Investment materials

•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 used in higher casting temperatures).

(GPT 8)

DEFINITION• 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)

•Acc to Craig ,-- An investment can be described as a

ceramic material that is suitable for forming a mold into which a metal or alloy is cast.

-- The operation of forming a mold is described as investing.

Introduction•The adoption of the casting practice in

dentistry for making crowns, bridges, gold alloy inlays and other restorations represent one of the major advancement in restorative dentistry.

•Following the production of a wax pattern either the direct or indirect method, the next stage in many dental procedures involves the investment of the pattern to form a mold.

•Lost wax casting procedure is adopted for fabrication of metal alloy appliances like inlays, crowns etc outside the mouth.

•An accurate wax pattern is prepared ,is invested in refractory mould materials.

•After wax burn out the mould is filled with alloy liquid, which solidifies & cools down. During this procedure the possible dimensional change taking place are to be suitably compensated.

Methods of compensation:Compensation is done by carefully

controlling the mould expansions, by availing large setting expansion & adjusting the thermal expansions of the investment materials.

Setting Expansion + Thermal Expansion

Properties required of an investment1. Easily manipulated.

2. Sufficient strength at room temperature: the inner surface of mold should not break down at higher temperatures due to force of molten metal.

3. Stability at higher temperatures: it should not decompose to give off gases that could damage the surface of the alloy.

4. Sufficient expansion: it must expand enough to compensate for shrinkage of the wax pattern and metal that takes place during the casting procedure.

5. Beneficial casting temperatures: preferably the thermal expansion versus temperature curve should have a plateau of the thermal expansion over a range of casting temperature.

6. Porosity: it should be porous enough to permit the air or other gases in the mold cavity to escape easily.

7. Smooth surface: fine detail and margins on the casting should be preserved.

8. Ease of divestment: The investment should break away readily from the surface of the metal and should not have reacted chemically with it.

9. Inexpensive.

Composition

•An investment is a mixture of three distinct types of materials: refractory material, binder material, and other chemicals.

•Refractory material-- It is usually a form of silicon dioxide,

such as quartz, tridymite, or cristobalite, or a mixture of these.

•Binder material-- As the refractory material alone do not

form a coherent solid mass, some kind of binder is needed.

-- Common binder used are: α-calcium sulfate hemihydrate.

-- Phosphate, ethyl silicate, and other similar materials also serve as binder for high temperature casting investments.

•Other chemicals-- Alone refractory material and binder

material cannot produce the desirable properties required.

-- Other chemicals , such as sodium chloride, boric acid, potassium sulfate, graphite, copper powder, or magnesium oxide are added in small quantities.

-- Small amounts of boric acid or chlorides enhance thermal expansion of investments bonded by calcium sulfate.

There are in general three types of investments:

•Gypsum bonded investment•Phosphate bonded investment•Silica bonded investment

•Gypsum bonded investments are the oldest materials and are used for casting conventional gold alloys.

•The phosphate bonded investments are used for base metal alloys in fixed partial prosthesis.

•Silica bonded investments are principally used for the casting of base metal alloy partial dentures.

Calcium Sulphate-Bonded Investments•They are used for casting metal inlays,

onlays, crowns and bridges.

•They are used for casting gold alloys.

•They can withstand temp up to 700oC.

• Acc to ADA specification no-2 there are three types of gypsum bonded investment materials:

-- Type 1: thermal expansion type; for casting inlays and crowns.

-- Type 2 : hygroscopic expansion type; for casting inlays and crowns

-- Type 3 : for casting complete and partial dentures.

•These materials are supplied as powders which are mixed with water and are composed of a mixture of silica (SiO2) and calcium sulphate hemihydrate.

•Other minor components including graphite, or powdered copper.

Composition

•25% to 35% - α-calcium sulfate hemihydrate.

•65% to 75% - quartz or cristobalite.

•2% to 3% - chemical modifiers.

Gypsum•α- hemihydrate form of gypsum act as a

binder to hold the other ingredients together and to provide rigidity.

•When this material is heated to the temperatures required for complete deydration and sufficiently high to ensure complete castings, it shrinks considerably and occassionally fractures.

•All forms shrink considerably after dehydration between 200oC and 400oC.

•A slight expansion takes place between 400oC and 700oC.

•And a large contraction then occurs.

•The shrinkage on heating is due to the dehydration of the set gypsum in two stages.

2CaSO4 · 2H2O (CaSO4)2 H2O + 3H2O

(CaSO4)2 · H2O 2CaSO4 + H2O

•Shrinkage is due to the transformation of calcium sulphate from the hexagonal to the orthorhombic configuration.

