1. HIGH TEMPERATURE INVESTMENTS
Stephen C. Bayne and
Jeffrey Y. Thompson
Department of Operative Dentistry
School of Dentistry
University of North Carolina
Chapel Hill, NC
Different investments are required for casting alloys that melt and are processed at higher temperatures. Low temperature investments (e.g., GBI, gypsum bonded investments) work fine for low temperature gold casting alloys but decompose and are unstable at much higher than normal usage temperatures. Information that follows is a quick review of the overall principles of investment design and reactions leading to discussion of the two main high temperature investment materials: phosphate-bonded investment (PBI) and silicate-bonded investment (SBI). [CLICK] IF YOU WANT TO SKIP ANY REVIEW THEN CLICK ON THE RED BOX IN THE NEXT 10 SECONDS.
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 2004, Bayne and Thompson, UNC School of Dentistry.

2. REVIEW and INTRODUCTION
A. Problem Analysis: impressions, casts, waxing, investing, casting, …
B. Requirements for Casting Investments:
1. Expansion: % compensation for alloy shrinkage
a. Gold alloy contraction: %
b. Base metal alloy contraction: %
2. Accuracy: Resistance to distortion during setting
3. Strength: Resistance to casting forces and high temperatures
a. Refractory: high mp that resistant to high temperature
C. General Formulation for Investment Materials:
1. Refractory FILLER: [66%] = Quartz, Cristobalite
2. BINDER (Matrix): [33%] = Gypsum, Phosphate, Silicate
3. ADDITIVES (Modifiers): [  1%]
D. Dimensional Changes = Setting + Hygroscopic + Thermal
Quickly review the basic considerations for the design of investment materials. In indirect dental restoration fabrication procedures, the most critical steps are investing and casting because of the large dimensional changes that are involved. Casting shrinkage must be compensated in advance by over-sizing the mold spaces for the castings. This is accomplished by expanding the investment materials during setting and heating prior to casting. For base metal alloys, the temperatures for casting are much higher than for gold alloys and range from 2.0 to 2.3% contraction. Investment materials are composed of 66% filler, 33% binder, and 1% additives. The binder undergoes a chemical setting reaction that can produce both setting and hygroscopic expansions. After setting the investment binder and filler both undergo expansion.

3. INVESTMENT REACTIONS A. Setting Reaction:
1. Chemical reaction (minor contraction)
2. Crystallization (xl impingement)
B. Hygroscopic Expansion:
Chemical setting reactions produce a net volumetric contraction but actual result in an expansion due to the impingement of growing crystals of reaction products on each other. This is facilitated by letting the investment set submerged in water so that water enters the matrix and allows the crystal pushing to occur more easily. The graph at the bottom left shows the difference (just for the matrix) between setting expansion alone (lower line) and hygroscopic expansion (upper line). During heating of the set investment, the matrix and filler expand. So the total expansion is the sum of the setting expansion + hygroscopic expansion + thermal expansion. The actual amounts of expansion involved can be balanced differently by different manufacturers but are designed to compensate for the average contraction of the metal alloy being cast.

4. SiO2 log TIME TEMPERATURE (C) ts tf ts tf ts tf LIQUID 1710 C
Liquid SiO2
1710 C ts
CRISTOBALITE
Glass tf
1410 C ts
High
Cristobalite tf
TRIDYMITE
TEMPERATURE (C)
867 C
High
Tridymite ts
High
Quartz tf
573
QUARTZ
160
Low
Cristobalite
Med Tridymite
Low
Quartz
Filler expansion is a critical contribution to the overall net expansion for high temperature alloys. Silica fillers are used in investment materials. Silica may exist as glass, cristobalie, tridymite, or quartz. The last 3 allotropic forms are crystalline and undergo inversions on heating (displacive transformations) as will be seen in the next figure.
105
room
temperature
Low Tridymite
log TIME
— = RECONSTRUCTIVE TRANSFORMATION
ts = Transformation Start (c-curve)
tf = Transformation Finish (c-curve)
- - = DISPLACIVE TRANSFORMATION (Temperature)
… = Cooling Path (e.g., P1)
SiO2

