COLLEGE OF DENTAL SCIENCES DEPARTMENT OF CONSERVATIVE DENTISTRY AND ENDODONTICS
Presented by : -
Dr. Niju Aelias
DENTAL INVESTMENTS INTRODUCTION : 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. This is mainly based on “Lost Wax Technique”. This process of casting involves some basic steps. 1. Preparation of a wax pattern. 2. Preparation of mold – It is done by pouring the mixed investment material around the wax pattern and allow it to set. Burnout : Wax is eliminated from he 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. HISTORY : This meticulous procedure of casting was used by various craftsmen to produce jewelary and ornaments. Its history can be traced back around 3000 B.C. But origin of lost wax technique, when viewed history makes its presence in the writings of Theophilus (11th century). 11th Century Theophilus described lost wax technique, which was a common practice prevailed in 11th century. 1558 Benvenuto Cellini claimed to have attempted, 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 Phillibrook 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.
DEFINITIONS : 1. 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, casting. 2. Dental Casting Investment : 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). 3. Refractory : Difficult to fuse / corrode, capable of enduring high temperatures. 4. Refractory investment : An investment that can withstand high temperature using a soldiering /casting. 5. Allotropic phase : Phases of similar composition but different crystallographic structures, with different properties. 6. Casting : Noun : Something that has been cast in a mold, an object formed by the solidification of a fluid that has been formed / injected into a mold. Verb : the art of forming an objet in a mold. IDEAL REQUIREMENTS OF AN INVESTMENT MATERIAL : 1. The powder should have a fine particle size to ensure a smooth surface on the casting. 2. The mixed unset material should have a smooth consistency. 3. It should be easy to manipulate â€“ easy to mix and also harden within a relatively short time. 4. It should have sufficient strength at room temperature -
Should exhibit sufficient strength at high temperature.
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.
5. Inner surface of mold should be smooth.
6. 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.
7. The material should be sufficiently porous enough to permit escape of air/other gases from the mold cavity during casting of molten metal. 8. It should show ease of divestment. -
It should not react with metal
It should easily break away from the surface of casting.
9. 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. CLASSIFICATION OF DENTAL INVESTMENT : I.
Based on Processing Temperatures : A. High temperature casting investments -
Phosphate bonded investments
Silicate bonded investments
B. Low temperature casting investments II.
Gypsum bonded investments (for low temperature gold alloy)
Depending on type of refractory used (Silica) -
Based on type of binder used A. Gypsum bonded investments : According to ADA Specification 2 Uses
Purely thermal expansion
Inlay and Crowns
Purely hygroscopic expansion
R.P.D. Frame work Thermal
B. Phosphate bonded investments C. Silicate bonded investments 4
BASIC COMPOSITION OF INVESTMENTS : In general, an investment is a mixture of the following 3 distinct types of materials, A. Refractory Materials : A material that will withstand high temperatures without decomposing on disintegrating. The most commonly used refractory material is silicon dioxide such as quartz, tridymite, or cristoballite or a mixture of these. Function : -
Resist the heat and forces of casting.
To expand and compensate for casting shrinkage
B. Binder : The refractory material alone do not form a coherent solid mass, so some kind of binder is needed. Commonly used binders are ; -
Îą - Calcium sulfate hemihydrate
Others are â€“ Sodium silicate, Ethyl silicate, Ammonium sulfate, Sodium phosphate.
C. Other Chemical Modifiers : Usually a mixture of refractory materials and a binder alone is not enough to produce all the desirable properties required of an investment. Other chemicals such as sodium chloride, boric acid, potassium sulfate, graphite, copper powder or magnesium oxide are often added in small quantities to modify various physical properties. Eg. Small amounts of chlorides or boric acid enhance the thermal expansion of investment bonded by calcium sulfate. INVESTMENTS FOR LOW CASTING TEMPERATURE : Gypsum Bonded Investments : The gypsum based material represent the type traditionally used for conventional gold alloys.
