The document provides a comprehensive overview of investment materials used in dental casting, including definitions, classifications, and properties of gypsum-bonded, phosphate-bonded, and silicate-bonded investments. It details the requirements for these materials, their composition, setting processes, and their thermal expansion characteristics to ensure accuracy in dental restorations. Additionally, the document addresses the use of newer materials for casting titanium alloys due to conventional investment materials' limitations.

1. Investment materials
Dr Lekshmy AR
1st
year MDS

2. Contents
• Definitions
• Requirements of investment materials
• Gypsum-bonded investment
• Phosphate-bonded investment
• Silicate-bonded investment
• Implication of material science during casting
• Other investment materials
• Summary
• References

3. Investing : The process of covering or
enveloping, wholly or in part, an object such as a
denture, tooth, wax form, crown, etc., with a
suitable investment material before processing,
soldering, or casting (GPT 9)
Definitions
Dental casting investment : A material
consisting principally 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 9)

4. Requirements of an investment material
 Must reproduce precisely the details form of the wax pattern
 Must provide sufficient strength to withstand the heat of burnout and
the impact and the pressure of cast molten alloy
 Must maintain the integrity at higher temperatures and should not
decompose to give off gases
 Must expand sufficiently to compensate for the solidification
shrinkage of the alloy
 Easily manipulated and setting time should be less
 Should be inexpensive
 Biological safety Fundamentals
Fundamentals of fixed prosthodontics,
3rd
edi.Herbert T. Shillingburg

5. Composition
An investment is a mixture of three distinct types of materials
 Refractory material
 Binder material
 Additives.

6. 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.

7. Common binder used are:
1. α-calcium sulfate hemihydrate
2. Phosphate
3. Ethyl silicate

8. Other chemicals
• Sodium chloride, boric acid, potassium sulfate, graphite, copper
powder, or magnesium oxide are added in small quantities.

9. Classification
Types Uses
1 Gypsum bonded investment Gold alloy inlays, onlays,
FPD
2 Phosphate bonded investment Base metal alloys FPD &
metal ceramic alloys
3 Silica bonded investment Co-Ni alloys, base metal
alloy partial dentures.

10. Gypsum bonded investment material
 They are the mold materials used in the casting of
dental gold alloys with temperature below 7000
C.
 Used for conventional casting of gold alloy inlays,
onlays, crowns and fpd.

11.  ADA specification-2; divided them further into
• Type I- for casting inlays/crowns.
Mode of expansion: thermal
• Type II-for casting inlays/crowns
Mode of expansion : hygroscopic
• Type III-for partial dentures with gold alloys

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

13. Binder
• Alpha hemi hydrate form of gypsum(25-45%)
• Strength of investment depends on amount of binder present.

14. Additives - (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.
Eg– Copper

15. Modifying chemicals: They regulate setting expansion
and setting time and also prevent shrinkage of gypsum when
heated above 200ºC .
Eg– Boric acid,
Soluble salts of alkali or alkaline earth metals

16. Setting reaction
Theories to explain setting reaction
1.Colloidal theory
2.Hydration theory
3.Dissolution –precipitation theory
Among these dissolution-precipitation theory most
accepted

17. Dissolution of calcium sulfate hemihydrate
Formation of saturated solution of calcium sulfate
Aggregation of calcium sulfate dihydrate
Precipitation of dihydrate crystals
The crystallization of calcium sulfate dihydrate

18. Properties
1. 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.

19. Factors controlling setting time
o Mixing time
o Water powder ratio
o Temperature
o Modifiers

20. 2.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: enlarging the mold to compensate for the
casting shrinkage of the gold alloy

21. a) Normal Setting Expansion
• Liner dimensional change as the investment sets
• A mixture of silica and hemihydrate gypsum results setting
expansion greater than that of gypsum product used alone
• Silica particles interfere with the intermeshing and
interlocking of crystals resulting in outward thrust of
crystals resulting in expansion
• Maximum setting expansion – 0.5%

22. b) Hygroscopic setting expansion
 Greater in magnitude than NSE.
 The gypsum product is allowed to set in contact with
water for mold expansion.
 Curve A-NSE;
 Curve B-Hygroscopic expansion
 Water is added 5 min after beginning of the mixing

23. • 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%.

24. Factors affecting expansion
 Composition: more finer silica particles -more HSE.
 α hemihydrate greater HSE than β hemihydrate.
 W:P ratio-lower W:P ratio-more HSE
 Spatulation: more mixing time- more HSE.
 Time of immersion: immerse in water before initial set more
expansion than immersion is delayed beyond the time of initial set.

25.  Confinement of the investment by the walls of the
container or the wax pattern reduces HSE.
 Added Water : HSE directly related to amount of water
added during the setting period until a maximum
expansion occurs.
 Shelf life: fresher investment-more HSE

26. c) Thermal setting expansion
• The thermal expansion is directly related to the amount and
type of silica present.
• Contraction of gypsum is balanced when quartz content
increases to 75%.
• Type 1 investments should have thermal expansion of not less
than1% and not greater than 1.6%.

