INTRODUCTION

HISTORY

DEFINITIONS

REQUIREMENTS

USES

CLASSIFICATION (TYPES)

BASIC STRUCTURE (COMPOSITION)

PROPRETIES OF INVESTMENTS

EXPANSION OF INVESTMENTS

INVESTMENTS FOR LOW TEMPERATURE CASTING PROCEDURES Hydrocolloids Gypsum investment Layered silicon rubber Gypsum bonded investmentINVESTMENTS FOR HIGH TEMPERATURE CASTING PROCEDURESPhosphate bonded investmentsEthyl silicate bonded investmentsINVESTMENTS USED TITANIUM BASED ALLOYSCalcia bonded investmentZirconia phosphoric acid systemModified phosphate bonded investments

DIVESTMENT

BRAZING INVESTMENTS

COMMERCIAL PRODUCTS

CONCLUSION

REFERENCES

The principal laboratory technique of making metal inlays, onlays, crowns and bridges, is based on casting practice. This application of casting practice is one of the major advances in restorative dentistry, which is mainly based on “Lost wax technique”.

This process of casting involve some basic steps:

1. Preparation of a wax pattern.2. Preparation of mold - pour the mixed investment

material around the wax pattern and allow it to set. Burn out: wax is eliminated from the investment by

boiling (or) burning it in oven.3. Then casting is done by melting the alloy and forcing

the molten metal into the mold cavity.

Casting

Casting since about 4000 BC…

Ancient Greece; bronzestatue casting circa 450BC

Iron works in early Europe,e.g. cast iron cannons fromEngland circa 1543

This meticulous procedure of casting was used by various craftsmen to produce jewelry and ornaments.

Its history can be traced back around 4000 B.C.

But origin of lost wax technique, when viewed history makes its presence in the writings of Theophilus (1 1 Century).

11 Century: Theophilus-Described lost wax technique, which was a common practice prevailed then.

1558 : Benvenuto Cellini - claimed to have attempted the use of wax and clay for preparation of castings.

1884 : Aguilhon de saran - Used 24K gold to form Inlay.

1887: J.R.Knapp - Invented Blowpipe.

1897 : Phillbrook - Described a method of casting metal filling.

1907 : Taggart - Devised a practically useful casting machine.

Various studies conducted on the properties of investment materials and casting alloys have led to a path for better, practical and useful processing methods.

Investing: The process of covering, enveloping, wholly (or) in part an

object such as denture, tooth, wax form, crown, etc. with a suitable material before processing, soldering, or casting.

Dental casting Investment: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 temperatures).

Refractory: Difficult to fuse/corrode, capable of enduring high temperatures.

Refractory investment: An investment that can withstand high temperature using a soldering or casting.

Allotropic phase:

Phases of similar composition but different crystallographic structures, with different properties.

Casting:

The act of forming an object in a mold.

Something that has been cast in a mold; an object formed by the solidification of a fluid that has been poured or injected into a mold.

Easy manipulation.Good flow.Detailed reproduction.Fine particle size to ensure a smooth surface on the casting.The mixed unset material should have a smooth consistency.Should have sufficient strength: Should exhibit sufficient strength at high temperatures. Inner surface of the mold should not break at a high temperature.Should exhibit sufficient strength, to withstand the force of molten alloy entering the mold.

At higher temperatures: It should be stable without any decomposition of investment.Should show sufficient expansion enough to compensate for shrinkage of wax pattern and solidification of molten metal.The material should be sufficiently porous (permeability) enough to permit escape of air/other gases from the mold cavity during casting of molten metal.It should show ease of divestment:It should not react with metal.It should easily break away (separable) from the surface of casting.

Should wet the surface of the wax pattern properly.Should be compatible with wax pattern, casting alloys or castable ceramics. Should not discolor the casting.Should not be bio-hazardous.Long shelf life.It should be economical.

No single material is known that can fulfill all the ideal requirements. So various ingredients/modifiers are added to get the desired properties!

