INVESTMENTS OF DENTAL MATERIALS
Investment can be described as ceramic
material that is suitable for forming a mold in
to which a metal or alloy is cast.
Or . also can defined as refractory mould
material used for embedding the wax pattern
and obtating corresponding mould for casting
Material such as gypsum. Phosphate. and
silicate. are used for investing
• Investing means capturing all the wax pattern
in a solid material which capable of
maintaining accurate details during burnout
procedure casting of metal alloy.
• The investment material is available as two
component system. Powder and liquid( water)
• Mixed before investing the wax pattern.
Ideal Requirement of an investment material
• It should be reproducing the shape, size, and
details record in the wax pattern.
• Easily manipulated and must readily wet the
surface of the wax pattern.
• Adequate setting time to allow the investing
procedure.
• It should be able to withstand the high
temperature during the burnout of the wax
and casting of the molten metal.
- Sufficient porosity to allow gases to escape .
-controlled expansion during setting to
compensate for the casting shrinkage during
cooling of the metal.
- Smooth surface finish of the inner surface of
the mold to have proper finish of the
restoration.
It should not react with wax and alloys.
Component of investment material
The investment material consist of a mixture of
1-Refractory material.2- Binder material, and
3-chemical modifiers,
Refractory material –it is form of silica such as
quartz,tridymite, or cristobalite, this is capable
of sustaining exposure to a temperature
without significant degradation,
• Binder material –it is capable of binding the
refractory material to form a coherent solid
mass. Commonly used binder are Alfahemihydrates, ethyl silicate, phosphate.
• Modifiers-Non oxidizing agent retarders,
accelerators, and coloring agent are added to
the refractory and binder material to enhance
their physical properties ,
Classification of investment material
• There are three main type of investment material used
in dentistry• Gypsum- bonded investment material- these material
used for conventional casting of inlay , onlays, crowns,
and partial denture made of gold alloys,
• Phosphate-bonded investment material-these are used
primarily for alloys that have to be cast at higher
temperature then gold based alloys,eg. Metal ceramic
prosthesis and for some base metal alloys,
• Ethyl Silicate- bonded investment material- these
are used in casting of removal partial denture
with base metal alloys,
•
•
•
•
Composition- gypsum bonded investment
Silica
- 60 to 65%
Alpha hemihydrates ,( dental stone) -30 to 35 %
Chemical modifier,
- 5%
• USE- For casting of inlays, fixed partial denture
removal partial denture ,and gold alloys and
low fusing alloys,
• Function of constituents – Alpha hemihydrates
-Binds and holds the silica particles
• Permit pouring of the mix in to the mold.
• Strength to mold
• Contributes to mold expansion( setting)
• Silica:Quartz or cristobalite- it act as refractory
during heating.
• Regulate thermal expansion
• Increases setting expansion of stone.
• Modifiers- used as coloring matter
• Reducing agent
• Modifying chemical –regulate setting
expansion and setting time and also prevent
shrinkage of gypsum when heat above 100 0c
• Manipulation- powder and water mixed
manually using a flexible rubber bowl and
spatula or in vacuum investment mixing
machine.
• Setting time- the setting time should not be less than 5
minute and more. The modern inlay investment set
initially in 9 to 18 minutes. This powder sufficient time
for mixing and investing the pattern.
•
•
•
•
•
Factors controlling setting timeManufacturing process
mixing time and rate
water- powder ratio temperature
modifier
PROPERTISE OF GYPSUM INVESTMENTS
Thermal behavior of gypsum – when heated silica
or quartz or cristobalite change its crystalline
form. this occurs at a transition temperature,
Quartz when heated invest from a low form k/a
alpha-quartz to high form called as beta- quartz
at a temperature of 375 0 c
Cristobalite when heated , invests from low or
alpha- cristobalite to a high or beta cristobalite at
temperature of 200- and 270 0c
EXPANSION OF GYPSUM INVESTMENT
• Expansion aids in enlarging the mold to
compensate for the casting shrinkage of the
gold alloys.
• Three type of expansion
• 1- Normal setting expansion
• 2- Hygroscopic setting expansion
• 3-Thermal expansion
Normal setting expansion
• A mixture of silica and dental stone in setting
expansion which is greater than when gypsum
product is used. the silica particle probably
interfere with the intermeshing of the crystal as
they form.
• Hygroscopic setting expansion-when gypsum
products are allowed to set in contact with water.