ADA Specification No.2 for casting investment for dental gold alloys comes in three types. Type I
: For inlay and crown
– Use mainly thermal expansion
: For inlay and crown
– Use mainly hygroscopic expansion
: For R.P.D. frame work – Use mainly thermal expansion
Common Brands : Baker’s Sterling, Begocast, Cristobalite, Inlay – Vest, Luster cast. Composition : 1. Binder : α - Calcium sulfate hemihydrate (25 – 45%) 2. Refractory Material Silica – Quartz or Cristoballite or a blend of two in varying proportions (65 – 75%) 3. Chemical modifiers a) Carbon / Copper powder (Reducing agents) (2-3%) b) Boric acid and sodium chloride (Balancing agents) Functions of Each Constituent : 1. Gypsum α Hemihydrate : -
The α hemihydrate form of gypsum is the binder for investments used in casting gold containing alloys with melting ranges below 1000oC.
Hold other ingredients together and provide rigidity.
Gypsum after heating undergoes dehydration with shrinkage and fracture of mold.
Effect Of Temperature On Calcium Sulfate Binders : The binder used for gold investments in dentistry is α - calcium sulfate hemihydrate.
During the investing process some of the water mixed with the investment reacts with the hemihydrate and in converted to calcium sulfate dihydrate, whereas the remainder of excess water is uniformly distributed in the mix. During the heating process the excess water is evaporated.
temperature rise about 105oC, calcium sulfate dihydrate starts losing water and expands, then shrink / contract considerably after dehydration between 200 oC and 400oC. A slight expansion then occurs between 400 oC and approximately at 700oC a large contraction occurs. This latter shrinkage in most likely caused by decomposition, and sulfur gases such as sulfurdioxide are emitted. This decomposition not only causes shrinkage but also contaminate the castings with the sulfides of the non noble alloying elements such as silver and copper. So gypsum bonded investment should not be heated above 700oC. These properties are explained as follows ; Upto about 105 oC, ordinary thermal expansion occurs.
Above 105oC, the calcium sulfate dihydrate is
converted to anhydrous calcium sulfate. Dehydration of the dihydrate and a phase change of the calcium sulfate anhydrate cause a contraction. The α form of tridymite (which might be present as an impurity) is expanding and sufficiently compensates for the contraction of the calcium sulfate to prevent the investments from registering a serious degree of contraction. At elevated temperature the α forms of silica present in the investment are converted to the β forms, which cause some additional expansion.
2. Silica : Silica (SiO2) is added to provide a refractory during the heating of the investment and to regulate the thermal expansion and also counter shrinkage of gypsum. 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. If the proper form of silica is employed in the investment, this contraction during heating can be eliminated and changed to an expansion. ď‚Ľ
Silica exist in four allotropic forms : -
Effect Of Temperature On Silicon Dioxide Refractories : The most commonly used refractory material is silica (SiO 2). Each of the polymorphic forms of silica, quartz, tridymite and cristoballite expands when heated, but the percentage of expansion varies from one type to another. Pure crystoballite expands to 1.6% at 400oC, whereas quartz expands about 1.4% at 600oC and the thermal expansion of tridymite at 600oC is less than 1%. The percentage of expansion of the 3 types of silica versus temperature shows, none of the three forms of silica expands uniformly, instead they all show a break (non linearity) in their thermal expansion.
In case of crystoballite the expansion is somewhat uniform to about 200oC. At this temperature its expansion increases sharply from 0.5% to 1.2%, and then above 250oC it again becomes more uniform. At 573o C quartz also shows a break in the expansion and tridymite shows a similar break at a much lower temperature. The breaks on the expansion versus temperature indicate that cristoballite and quartz, each exist in two polymorphic forms. One of which is more stable at a higher temperature and the other at a lower temperature. The form that is more stable at room temperature is called the α form, and the more stable form at higher temperature is designated as the β from. Tridymite has three stable polymorphic forms. Thus the temperatures of 220oC for cristobalite, 573o C for quartz and 105o and 160o C for tridymite are displacive transition temperatures. A displacive change involves expansion or contraction in the volume of the mass without breaking any bonds. In changing from the α form (which is the more stable form at room temperature) to the β form (which is stable at higher temperatures), all three forms of silica expand. The amount of expansion is highest for cristoballite and lowest for tridymite. The quartz form of silica is found abundantly in nature, and it can be converted to cristobalite and tridymite by being treated through a reconstructive transition during which bonds are broken and a new crystal structure is formed.
