Investment materials

PRESENTED BY Dr EKTA GARGMDS 1st YEAR

DEPARTMENT OF CONSERVATIVE DENTISTRY &

ENDODONTICS

An investment can be described as a material which is suitable for forming a mold into which a metal or alloy is appropriately cast.

The procedure for forming the mold is described as “investing”.

Easily Manipulated – not only should it be possible to mix & manipulate the

mass readily & to paint the wax pattern easily, but the investment should also

harden within a short time.

Sufficient strength at room temperature – To permit ease in handling &

provide enough strength at higher temperature to withstand the impact force of

the molten metal.

Stability at higher temperature – Investment must not decompose to give off

gases that could damage the surface of the alloy.

Sufficient Expansion – Enough to compensate for shrinkage of the wax

pattern & metal that takes place during the casting procedure.

Porosity – Porous enough to permit the air & other gases in the mold cavity to escape

easily during the casting procedure.

Smooth surface – Fine details & margins on the casting.

Ease of Divestment – The investment should break away readily from the surface

of the metal & should not react chemically with it.

Inexpensive

Not be bio hazardous

Reasonable setting time

Long shelf life

REFRACTORY MATERIAL

BINDER

OTHER CHEMICALS

Refractory material: Usually a form of silicon dioxide, such as

quartz, tridymite, cristobalite or a mixture of these.

Binder material: Common binder used for dental casting gold

alloy is α calcium sulphate hemihydrate, phosphates and ethyl

silicate.

Other Chemicals: Such as sodium chloride, boric acid

potassium sulphate, graphite, copper powder or magnesium

oxide.

• Phosphate bonded

• Silicate bonded

High temperature

casting investment

• Gypsum bonded

Low temperature

casting investment

CLASSIFICATION

1. Based on processing temperature :-

2. Based on type of

Binder Used

Gypsum bonded

investment

Type I

Type II

Type III

Phosphate bonded

investments

Ethyl silicate

Bonded

investments

3. BASED ON TYPE OF REFRACTORY USED

a.) Silica -

quartz

cristobalite

b.) Magnesium oxide

c.) Zirconia based investments

The Gypsum based materials represent the type traditionally used for conventional casting of gold alloy inlays, onlays, crowns, & fixed partial dentures (FPDs).

ADA specification No. 2 for casting investments for dental gold

alloys encompasses three types of investments.

Type I: Employed for the casting of inlays or crowns when the

alloy casting shrinkage compensation is accomplished principally

by thermal expansion of the investment.

Type II: Investments are also used for the casting of inlays or

crowns, but the major mode of compensation is by the hygroscopic

expansion of the investment.

Type III: Used in the fabrication of partial dentures with gold

alloys.

Quartz or Cristobalite – Allotropic forms of silica [ 55 – 75 % ]

alpha– hemihydrate – provides strength and rigidity serves as a binder [ 25 – 35 %]

Chemical modifiers – 5 %

The α-hemihydrate form of gypsum is generally the

binder for investments used in casting gold

containing alloys with melting ranges below

1000°C.

When this material is heated to the temperature

required for complete dehydration and sufficiently

high to ensure complete castings, it shrinks

considerably and frequently fractures.

Dimensional Change

of three forms of

gypsum when heated

Silica (SiO2) is added to provide a refractory component

during the heating of the investment and to regulate the

thermal expansion.

During the heating, the investment is expected to

expand thermally to compensate partially or totally for

the casting shrinkage of the gold alloy.

If proper forms of silica are employed in the investment,

the contraction of gypsum during heating can be

eliminated and changed to an expansion.

Thermal expansion of

four forms of silica

Quartz 575°c

Cristobalite b/w

200°c & 270°c

Tridymite two

inversions occur at

117°c & 163°c

Cristobalite

Tridymite

Fused Quartz

Modifying agents, coloring matter, & reducing agents,

such as carbon & powdered copper.

Reducing agents provide a nonoxidizing atmosphere in

the mold when gold alloy is cast.

Some of the added modifiers, such as boric acid, and Na

chloride, not only regulate the setting expansion and the

setting time, but they also prevent most of the shrinkage

of gypsum when it is heated above 300°C.

According to ADA specification No. 2 for dental inlay

casting investment, the setting time should not be

shorter than 5 minutes nor longer than 25 minutes.

Usually, the modern inlay investments set initially in 9

to 18 minutes.

Sufficient time should be allowed for mixing and

investing the pattern before the investment sets.

The purpose of setting expansion is to aid in enlarging the mold

to compensate partially for the casting shrinkage of the gold.

ADA specification No. 2 for Type I investment permits a

maximum setting expansion ‘in air’ of only 0.6%.