INVESTMENTS USED IN MAKING MOLD FOR ACRYLIC DENTURES

PLASTER OF PARISDENTAL STONEHYDROCOLLOID (GLYCERIN MODIFIED AGAR)

INVESTMENTS USED FOR CASTING GOLD ALLOYSGYPSUM BONDED INVESTMENTTYPE I (THERMAL EXPANSION)TYPE II (HYGROSCOPIC EXPANSION)TYPE III (FOR RPD FRAMEWORKS) PHOSPHATE BONDED INVESTMENTTYPE I (INLAYS & CROWNS)TYPE II (RPD FRAMEWORKS)

INVESTMENTS USED FOR CASTING BASE METAL ALLOYS

PHOSPHATE BONDED INVESTMENTTYPE I (INLAYS & CROWNS)TYPE II (RPD FRAMEWORKS)SILICATE-BONDED INVESTMENTSODIUM SILICATE BASEDETHYL SILICATE BASED

INVESTMENTS USED FOR CASTING CPTI AND TITANIUM ALLOYSPhosphate-bonded investments Ammonia free PBIZircon (ZrO2.SiO2) coated PBISilicate bonded investmentsCemented investmentsSilica (SiO2) based investmentsMagnesia (MgO) based investmentsZirconia (ZrO2) based investmentsAlumina (Al2O3) based investmentsResin based calcia investment INVESTMENTS USED FOR SOLDERING AND BRAZING PROCEDURES

Gypsum-bonded investmentsPhosphate-bonded investments

BASIC STRUCTURE OF INVESTMENTS:

INVESTMENT MATERIALS CONSIST OF A REFRACTORY AND A BINDER.

REFRACTORY:

Resists high temperature

Provides thermal expansion

The refractory is one or more of the forms of silica, usually quartz or cristobalite in powdered form. When heated the forms of silica undergo displacive transformations, which cause them to expand significantly. This displacive transformation coupled with the setting expansion accounts for the total expansion required, to compensate for the shrinkage of the alloy as it solidifies.

The amount of expansion and the temperature at which it occurs depends on the type and amount of silica used in the investment.

Binder:

Binds the refractory particles

Provides strength and rigidity to investment

Gypsum-bonded investments consist of gypsum products (autoclaved calcium sulphate hemihydrate, α-form) as the binder, which reacts with water to form calcium sulphate dihydrate. There is no interaction between the refractory (silica) and the binder.

Phosphate-bonded investment materials consist of ammonium phosphate and magnesium oxide as the binder. On setting magnesium oxide reacts with the phosphate ions to produce magnesium ammonium phosphate. In these investments the binder does interact with the surface of the silica particles, which gives, rise to a stronger set material.

Additives:

Modifiers such as boric acid and sodium chloride are added to regulate the setting time and expansion.

Reducing agents like carbon (graphite) and powdered copper are present in the investment to produce a reducing atmosphere in the investment mould and thus minimize the oxidation of the alloy.

Coloring agents may be added to the investments.

Liquid:

Some investment materials are marketed with an accompanying high-expansion liquid, which either replaces water or is used in addition to water to control expansion on heating. Such liquids contain alkaline colloidal silica giving them improved strength when set.

Setting time:

The setting time of these investments is dependent upon the gypsum content and upon the type of gypsum employed. It may also be varied by the conditions of mixing. Initial setting times vary between 8 and 15 minutes and final setting times between 12 and 25 minutes.

According to ADA specification no.2, the setting time should neither be shorter than 5 minutes nor longer than 25 minutes.

Finer the particle size faster is the set.

More the w/p ratio, fewer nuclei per unit volume and hence setting time is prolonged.

The longer and the rapidly the material is mixed, the shorter is the setting time.

No much change in setting time when water between 0 C (32 F) and 50 C (120 F) is used, but increased more than 50 C (120 F), a gradual retardation occurs. As the temperature reaches 100 C (212 F), no reaction takes place.

Accelerators such as potassium sulphate in concentrations higher than 2% produces syngenite [K2Ca(SO4)2.H2O], which crystallizes rapidly. Sodium chloride <2% acts as accelerator but when used >2% it retards the setting reaction. Sodium sulfate < 3.4% acts as accelerator.

Retarders such as citrates, acetates and borates are used to prolong the setting time.

Compressive strength:In practice it is the compressive strength of the investment material at the casting temperature that is significant in resisting the possible distortion caused by the inrush of molten alloy. If the material is mixed with a low water/powder ratio (thick mix) then the compressive strength will be increased.

It appears both from laboratory testing and practical experience, that the majority of the materials on the market are satisfactory if used in accordance with the recommended proportions.

Compressive strength of PBI type I is 2.5 Mpa and that of type II is 10 Mpa.

Compressive strength of ESBI is 1.5 Mpa.

Expansion:

The linear casting shrinkage of gold alloys will vary from one alloy to another, but it will not be less than 1.5 per cent and may be as high as 2 per cent. In order that dental castings shall not be that much too small that it is necessary to expand the mould into which they are cast.