27. • Type II investments should have thermal expansion of
between 0% to 0.6% at 500°C.
• Maximum thermal expansion should be attained at
temperature not higher than 700 degree centigrade
Factors affecting thermal expansion
• W/P ratio-more water less expansion
• Related to the amount and type of silica used
• Chemical modifiers : NaCl, KCl, LiCl increases TE

28. 3.Setting contraction
• When an investment is cooled from 700°C, it contracts but
less than its original dimension.
• On reheating it expand thermally to the same maximum
reached before.
Precaution – Investment should not be heated a second time
because internal cracks may develop.

29. 4.Compressive strength
• According to ADA sp no 2 the compressive strength should not
be less than 2.4mpa when tested 2 hr after setting
5.Fineness
• Surface roughness of the casting and setting time depend upon
the fineness of the material.
• Fine Silica- more hygroscopic expansion.

30. 6.Porosity
• The material should be porous to allow escape of air from mold space
while casting.
• The common method of venting the mold is through the pores of the
investment.

31. Storage
• Should be stored in airtight and moisture proof
containers.
• Purchase in small quantities.

32. Phosphate bonded Investment material
• Most palladium and base metal alloys used for partial dentures
and porcelain fused to metal restorations have high melting
temperatures.
• They should be cast at a mold temperature higher than 700ºc.

33. • To withstand these high temperatures ,molds require
different types of binders such as phosphate compounds.
• They are used in construction of high melting temperature
dental alloys.
• Soldering and porcelain veneering

34. Types
ADA Sp No.42
Type 1
• For casting of inlays crowns and other fixed restorations
Type 2
• For casting of removable partial dentures

35. Composition
Refractory materials (Approximately 80%)
Silica in quartz , cristobalite or a mixture of two .
Purpose
• To provide high temperature thermal shock resistance
• To provide high thermal expansion.

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

37. Modifiers
• Carbon is often added -clean casting.
• Facilitates easy divesting of casting and mold.
• Generally added when casting alloy is gold.
• Not used with Ag-palladium alloys or base metal alloys as;
o palladium reacts with carbon at temperatures
above15040
C
o carbon embrittles the alloys.

38. It is available as two component systems
1. Powder which contains refractory materials, 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 Metal Ceramic & newer gold
alloys.

39. Setting reaction
• The chemical reaction that causes the investment to set and
harden is
NH4H2PO4 +MgO+5H2O→NH4MgPO4 +6H2O
• The product formed is predominantly colloidal multimolecular
(NH4MgPO4·6H2O)n aggregate around excess MgO and fillers.

40. Properties
• Compressive strength
o Type 1- 2.5 Mpa
o Type 2- 3 Mpa
• Thermal expansion
o 0.8% when 50:50 mixture of liquid and water

41. Working & setting time
o Temperature
Warmer the mix faster it sets
o The setting reaction also liberate the heat and
accelerates rate of setting
Mixing time
o Increased mixing time and mixing efficiency result
in faster set.

42. Surface Quality of cast metal
 Detail reproduction and surface smoothness of a metal-
ceramic gold alloy restoration cast in a phosphate
bonded investment were considered inferior to those
with gypsum-bonded investment.
 Due to improvement in technique and in composition of
material Phosphate bonded investments now approach
the surface quality or fineness of the gypsum
investments,

43. Advantages
 They have high fired strength.
 They also provide high setting and thermal expansion enough to
compensate cast metal shrinkage.
 3They can withstand temp more than 700°C

44. Disadvantage
• Rougher surfaces on casting.
• Difficult in divesting.
• Less porous than gypsum bonded investment .

45. Modifications
1. Phosphate bonded refractory cast for RPD
2. Investment for ceramics
Eg: Polyvest and VHT-whipmix
o Fine grained
o Working time 4-5 min
o Withstand repeated firing up to 12000
c
o Used with high expanding porcelains.

46. Casting
Investing procedure-common for both gypsum and Phosphate
investment
1. Mixing by-
1.Hand mixing
2.Vacuum mixing.
(In vacuum mixing-removes air bubbles, texture somewhat smoother
and better details reproduction)

47. • Excessive vibration should be avoided as-
o Causes solids in the investment to settle and may
lead to free water accumulation adjacent to wax
pattern
o Surface roughness
o May dislodge small pattern produce a miscast

48. 3. Wax elimination and Heating
after investment set, approximately-1hr,it
is ready for burnout
Burnout temperature
• Gypsum investment
o 5000
C for hygroscopic technique
o 7000
C for thermal expansion technique
• Phosphate bonded investment
o Range from 7500
C to 10300
C depending on
the type of alloy

49. • The temperature setting is more critical with gypsum-
bonded investments than for the phosphate type because
the gypsum investments are more prone to investment
decomposition.
• As temperature rises above 7000
C ,calcium sulfate reduce
as following reaction-
CaSO4 + 4C CaS + 4CO
3CaSO4 + CaS 4CaO+ 4SO2

50. Heating in burnout furnace
• For gypsum bonded investment
o The molds are usually placed in a furnace at room
temperature, slowly heated to 650 °C to 700 °C in 60
minutes, and held for 15 to 30 minutes at the upper
temperature.
• For phosphate bonded investment
o The heating rate is usually slow to 315 °C and is quite
rapid thereafter, reaching completion after a hold at the
upper temperature for 30 minutes.