The setting expansion of such modern investment is

approximately 0.4%. It can be regulated by retarders and

accelerators.

A mixture of silica and gypsum hemihydrate results in setting

expansion greater than that of the gypsum products when it is

used alone.

Setting expansion is influenced by the exothermic heat

transferred to the pattern.

Variables other than the exothermic heat of reaction also

influence the effective setting expansion.

• As the investment sets, it essentially gains sufficient

strength to produce a dimensional change in the wax

pattern & mold cavity as setting expansion occurs.

• Also, the softer the wax, the greater the effective setting

expansion, because the softer wax is more readily moved

by the expanding investment.

One of the methods for expanding the casting mold to compensate

for the casting shrinkage of the gold alloy.

Occurs when the gypsum product is allowed to set when placed in

contact with water.

Greater in magnitude than normal setting expansion

ADA specification No. 2 for Type II investments requires a

minimum setting expansion in water of 1.2% while the maximum

allowed is 2.2%.

Factors influencing hygroscopic expansion-

a. Effect of composition-

The magnitude of setting expansion of a dental

investment is generally proportional to the silica content

of the investment.

• Finer the particle size of silica greater the

expansion.

• α-hemihydrate will produce a greater expansion

than β-hemihydrate.

b. Effect of water:powder ratio:

The highest the W:P ratio of the original investmentwater mixture, the less the hygroscopic settingexpansion.

c. Effect of spatulation:

With most investments, as the mixing time is reduced,the hygroscopic expansion is decreased.

d. Shelf life of the investment:

Older the investment, the lower its hygroscopicexpansion.

e. Effect of time of immersion:

The greatest amount of hygroscopic setting expansion isobserved if the immersion takes place before the initialset.

f. Effect of the amount of water added:

The magnitude of hygroscopic expansion is in direct proportion to

the amount of water added during the setting period until a

maximum expansion occurs, no further expansion is evident

regardless of any amount of water added.

Expansion can be detected when water is poured into a vessel

containing only small, smooth quartz particles. The water is drawn

between the particles by capillary action and thus causes the particle

to separate, creating an expansion.

The effect is not

permanent after the water is

evaporated, unless a binder is

present.

The greater the amount of

the silica or the inert filler,

the more easily the added

water can diffuse through the

setting material and the

greater is the expansion.

The thermal expansion of a gypsum bonded investment is

directly related to the amount of silica present and to the type of

silica employed. A considerable amount of quartz is necessary to

counterbalance the contraction of gypsum during heating.

The contraction of the gypsum is entirely balanced when the

quartz content is increased to 75%.

The investments containing cristobalite expand earlier and to a

greater extent than those containing quartz.

The desirable magnitude of the thermal expansion of a

dental investment depends on its use.

If hygroscopic expansion is to be used to compensate

for the contraction of the gold alloy, as for the Type II

investment. ADA specification No. 2 requires that the

thermal expansion be between 0% and 0.6% at 500°C.

However, for Type I investment, which rely principally

on thermal expansion for compensation, the thermal

expansion must be not less than 1% nor greater than 1.6%.

Another desirable feature of an inlay investment is that

its maximum thermal expansion be attained at a

temperature not higher than 700°C. Thus when a thermal

expansion technique is employed, the maximum mold

temperature for casting of gold alloy should be less than

700°C.

Factors affecting thermal expansion-

a. Effect of Water:Powder ratio

The magnitude of thermal expansion

is related to the amount of solids

present. Therefore it is apparent that

the more water that is used in mixing

the investments, the less is the

thermal expansion that is achieved

during subsequent heating.

b. Effect of chemical modifiers:

The addition of small amounts of sodium, potassium, or

lithium chlorides to the investment eliminates the

contraction caused by the gypsum and increases the

expansion without the presence of an excessive amount

of silica.

c. Thermal contraction/cooling

of investment:When the investment is cooled from

700˚c, its contraction follows the expansion

curve during the inversion of the beta-

quartz or beta crytobalite to its stable alpha

form at room temperature.

On cooling – investment shows overall

contraction as compared to its dimensions

before heating .

On reheating – does not expand to its

previous level – also it can cause cracks &

affect quality of casting .

d. Strength:

According to ADA specification No. 2, the compressive

strength for an inlay investment should not be less than

2.4Mpa tested 2 hours after setting.

Heating the investment to 700°C may increase or

decrease the strength as much as 65%, depending on the

composition. The greatest reduction in strength on heating

is found in investments containing sodium chloride.

Other gypsum investment considerations:Fineness:- Finer the investment, the smaller are the surface

irregularities on the casting.

Porosity:- More gypsum crystals present in the set

investment,

less porosity.