There are three possible ways by which this may be done:

a) By setting expansion

b) By hygroscopic expansion

c) By thermal expansion

NORMAL SETTING EXPANSION IS THE EXPANSION THAT OCCURS DIRECTLY AFTER MIXING AND PRIOR TO IMMERSION IN WATER OR HEATING. IT OCCURS DURING THE SETTING OF THE INVESTMENT AND HENCE THE NAME.

Hygroscopic expansion occurs if the investment is allowed to set in contact with water. It is of two types:

That achieved by immersing the casting ring in water.That achieved by placing measured amount of water on the investment within the ring.

Thermal expansion occurs on heating the investment through burnout of the wax pattern and to the temperature at which molten metal is cast into the mould. The refractory is responsible for this type of expansion.

Permeability:

This is necessary to allow the air to pass out as the molten metal enters the mould space. The greater the proportion of gypsum the less will be the permeability, but the most significant factor in this respect is the uniformity of the particle sizes of the investment.

If all the particles are of equal sizes, the permeability will be greater than that of a mixture of large and small sizes. But greater the permeability of the mould the rougher will be the casting.

Phosphate and gypsum-bonded investments are more porous as compared to the silicate-bonded investments. [As the silicate-bonded investments contain admix of small and large particles and also they are condensed sufficiently to obtain good strength, the porosity decreases.]

Graphite in some investments renders more permeability after burnout.

Expansion of investments:

This is an anomalous use of the word expansion because gypsum products, without refractory, contract or shrink on heating. Between 200°C and 400°C shrinkage occurs as a result of dehydration.

Between 400°c and 700°c a slight expansion occurs and above 700°c the materials shrinks severely as it decomposes. Gypsum bonded investment can be made to expand sufficiently on heating with the addition of either quartz or cristobalite by counterbalancing the shrinkage of the gypsum binder.

Normal Setting Expansion:

Normal setting expansions are seen in Gypsum-bonded (0.06-0.6%) and Phosphate-bonded (0.5%) investments. The casting ring may restrict the expansion, but when the ring is lined with wet absorbent kaolin impregnated paper, the expansion markedly increases.

• Lower the Water/Powder ratio greater is the expansion

• Longer the Mixing time within the practical limits, greater is the expansion

• Chemical accelerators and Retarders generally reduce the setting expansion

No setting expansion will occur with any of the investment materials bonded with ethyl silicate. In certain materials of this type a contraction of about 0.2% to 0.4% may take place.

Hygroscopic Setting Expansion:

If the material is allowed to set in contact with water a greater expansion occurs. This is known as hygroscopic expansion. The extent to which the investment expands hygroscopically varies according to their composition.

Hygroscopic expansion is inversely proportional to the percentage of silica in the investment.

Finer the silica particle size of silica greater the expansion.

Higher the water content in the original mix, lesser is the hygroscopic expansion.

•The magnitude of hygroscopic expansion is directly proportional to the amount of water added during the setting.•Reduced mixing time decreases the hygroscopic expansion.•Older materials show less expansion.•Greatest amount of expansion is observed if the investment is placed in water before its initial setting. •Less expansion seen when confined to the casting ring.

The common method of obtaining hygroscopic expansion is to immerse the ring filled with investment into water at 37 C, at the initial setting time of the material and allows it to remain there for 30 minutes. A controlled expansion can be obtained by adding measured amount of water on to the surface of the investment, within the casting ring.

Hygroscopic expansion of about 0.6 to 0.8 % can be seen in PBI.

Thermal expansion:

Thermal expansion occurs during the heating period, first when the wax is being eliminated, and subsequently during the time necessary to reach the temperature suitable for casting to be carried out. The percentage thermal expansion for any material depends upon:

The water/powder ratio of the mix. Greater the water/powder ratio lesser is the expansion.

The proportion of silica in the material. When silica content increases the expansion increases.

The variety of silica, either quartz or cristobalite. Cristobalite gives more thermal expansion than quartz even at a lower temperature (200 C).

When Gypsum alone is heated it expands slightly up to 118C but then contracts markedly, as much as 2 % at 600C. If the silica content is increased the expansion of silica counterbalances the thermal contraction of gypsum. But there may be weakening of the investment. The addition of small amounts of modifiers such as sodium, potassium, or lithium chlorides may eliminate the contraction caused by the gypsum.

With quartz owing to its higher inversion temperature, the optimum thermal expansion does not occur until approximately 650-700 C. This means that casting must not be carried out bellow that temperature when using quartz-type investments.