52. Ethyl silicate bonded Investments
• Though losing popularity because of the more complicated and
time consuming procedures, still used in the construction of high
fusing base metal partial denture alloy.

53. Composition
• Refractory material – Silica
• Binder –Silica gel that reverts to silica
(cristobalite) on heating.
• Modifier – Magnesium oxide (strengthen the gel)
Ammonium chloride - accelerator

54. Binder
 Ethyl silicate
o Colloidal silicic acid is formed by hydrolysing ethyl
silicate in presence of HCl, ethyl alcohol& water.
Si(OC2H5)+4H2O Si(OH)4+4C2H5OH

55. • It is then mixed with quartz or cristobalite and magnesium oxide
(alkaline).
• Coherent gel of polysilicic acid formed(accompanied by a setting
shrinkage) ,dried at temp 168°C, loses alcohol & water to form
concentrated hard gel .
• Volumetric contraction during drying is known as green shrinkage.
• As the gelation process is slow and time consuming , amines are
added to ethyl silicate- hydrolysis and gelation occurs
simultaneously.

56. It is supplied as a powder and liquid
 Powder consists of refractory particles of silica and
glasses along with the magnesium oxide and some
other refractory oxides in minor amounts
 Liquid contains stabilized alcohol solution of silica
gel

57. Manipulation
• The powder is added to hydrolysed 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 .

58. • 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%

59. • Can be used for higher temp castings (1090°C-
1180°C) and compactible with higher fusing alloys.
• Care should be taken while handling and burnout as
inflammable alcohol is given off.

60. Disadvantage
• Non porous material.
• Complicated manipulation
• Cannot be used for titanium and its
alloys as silica can oxidize titanium or its
alloys easily.

61. Other investment materials
Soldering investment
Divestment materials
Investment material for titanium alloys
 Investments for all ceramic restoration

62. Soldering investment
• ANSI/ADA Specification No. 93 (IS0 11244) for dental
brazing investments defines two types of investment:
o Type 1: Gypsum-bonded dental brazing
investments
o Type 2: Phosphate-bonded dental brazing
investments

63. Soldering investment differ from casting investment by
• lower setting and thermal expansion
• most often ingredients do not have as fine particle as
casting investment
Uses
Type 1- for soldering of low melting alloys
Type 2 - for soldering of high melting alloys

64. Divestment
• Die stone and investment combination.
• These mixed with colloidal silica liquid
• The die is made from this mix and the wax pattern is then
constructed on it.
• The whole complex is then invested in a mixture of divestment
and water
• This combination used to compensate distortion of wax pattern
of long span bridges or RPD frameworks during removal from
die.

65. Newer investments for casting titanium based alloys
Problems with conventional investment materials-
• Molten titanium is highly reactive with the oxygen and is capable of
reducing some of the oxides commonly found in the investment.
• Titanium can also dissolve residual oxygen, nitrogen, and carbon from
the investments.
• These elements can also embrittle titanium in the solid state.

66. Objective for a titanium investment
• To reduce breakdown of the investment
• To reduce contamination of the titanium – refractory
materials that are less easily reduced by titanium
should be used

67. Investments used for casting Titanium alloy
Silica based investment
Magnesia based investment
Zirconia based investment
Alumina based investment
Resin based calcium investment

68. Conclusion
• Due to increase in the use of the higher melting alloys,
the use of phosphate bonded investment materials
increased.
• Gypsum bonded investment cannot withstand
temperature higher than 7000
C and it can be used only
with conventional gold alloys.
• The complex procedures limited the use of silicate
bonded investments.
• Due to highly reactive of titanium with the oxygen in
the mold cavity , newer investment materials are
developed.

69. • Gypsum bonded investment
(7000
C)
• Phosphate bonded investment
(700-10300
C)
• Silica bonded investments
(1090-11800
C)
• Mold for gold casting alloys
• Mold for base metal and gold
casting alloys, cast ceramic and
glasses
• Mold for base metal casting
alloys

70. References-
• Phillips Science of Dental material, 11th edition
• Restorative Dental materials, Robert G. Craig.11th
edition
• Fundamentals of fixed prosthodontics,3rd
edi.
Herbert T. Shillingburg