More uniform the particle size, greater is its porosity.

Storage:- The investment should be stored in airtight &

moisture

proof containers.

As suggested by Skinner (1963) “The definite advantage of thistype of investment is that there is less chance for contamination of gold

alloy during casting and hence could be the investment of the future.

The present trend is towards the use of less expensive base metal Alloys, most of which require phosphate investments.

The rapid growth in use of metal ceramic restorations and a higher melting point alloys have resulted in an increased use of “phosphate bonded investment.”

Refractory fillers

Binders

Filler: silica, in the form of cristobalite, quartz, or a mixture of

the two and in the concentration of approximately 80%.

• The purpose of this filler is to provide high temperature thermal

shock resistance (refractoriness) and a high thermal expansion.

The binder: consists of magnesium oxide and a phosphate

(Monoammonium phosphate).

Colloidal silica liquid suspensions are available for use with the phosphate

bonded investments in place of water. For base metal alloys, a 33% dilution of the

colloidal silica is required.

Carbon is often added to the powder to produce clean castings, and facilitate the

‘divesting’ of the casting from the mold.

Water

Exhibit less SE

Hygroscopic expansion negligible

Colloidal silica

- Exhibit higher SE – useful as shrinkage more in base metal alloys

.

Can expand hygroscopically

Strength of investment more

Setting Reaction

The chemical reaction for the binder system that causes the investment to set and harden is

NH4H2PO4 + MgO + 5H2O NH4 MgPO4 6H2O

MgO+NH4H2PO4+ 5H2O

(NH4MgPO4 .6H2O)n Room TemperatureMgO

NH4H2PO4 Colloidal-type particlesH2O

Prolonged setting at 25°cor dehydration at 50°c

(NH4MgPO4 .6H2O)nDehydrated at 160°c

(NH4MgPO4 .H2O)nHeated from 300-650°c

(Mg2P2O7)n

Noncrystalline polymeric phaseMg2P2O7

Heated above 1040°c

Mg3 (P2O4) 2

On heating the binder undergoes the following thermal reactions :

When phosphate bonded investments are mixed with water they exhibit a shrinkage within essentially the same temperature range as gypsum bonded investments i.e. 200°c-400°c

This contraction is practically eliminated when a colloidal silica solution replaces the water.

The early thermal shrinkage of phosphate investments is associated with the decomposition of the binder, magnesium ammonium phosphate and is accompanied by the evolution of ammonia, which is readily apparent by its odor.

Setting and Thermal Expansion

Influence of liquid concentration on setting & thermal expansion.

Thermal expansion of the investmentwhen mixed with water as compared

to special liquid.

Phosphate investments are markedly affected bytemperature. The warmer the mix, the faster it sets.The setting reaction itself gives off heat, and thisfurther accelerates the rate of setting. The moreefficient the mixing better the casting in terms ofsmoothness and accuracy.

The ideal technique is to mix, as long as possible, yethave enough time for investing. Mechanical mixing undervacuum is preferred.

Ethyl Silicate Bonded Investment

ETHYL SILICATE bonded investments are being used inthe construction of the high fusing base metal partial denture

alloys.

These investments are losing popularity because of the morecomplicated and time consuming procedures involved.

The silica is used as the binder which may be derived from ethyl silicate or sodium silicate.

POWDER LIQUID It’s a mixture of

Powder – refractory particles of silica , MgO & other oxides .

For these systems – 2 or more liquids – reacted together – before being mixed with powder .

The REACTIONThe silica is first formed by the hydrolysis of ethyl

silicate in the presence of hydrochloric acid, ethyl alcohol & water. The reaction can be expressed as:

Si (OC2H5) + 4H2O HCl Si(OH)4 + 4C2H5OH

Because a polymerized form of ethyl silicate is used, a colloidal sol of polysilicic acids is expected instead of the

simpler silicic acid sol shown in the reaction- Stage

called - HYDROLYSIS

The second stage of reaction – GELATION .Sol – mixed with quartz or cristobalite + small amount of MgO to render the mixture alkaline .Coherent gel of polysilicic acid formed – accompanied by slight ‘setting shrinkage’.

Third stage – DRYING

Soft gel – dried to a temp. below 168 °C .During drying – Gel loses alcohol & water – to form hard concentrated gel of silica particles – tightly packed together .Volumetric contraction accompanies drying – “ green shrinkage” .

Process of ethyl silicate-bonded investment is a little more complicated than that of phosphate type in that care must be exercised during handling & burnout because inflammable alcohol is given off.

This type of investment can be heated between 1090°c and 1180°c and is compatible with the higher fusing alloys. Its low setting expansion minimizes distortion.