The α quartz is converted to β quartz at a temperature of 573oC.
If the β quartz is heated to 870oC and maintained at that temperature, it is converted to β tridymite.
From β tridymite obtaining either α tridymite or β cristoballite is possible. 9
If β tridymite is cooled rapidly to 120oC and hold at that temperature, it is changed to α - tridymite, which is stable at room temperature. On the other hand, if β tridymite is heated to 1475o C and hold at that
temperature, it is converted to β - cristoballite.
Further heating of β
cristoballite produces fused silica, but if it is cooled to 220 oC and held at that temperature, α cristoballite is formed. 870o C
1700oC Fused Silica
Middle tridymite 105oC α-tridymite
All forms of silica are in either α forms in the investment and during the heating process they are converted completely or in part to their corresponding β forms. This transition involves an expansion of the mass, which helps to compensate for casting shrinkage.
Fused quartz is amorphous and glass like 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.
Quartz, cristobalite or a combination of the two forms may be used in a dental investment. Both are now available in pure from.
Tridymite is no longer an expected impurity in cristobalite.
3. Modifiers : a) Reducing agents : Carbon and powdered copper provide a non oxidizing atmosphere in the mold when the gold alloy is cast. b) Balancing agent : Boric acid and sodium chloride 10
Prevent shrinkage of gypsum when it is heated above 300oC
Setting Reaction of Gypsum Bonded Investments : Same as Dental stone. CaSO4 ½ H2O + ½ H2O CaSO4•2H2O + 3900 Cal / gmol. (Ca. Sulfate hemihydrate) (Ca. Sulfate dihydrate) When calcium sulfate hemihydrate is mixed with water, calcium sulfate hemihydrate is converted back to calcium sulfate dihydrate which sets to form a solid mass which binds the silica particles together. The reaction is exothermic and whenever 1 gm mol of calcium sulfate hemihydrate is reacted with 1.5 gm mol of water. 1 gm mol of calcium sulfate dihydrate is formed and 3900 calories of heat are developed. The microstructure of set material shows; rod like particles of gypsum intermeshed with large irregular particles of silica refractory. Setting Time : It is dependent on the gypsum content and upon the type of gypsum employed. Initial setting time : 8 – 15 min Final setting time
: 12-25 min.
This can be altered by addition of K 2SO4, NaCl (Increase setting time), Borax and potassium citrate (decrease setting time). ADA Specification No.2 ; States that setting time should not be less than 5 minutes and nor longer than 25 minutes. Modern inlay investments set initially in 9-18 minutes.
This provides sufficient time for mixing and
investing the pattern. 11
Factors Controlling Setting Time : • Manufacturing process – finer the particle size – faster setting • Mixing time and rate - ↑ mixing time - ↓ setting time. • Water / powder ratio - ↑ water : powder ratio - ↑ setting time • Temperature - ↑ temperature - ↑ setting time • Modifier – Accelerator and retarders – Accelerates ↑ setting time, Retarders ↓ setting time Properties of Gypsum Bonded Investment : 1. Expansion 2. Contraction 3. Strength 4. Other consideration 1. Expansion : Expansion aids in enlarging the mold. This property of investment is needed for compensation of casting shrinkage of alloy. Expansions are of 3 types : 1. Normal setting expansion. 2. Hygroscopic setting expansion 3. Thermal expansion Normal Setting Expansion : The setting expansion of an investment, is the linear expansion that take place during the normal setting of the investment in air. A mixture of silica and gypsum hemihydrate results in setting expansion greater than that of the gypsum product when it is used alone. It is because of interference of silica with the growing crystals.
