1. Dr. Raied Al-Attar 2016-2017 4th lecture
Investment Material
Dr. Raied Al-Attar
4th lecture

2. DEFINITIONS
Investing: the process of covering or enveloping, all or in part, an object such as denture, tooth, wax form, crown, etc. with a suitable investment material before processing, soldering or casting.
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).

3. REQUIREMENTS OF INVESTMENT MATERIALS
Must reproduce precisely the details form of the wax pattern
Must provide sufficient strength to withstand the heat of burnout and the impact and the pressure of cast molten alloy
Must maintain the integrity at higher temperatures and should not decompose to give off gases
Must expand sufficiently to compensate for the solidification shrinkage of the alloy
Easily manipulated and setting time should be less
Should be inexpensive
Biological safety

4. MATERIAL Component Investment materials are basically composed of
1. A refractory material
2. A binder material
3. Additives .

5. REFRACTORY MATERIAL
SILICA (silicon dioxide) is used as refractory material & to regulate the thermal expansion.
It exists in four allotropic forms such as
Quartz
Tridymite
Cristobalite
Fused quartz
REFRACTORY MEANS-HEAT RESistance material ,it must be chemically and physically stable at higher temoerature.there are many refractory materials like mgo.alo.but here we chose silica as its themal expansion is hiher than others which we also require for compensention of casting shrinkage.
Each form of silica exists in two phases.
Low temperature phase or alpha phase
High temperature phase or Beta phase
On heating the change between the two phases is rapid and readily reversible on cooling .this change is known as inversion

6. BINDER It binds the refractory materials together.
According to the binder used investment materials are of three groups.
1. GYPSUM-BONDED INVESTMENTS
2. PHOSPHATE BONDED INVESTMENTS
3. SILICA BONDED INVESTMENTS
Provide strength to the investment.

7. ADDITIVES
Used are
Reducing agents –provides a non oxidizing atomsphere in the mold cavity
Modifying chemicals –regulates physical properties like setting reaction, setting expansion
Coloring matter

8. GYPSUM BONDED INVESTMENT MATERIAL
They are the mold materials used in the casting of dental gold alloys with temperature below 7000C.
Used for conventional casting of gold alloys inlays, onlays, crowns.

9. ADA SPECIFICATION-2; divided them further into
TYPE I- for casting inlays/crowns.
mode of expansion: Thermal
TYPE II-for casting inlays/crowns
mode of expansion : Hygroscopic
TYPE III-for partial dentures with gold alloys
According to their mode of expansion and appliance to be fabricated is fixed or removable ,gypsum bonded investment material divided into

10. Composition Refractory
Crystalline polymorphs of silica (quartz or cristobalite)
55-75%
Silica is added to provide a refractory component during the heating of the investment and to regulate the thermal expansion.

11. BINDER – Alpha hemi hydrate form of gypsum(25-45%)
Strength of investment depends on amount of binder present.

12. ADDITIVES - (4-7%) Used are Reducing agents Modifying chemicals
Coloring matter
Reducing agents : they reduce any metal oxides formed on the metal by providing a non oxidizing atmosphere in the mold when the alloy enters mold.
Ex– Copper
Modifying chemicals: They regulate setting expansion and thermal expansion and also prevent shrinkage of gypsum when heated above 200ºC .
Ex– Boric acid
Soluble salts

13. PHOSPHATE BONDED INVESTMENT MATERIAL
Most palladium and base metal alloys used for partial dentures and porcelain fused to metal restorations have high melting temperatures. They should be cast at a mold temperature higher than 700 ºc. To withstand these high temperatures ,molds require different types of binders such as phosphate compounds.
Rapid growth in the use of metal-ceramic and hot-pressed
ceramic prostheses has resulted in the increased use of
phosphate-bonded or silicate-bonded investments, fusion temperatures of Ni-Cr and Co-Cr alloys are
in the range of 1150 °C to 1500