If cristobalite is used the expansion takes place at a lower temperature and to a greater degree. Expansions of 1.2 % may be obtained at 500 C, remaining constant up to 900 C if necessary. This means that the casting temperature range is much less critical if cristobalite is used, than in case of quartz.

Whatever type of investment is used, the heating of the ring must be gradual to obtain the maximum thermal expansion.

Thermal expansion of 0.8% is seen in PBI if 50:50 mixtures of liquid and water are used. Thermal expansion may increase to 1.0-1.2% if undiluted liquid is used.

In ESBI the thermal expansion may be 1.5 to 1.8% when heated from room temperature to 1000-1177c.

Hydrocolloids:

Fluid resin technique (Shepard and Winkler 1967). Makes use of hydrocolloid as investment material. The waxed- up denture is sealed and positioned in a specially designed flask. Which is then filled with reversible hydrocolloid investment medium. After gelation of hydrocolloid, the cast with the attached waxed up denture is removed then vents and sprues are cut from outside the flask into the mold space. The wax and base plate are eliminated, then the teeth and the cast are replaced in the vented flask.

Fluid resin is mixed and poured into the mold via the sprue openings (28 ml powder of 13 ml of liquid). Then filled flask is held in a pressurized (0.14 Mpa) chamber at room temperature until the resin cures completely . The set resin is removed and trimmed.

Advantages:

1) Better tissue fit

2) Fewer open bites

3) Less fracture of porcelain teeth during deflasking

4) Decreased material cost

5) Simple processing method.

Disadvantages :

1) Air Inclusions

2) Shifting of teeth

3) Decrease in occlusal vertical dimension

4) Occlusal imbalances

5) Incomplete flow of denture base material

6) Technique sensitive

Gypsum Investment:

They are used in investment of conventional heat and cold cure dentures, and also in fluid resin denture preparation

Basic reaction of gypsum products

Gypsum products used for processing dentures are : Type II, III plaster

Layered Silicon rubber :This was given by Macroft K. R, Tencate I. L and Hurst W. W.(1961).

This technique showed a good accuracy in reproducing

vertical dimension of denture.

Technique1 pour - is done with artificial stone,2 pour - thin layer of silica rubber is coated on teeth.3 pour - Investment completed with artificial stone.

“A comparative study of changes in vertical dimension of occlusion using different investing mediums” was done by Bennie et a! by recording the mean opening at incisal guidance pin (inch) (J. Prosthet dent-49 :568-57 1, 1983)

They concluded that artificial stone with a layer of silicone rubber, accurately reproduced the vertical dimension of occlusion during processing than other investing mediums used in investigation.

INVESTMENTS FOR GOLD ALLOYS:

The materials are supplied in the form of powder, which is mixed with water. The binding agent is usually the α-hemihydrate of gypsum. Since it gives a greater crushing strength to the investment; the binder content is usually between 25 and 40 percent by weight.

In addition to silica there may be incorporated small amounts of reducing agents such as carbon, which produce a reducing atmosphere in the investment mould and thus minimize the oxidation of the alloy. Silica may be present in of its forms, quartz or cristobalite.

When these substances are heated to temperature known as their “inversion points”, they undergo an inversion from the α-form to the β-form. This change is accompanied by a marked expansion. The inversion of quartz occurs at 575 c, and of cristobalite at between 200 c and 270 c.

ADA 2

Type I - For inlay and crown (thermal expansion)

Type II - For inlay and crown (hygroscopic)

Type III – RPD framework

Cannot be used in conjunction with cobalt chromium alloys whose melting point is approximately 1425 c; decomposition of gypsum takes place in the presence of silica. This causes rapid evolution of oxides of sulphur and consequent porosity and embitterment of casting occurs.

However cobalt-chromium alloys with lower melting points around 1260oc are used, then gypsum bonded investments of the type used for gold casting can be employed.

WHEN MIXED WITH WATER,

(CASO4)2.H2O + 3 H2O 2CASO4.2H2O + HEAT

• WHEN HEATED TO 110-130 C,

CaSO4.2H2O (CaSO4)2.H2O

WHEN HEATED TO 130-200 C,

(CASO4)2.H2O CASO4

WHEN HEATED TO 200-1000 C,

Hexagonal Anhydrate Orthorhombic Anhydrate

INVESTMENTS FOR BASE-METAL ALLOYS:

They must withstand temperatures of the order of 1000 C without cracking or distortion.