The silica particles interfere with the intermeshing and interlocking of the crystals as they form. Thus, the thrust of the crystals is outward during growth and therefore more effective in the production of expansion. ADA Specification No.2 for Type I investment permits a maximum setting expansion in air of only 0.6%. Modern investments show setting expansion of approximately 0.4%. It is regulated by retarders and accelerators. Effect of Wax Pattern on Normal Setting Expansion : The setting reaction of gypsum bonded investment is exothermic in nature (3900 cal/gmol). The heat causes expansion of wax pattern leading to expansion of the mold. The amount of heat liberated depends on The ratio of gypsum – ↑gypsum ↑heat - ↑ S.E. W:P Ratio ↓ W:P ratio ↑ Heat ↑ S.E. B. Hygroscopic Setting Expansion : Hygroscopic expansion is the linear expansion of the investment that occurs if the investment is in contact with water from any source during the setting process, after investing the wax pattern. Contact with water can be achieved by placing the casting ring in a water bath, before initial set is complete or by putting some water on the surface of the investment in the ring or by using a wet liner inside the casting ring. Distinguishing between a setting expansion and hygroscopic expansion is difficult because both take place almost at the same time and end at the same time.
In practice a sum of the hygroscopic and setting expansion of the
investment is obtained, which is six times more than the normal setting expansion.
Hygroscopic expansion may occur because of • Gypsum • Refractory Gypsum : According to some theories, • Addition of water during the setting of an investment increases the surface film thickness on the inert particles and gypsum crystals, thereby forming them apart. • Added water during the setting process permits further hydration of calcium sulfate, thus causing expansion of the investment. • Added water may force gypsum gel to swell. • Addition of water provides additional volume in to which gypsum crystals can grow. Refractory : The water physically seperates the fine parities of silica by capillary action leading to expansion of mass. This is reversible in case of absence of binder. But if binder is present and set, the expansion is retained. The decreased expansion is affected by : • Amount of silica ↑ amount of silica - ↑ expansion • W:P ratio ↑ W:P ratio ↓ Expansion • Time of insertion – Investment immersed in water after initial setting cause decreased expansion. ADA Specification No.2 for Type II Investments requires a Minimum expansion of 1.2% Maximum expansion of 2.2%
FACTORS AFFECTING HYGROSCOPIC EXPANSION : 1. Composition : HSE is directly proportional to the amount of silica present. Finer the particle size of silica : Increased hygroscopic expansion HSE is greater in α hemihydrate than β hemihdyrate. A dental investment should have enough hemihdyrate binder with the silica to provide sufficient strength after hygroscopic expansion. Otherwise, a shrinkage occurs during the subsequent drying of the set investment. At least 15% of binder is necessary to prevent a drying shrinkage. 2. Water Powder Ratio : Increased water powder ratio Decreased Hygroscopic exposure. 3. Spatulation : Increased missing time : Increased hygroscopic expansion. 4. Ratio of Spatulation : Increased spatulation : Increased hygroscopic expansion. 5. Shelf life of investment : Old investment : Decreased hygroscopic expansion. 6. Time of investment : Immersion of setting investment. Before initial setting : Increased hygroscopic expansion After initial setting : Decreased hygroscopic expansion 7. Effect of confinement : Both the normal and the hygroscopic setting expansion are confined by opposing forces, such as the walls of the container in which the investment is placed or the walls of the wax pattern. The confining effect in the hygroscopic expansion is more pronounced than the similar effect on the normal setting expansion. 8. Water bath temperature: Although the water bath temperature has little effect on the hygroscopic expansion of the investment, it has a definite effect on the wax pattern. At higher water bath temperature, the wax pattern expands, requiring less expansion of the investment to compensate for the total casting shrinkage. In addition higher water bath temperature soften the wax. The softened wax then gives less resistance to the expansion of the investment, thus 15
