3. Introduction
■ A material which is suitable for forming a mould into which molten metal or alloy is
cast.
■ These materials can withstand high temperatures.
■ Also known as refractory materials.
■ In general, an investment is a mixture of three distinct types of materials
–Refractory material,
–Binder material
–Other chemicals
4.
5. Components of Investment
■ Refractory Material
–a form of silicon dioxide, such as quartz, tridymite, or cristobalite, or a mixture of
these
–They serve two functions:
■ Acts as a material that can withstand high temperatures.
■ Regulates the thermal expansion.
6. Components of Investment
■ Binder Material
–refractory materials alone do not form a coherent solid mass,
–Some kind of binder is needed. Ex: Gypsum, phosphate, ethyl silicate
■ Other Chemicals
–sodium chloride, boric acid, potassium sulfate, graphite, copper powder, or
magnesium oxide,
–Small quantities -modify various physical properties
7. Properties Required of an Investment
■ Easily manipulated
■ Sufficient strength at room temperature
■ Stability at higher temperatures
■ Sufficient expansion: compensate for shrinkage of the wax pattern and metal.
■ Porosity: porous enough to permit the air or other gases in the mold cavity to escape.
■ Smooth surface: Fine detail and margins on the casting.
■ Ease of divestment
■ Inexpensive.
8. ■ At higher temperatures, the investment must not
decompose to give off gases that may corrode the
surface of the alloy.
■ Casting temperatures should not be critical
9. Types of Investment
■ Based on the nature of Binder, its classified on 3 types
–Gypsum bonded investments: used for casting gold alloys, withstand temperature up to
700°C.
–Phosphate bonded investments: For metal ceramic and cobalt-chromium alloys,
withstand higher temperatures.
–Ethyl silica bonded investments
■ alternative to the phosphate bonded investments, for high temperature casting.
■ Principally used in the casting of base metal alloy partial dentures
10. Based on processing temperature
•Silicate bonded
•Phosphate
•bonded
High temperature
Casting investment
•Gypsum bonded
Low temperature
casting
investment
11. BASED ON TYPE OF REFRACTORY USED
1:Silica
2:Magnesium oxide
3: Zirconia based
investments
•quartz
•cristobalite
12. based on application (ISO 15912:2006)
■ Type 1: for the construction of inlays, crowns and other fixed
restorations
■ Type 2: for the construction of complete or partial dentures or other
removable appliances
■ Type 3: for the construction of casts used in brazing procedures
■ Type 4: for the construction of refractory dies
13. Gypsum Bonded Investments
■ ADA Sp. No. 2 for gold alloy casting investments again classify it 3
types.
■ Constituents
–65% to 75%: quartz or cristobalite, or a blend of the two,
–25% to 45%: α-calcium sulfate hemihydrate
–2% to 3% chemical modifiers
14. Gypsum Bonded Investments
investment use compensation for alloy casting shrinkage
Type I casting of inlays or crowns Thermal expansionof the investment
Type 2 casting inlays, onlays, or crowns hygroscopic expansionachieved by
immersingthe investedring in a warm
water bath
Type 3 casting gold alloys Thermal
15. Gypsum: α-calcium sulfate hemihydrate
■ Casting gold-containing alloys with melting
ranges below 1000 °C
■ Material is heated at temperatures
sufficiently high
➢ 200-400 °C: shrink considerably
➢ 400 °C -700 °C: slight expansion takes
place between
➢ Above : decomposition and the release of
sulfur dioxide causing contamination of alloy
16. Silica
■ It exists in 4 allotropic forms: quartz, tridymite, cristobalite and fused
quartz.
–Quartz and cristobalite are of dental importance
■ Investment should expand thermally -compensate partially or totally for
the casting shrinkage of the solidifying alloy.
■ Gypsum -shrinks considerably when it is heated,
■ so it should be silica which should expand to cause overall expansion of
investment
17. Silica
■ When heated allotropes of
silica changes from α(low
room temperature form) to
β(high temperature form)
–Phase transformation is called
an inversion.
–Causes linear expansion
–overall causing volume
expansion
18. MODIFIERS
■ Certain modifying agents
–Coloring matter,
–Reducing agents, such as
carbon and powderedcopper,
■ Alkali-earth and transition-
metal chlorides, boric acid,
and sodium chloride
–regulate the setting
expansion and the setting
time
19. Functions of Constituents
■ Alpha hemihydrate
■ It binds and holds the silica particles together.
■ Permits pouring of the mix into the mold.
■ It imparts strength to the mold..
■ Contributes to mold expansion (by setting expansion).
22. Properties
■ Setting time: set initially in 9 to 18 mins
–It can be manipulated in the same way as of gypsum product
■ NORMAL SETTING EXPANSION
–silica particles probably interfere with the intermeshing and
interlocking of the crystals
–thrust of the crystals is outward during growth,
–Soft wax and thin pattern may get distorted,
23. Properties: HYGROSCOPIC SETTING
EXPANSION
■ Expansion when contact with heated
water.
■ Greater in magnitude than normal
setting expansion
■ Type II investments : 1.2% -2.2%
■ Directly proportional
–Silica content of the investment
–Water mixed during setting reaction
•Indirectly proportional : size of silica
particles
A, Normal setting expansion
B, Hygroscopic setting expansion
24. Properties: THERMAL EXPANSION
■ Thermal expansion of silica must be increased to counterbalance the contraction of
the gypsum
■ Contraction of the gypsum is entirely balanced when the quartz content is increased to
75%
■ Type I: 1% -1.6%, Type II: -0% and 0.6% at 500 °C
■ It depends on
–Particle size of the quartz,
–Type of gypsum binder: αor β
–Resultant W/P ratio
–Allotropes of quartz : Cristobalite> quartz
25. Properties
■ THERMAL CONTRACTION
–Inversion of the β form to its stable α form at room temperature.
