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) 

1. INVESTMENT MATERIAL
Presented by-
Dr. Nakul Patidar

2. INTRODUCTION
 Casting procedure :- It is a principal laboratory method used to form
metal inlays, onlays, crowns and bridges by casting molten alloys,
under pressure, or under vacuum with the help of centrifugal force.

3.  It involves:-
(1) A wax pattern of the object to be reproduced
(2) A suitable mold material, known as investment, which is placed
around the pattern and permitted to harden
(3) Suitable furnaces for burning out the wax patterns and heating
the investment mold
(4) Proper facilities to melt and cast the alloy.

4. DEFINATION
 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 the mold is described as investing.
(CRAIG’S 13 edition)

5. DEFINATION
 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)

6. Ideal requirements
 Easily manipulated
 Should not separate when mixed.
 Smooth surface
 Appropriate setting time
 Sufficient strength during casting
 Sufficient expansion:
 Porosity:
 Ease of divestment
 Inexpensive

7.  In general investment material contain:-
Refractory material:-
Silica based regulate thermal expansion
Quartz investment • Cristobalite investment • Tridymite investment •
Mixture of quartz, cristobalite and tridymite • Mixture of alumina,
magnesia, zirconia
Binder :-
Gypsum(α-calcium sulfate hemihydrate), Phosphate, Ethyl-Silicate To
hold or bind the refractory material particles
Modifier:-
Modifiers NaCl, boric acid, potassium sulfate, graphite, copper
powder,..etc.

8. CLASSIFICATION
I. Based on the binder used: 3 types
Rosensteil
Types Uses
1. Gypsum Bonded Investments, Or Calcium
Sulphate Bonded Investments
Gold alloy inlay,
onlay,FPD
2. Phosphate Bonded Investments Base metal alloy &
Metal ceramic alloy
3. Ethyl Silicate Bonded Investments, Or Silica
Bonded Investments
Co-Ni alloy

9. CLASSIFICATION
II. On the basis of the type of silica principally employed, dental
investments are often classified as
Quartz investments
Cristobalite investments
Phillips 10th ed

10. GYPSUM BONDED INVESTMENTS, OR
CALCIUM SULPHATE BONDED INVESTMENTS
ADA spec No. 2
 Type I investments(Thermal expn.) – INLAY AND CROWN
 Type II investments(Hygroscopic expn.)- INLAY, ONLAY AND CROWN
 Type III investments- CAST PARTIAL DENTURE WITH GOLD
ALLOY

11. GYPSUM INVESTMENT - COMPOSITION
 Gypsum: Binder+ strength+ setting expansion (25-35%)
 Silica: regulate the thermal expansion. (65-75%)
 Modifiers (2-3%)
Reducing agents – provide non oxidizing enviroment
Coloring matter
Boric acid and sodium chloride - Regulate setting expansion,
setting time, & prevent shrinkage of gypsum when heated above
3000 C.

12. Advantages:
 Adequate strength
 Adequate porosity
 Controlled large setting and thermal expansion
 Simple methods of manipulation and casting procedure
 Not very expensive.

13. Disadvantages:
 Sulphur dioxide gas produced; during heating causes; tarnishing or
discoloration of gold alloys.
 Hence this is not suitable for casting of high fusing Noble and High
Noble alloys. Thus phosphate-bonded investment material is
preferred.

14. SETTING TIME
 According to ANSI/ADA specification No. 2
for dental inlay casting investment 5-25 min

15. Setting reaction
 Gypsum bonded investment (GBI) shows shrinkage when heated above 700o C
and is most likely due to decomposition and release of sulfur gases.
 CaSO4 ---------> SO2 + SO3 (above 700o C )
 This decomposition not only causes shrinkage but also contaminates the castings
with the sulfides of the non- noble alloying elements. •
 Hence gypsum investments should not be heated above 700o C.

16. Effect of Temperature on Silicon Dioxide Refractories
Each of the polymorphic forms of silica—quartz, tridymite, and cristobalite—
expands when heated, but the percentage of expansion varies from one type to
another.
Pure Cristobalite expands to 1.6% at 200°C- 270° C
Quartz expands about 1.4% at 573°C
Tridymite expands to less than 1% at 600°C.
The amount of expansion is highest for cristobalite and lowest for
tridymite.