•This later shrinkage is most likely caused by decomposition and the release of sulfur gases, such as sulfur dioxide.

•This decomposition not only causes shrinkage but also contaminates the castings with the sulfides.

•Thus it is imperative the gypsum products not be heated above 700oC and these effects can be minimized by ‘heat soaking’ the mould at 700°C for at least an hour to allow the reactions to be completed before casting commences.

•The gypsum products containing carbon should not be heated above 650oC.

•In this way , proper fit and uncontaminated alloys are obtained.

Silica (SiO2)• It is added to provide a refractory component during the

heating of the investment and to regulate the thermal epansion.

• Usually, the wax pattern is eliminated from the mold by heat.

• During the heating, the investment is expected to expand thermally to compensate partially or totally for the casting shrinkage of the gold alloy.

• Gypsum shrinks considerably when it is heated,

regardless of whether it is set plaster or stone.

• If the proper forms of silica are employed in the investment, this contraction during heating can be eliminated and changed to an expansion.

• Silica exists in at least four allotropic forms: quartz, tridymite, cristobalite, and fused quartz.

•When quartz, tridymite, or cristobalite is heated, a change in crystalline form occurs at a transition temperature characteristic of the particular form of silica.

•For example, when quartz is heated, it inverts from a "low" form, known as α-quartz, to a "high" form, called β-quartz, at a temperature of 575o C (1067o F).

•In a similar manner, cristobalite undergoes an analogous transition between 200o C (392o F) and 270o C (518o F) from "low" (α-cristobalite) to "high" (β-cristobalite).

•Two inversions of tridymite occur at 117o C (243o F) and 163o C (325o F), respectively.

•The β-allotropic forms are stable only above the transition temperature noted, and an inversion to the lower α form occurs on cooling in each case.

•In powdered form, the inversions occur over a range of temperature rather than instantaneously at a specific temperature.

•The density decreases as the α form changes to the β form, with a resulting increase in volume that is exhibited by a rapid increase in the linear expansion.

The density decreases when the alpha form changes to beta form with a resulting increase in the new volume

•Fused quartz is amorphous and glasslike in character, and it exhibits no inversion at any temperature below its fusion point.

•It has an extremely low linear coefficient of thermal expansion and is of little use in dental investments.

•Thermal expansion of three forms of silica

•Quartz, cristobalite, or a combination of the two forms may be used in a dental investment.

•Both are now available in pure form.

•Tridymite is no longer an expected impurity in cristobalite.

•On the basis of the type of silica principally employed, dental investments are often classified as quartz or cristobalite investments.

Modifiers •The reducing agents are used in some

investments to provide a nonoxidizing atmosphere in the mold when the gold alloy is cast.

•Some of the added modifiers, such as 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 100oC (572o F)

•They are limited to the usual balancing agents to regulate the setting time and setting expansion.

Setting Time•The setting time should not be shorter

than 5 mins or longer than 25 mins.

•Usually, the modern inlay investments set initially in 9 to 18 mins.

Normal setting expansion:linear dimensional change as the investment setsSilica particles result in greater setting expansion HOW??silica particles interfere with the intermeshing and interlocking of crystals

resulting in outward thrust of crystals

resulting in expansion.

According to ADA sp no 2:

FOR TYPE I investments:0.6%Value of setting expansion for modern

investments is 0.4%,regulated by retarders and accelerators.

Purpose : To aid in enlarging the mold to

compensate partially for the casting shrinkage of gold.

Factors determining the effective setting expansion

• Greater the gypsum content of the investment, greater the exothermic heat transmitted to the wax pattern and greater the mould expansion.

• Lower the W/P ratio for the investment, greater the exothermic heat and greater the setting expansion.

•Thinner the walls of the wax pattern, greater the setting expasion of the investment.

•Softer the wax, greater the setting expansion. If a wax softer than Type B inlay wax is used, the setting expansion may cause a serious distortion of the pattern.

2).HYGROSCOPIC SETTING EXPANSION-expansion that occurs as the investment hardens while immersed in water

• This is one of the methods for expanding the casting mold to compensate for casting shrinkage.

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

• The water is drawn between the refractory particles by the capillary action and thus causes the particles to separate creating an expansion

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

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

• In one, method known as the ‘water immersion’ technique, the investment mould is placed into water.

• Another method is the ‘water added’ technique.

• Here a measured volume of water is placed on the upper surface of the investment material within the casting ring.