They must have sufficient expansion to compensate for the thermal contraction of the cobalt-chromium alloy, which has been established to be about 2.2 %.

PHOSPHATE-BONDED:

This investment material is essentially a powder containing the silica refractory, a soluble acid phosphate and a metallic oxide. When this powder is mixed with water, the following reaction occurs:

NH4H2PO4 + MgO NH4MgPO4 + H2O

OR

NH4H2PO4 + MgO +H2O NH4MgPO4.6H2O

This chemical reaction is accompanied by a physical reaction in which the slurry changes into a solid; this gives the initial or green strength to the investment.

On heating:

Undergoes dehydration at 160 C, NH4MgPO4.6H2O NH4MgPO4.H2O + 5H2O

And decomposition between 300 C and 650 C,2NH4MgPO4.H2O Mg2P2O7 + 3H2O + 2NH3

Further reaction with excess mgo if heated above 1040 C, Mg2p2o7 + mgo mg3(p2o4)2

When ammonium magnesium phosphate and silica react to form complex silico-phosphates, it gives greater strength to the investment at higher temperature.

SILICATE-BONDED:

1.Binder based on sodium silicate:This can be used for dip or spray coating for wax patterns. An aqueous solution of sodium silicate is acidified by the addition of hydrochloric acid and abounding silicic acid gel is formed. However such investments are not generally used.

2.Binder based on ethyl silicate:Once it was widely used but now used mainly in large laboratories, where its inconvenience of preparation is offset by its lower cost.

Ethyl silicate is a colorless liquid, of specific gravity 1.06, insoluble in water but soluble in alcohol and other organic solvents. Its important chemical property is its ability to undergo hydrolysis to give silicic acid as follows:

Si(C2H5O)4 + 4H2O Si(OH)4 + 4C2H5OH

nSi(OH)4+ MgO MgO[Si(OH)4]n

This can be brought about by addition of a small amount of dilute hydrochloric acid to a mixture of ethyl silicate, industrial spirit and water.

500 ml ethyl silicate1500 ml industrial spirit50 ml water10 ml dil. Hydrochloric acid

The addition of hydrochloric acid is necessary in order to accelerate the hydrolysis. Solution, such as that above, are prepared and allowed to stand for 12 to 24 hours before use.

The silica investment powder, which may be quartz or cristobalite or a mixture of the two, is then mixed with the solution and setting takes place, usually within hour after mixing.

THE MECHANISM OF THE SET IS AS FOLLOWS:

Hydrolysis of the solution is commenced by the addition of the acid and if the solution is allowed to stand at room temperature for about a week, hydrolysis proceeds to complete gelation and the solution turns into a jelly. If silica powder is added to the solution before this occurs, say 24 hours after adding the acid, then this silica speeds up the hydrolysis, produces gelation and hence the investment sets.

The end products are soft silica gel and ethyl alcohol; on subsequent heating of the ring, water and alcohol are driven out and a hard gel of amorphous silica remains.

It has been found that the particle size and shape of the silica powder are important factors in the bonding process. If particles are of uniform size, bonding with the hydrolyzed ethyl silicate solution will not take place. Consequently varying grades of silica powder must be used and their structure should be subangular and porous.

However in order to obtain good surfaces on the castings some fine grade must be used and it is recommended that 70% coarse and 30% fine grade is employed. Above this percentage the mould is liable for crack on heating.

INVESTMENTS FOR CASTING TITANIUM:

There are various investments on the market for casting titanium. Phosphate-bonded investments have been used but are usually modified forms that give sufficient expansion to compensate for the shrinkage of the metal at lower mould temperatures. To avoid contamination and surface degradation of the casting the mould is heated to a much lower temperatures prior to casting.

In some cases the metal is cast into a mould at room temperature. In this case, all the expansion of the mould must be achieved by setting through the use of high expansion liquids. Other types of materials for casting titanium include alumina-based, spinel-based, zirconia-based and magnesia-based investments. One such magnesia-based investment utilizes magnesia as the refractory and aluminous cement as the binder.

Recently a new investment material for titanium casting has been developed, which contains calcia (cao2) as refractory and cold-cure acrylic resin (PMMA) as binder, known as “resin-bonded calcia investment”.

This is a the recently developed investment for casting titanium inlay, crown and bridge.