17. Effect of Temperature on Calcium Sulfate Binders
Up to about 105° C, ordinary thermal expansion occurs.
Above 105° C, the calcium sulfate dihydrate is converted to anhydrous
calcium sulfate by loosing water . Dehydration of the dihydrate and a
phase change of the calcium sulfate anhydrite cause a contraction.
However at elevated temperatures, α-forms of silica present in the
investment are converted to the β-forms, which causes some expansion
and compensates for the contraction of calcium sulfate.

18. Factors controlling setting time:-
Setting time decrease:-
 A. Calcination is incomplete and considerable gypsum is left
 B. Mixing time and rate is more
 C. Water/powder ratio is more
 D. Temperature increases
 E. Sodium chloride 2%
 F. Sodium sulfate at 3.4%

19. Factors controlling setting time:-
Setting time increase
 A. Sodium chloride > 2%
 B. Sodium sulfate at > 3.4%
 C. Glue, gelatin, and some gums
 D. Citrate, acetate, borate

20. SETTING EXPANSION
 Normal or linear setting expansion
 Hygroscopic expansion
 Thermal expansion

21. Normal or linear setting expansion
 Setting surrounded by air
 Compensate casting shrinkage
 Occur btw 0.4-0.6 %
 Increase expansion
Gypsum Content = more
Water powder ration = less

22. HYGROSCOPIC EXPANSION
 Occur when it set under water
 6 time more than normal
 1.2-2.2%

23.  Greater in magnitude than
the normal setting expansion
(> 6 times).

24. FACTORS AFFECTING HYGROSCOPIC
EXPANSION
 Propotional to silica content
 Finer particle – more expansion
 α particle produce more expansion
 As water added during setting – more expansion
 Increase W/P ratio- Less expansion
 Reduced mixing time- less expansion
 Older investment- less expansion

25. THERMAL EXPANSION
 Directly related to amount and type of silica
 1.0-1.6%
 Cristobalite more expansion > Quartz
 It is achevied upto 700 0 C.
 Type I investment rely upon this

26. Factors to Control Thermal Expansion
 More silica more expansion
 More water less expansion
 On cooling little contration

27. STRENGTH
 2-4 Mpa
 Factor affecting strength
 1. W/P ratio
 2. Heating >700 C
 3. On cooling

28. Other gypsum investment considerations
 Fineness: Finer particle size- smooth porosity
 More gypsum crystal- more porosity
 Storage- must be stored in airtight and moisture- proof containers.

29. Phosphate Bonded Investments
 Metal ceramic alloy invested
 High temperature investments
 Need to be heated to casting temperatures of about 1200°C to 1300°C.
 High-gold and palladium-based alloys.

30. PHOSHPHATE INVESTMENT - COMPOSITION
 Refractory filler; 80% silica
 to provide high temperature thermal shock resistance (refractoriness) and
a high thermal expansion.
 Binder- <20%
 Crystoballite, quartz (MgO+ Monoammonium phosphate)
 Carbon; clean and devesting the casting

31. Setting Reaction

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

33.  Modifiers :
 Carbon is often added - to produce clean casting and facilitate divesting.
 Palladium reacts with carbon even at temp 1500°C, so carbon free
phosphate bonded investment began to be used for higher temp.

34. Setting and Thermal Expansion
 When PBI are mixed with water- initial shrinkage is seen same that of
GBI within the temperature range of (200˚ C to 400°C).
 The early thermal shrinkage of phosphate investments is associated with
the decomposition of the binder, magnesium ammonium phosphate, and is
accompanied by evolution of ammonia, which is readily apparent by its
odour.

35. • This contraction is practically eliminated when a colloidal
silica solution is used for mixing phosphate bonded
investment instead of water.
61

36.  Expansion and strength of these
investment can be modified by:-
 altering the liquid:powder ratio or
 Increasing the concentration of the
special liquid.
But if more liquid is used it
causes dense nonporous investment which
can effect casting. Influence of liquid concentration on
the setting and thermal expansion

37. Working and Setting Time
• Markedly affected by temperature
 The warmer the mix, the faster it sets.
• Increased mixing time and mixing efficiency result in a faster set and a
greater rise in temperature.
• In general, the more efficient the mixing, the better the casting in terms of
smoothness and accuracy.
• Mechanical mixing under vacuum is preferred. 63

38. Other Properties
• Detailed reproduction and surface smoothness of phosphate-bonded
investment is inferior than gypsum- bonded investment.

