65%, depending on the
composition. The greatest reduction in strength on heating
is found in investments containing sodium chloride.

37. Other gypsum investment considerations:
Fineness:- Finer the investment, the smaller are the surface
irregularities on the casting.
Porosity:- More gypsum crystals present in the set
investment,
less porosity.
More uniform the particle size, greater is its porosity.
Storage:- The investment should be stored in airtight &
moisture
proof containers.

39. As suggested by Skinner (1963) “The definite advan
type of investment is that there is less chance for c
alloy during casting and hence could be the investmen
The present trend is towards the use of less expens
Alloys, most of which require phosphate investments.
The rapid growth in use of metal
ceramic restorations and a higher melting
point alloys have resulted in an increased
use of “phosphate bonded investment.”

40. Refractory
fillers
Binders

41. Filler: silica, in the form of cristobalite, quartz, or a mixture of
the two and in the concentration of approximately 80%.
• The purpose of this filler is to provide high temperature thermal
shock resistance (refractoriness) and a high thermal expansion.
The binder: consists of magnesium oxide and a phosphate
(Monoammonium phosphate).

42.  Colloidal silica liquid suspensions are available for use with the phosphate
bonded investments in place of water. For base metal alloys, a 33% dilution of the
colloidal silica is required.
 Carbon is often added to the powder to produce clean castings, and facilitate the
‘divesting’ of the casting from the mold.
Water
Exhibit less SE
Hygroscopic expansion
negligible
Colloidal silica
- Exhibit higher SE – useful as
shrinkage more in base metal alloys
.
Can expand hygroscopically
Strength of investment more

43. Setting Reaction
The chemical reaction for the binder system that
causes the investment to set and harden is
NH4H2PO4 + MgO + 5H2O  NH4 MgPO4 6H2O

44. MgO+NH4H2PO4+ 5H2O
(NH4MgPO4 .6H2O)n Room Temperature
MgO
NH4H2PO4 Colloidal-type particles
H2O
Prolonged setting at 25°c
or dehydration at 50°c
(NH4MgPO4 .6H2O)n
Dehydrated at 160°c
(NH4MgPO4 .H2O)n
Heated from 300-650°c
(Mg2P2O7)n
Noncrystalline polymeric phase
Mg2P2O7
Heated above 1040°c
Mg3 (P2O4) 2
On heating the binder undergoes the
following thermal reactions :

45. When phosphate bonded investments are mixed with water they
exhibit a shrinkage within essentially the same temperature range
as gypsum bonded investments i.e. 200°c-400°c
This contraction is practically eliminated when a colloidal silica
solution replaces the water.
The early thermal shrinkage of phosphate investments is
associated with the decomposition of the binder, magnesium
ammonium phosphate and is accompanied by the evolution of
ammonia, which is readily apparent by its odor.
Setting and Thermal Expansion

46. Influence of liquid concentration
on setting & thermal expansion.
Thermal expansion of the investment
when mixed with water as compared
to special liquid.

47. Phosphate investments are markedly affected by
temperature. The warmer the mix, the faster it sets.
The setting reaction itself gives off heat, and this
further accelerates the rate of setting. The more
efficient the mixing better the casting in terms of
smoothness and accuracy.
The ideal technique is to mix, as long as possible, yet
have enough time for investing. Mechanical mixing under
vacuum is preferred.

48. Ethyl Silicate Bonded
Investment

49. HYL SILICATE bonded investments are being us
nstruction of the high fusing base metal partial d
alloys.
e investments are losing popularity because of th
mplicated and time consuming procedures involve
The silica is used as the binder
h may be derived from ethyl silicate or sodium sil

50. POWDER LIQUID It’s a mixture of
 Powder – refractory particles of silica , MgO & other
oxides .
 For these systems – 2 or more liquids – reacted together
– before being mixed with powder .

51. The REACTION
The silica is first formed by the hydrolysis of ethyl
silicate in the presence of hydrochloric acid, ethyl
alcohol & water. The reaction can be expressed as:
Si (OC2H5) + 4H2O HCl Si(OH)4 + 4C2H5OH

52. Because a polymerized form of ethyl silicate is used, a
colloidal sol of polysilicic acids is expected instead of the
simpler silicic acid sol shown in the reaction- Stage
called - HYDROLYSIS
The second stage of reaction – GELATION .
Sol – mixed with quartz or cristobalite + small amount
of MgO to render the mixture alkaline .
Coherent gel of polysilicic acid formed – accompanied
by slight ‘setting shrinkage’.

53. Third stage – DRYING
Soft gel – dried to a temp. below 168 °C .
During drying – Gel loses alcohol & water – to form
hard concentrated gel of silica particles – tightly
packed together .
Volumetric contraction accompanies drying – “ green
shrinkage” .

54.  Process of ethyl silicate-bonded investment is a little
more complicated than that of phosphate type in that care
must be exercised during handling & burnout because
inflammable alcohol is given off.
This type of investment can be heated between 1090°c
and 1180°c and is compatible with the higher fusing
alloys. Its low setting expansion minimizes distortion.