Invest Final 3

Investment Materials

Dental casting investment (According to GPT 7)

A material consisting principally of an allotrope of silica and

a bonding agent. The bonding substance may be gypsum (for use in

lower casting temperature) or phosphates and silica (for use in

higher casting temperature).

Refraction investment (Acc to GPT 7)

An investment material that can withstand the high

temperature used in soldering or castings.

Investing (Acc to GPT 7)

The process of covering or enveloping wholly or in part, an

object such as a denture, tooth, wax form, crown etc with a suitable

investment material before processing, soldering or casting.

Requirements

It should be easily manipulated.

The investment should be capable of reproducing the shape,

size and detail recorded in the wax pattern.

The investment mould should be capable of maintaining its

shape and integrity at high temperature.

It should have high compressive strength to withstand the

stresses set up when the molten metal enters the mould.

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Should compensate for casting shrinkage by its setting

expansion.

The investment should produce a smooth surface and fine

detail and margins on the casting.

After the casting is completed it should be easily removed

from the surface of the metal.

It should be inexpensive.

Factors involved in the selection of investment materials

1. The casting temperature to be used.

2. Type of alloy to be cast

The investment which is able to retain its integrity at the

casting temperature and able to provide the necessary compensation

for casting shrinkage is chosen.

TYPES

Gypsum bonded

Phosphate bonded

Ethyl silica bonded investment

Applications of the various types of investment

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Gypsum bonded materials mould for gold casting alloys

Phosphate bonded materials high fusing, noble metal alloys

and base metal alloys

Silica bonded materials mould for base metal casting alloys

(rarely used).

COMPOSITION (IN GENERAL)

All investment materials must contain a refractory material

and a binder.

Refractory material

A material that will withstand high temperature without

decomposition or disintegration (e.g.) silica

Silica exists in four allotropic forms

Quartz

Tridymite

Cristobalite

Fused quartz

Functions

It withstand increased temperature

Regulates the thermal expansion

Binder

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It is used because the refractory materials alone do not form a

coherent mass, some kind of binder is needed. e.g.:

Gypsum

Phosphate

Silicate

Other chemicals

Sodium chloride

Boric acid

Potassium sulfate

Graphite

Copper powder or magnesium oxide

Functions

To achieve desirable properties required for the investment

E.g.: chloride or boric acid enhance thermal expansion

GYPSUM BONDED INVESTMENTS

Classification

ADA specification number 2 for gold alloy casting

investment, there are three types based on use and type of

expansion used

Type I inlay, thermal expansion

Type II inlay or crowns, hygroscopic expansion

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Type III partial denture, thermal expansion

Supplied: As powder in bulk or preweighed packs

Composition

Silica 60 to 65% (quartz, or cristobalite)

Alpha hemihydrate 30 to 35%

Chemical modifiers – 5%

Functions of each constituent

Silica (quartz or cristobalite)

Refractory material

Regulates thermal expansion

Prevents contraction of gypsum

Modifiers

Coloring agents

Reducing agents – they reduce any oxides formed on the metal

by providing a non-oxidising atmosphere in the mould (e.g.)

carbon or copper powder

Modifying chemicals

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They regulate setting expansion and setting time and also

prevent shrinkage of gypsum when heated above 300 0C. e.g.:

1. Boric acid,

2. Sodium chloride.

PROPERTIES OF GYPSUM INVESTMENTS

I. Setting time

According to ADA specification no 2,

Setting time should not be shorter than 5 minutes not longer

than 25 minutes.

Modern investment material set initially in 9 to 18 minutes.

II. Storage

It should be stored in air tight and moisture proof containers

Advisable to purchase in small quantities

III. Strength

Factors controlling it are

hemihydrate increased strength

Chemical modifiers increased strength

Increased water decreased strength

Heating to 700 0C increase or decrease strength up to 65%

depending on composition

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Cooling decreased strength

IV. Thermal stability

One of primary requirement is that it should withstand its

integrity at casting temperature.

It decomposes above 1200 0C to liberate sulphurtrioxide

CaSO4 + SiO 2 CaSiO 3 + SO3

Adverse effect

Weakening of investment

Porosity in the castings

So, gypsum bonded investment are generally restricted to use

well below 1200 0C (e.g.) (1) Majority of gold alloys and (2) lower

melting base metal alloys.

Second reaction

CaSO4 + 4C CaS + 4CO

The carbon may be derived from the residue left after burning

out the wax pattern or may be present as graphite in the investment.

Further leads to

3CaSO4 + CaS 4CaO + 4SO 2

These reactions take place above 700 0C.

Method of minimizing

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“Heat soaking” the investment mould at the casting

temperature to allow the reaction to be completed before casting

commences.

V. Porosity

Gypsum bonded are sufficiently porous to allow escape of air

and other gases from the mould during casting.

VI. Setting expansion

It is of three types

Normal

Thermal

Hygroscopic.

NORMAL SETTING EXPANSION

A mixture of silica and dental stone results in a setting

expansion which is greater than gypsum product is used alone. The

silica particles probably interfere with the intermeshing of the

crystals as they form. This creates an outward thrust which results

in expansion. According to ADA specification no 2,

Type I – 0.5% in air

Modern investment – 0.4%

It can be regulated by

Retarders

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Accelerators

Factors affecting setting expansion

Lower water: powder ratio greater in the setting expansion.

More gypsum content increase setting expansion

HYGROSCOPIC SETTING EXPANSION

When gypsum products are allowed to set in contact with

water, it leads to outward growth of crystals.

This results in increased expansion than the normal setting

expansion.

ADA specification no 2 for type II investments require 1.2%

and maximum 2.2% expansion.