14. They are used in construction of high melting temperature dental alloys .
Soldering and porcelain veneering

15. Types
Type 1
For casting of inlays crowns and other restorations especially for alloys like gold, platinum ,palladium cobalt chromium and nickel chromium
Type 2
For casting of removable partial dentures

16. COMPOSITION
Refractory materials – (concentration of approximately 80%)silica in quartz , cristobalite or a mixture of two .
Purpose
To provide high temperature thermal shock resistance
To provide high thermal expansion
Binder (<20%)
Magnesium oxide (acid) and a phosphate (base)
Originally phosphoric acid was used but mono ammonium phosphate has replaced it as it can be incorporated in powder form

17. Carbon is often added -clean casting.
Modifiers :CARBON
Carbon is often added -clean casting.
Facilitates easy divesting of casting and mold.
Generally added when casting alloy is gold.
Not used with Ag-palladium alloys or base metal alloys as;
-palladium reacts with carbon at temperatures above 15040C
CLINICAL CONSIDERATION- So for base metal alloys and palladium containing alloys carbon free phosphate bonded investment should use. Also, a carbon crucible should not be employed for melting the alloy.
When silver palladium or base metal alloys are invested with the investment containing carbon, it embrittles the alloys even though the investment is heated to the temperature that burn out the carbon.
Palladium reacts with carbon even at above temp 1500°C, so carbon free phosphate bonded invst used for higher temp.

18. It is available as two component systems
1- It is a Powder which contains refractory materials and binders and modifiers
2- Aqueous solution stabilized with colloidal silica
Colloidal silica suspension facilitate greater expansion of the investment which can compensate the greater casting shrinkage of alloys used in Metal Ceramic & newer gold alloys. .
thermal expansion of a
typical phosphate investment mixed with water as compared
with the same investment mixed with its accompanying
special liquid. When phosphate investments are mixed
with water, they shrink over the same temperature range
as gypsum-bonded investments (200 °C to 400 °C). This
contraction is practically eliminated when a colloidal silica
solution replaces water.

19. Advantages
1.They have high fired strength. This make them handle without breaking before they are placed in a furnace for the wax burn out process and strong enough to withstand the impact and the pressure of centrifugally cast molten alloy
2.They also provide high setting and thermal expansion enough to compensate cast metal prosthesis or porcelain veneers during cooling
3. They can withstand temp more than 700°C

20. Disadvantage
When used with alloys having casting temperature greater than 10300C results in mold breakdown & rougher surfaces on casting.
Less porous than gypsum bonded investment(The material should be porous to allow escape of air from mold space while casting) .

21. ETHYL SILICATE BONDED INVESTMENTS
losing popularity because of the more complicated and time consuming procedures, still used in the construction of high fusing base metal partial denture alloy.

22. Composition Refractory material – Silica
Binder –Silica gel that reverts to silica (cristobalite) on heating.
Modifier –
Magnesium oxide (strengthen the gel)
Ammonium chloride - accelerator

23. Binder Methods used to produce the Binder
Lowering the pH of sodium silicate by addition of an acid or an acid salt.
Converting colloidal silica to gel by adding ammonium chloride
Ethyl silicate: colloidal silicic acid is formed by hydrolyzing ethyl silicate in presence of Hcl, ethyl alcohol & water.
Si(OC2H5)+4H2O Si(OH)4+4C2H5OH
Since an ethyl silicate polymer is used, a colloidal sol of
polysilicic acids is expected instead of the simpler silicic acid
sol shown in the reaction.

24. It is supplied as a powder and liquid
Powder consists of refractory particles of silica and glasses along with the magnesium oxide and some other refractory oxides in minor amounts
Liquid contains stabilized alcohol solution of silica gel

25. Can be used for higher temp castings ( 1090°C-1180°C) and compactible with higher fusing alloys.
Care should be taken while handling and burnout as inflammable alcohol is given off.

26. Disadvantage Non porous material. Complicated manipulation
Cannot be used for titanium and its alloys as silica can oxidize titanium or its alloys easily.

27. Other investment materials
Soldering investment
Divestment materials
Investment material for titanium alloys
Investments for all ceramic restoration