39. ADVANTAGE-PBI
• Higher strength
• High fired strength – less mold cracking and few fins on casting
• Can withstand temperature up to 1000°C for short period of time (useful
or performing metal-joining operations).
 They can also provide setting and thermal expansions high enough to
compensate for the thermal contraction of cast-metal prostheses or
porcelain veneers during cooling.

40. DISADVANTAGE- PBI
 When used with higher-melting alloys (casting temp. higher than 1375°C)
- results in mold breakdown and roughen surface of castings.
 Difficult to divest (removal of casting from the investment)
 When higher expansion is required, more of the silica
 Liquid is used with the result that a more dense and less porous mold is
produced. This can result in incomplete castings if a release for trapped
gases is not provided.

41. ETHYL SILICATED BONDED
INVESTMENT

42.  Second type of investment used in dentistry.
 Used since the early 1930’s
 High fusing base metal partial denture alloys
 Cobalt chromium or nickel titanium alloy
 Disadv- more complicated and time consuming
 Ethyl alchol- flammable content

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

44. BINDER
 Binder silica gel reverts to silica (cristobalite) on heating.
 Methods to obtain silica or silicic acid gel binders:-
1)By adding an acid or an acid salt to sodium silicate.
2)By the adding an accelerator, such as ammonium chloride to an aqueous
suspension of colloidal silica.
3)By hydrolyzing ethyl silicate in the presence of HCl, ethyl alcohol, and
water.

45. SETTING REACTION
During drying at temp below 168°C , soft gel loses alcohol and water to
form a concentrated, hard gel which lead to volumetric contraction
(GREEN SHRINKAGE) which ultimately reduces the size of the mold.
This shrinkage occurs in addition to setting shrinkage.

46.  Powder consists of refractory particles of silicas and glasses along with
magnesium oxide and some other refractory oxides in minor amounts.
 Liquid contains stabilized alcohol solution of silica gel
 A. 1 bottle contains diluted water-soluble silicate solution.
 B. 2 bottle contains a properly diluted solution of hydrochloric acid.

47. ADVANTAGE
 It offers the ability to cast high temp Co-Cr and Nickel –Cr alloys, and
attain good surface finishes, low distortion, and high thermal expansion
(good fit).
 High permeability(gel form), yields sharply defined castings
 Low fired strength – easy removal of casting.

48. Disadvantage
 It gives off Inflammable components during processing.
 Expensive.
 Strength is low so precautions should be taken.
 More time consuming and complicated procedures involved.
 Manipulation is very technique sensitive.
Satish Chandra

49. Divestment (whip-mix, Louisville, KY)
 It is a combination of Die stone and gypsum bonded investment material.
The powder is mixed with colloidal silica.
 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 &
water.
 This combination used to compensate distortion of wax pattern of long
span bridges or RPD frameworks during removal from die.
 Special GBI or PBI investment materials are used.

50. PROPERTIES
 Setting expansion - 0.9%
 Thermal expansion - 0.6% (at 977°c)
Advantages
 The wax pattern and die are invested simultaneously with out
 removal of pattern. Useful with gold alloys
DIVESTMENT PHOSPHATE(DVP)
 •Similar to divestment, but used for casting post and core, crowns
 of base metal alloys without any need of removal of wax pattern.

51. Different type of wax pattern
Pattern Processing Impression
Inlay
Casting
Base plate
Ivorian
Beading &
Boxing
Utility
Sticky
Block out
Carving
Corrective
Bite
registration

52.  Rise in temperature- Wax expands
 Lower temperature- contract
 Dental wax has largest coefficient of thermal expansion
 A wax pattern prepared directly in a patient's mouth will shrink about 0.4% when
cooled from oral temperature. In the indirect method of preparing the wax pattern
on a die, the wax shrinkage is about 0.2%.

