first describe by Philbrook D, & Taggart WH

Mould/ die for pattern

Sprue attached to patterns

Patterns invested

Burnout in furnace

Flow of molten metal

Fundamentals of Spruing, Investing & Casting

After casting Finished & polished

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

(GPT 8) 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).

(GPT 8)

1. should be capable of reproducing the shape, size and detail in the wax pattern.

2. easily manipulated.3. setting time should be less.4. should maintain the integrity at higher temperatures and

should not decompose to give off gases.5. Possess sufficiently high value of compressive strength at the

casting temperature to withstand the stresses set up when the molten metal enters the mould.

6. should expand to compensate for the casting shrinkage

7. Investment should be porous enough to permit the air or other gases in the mold cavity to escape easily during the casting

8. Investment should produce a smooth surface and fine detail and margins on the casting.

9. should be inexpensive.

Wax shrinkage- 0.3-0.4%.

Alloy shrinkage- 1.3- 1.6% ( noble alloys), 2.0- 2.3%( Predominantly base metal alloys)

Investment materials are basically consist of

1. A refractory material 2. A binder material

3. Additives used to improve the

handling character.

SILICA (silicon dioxide) is used as refractory material & to regulate the thermal expansion.

It is available in four allotropic forms such as Quartz Tridymite Cristobalite Fused quartz Quartz and Cristobalite are used extensively in dental

investments

Cristobalite occurs naturally as a rare mineral but is normally manufactured by prolonged heating of the quartz at high temperatures to induce the appropriate slow inversion.

Each form of silica exists in two phases.1. Low temperature phase or alpha phase

2. 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 high – low inversion .

Depending on type of silica used the investment materials are classified as

1. Quartz investments

2. Cristobalite investments

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

Other chemicals such as sodium chloride, boric acid, graphite, copper powder, are often added in small quantities to modify physical properties.

boric acid and sodium chloride, not only regulate the setting expansion and the setting time, but also prevent most of the shrinkage of gypsum when it is heated above 300 ºC (572° F).

They are the mold materials used in the casting of dental gold alloys with liquidus temperatures no more than 1080 ºc.

Used for conventional casting of gold alloys inlays, onlays, crowns and Fpd.

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.

contracts

MODIFIER - (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 300 ºc .

They act by reducing the two large contractions of gypsum binder on heating to temperatures above 300 ºc .

Ex– Boric acid Soluble salts of alkali or alkaline earth metals

SETTING TIME According to ANSI/ADA Specification No. 2 for

dental inlay casting investment, the setting time should not be shorter than 5 min or longer than 25 min. Usually, the modern inlay investments set initially in 9 to 18 min.

Should allow sufficient time for mixing and investing the pattern.

SETTING EXPANSION 1. normal setting expansion: the expansion takes

place when the material sets in air 2. hygroscopic setting expansion: when setting occurs

under water. 3. thermal expansion: when it is affected by heat

Purpose: to enlarge the mold to compensate for the casting shrinkage of the gold alloy

ADA sp no 2 for type 1 investment permits a maximum setting expansion in air of 0.6% setting expansion of modern investments is 0.4%which can be regulated by accelerators and retarders .

It is enlarging the mould containing the wax pattern and it depends on the thermal expansion of pattern caused by heat of reaction.

High gypsum content- high expansion.

Other variables – lower w/p ratio, drier mix of investment- high NSE.

Thin wall pattern- more expansion than thick walls.

Soft wax- more NSE. Wax softer than type II inlay wax- NSE

causes serious distortion of the pattern.

Greater in magnitude than NSE. the gypsum product is allowed to set under or in

contact with water for mould expansion.

The hygroscopic setting expansion may be 6 or more times greater than the normal setting expansion of a dental investment

The increased amount of expansion is because the water helps the outward growth of crystals

The investment should be immersed in water before the initial set is complete.

ADA sp no 2 for such type 2 investments require minimum setting expansion in water of 1.2% and maximum 2.2%.

Composition: more finer silica particles -more HSE. α hemihydrate greater HSE than β hemihydrate. W:P ratio: less water, more powder in mix. Spatulation: more mixing time- more HSE. Time of immersion: immerse in water before initial

set. Confinement: less opposing force from walls of

casting ring (wet cellulose), immersion in water bath at 37.7°C- expansion of wax pattern.

Water: more immersion water. Shelf life: fresher investment.

The thermal expansion is directly related to the amount and type of silica present.