–Contracts to less than its original dimension
■ EFFECT OF CHEMICAL MODIFIERS
–Increasing the silica content –reduces the strength of investment
–Small amounts of sodium, potassium, lithium chlorides or boric acid –
eliminates the need for adding silica to cause expansion
26. Properties
■ STRENGTH
–Adequate to prevent cracking, bulk fracture, or chipping of the mold
–affected by the W/P ratio same way as gypsum.
–It decreases after heating to 700 0C –microcracks
■ FINENESS
–Fine silica results in a higher hygroscopic expansion
–Also it will give finer details accurately and minimal surface
roughness.
27. MANIPULATION
■ The measured quantity of powder and
water is mixed manually using a flexible
rubber bowl and spatula or in a vacuum
investment mixing machine.
28.
29. PHOSPHATE-BONDED INVESTMENT
■ Increased use of metal-ceramic, hot-pressed ceramic
prostheses and base metal alloy –requires investment which
can bear higher temperature
■ Phosphate-bonded or silicate-bonded investments can be
used in such condition
■ But they are difficult to disinvest –however these problem has
been sorted out recently.
30.
31. COMPOSITION
■ Refractory filler: silica (80% by weight) in the form of cristobalite, quartz, or a mixture of the two
forms.
–particle size varies from a submicron level to that of a fine sand
■ Binder: magnesium oxide (basic) and a phosphate (acid)
–Originally phosphoric acid was used, but monoammonium phosphate has replaced
■ Other: Carbon
–Produce clean castings and facilitate the divesting of the casting from the investment mold
–Appropriate when the casting alloy is gold
–Others: palladium reacts with carbon at temperatures above 1504 °C to make it brittle
■ Colloidal silica suspensions in place of water
–Greater expansion: newer gold-containing alloys and other alloys -higher melting temperature
ranges than traditional gold alloys,
32. SETTING REACTION
■ Ammonium diacid phosphate reacts with magnesium oxide-
green strength, or room temperature strength.
■ ammonium diacid phosphate is used in a greater amount.
■ additional amount can react with silica at an elevated
temperature
–P2O5and SiO2forms silico phosphate
–increases the strength of investment
33. SETTING AND THERMAL EXPANSION
■ slight expansion occurs compared to gypsum bonded
■ increased considerably by using a colloidal silica solution in
place of water
■ early thermal shrinkage -decomposition of the binder,
accompanied by evolution of ammonia.
■ Gypsum investments the shrinkage is caused by the
transformation of calcium sulfate from the hexagonal to the
rhombic form.
34. WORKING AND SETTING TIME
■ markedly affected by temperature
–warmer the mix, the faster it sets
–setting reaction itself gives off heat, and this further accelerates the rate
of setting
■ Increased mixing time and mixing efficiency,
–result in a faster set
–Better smoothness and accuracy of the casting.
–Mechanical mixing under vacuum is preferred
■ L/P ratio
–increase in the L/P ratio increases the working time
35. Manipulation
■ Powder/liquid ratio - 16 to 23 ml/100 gm. (The liquid is usually
diluted with water. The amount of liquid to water ratio varies
with the particular brand of investment and type of alloy used.
The amount of water used ranges from 0 to 50% depending on
the expansion required).
■ The powder is mixed with a measured amount of liquid using a
bowl and spatula. Following hand mixing for 20 seconds
mechanical mixing under vacuum is done for a further 90
seconds
Working time is around 8-9 minutes.
36. Manipulation
■ The mixed material is
vibrated into the casting
ring or agar mold (RPD
framework). The material
is allowed to bench set
for a minimum 30-45
minutes depending on
the particular investment.
Vacuum investment mixer.
37. ETHYL SILICATE–BONDED INVESTMENT
■ Its use has declined -more complex and time-consuming
procedures
■ But still used in the construction of high-fusing base metal
partial denture alloys.
■ Here the binder is silica gel.
■ Its made either by 2 method from
–Sodium Silicate
–Ethyl silicate
38. Types
■ Sodium silicate
–pH lowered by addition of an acid or an acid salt,
–a bonding silicic acid gel forms.
–Accelerator: ammonium chloride
■ Ethyl silicate
■ It is hydrolyzed in the presence of hydrochloric acid, ethyl alcohol, and water.
■ Sol is then mixed with quartz or cristobalite,
■ Finely powdered magnesium oxide is added -keep the mixture alkaline.
■ A coherent gel of polysilicic acid then forms, accompanied by a setting shrinkage
39. Ethyl silicate
■ When this soft gel is dried at a temperature below 168 °C
–loses alcohol and water to form a concentrated, hard gel
–Green shrinkage, which is additive to the setting shrinkage
–As well it takes longer time to hydrolyze and gelation–amines can be
added to faster the reaction
■ Thus in this type of investment, mold enlargement should compensate
not only casting shrinkage, but green shrinkage and setting shrinkage
40. Manipulation
■ The powder is added to the 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 the platform of a special type of vibrator that
provides a so-called tamping action.
■ This allows the heavier particles to settle quickly while the excess liquid
and some of the fine particles rise to the top.
■ In about 30 minutes, the accelerator in the powder hardens the settled
part and the excess at the top is poured off.
■ Thus, the L/P ratio in the settled part is greatly reduced and the setting
shrinkage is reduced to 0.1%.
■ The remaining cast is somewhat fragile because the amount of binder is
quite low and it is essentially composed of silica.