making the setting and hygroscopic expansion more effective. The net effect is higher expansion of the mold with higher water bath temperature. During the setting process, dental casting investments actually absorb water from their surroundings and expand. If during setting more water, an investment is permitted to take up from any source, the higher the hygroscopic expansion, upto a point where further addition of wax do not create any additional expansion. This degree of expansion or its corresponding quantity of water is called the critical point. For an investment to reach its maximum hygroscopic expansion, sufficient water should be available. If hygroscopically expanding investments are in contact with less water than they are able to absorb, but they will not exhibit their maximum hygroscopic expansion. 9. Amount of water added: More amount of water added during setting period, more is the expansion. Thermal Expansion : In case of gypsum investments, thermal expansion is achieved by placing the mould in a furnace at a temperature not greater than 700 oC. When the investment ring with the investment is heated two events take place. a. Gypsum undergoes shrinkage, as it becomes calcium sulfate anhydrate losing water. b. Silica undergoes a thermal expansion. Cristobalite contributes more to such expansion. 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. â€˘ When the quartz content of the investment is increased to 60% with the balance being hemihydrate binder, the initial contraction of gypsum is not eliminated. 16
• The contraction of gypsum is entirely balanced when the quartz content is increased to 75%. • If a sufficient amount of setting expansion had been present, the casting made at 700oC would have fit the die reasonably well. • Quartz expands to 1.4% at 575oC • Cristobalite expands to 1.6% at 250oC Cristobalite produce adequate mold expansion at lower temperature because of the lower inversion temperature of the cristobalite in comparison with that of quartz. Thus the normal contraction of the gypsum during heating is easily eliminated. A reasonably good fit of the casting is obtained when the gold alloy is cast into the mold at temperature of 500oC and higher. The amount of thermal expansion of a dental investment depend on it use. If the hygroscopic expansion is used to compensate for the contraction of the gold alloy for Type II investments, ADA specification No.2 requires that the thermal expansion be between 0% and 0.6% at 500oC. However, for the Type I investments 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 maximal thermal expansion be attained at a temperature not higher than 700 oC. Thus when a thermal expansion technique is used, the maximum mold temperature for the casting of gold alloy should be less than 700oC.
The gold alloys can
become contaminated at a mold temperature higher than this.
FACTORS AFFECTING THERMAL EXPANSION : 1. Water Powder Ratio : The magnitude of thermal expansion is related to the amount of solids present. ↑ W:P ratio ↓ thermal expansion. 2. Effect of chemical modifiers : Disadvantage of an investment that contains sufficient silica to prevent any contraction during heating is that the weakening effect of silica in such quantities is likely to be too great. • Addition of small amounts of sodium, potassium or lithium chlorides to the investment. -
Eliminate the contraction caused by the gypsum.
Increases expansion without the presence of an excessive amount of silica.
• Boric acid has a similar effect -
Hardens the set investment
It apparently disintegrates during the heating of the investment and a roughened surface on the casting may result.
• Silica do not prevent gypsum shrinkage but counter balance it. • Chlorides actually reduce gypsum shrinkage below temperatures of approximately 700oC. 2. Thermal Contraction : When the investment is allowed to cool from 700oC the refractory and binder contact (This contraction is because of gypsum when it is first heated). On cooling to room temperature, the investment exhibits an overall contraction compared with its dimension before heating. On reheating to the temperature previously attained, the investment does not expand thermally to the previous level. Moreover the process of cooling and reheating causes internal cracks in the investment that can affect the quality of the casting.
