Investment material

Investment Material

Dr. Raied Al-Attar
2016-2017
4th lecture

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).

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


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

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

• 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

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

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

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.

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

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.

BINDER – Alpha hemi hydrate form of gypsum(25-45%)

Strength of investment depends on amount of binder present.

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


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.

They are used in construction of high melting temperature dental alloys .

Soldering and porcelain veneering

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







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

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.

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.


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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

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) .

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.

Composition

Refractory material – Silica

Binder –Silica gel that reverts to silica (cristobalite) on heating.

Modifier –
Magnesium oxide (strengthen the gel)
Ammonium chloride - accelerator


BinderMethods 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

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

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.

Disadvantage

Non porous material.

Complicated manipulation

Cannot be used for titanium and its alloys as silica can oxidize titanium or its alloys easily.


Other investment materials
Soldering investment

Divestment materials

Investment material for titanium alloys

Investments for all ceramic restoration