5. INVESTMENT REACTIONS (continued)
C. Thermal Reactions:
1. Review of Phase Changes for Solid Phases:
a. Reconstructive Phase Transformation: very SLOW
b. Displacive Phase Transformation: very FAST
2. Review of Equilibrium and TTT Diagrams for SiO2:
3. Silica Filler:
a. 3 forms undergo displacive transformations and inversions
b. Unique thermal expansion coefficients for each form
Form: Expansion:
Fused Silica:
Cristobalite: 1.7% ( C)
Tridymite: 0.9% ( C)
Quartz: 1.5% ( C)
Reconstructive transformations are extremely slow and require diffusion. They are not involved during heating processes of fillers because there is insufficient time. Displacive transformations are fast distortions of the crystal structure to accommodate for strains of non-equilibrium phases and can be quickly reversed on heating. For silica, the inversions produce expansions as shown above. Cristobalite is commonly used because it expands 1.7% on heating to 600°C. Most fillers are a blend of both cristobalite and quartz to provide the exact amount of desired thermal expansion.

6. INVESTMENT MATERIALS
A. Gypsum Bonded Investment, GBI: (Low Temperature Investment)
B. Phosphate Bonded Investment:
1. Chemical Composition:
a. Binder:
(1) MgO + Acidic Phosphate
(Mono-ammonium phosphate) as powder
(2) Aqueous colloidal silica suspension
generally as liquid
b. Filler: Quartz and/or Cristobalite
c. Modifiers:
2. Setting Reaction:
NH4H2PO4 + MgO + 5H2O ---> NH4MgPO4.6H2O
(Many more complex reactions take place during heating)
3. Dimensional Changes: Parallels GBI investment
4. Applications: P/M Restorations (and also Gold Alloys)
5. Commercial Examples (Ceramigold, Fast-Fire, Powercast)
The last semester, we examined the composition and setting reaction of gypsum-bonded investment (GBI). The two high temperature investment materials parallel GBI but operate to higher temperatures. PBI is an excellent investment for most base metal alloys (e.g., Ag-Pd) but not adequate for the very high temperatures required for Ni-Cr, Co-Cr, and Fe-Cr alloys, when SBI is used.
[CLICK] In PBI, the binder must be designed for high temperature stability and so MgO is blended with acidic phosphate and mixed with aqueous colloidal silica suspension to produce a phosphate reaction product (hence, PBI). [CLICK] The actual setting reaction is fairly complex but the reaction products are similar to GBI in that they produce crystals that cause expansion during impingement and that effect is enhanced by hygroscopic setting.
PGI includes normally a finer-grained powder than GBI and therefore the surfaces of PBI castings are smoother. While PBI is more expensive than GBI, some dental technicians use it to cast gold alloy as well since it generates a casting that requires less work for finishing and polishing. [CLICK] There are many commmercial examples but Ceramigold is a popular one from Whipmix. If you click on the image above, it will take you to the Whipmix website for this product.

7. INVESTMENT MATERIALS (continued)
C. Silicate Bonded Investment:
1. Chemical Composition:
a. Binder: Sodium Silicate + Acid
b. Filler: Quartz or Cristobalie
c. Modifiers:
2. Setting Reaction:
Sodium Silicate + Acid ---> Silicic Acid Gel
(Silicic acid gel reverts to silica during burnout)
3. Dimensional Changes:
(Same as for GBI case)
(Better -- matrix and filler both silica for inversion)
4. Applications: High temperature casting alloys
5. Commercial Examples: (Hi-Temp)
SBI is required for the highest melting alloys (Co-Cr, Ni-Cr, and Fe-Cr). Still other special investments are used for other materials that are not discussed here such as Ti casting. [CLICK] The components are similar to GBI with a binder, filler, and modifiers. [CLICK] A sodium silicate and acid reaction generates a silicic acid gel for a matrix. On heating, the matrix reverts to silica. In essence the investment is silica filler surrounded by silica matrix in the end. [CLICK] A commercial example is Hi-Temp from Whipmix. For both SBI and PBI the liquid component is a special reactant instead of water that is used for GBI.

8. Thank you.
THANK YOU