The amount of expansion exhibited is much
greater than normal setting expansion.
• Thermal expansion-obtained due to
cristobalite as the temperature rise if its
content is greater than 75% by wt.it can
counteract the initial contraction of the
gypsum binder but larger amount of silica may
decrease the strength.
• Advantages of GBI-Adequate strength
-Adequate porosity
-Controlled large setting and thermal
expansion
-Simple method of manipulation and
casting procedure.
-No very expensive
• Disadvantages of GBI• Can not used for titanium alloys,
• GBI powder is hygroscopic. Large quantity
should not be purchased( short time age time)
store carefully in air tight containers.
• Too high costing force and careless wax
burnout method
PHOSPHATE BONDED INVESTMENT
• Most widely used investment in dentistry
• Use of high fusing noble or base metal alloys.
• For casting of high fusing alloys. Metal ceramic alloys and
base metal alloys like nickel- chromium and cobaltchromium
• This is supplied in sealed sarchet,wet.200gm.or 400gm.it is
mixed with distilled wateror water containing freeze-silica
gel
• Classification- type 1: for inlays, crowns and other fixed
restoration.
• Type 2: for partial denture and other cast removable
restoration.
• Composition –Refractory material :quartz or
cristobalite or their mixture 80%
• Function-high temp.gives large setting expansion.
• it gives strength at room temperature.
• It is soluble in water and provides phosphate
ions.
• It reacts with silica at high temperatures to
increase strengths at casting temperature
• Binder-mixture of basic mgo and acidic
(nh4h2po4)
- 20%
• Magnesium oxide: Reacts with phosphate ions
strength and setting and thermal expansion
• Carbon: it helps produce clean casting and
easier divestment from the mold.
• Liquid- this is form of silica sol in water, which
gives higher thermal expansion.
• Manipulation-p/L –ratio- 16 to 23ml/100gm.
• The powder is mixed with a measured amount
of liquid using a bowl and spatula. Hand
mixing 20 sec.mechanical mixing under
vacuum is done for 90 sec. working time is 8-9
minutes. The mixed material is vibrated in the
casting ring the material is allowed to bench
set for a minimum 30-45 minutes depending
the investment.
• Factors affecting setting time• 1- Temperature of the mix and environment.
• Warmer temp. accelerate the setting , cooling
the liquid prolong the working time.
• Increased p/L ratio delays setting and gives
more working time.
• Increasing the mixing time accelerates the
setting time.
• Properties – Expansion- expansion are three
type
• wax pattern expansion – during setting allows
a significant expansion of the wax pattern
• Setting expansion-this is around 0.7 to 1%
• Thermal expansion- around 1 to 1.5%.
• Strength- these are two type – wet strength
range from 4-19 Mpa it is important for
handling the set material to casting
• Dry strength-investment under high
temperature
• Flow-they appear to low flow when mixed
however they flow readily and envelope the
pattern when poured in to mold under
vibration
• Surface smoothness-PBI produced rough
casting when compared to gypsum bonded
investment
Silica Bonded investment material• The silica is the binder it derived from ethyl
silicate or aqueous dispersion of colloidal silica
or sodium silicate. The are less commonly
used.
• Refractory material- powdered are quartz and
cristobalite
• Small amount of magnesium oxide is added to
the powder to reduce the ph of silica gel
during manipulation
• Binder – silica gel in the form of sodium silicate
and colloidal silica is the commonly used binder
that convert to silica at high temperatures.
• Investing procedures- wax pattern is fabricated to
required dimension , the next step is to invest the
wax pattern . This is done following steps.
• 1- attachment of the sprue former and removal
of wax pattern from the master cast.
• Inspection of the wax pattern for proper
verification of details.
• Attaching to the crucible former and investing
of the wax pattern
• Burnout procedure to remove the wax
• Casting of molten metal the mold
• Removal of the cast metal framework from
the investnment.
• Finishing and polishing of the metal
• Try-in the master cast
• Sprue former/sprue.can be defined as the
channel in the investment through which the
molten metal enters the mold space created
by the burnout of the wax pattern, sprue
formers may be made of wax, metal or
plastic/resin
• Dimension of sprue former-the diameter depend
on the thickness of the wax pattern . The
diameter should be approximately equal to the
thickest portion of the wax pattern
• Location-should be attach to the widest part of
the wax pattern to allow proper flow of the
molten metal.
• Direction-the sprue former is attached at an
angle of 45 degree to thickest part of wax
pattern.