1. DENT204, UNC School of Dentistry
LOW TEMPERATURE
INVESTMENTS
DENT204, UNC School of Dentistry
Stephen C. Bayne
Department of Operative Dentistry
School of Dentistry
University of North Carolina
Chapel Hill, NC
Mold making materials for castings are called INVESTMENTS. The mold (or investment) must be accurate, be oversize by the exact amount of any casting alloy shrinkage during cooling, must not undergo detrimental reactions with the hot casting alloy, and must be easily removed to reveal the solid casting. Investments are ceramic materials.

2. INDIRECT RESTORATIONS Overview of Potential Errors
Impressions
Casts/Dies
Waxing
Investing
Casting
Finishing/Polishing
Cementing
0.1 to 0.2%
------
+1.5 %
1.5 %
Gold casting alloys = 1.4-to-1.7%
Base metal alloys = 2.0-to-2.3%
All of the steps involved in INDIRECT RESTORATIVE PROCEDURES (impressions, casts/dies, waxing, investing, casting, finishing/polishing, and cementing) potentially involve errors that can be compounded. However, the most critical stages of error management involve the investing and casting steps. Because casting includes significant dimensional changes during cooling from the casting temperature to room temperature, it is crucial that the investing process produce the proper oversized mold space to cancel out future shrinkage effects. [CLICK]
The actual amount of alloy shrinkage depends on the type of alloy being cast. Gold casting alloys (or substitutes) typically generate about 1.4 to 1.7% shrinkage. Base metal alloys, because of their much higher melting range, produce 2.0 to 2.3% shrinkage.

3. General Formulation and Reactions
INVESTMENT MATERIALS
General Formulation and Reactions
Three major components:
BINDER (or matrix) = 33%
Refractory FILLER = 66%
ADDITIVES = 1%
0.6% Expansion +
0.9% Expansion
1. Setting Reaction: Chemical Reaction (Formation) = []
Physical Reaction (Xl Growth) = [+]
2. Hygrosopic Reaction: Physical Reaction (Xl Growth) = [+]
3. Heating the Mold: Chemical Reaction (Matrix Decomposition) = []
Physical Reaction (Thermal Expansion) = [+]
Physical Reaction (Silica Inversions) = [+]
4. Casting Shrinkage: Physical Reaction (Thermal Contraction) = []
TOTAL: = [0]
Investment materials are composed of three principal things, 33% BINDER (or matrix), 66% FILLER, and a small amount or about 1% ADDITIVES. The binder is initially fluid and allows the investment to be poured around a wax pattern to create the mold. After setting, the filler provides some strength but primarily is added to contribute to thermal expansion during heating of the investment mold. Additives provide reaction control.
During the process of both forming the investment mold and heating it to the casting temperature, there are several reactions that take place, which all contribute to producing the 1.5% expansion required to compensate for subsequent casting alloy shrinkage. [CLICK] These are listed in the table below.
Briefly, here are the dimensional changes taking place. Chemical setting produces a volumetric contraction (i.e., the volume of product is smaller than the volume of reactants). However, the growing reaction product crystals impinge on one another and push to cause an expansion. If the settnig is done with the investment submerged in water, then this dramatically enhances the expansion. During heating some of the matrix can thermally decompose and cause some shrinkage. For the most part, heating induces thermal expansion and inversion in the silica filler. [CLICK] The NET change should be about 1.5% expansion and compensate for the gold alloy shrinkage of 1.5%.

4. Summary of Major Expansions and Contractions
INVESTMENT MATERIALS
Summary of Major Expansions and Contractions Tm A l o y
- DV
from
LCTE on
cooling
of alloy
+ DV
from
LCTE I n v e s t m
+ DV
from
inversion
of the
fillers
If you ignore the chemical changes in the material and just look at the expansions, there are 4 things happening: [CLICK] (1) regular setting expansion from the matrix, [CLICK] (2) hygroscopic setting expansion from the matrix, [CLICK] (3) thermal expansion from the filler, and (4) inversion (i.e., displacive expansion) of the filler. Normally the investment is heated up to a point just below the melting temperature of the alloy, [CLICK] so that the internal mold surfaces are slightly colder than the alloy. That facilitates solidification by encouraging nucleation and growth to the solid. [CLICK] The alloy is heated to the point where it is just above the melting temperature and cast.
The amount of cooling shrinkage can be directly calculated from the coefficient of thermal expansion/contraction for the alloy and the difference from the solidification temperature and room temperature.
+ DV
from
hygro-
scopic
expansion
+ DV
from
setting
Trt

5. Mixing of Investment Materials
Casting
Ring
Ring Base
Liner
The manipulation steps are briefly summarized above for using investment. A casting ring is pressed against its base, which is shaped like a funnel. Together they hold the investment. A wide sprue is attached to the thickest portion of the wax pattern and then connected to the foot of the funnel. [CLICK] This places the wax pattern approximately in the center of the ring when the two pieces are assembled. Since the investment material is undergoing expansion, a paper liner is placed along the wall of the investment ring allow movement. [CLICK] Investment material is mixed in a closed cup using a mixing paddle under vacuum to minimize porosity. A side arm of the mixing cup contains an opening to attach to the top of the investment ring. [CLICK] Mixed material flows into the ring when the cup is inverted. The mixing paddle stem is vibrated to facilitate flow and eliminate bubbles. The ring is removed and set in a water bath until setting (and hygroscopic expansion) is complete. After setting the casting ring base is removed.