• This produces a more readily controlled expansion.

FACTORS AFFECTING HYGROSCOPIC SETTING EXPANSION

1. Effect of composition v     The finer the particle size of the

silica, the greater is the hygroscopic expansion.

v     Higher the silica content greater is the expansion.

Alpha hemihydrate produces more expansion in the presence of silica, than beta hemihydrate.

HYGROSCOPIC SETTING EXPANSION & NORMAL SETTING EXPANSION

•The hygroscopic setting expansion of stone or plaster alone is very slight.

•The investment should have at least 15% binder to provide strength after hygroscopic setting expansion, and to prevent drying shrinkage

FACTORS AFFECTING HYGROSCOPIC SETTING EXPANSION

1. Effect of composition v     The finer the particle size of the

silica, the greater is the hygroscopic expansion.

v     Higher the silica content greater is the expansion.

2. Effect of w/p ratiov     The higher the W/p ratio of the

original investment water mixture, the less is the hygroscopic setting expansion.

.3.Effect of temperaturev     Higher the temperature of immersion water,

less is the surface tension and hence greater is the expansion

4. Effect of time of immersionv     Immersion before the initial set causes

greater expansion.

5. Effect of spatulation v     The shorter the mixing time, the less is the

hygroscopic expansion.

6. Effect of shelf life of the investment v     The older the investment, the less is the

hygroscopic expansion. The material should be stored in air tight containers and should not be exposed to humidity. It is better to purchase small amounts of the investment at a time.

7. Effect of confinement v     Both the normal and the hygroscopic setting

expansions are confined by opposing forces, such as the walls of the container in which the investment is poured or the walls of a wax pattern.

This confinement can be avoided largely by placing damp asbestos as a liner on the inner wall of the ring.

The water in the asbestos also is utilized for hygroscopic expansion.

8. Amount of added waterAn increase in the amount of water

added, increases the hygroscopic setting expansion upto a certain point, after which further addition of water does not create any expansion.

•This degree of maximum expansion is called the “critical point”.

•This critical point can be raised or lowered easily by changing the manipulative conditions like W/P ratio, time of spatulation, age of investment etc.

Particle size of silica

•Finer particles of silica produce greater hygroscopic expansion.

•The hemihydrate particles have little effect on this expansion.

Silica/binder ratio

•If this ratio increases, greater will be the hygroscopic expansion and lesser the strength.

•This is because the added water can easily diffuse through the silica particles.

Thermal Expansion•Thermal expansion is directly related to

the amount and type of silica employed.

•The contraction of gypsum is entirely balanced when the quartz content is about 75%.

•Type I investments – thermal expansion of not less than 1.0% nor greater than 1.6%.

•maximal thermal expansion be attained at a temperature not greater than 700°C.

•Gold alloys are apt to be contaminated above this temperature.

Factors affecting thermal expansion

•W/P ratioMore the water used for mixing – less will

be the thermal expansion.

• Effect of chemical modifiers- The addition of small amounts of chlorides of sodium, potassium or lithium to the investment eliminates the contraction and increases expansion.

Strength

•ADA specification no. 2 requires a minimum compressive strength of 2.5 Mpa, 2 hours after setting of the investment

•Modifiers aid in increasing the strength as more of the binder can be used without much reduction in thermal expansion.

•Use of alpha hemihydrate increases compressive strength( than beta hemihydrates).

Fineness

•Fineness of the investment affects the setting time and surface roughness of the casting. Fine silica particles increase the hygroscopic setting expansion and gives smoothness to the casting.

Porosity •More gypsum crystals present in the set

investment – less is the porosity.

•Less the hemihydrate content – greater the amount of gauging water – more is the porosity.

•Mixture of coarse and fine particles exhibits less porosity than an investment composed of uniform particle size.

Storage

•Air tight & moisture proof containers.

•Purchase in smaller quantities.

Phosphate bonded investment material•The use of phosphate or silica boned

investment is increased as rapid growth in use of metal ceramic restorations and increased use of higher melting alloy.

•Base metal alloys and higher melting range gold alloys require phosphate bonded investment material.

Composition This type of investment consists of three types of

components, each of which is responsible for certain characteristic properties:

The components are:1. Ammonium Diacid phosphate• It gives strength, soluble in water, provide

phosphate ions, reacts with silica at high temp to increase strength.

2. Refractory filler : silica in the form of cristobalite, quartz or a mixture of the two and in concentration of approx 80%.

•Purpose of the filler: to provide high temperature thermal shock resistance and a high thermal expansion.