Contraction of gypsum is balanced- quartz 75% More Expansion in the presence of cristobalite and

initial contraction of gypsum is eliminated. Type 1 investments should have thermal expansion of

not less than1% and not greater than 1.6%. Type 1I investments should have thermal expansion of

between 0% to 0.6% at 500°C.

SETTING CONTRACTION: When an investment is cooled from 700°C, it contracts. On reheating it expand again but not recommended as investment may crack on heating.

COMPRESSIVE STRENGTH: According to ADA sp no 2 the compressive strength

should not be less than 2.5MPA to withstand force of molten metal.

FINENESS: Surface roughness of the casting and setting time depend upon the fineness of the material. Fine silica- more hygroscopic expansion.

POROSITY: The material should be porous to allow escape of air from mold space while casting. Lower content of hemihydrate- more water gauging- more porous.

It is a gypsum bonded material mixed with colloidal silica

Setting expansion is 0.9% Thermal expansion is 0.6% when it is heated to 677 ºc As it is a gypsum bonded material it is not

recommended for high fusing alloys. Divestment phosphate is a phosphate bonded

investment used as a divestment for high fusing alloys.

Advantages Disadvantages

Easier to manipulate. Adequate strength. Adequate porosity. Less expensive. Amount of

dimensional change easier to control.

Thermally unstable at high temp.

Powder is hygroscopic. Improper wax burnout

& high casting force may produce cracks in investment leading to fins.

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

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

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 is often added -clean casting. Facilitates easy divesting of casting and mold. Generally added when casting alloy is gold. 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.

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 MC & newer gold alloys.

.

Expansion can be varied by the proportions of silica and water. More silica and less water – more expansion.

When mixed with water- initial shrinkage is seen same as that of GBI. Completely eliminated with colloidal silica.

Liquid can be used as full strength or diluted with water to provide some degree of control over setting or thermal expansion. But if more liquid is used it caused dense nonporous investment which can effect casting.

Early thermal expansion is associated with- decomposition of binder, magnesium ammonium phosphate and evolution of ammonia gas.

Some shrinkage is masked by expansion of refractory filler ( crystobalite)

1 Compressive strength Type 1- 2.5 mpa Type 2- 3 mpa2 Thermal expansion 0.8% when 50:50 mixture of liquid and water

Temperature Warmer the mix faster it setsThe setting reaction liberate the heat andaccelerates rate of setting Mixing timeincreased mixing time and mixing efficiency result in faster set.

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 with stand 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 900°C

These are used for high fusing base metal alloy

Refractory material – Silica

Binder –Silica gel or ethyl silicate

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

Silica gel: pH of sodium silicate lowered by addition of an acid or acid salt.

Colloidal silica converted to gel by adding ammonium chloride.

Ethyl silicate: colloidal polysilicic acid is formed by hydrolizing ethyl silicate in presence of Hcl, ethyl alcohol & water. It is then mixed with quartz or cristobalite and magnesium oxide ( alkaline). Coherent gel of polysilicic acid formed, dried at temp 168°C, loses alcohol & water to form concentrated hard gel . Volumeric contraction during drying is green shrinkage.

Amines added to ethyl silicate- hydrolysis and gelation occurs simultaneously.

It is supplied as a powder and liquid or two liquidsIf supplied as a powder and liquid Powder consists of refractory particles of silicas

and glasses along with the calcined magnesium oxide and some other refractory oxides in minor amounts

Liquid contains stabilized alcohol solution of silica gel

If supplied as 2 liquids One is ethyl silicate Certain types of amines are added for hydrolysis and

gelation to occur simultaneously Other is acidified solution of denatured alcohol

The powder is added to hydrolyzed ethyl silicate liquid, mixed quickly and vibrated into a mold , that has an extra collar to increase the height.

The mold is placed on a vibrator that has a tamping action

This allows the heavier particles to settle while the excess liquid and some of the finer particles rise to the top .

The top of the mold is prone to cracking due to greater drying shrinkage from evaporation of the ethyl alcohol

In about 30 minutes the accelerator in the powder hardens the settled part, and the excess is poured off (to avoid crack formation).

The liquid powder ratio in settled part is greatly reduced and the setting shrinkage is reduced to 0.1%

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.

Investment can be mixed in 2 ways

1. Hand mix: The liquid is added to a clean, dry mixing bowl, and the powder is gradually added to the liquid, using the care and caution to minimize air entrapment. Mixing is formed gently until all the powder has been wet.

2. Vacuum mixing: this is a type of mechanical mixing done under vacuum created by a vacuum mix machine.

Patterns invested

Burnout in furnace

Flow of molten metal