6. Setting Reaction and Hygroscopic Reaction
INVESTMENT MATERIALS
Setting Reaction and Hygroscopic Reaction 10 20 30 40
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
TIME (Minutes)
EXPANSION (%)
Hygroscopically
Setting
Investment
Investment matrix alone produces about 0.36% expansion. [CLICK] Remember that it only represents about 33% of the entire investment so the true expansion is only about 0.12%. [CLICK] If the investment is set under water (i.e., hydroscopic expansion), the water runs into the spaces between growing crystals and dramatically facilitates setting expansion. [CLICK] Four to five times as much expansion occurs. For the actual investment this represents about 1.8% x 33% = 0.6%.
To get to the necessary 1.5%, the investment relies on thermal expansion and inversion of the silica filler particles (66%). That is considered next.
Normal
Setting
Investment

7. Time-Temperature-Transformation Diagram for SiO2
INVESTMENT MATERIALS
Time-Temperature-Transformation Diagram for SiO2
SiO2
LIQUID
CRISTOBALITE
TRIDYMITE
QUARTZ
1710 C
1410 C
867 C
Reconstructive Transformations
log TIME
Liquid SiO2
High
Cristobalite
Glass
Low
Tridymite
Med Tridymite
Low Tridymite
Quartz
160
105
573 ts tf P1 P2 P3 P4 P5
T (C)
room
temperature
Displacive Transformations
You will remember the TTT diagram for SiO2 (silica). [CLICK] Depending on the cooling rate in the formation of silica, it can end up as a glass, cristobalite, tridymite, or quartz. Each of the 3 crystalline forms undergoes a displacive transformation on cooling to room temperature. [CLICK] That displacive transformation represents a rapid contraction of the material. We take advantage of this on heating. The displacive transformation is reversed and the material dramatically expands. We take advantage of this by choosing the right mixture of forms of silica to produce expansion.

8. Thermal and Displacive Expansion of SiO2 Allotropes
INVESTMENT MATERIALS
Thermal and Displacive Expansion of SiO2 Allotropes
0.2
0.6
0.4
1.4
0.8
1.0
1.2
1.8
1.6
2.0
EXPANSION (%)
TEMPERATURE (C)
100
200
500
300
400
600
700
Cristobalite
Quartz
Tridymite
The curves above show the combination of normal thermal expansion and displacive expansion for each of the forms of silica. [CLICK] Notice that amorphous silica has a low thermal expansion rate and no displacive expansion. So, for all practical purposes, it is not useful for a filler for investment. [CLICK] Cristobalite produces the largest displacive transformation. [CLICK] Tridymite and [CLICK] quartz produce displace expansions that are smaller and occur before and after cristobalite, respectively. Some combination of these materials is chosen as the investment filler to gain the additional 0.9% that is needed to get to a total of 1.5% for the system.
Fused Silica
(Amorphous)

9. Gypsum Bonded Investment (Low Temperature)
INVESTMENT MATERIALS
Gypsum Bonded Investment (Low Temperature)
Crystal expansion
and interlocking
1. Chemical Composition:
a. Binder: % alpha calcium sulfate hemihydrate
b. Filler:
(1) Quartz, or
(2) Cristobalite
c. Modifiers
(1) Colorants:
(2) Reducing Agents:
(3) Shrinkage Controllers: Boric Acid, NaCl
2. Setting Reaction:
CaSO4-(1/2)H2O + (3/2)H2O
----> CaSO4-(2)H2O
3. Dimensional Changes:
a. Contraction due to the chemical reaction
b. Expansion due to impinging crystal growth
c. Expansion facilitated by hygroscopic technique
d. Contraction due to calcium sulfate dehydration (~200C)
e. Contraction due to calcium sulfate de-sulfonation (~700C)
f. Expansion due to thermal expansion
g. Expansion due to inversions of cristobalite (and quartz)
h. Contraction due to cooling of casting alloy
4. Applications: Gold casting alloys
For casting gold alloys for crowns and bridges, the least expensive investment is GYPSUM-BONDED INVESTMENT or GBI. Since gold alloys can be cast at a low temperature, and GBI is only stable to low temperatures, this combination works. GBI is also called LOW TEMPERATURE INVESTMENT. Investment is produced by mixing water with the investment powder at a set ratio. For other alloys that require higher temperatures, different materials are needed. However, their formulations and expansion reactions are very similar.
Gypsum is the BINDER. Remember from your information about the setting of gypsum products that the crystals [CLICK] press on each during formation [CLICK] and also are susceptible to hydroscopic expansion.
As GBI is heated it also undergoes some degradation, including loss of its water and de-sulfonation. However, if it is heated quickly then not too much decomposition has time to happen. The surfaces of the casting will tend to be discolored or black. This is due to the decomposition products reacting with the gold. This film is removed by PICKLING the casting in AQUA REGIA acid to dissolve it.

10. THANK YOU
In a later module, you will be introduced to HIGH TEMPERATURE INVESTMENTS. The two major ones are phosphate bonded investment or PBI and silicate bonded investement or SBI.
Thank you.