FACTORS AFFECTING HYGROSCOPIC SETTING

EXPANSION

Composition

It depends on

Increased silica content increased expansion

Finer the particle size increased expansion

Water: powder ratio

The higher the water: powder ratio→ decreased expansion

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Temperature

Increased temperature of immersion water, less is the surface

tension increased expansion

Effect of spatulation

Decreased mixing time decreased expansion

Shelf life of the investment

The older the investment decreased expansion

Effect of time of immersion

The greatest amount of expansion is seen when immersion is

before the initial set.

Effect of confinement

The confinement of the investment by the walls of the

container and the wax pattern reduces hygroscopic expansion.

Effect of the amount of added water

Magnitude of the hygroscopic expansion is in direct

proportion to the amount of water added during the setting period

until a maximum expansion occurs. No expansion occurs or further

adding of water when a maximum level is reached.

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Fineness

The finer the investment, greater the smoothness of the

casting

THERMAL EXPANSION

In gypsum bonded investment thermal expansion is achieved

at temperature of 700 0C. The amount of thermal expansion required

depends on which method is used for casting shrinkage

compensation.

For hygroscopic expansion technique the required thermal

expansion is 0.5 to 0.6%.

For normal setting expansion technique 1.0 to 2.0%.

Type I investment should have a thermal expansion of not less

than 1.0% or more than 1.6%.

Factors controlling

Amount and type of silica used

W: P ratio. More water decreased expansion

Chemical modifiers e.g.: NaCl, KCl, Lithium chloride

increases thermal expansion and prevents contraction of the

mould.

PHOSPHATE BONDED INVESTMENTS

TYPES

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Type I for inlays crowns and other fixed restorations

Type II for partial dentures and other cast removable

restorations

Uses

Metal – ceramic restoration

High fusing, noble metal alloys and base metal alloys like

nickel chromium and cobalt chromium

Supplied as

Powder with special liquid

Composition

Powder

Ammonium diacid phosphate (NH 4H2PO4)

Functions

Provides Strength

It is soluble in water and provides phosphate ions

It reacts with silica at high temperature to increase strength at

casting temperature

Silica: refractory (Cristoballite or quartz or mixture)

Magnesium oxide : reacts with phosphate ion

Liquid

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The phosphate bonded investments are mixed with a special

liquid supplied by the manufacturer. This liquid is a form of silica

sol in water, which gives higher hygroscopic expansion.

PHOSPHATE BONDED INVESTMENT

Setting reaction

NH4H2PO4 + MgO + 5H 2O NH4MgPO4 . 6H2O

The above reaction occurs at room temperature, ammonium

diacid phosphate reacted with magnesium oxide to give the

investment green strength.

The ammonium diacid phosphate is used in greater amount

than is necessary for this reaction, so that the excess react with

silica at an elevated temperature.

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I. Working and setting time

Warmer the mix, faster its set

Setting reaction itself produces heat which further accelerate

the rate of setting

Increased mixing time and mixing efficiency faster set and

greater rise in temperature

l iquid : powder ration increases the working time

II. Strength

The strength of the phosphate bonded materials is aided by

the formation of silicophosphates on heating.

The cohesive strength of the phosphate investment is so high

that no casting ring is needed while heating.

III. Porosity

They are sufficiently porous to allow escape of air and other

gases from the mould during casting.

IV. Setting and thermal expansion

When mixed with water it showed contraction at 200 0C to

4000C, but the use of silica solution instead of water the contraction

is eliminated and expansion is achieved.

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SILICA BONDED INVESTMENTS

Uses

It is used in the construction of the high fusing base metal

partial denture alloys.

Composition

Silica is the binder, it is derived from the ethyl silicate or

aqueous dispersion of colloidal silica or sodium silicate.

Two systems are available

One system consists of silica refractory which is bonded by the

hydrolysis of ethylsilicate in the presence of HCl. The final

product is colloidal solution of silicic acid and ethyl alcohol.

Si (OC 2H5)4 + 4H 2O Si(OH) 4 + 4C 2H5OH

The other system uses sodium silicate. They are supplied

along with two bottles of special liquid. One bottle contains

sodium silicate and other contains diluted HCl.

Disadvantage

It is loosing its popularity because of its complication like

inflammable components and time consuming procedure.

The silica bonded materials are so closely that they are

nearly porosity free and there is a ‘bulk pressure’ building

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up which cause the mould to be porous. These problems can

be overcome by making vents in the investment which

prevent the pressure from increasing.

SOLDERING INVESTMENT

Composition

Quartz and a calcium sulfate hemihydrate binder

Properties

Decreased setting and thermal expansion to prevent

misalignment of soldering parts.

Increased fineness of the particle size is not recommended

Uses

Soldering of clasps in removable partial dentures etc.,

INVESTMENT FOR ALL CERAMIC

Two types are developed

- Cast glass technique

- Refractory die material

Cast glass technique

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It consists of casting ring lined with ceramic paper and the

pattern is invested in phosphate bonded investments.

Refractory die material

They are phosphate boned investment material

They are poured into the impression to make refractory dies

The ceramic is builded up over the refractory die and finally

it is subjected to firing

Conclusion

It is the duty of the dentist to have a sound knowledge on

investment materials and their properties for a successful casting.

References

1. Mccabe JF:Applied Dental Materials. 8th Edition.

2. Craig RG, Powers JM, Wataha JC: Dental Materials,

Properties and Manipulation. Seventh Edition, India 2001;

Harcourt private limited.

3. Anusavice: Philips science of Dental materials. Eleventh

Edition, India 2003; Saunders Publication.

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Documents

Invest Final 3