53. Metals- Casting Shrinkage
 Gold alloy –
 Alloy shrink from liquid to solid state
 1.25-1.65%
Base metal alloy – (Nickel based)
Higher shrinkage than gold alloy -2.0%

54. SETTING EXPANSION
Gypsum bonded Phosphate bonded Ethyl silicate
Normal-0.4-0.6% Normal-0.9%
Thermal-1.0-1.6% Thermal-0.8% Thermal-0.6%
Hygroscopic expansion-1.2-2.2%

55. Setting reaction

56. Gypsum bonded investment56
 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.
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.
• 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.

57. Phosphate bonded investment
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.
2(Mg·NH4·PO4·6H2O) → Mg2·P2O7 + 2NH3 + 13H2O
At a higher temperature some of the remaining phosphate reacts with silica
forming complex silicophosphates.
These cause a significant increase in the strength of the material at the
casting temperature.
57

58. THE SETTING REACTION -
Stage 1 – Hydrolysis :
The silica is bonded by the hydrolysis of ethyl silicate in the
presence of hydrochloric acid.
The product of the hydrolysis is the formation of a colloidal
sol of polysilicic acid and ethyl alcohol.
(C2H5O)4Si + 4H2O HCl Si(OH)4 + 4C2H5OH
ETHYL SILICATE SETTING REACTION-
58

59. ETHYL SILICATE SETTING REACTION-
Stage 2 – Gelation :
The solution is then mixed with the quartz or cristobalite to
which is added a small amount of finely powdered magnesium
oxide to render the mixture alkaline.
A coherent gel of polysilicic acid then forms accompanied by a
shrinkage.
59

60. Stage 3 – Drying :
The soft gel is dried at a temperature below 168°C. During the drying
process, the gel loses alcohol and water to form a concentrated, hard
gel.
A volumetric contraction accompanies the drying, which reduces the
size of the mold. This contraction is known as “green shrinkage”,
and it occurs in addition to the setting shrinkage.
ETHYL SILICATE SETTING REACTION-
60

61. PROBLEM CAUSE SOLUTION
Rough surface Don’t overheat mold or alloyBreakdown of
investment
Weak surface of
investment
Avoid use of high
water/powder ratio of
investment.
Avoid dilution of the
investment material from
application of too much
wetting agent
Fins Cracking on
investment
Avoid too rapid heating
investment.
Casting defects because of investment material:

62. PROBLEM CAUSE METHOD OF
AVOIDING FAULT
Casting too large Excessive mold
expansion
•
•
Use correct
temperature
Use correct type of
investment material
Casting too small Too little mold
expansion
Heat the mold
sufficiently.

63. PROBLEM CAUSE
Irregular voids Inclusion of particles of
investment material
Rounded margins;
Regular large voids
Back-pressure effect;
air unable to escape
from mould
METHOD OF
AVOIDING FAULT
Heat mold upside down so
that particles fall out of the
mold
1.Use sufficient casting
force
2.Use investment of
adequate permeability
3.Avoid presence of
residue of wax in mould
4.Place pattern no more
than 6-8mm away from
end of casting ring.

64. • Newer investments have been aimed at the casting of titanium and
titanium based alloys.
• This is because molten titanium is highly reactive with oxygen and
is capable of reducing some of the oxides commonly found in
investment.
• Titanium can react with residual oxygen, nitrogen and carbon from
investment. These elements harden and embrittles titanium in the
solid state.
Recent advances:

65. As a result, either modifications of existing refractory formulations
and binder or new refractory formulations and binder system are
required.
A variety of investment formulations for the casting of titanium have
been developed over the past several years.
These investments might be classified as PBI and Cemented
(magnesia bonded by an aluminous cement, which contains a mass
fraction of 5% zirconia)
In these investments many kinds of refractories such as silica,
alumina, magnesia and zirconia have been used.

66. Titavest, an aluminia-magnesia-system. Mixed with a
special liquid the material becomes a spinel structure.

67. References
• Text book dental material
• Craig’s Restorative Dental Materials
 13th edition by Ronald L. Sakaguchi and John M. Powers.
• Philip’s science of dental materials, 11th edition by Kenneth J. Anusavice.