Binder — calcia

Refractory —Zirconia

There are 2 types of CaO and mixing liquid.1. Saturation type (total expansion 2 ± 3%)2. Delayed expansion type

Properties1. Total thermal and setting expansion found was -1 .5 -

2.5%2. The maximum thermal expansion is found at - 900 -

1200°c

Composition:

Powder:Zirconia (Zr sio4) MixtureZirconia flower

Properties:1. High refractoriness 2. High thermal conductivity3. Low co-efficient of thermal expansion4. Low reactivity with molten titanium

Advantages:

1. Quick casting procedure

2. Increased accuracy of casting

They usually contain phosphate binder and

Mgo /quartz refratories.

1. Magnesia bonded investmentPhosphate binderAlumina /Mgo refractories

good heat resistance

Low thermal expansion

2. Phosphate bonded investment:

Phosphate binder

Mgo + Al2O3 →Mgo –Al2O5 (spinel→ highly refractory.)

Show large expansion due to spinel reaction.

3. Spondumen (H2O - A12O5 - Sio2) Expand irreversible on heating at 900 — 1100°c

4. Aluminous cement :

(CaO - A12O5 + Mgo (refractory) + 5% Zirconia

Burnout (o)ZrSio4 ---------------------› Zr ↑Expansion

(By whipmix corporation)

It is a combination of Die stone and gypsum bonded investment material. The powder is mixed with colloidal silica.

Properties:

Setting expansion - 0.9%

Thermal expansion - 0.6% (at 977°c)

Advantages:

The wax pattern and die are invested simultaneously with out removal of pattern. Useful with gold alloys

Divestment phosphate(DVP):

Similar to divestment, but used for casting post and core, crowns of base metal alloys without any need of removal of wax pattern.

ADA Sp No 93

Type I - Gypsum bonded dental brazing investment

Type II - Phosphate bonded.

STARVESTMAXAVESTMICROFINE DURACAST-XECERAFINAOMNICAST-LPPOWER CAST QWESTFASTFIRE 15 HI-THERMFUJIVEST TOTALCASTDEGUVEST 1700CHROMECASTJELVEST CHROMECAST-XL

STARVEST • Complete castings done in less than 60 minutes by fast fire

(accelerated casting) method. • Longer working time • Smooth easy mix • Smooth bubble-free castings • Universal investment suitable for all type of alloys including

platinum for jewelry and titanium • Perfect for pressable ceramic castings • High batch-to-batch consistency • Suitable for casting against titanium implants • Used as soldering investment

MAXAVEST is an excellent all-purpose high heat casting investment for crown and bridge castings in non-precious nickel-chrome alloys and gold-silver-palladium alloys. When used according to instructions, maxavest will yield most consistent castings in semi-precious and non-precious alloys of all types.

Properties at 100% liquid: Liquid to powder ratio: 25 to 26 /100 Working time: 4 to 9 minutes Setting time: 7 to 14 minutes Setting expansion: 2.0 to 3.0 % Thermal expansion: 1.3 % Compression strength at 60 minutes: 1000 psi (70 kg/cm2) Recommended liquid dilution: Semi-precious alloys: 2 part liquid to 1 part water Non-precious alloys: 4 part liquid to 1 part water

CHROMECAST-XL Less coarse than similar investments. It mixes to a nice and creamy texture and has longer working time ( 4 to 6 minutes ). It achieves strength very quickly, which allows casting ring placement into even hot burnout ovens. batch-to-batch consistency, and accuracy provided by the fine texture of the investment is an important property of chromecast.It can also be used for other castings in gold, palladium, platinum, and non-precious alloys when it is used with special liquid. the selective use of the special liquid provides controlled expansion required for other applications such as crown & bridge castings. Physical properties @ 72of (22oc): (All tests conducted with full strength liquid) Water-to-powder ratio 15 / 100 Working time (min) 4-6 Setting time (minutes) 15 max Expansion: Setting 1.0 % Thermal 1.6 %

As Prosthodontists, our aim is to make a restoration as accurate as possible. Most of the restorations what we are making are cast restorations and that is why we should have knowledge about various materials and techniques used in casting a dental restoration.

Investment materials are to be selected based on the type of restoration, the type of metal or alloy to be casted, as well as technique used for casting and that is how we can achieve a better restoration.

William J. 0’Brien: Dental materials and their selection

Robert G. Craig: Restorative dental materials.

John F McCabe: Applied dental materials.

E.C.Coombe: Notes on dental materials.

Kenneth J Anusavice: Science of dental materials.

E.H. Greener: Material science in dentistry.

Bernard G. N. Smith: The clinical handling of dental material.

Malvin: Dentistry an illustrated history.