•3. Binder - Magnesium oxide and phosphate that is acid in nature.

•Initially phosphoric acid was used but monoammonium phosphate has replaced it, because it can be incorporated into the investment powder.

•Colloidal silica suspensions are available for use with phosphate investments instead of water.

•These suspensions can freeze and become unusable – so should be stored in frost free environment.

•Phosphate bonded investments are mixed with a special liquid .

•The liquid is a form of silica sol in water, which gives higher thermal expansion.

• Carbon is added to the powder to produce clean casting and facilitate the divesting of the casting from the mold.

• This addition is appropriate when casting alloy is gold but not with base metal alloys.

• Carbon embrittles the alloy even though the investment is heated to temp that burn out the carbon.

•The chemical reaction is as follows that causes the investment to set and harden:

NH4H2PO4 + MgO + 5H2O NH4MgPO4 6H2O

Working and Setting time

• Unlike gypsum investments, phosphate investments are markedly affected by temperature.

• Warmer the mix, faster the set. • The reaction gives off heat, which further

accelerates the setting. Increased mixing time and mixing efficiency result in a faster set; these two factors give smoothness and accuracy to the casting.

• Mechanical mixing under vacuum is preferred.• An increase in liquid/powder ratio increases the

working time.

Setting and thermal expansion•As powder(gypsum) and liquid is mixed

there is slight expansion. This can be increased by using colloidal silica solution instead of water.

•When phosphate bonded investment material is mixed with liquid containing silica first there is early thermal shrinkage.

•This is associated with the decomposition of the binder, magnesium ammonium phosphate and is accompanied by evolution of ammonia.

•Shrinkage is masked because of the expansion of the refractory filler, like cristobalite.

Setting and thermal expansion•The combined setting and thermal

expansion for phosphate investments is around 2% if the special silica liquid is used

Ethyl Silicate bonded investments•Less popular because of more

complicated and time consuming procedures.

•Used in construction of high fusing base metal partial denture alloys.

•Binder is silica gel that reverts to silica on heating.

•Several methods are used to produce silica or silicic acid gel binders.

•When a pH of sodium silicate is lowered by the addition of an acid or an acid salt such as monoammonium phosphate, a bonding silicic acid gel forms. The addition of Magnesium oxide will strengthen the gel.

•An aqueous solution of colloidal silica can be made to gel by the addition of an accelerator, such as ammonium chloride

•A colloidal silicic acid is first formed by hydrolyzing ethyl silicate in the presence of hydrochloric acid, ethyl alcohol and water, as follows:

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

•Because a polymerized form of ethyl silicate is actually used, a colloidal sol of polysilicic acids is formed instead of the simpler silicic acid shown in the above reaction.

•The formation of poly silicic acid constitutes the 1st stage of the setting reaction, called “hydrolysis”.

•Stage 2 is called “gelation”. Here the sol is mixed with quartz or cristobalite to which is added a small amount of finely powdered MgO to render the mixture alkaline. A coherent gel of polysilicic acid then forms accompanied by a slight ‘setting shrinkage’.

•Stage 3 is called “drying”.

•Here the soft gel is dried to a temperature below 168°C. During drying, the gel loses alcohol and water to form a hard, concentrated gel of silica particles tightly packed together.

• A considerable volumetric contraction accompanies the drying. Which reduces the size of the mould. This contraction is known as “green shrinkage” and it occurs in addition to the setting shrinkage.

•A faster method to obtain silica gel is by the addition of amines such as piperidine to the solution of ethyl silicate. Here hydrolysis and gelation occurs simultaneously. But an unacceptable shrinkage may occur, mainly in the stage of hydrolysis.

•Stock solutions of hydrolysed ethyl silicate binder may be prepared and stored in dark bottles.

•The solution gels slowly on standing and its viscosity may increase noticeably after 3-4 weeks when it has to be discarded.

•Silica-bonded investments being more refractory than phosphate-bonded investments, can tolerate higher burn-out or mould-casting temperatures.

•Temperatures between 1090 and 1190°C are employed when the higher fusing chromium containing alloys are cast.

Investments for small castingOBJECTIVE

-castingTo provide metallic duplication of

missing tooth structure with accuracy.

•Steps in making metal inlays, onlays crowns and bridges

case selection tooth preparation gingival retraction making impression die preparation

•Wax pattern preparation•Investment of wax pattern•Burnout procedure•Casting• finishing and polishing•Cementation•Recall.

Wax pattern with sprue attached