3. Strength : The strength of the investment must be adequate to prevent fracture or chipping of the mold during heating and casting of the gold alloy. The total thermal contraction of the investment is similar to that of the gold alloy from the casting temperature to room temperature, the contraction of the investment is fairly constant, until it cools to below 550 oC. Thus, when the alloy is still quite hot and weak, the investment can resist alloy shrinkage by virtue of its strength and constant dimension. This can cause distortion and even fracture in the casting if the hot strength of the alloy is low. Although this is rarely a factor with gypsumbonded investments, it can be important with other types of investments. The compressive strength of the investment mold is a primary factor to be considered in addition to the expansion when evaluating the dimensional accuracy of dental castings. Ideally, the investment should have sufficient expansion to compensate for all of the thermal contraction of the alloy. However, after burnout of the mold, the strength need be no greater than that required to resist the impact of the metal entering the mold. According to ADA Specification No.2, the compressive strength for the inlay investment should not be less than 2.4 MPa (350 psi) when tested 2 hours after setting. Factors Affecting Strength : 1. Increased W:P ratio :decreased compressive strength. 2. Heating the investment to 700oC may increase or decrease strength by 65%. Greatest reduction in strength on heating is found in investments containing NaCl. 3. After the investment has cooled to room temperature, its strength decreases considerably, because of fine cracks that form during cooling.
4. The use of α hemihydrate instead of plaster increases the compressive strength. 5. The use of chemical modifiers increases the strength because more of binder can be used without a marked reduction in the thermal expansion. 4. OTHER CONSIDERATIONS : a. Fineness : Fineness of the investment affects its setting time, surface roughness of the casting. Fine the silica particle – greater the hygroscopic expansion than a coarse silica. Finer the investment – Smaller are the surface irregularities on the casting. b. Porosity : During the casting process, the molten metal is forced into the mold under pressure. As the molten metal enters the mold, the air must be forced out ahead of it.
If the air is not completely eliminated, a
backpressure builds up, which prevent the gold alloy from completely filling the mold. The common method for venting the mold is through the pores of the investment. The amount of porosity depends on : • Gypsum crystals. More gypsum crystals are present in the set investment less porosity. Therefore lower the amount of hemihdyrate and greater the amount of water used to mix the investment more porous it becomes. • More uniform the particle size Greater is the porosity. A mixture of coarse and fine particles Less porosity than an investment composed of a uniform particle size. c. Storage : Under conditions of high relative humidity, the setting time may change. Under such conditions, the setting expansion and the hygroscopic expansion may be altered so that the entire casting procedure may be adversely affected. Therefore the investments should be stored in airtight 20
and moisture proof containers. During use, the containers should be opened for short time as possible. All investments are composed of a number of ingredients each of which possess a different specific gravity. Therefore there is tendency for these components to separate according to the specific gravity. This separation may sometimes influence the setting expansion and other properties of the investment so it is advisable to purchase the investment in relatively small quantities. The investment should be weighed and the water should be measured according to the proportion of the investment mix. Only in this manner can one expect to control the setting or the thermal expansion in relation to the compensation needed for the casting shrinkage and other important properties. Some manufacturers supply their investment in preweighed packages so that one needs only to measure the gauging water. Uses : For casting of inlays, bridges, removable partial denture frame works using gold alloys and other low fusing alloys. MODIFIED TYPES OF GYPSUM â€“ BONDED INVESTMENTS : Hygroscopic Thermal Inlay Casting Investment : A new inlay casting investment that can be used as a hygroscopic or thermal type has became available. This investment contains a blend of quartz and cristobalite as the refractory. When hygroscopic casting technique is used, the investment is heated to 482oC after setting in accordance with the normal water immersion technique. When used in the thermal casting technique, the investment is not immersed in water, but after setting it is heated to 644 oC, then the appropriate expansion is achieved.
INVESTMENTS USED FOR HIGH TEMPERATURE CASTING PROCEDURE : Most palladium and base metal alloys used for partial dentures and porcelain fused-to-metal restoration have high melting temperatures. They are cast in moulds at 850 to 1100oC. So, gypsum bonded investments cannot be used, because it disintegrates at such high temperature. To withstand these high temperatures, the molds require different types of binders such as silicate and phosphate compound. The investment used for this purpose are ; -
Phosphate bonded investments
Silica bonded investments
Phosphate Bonded Investments : The most common type of investment for casting high melting alloys is the phosphate bonded investment. Common Brands : Aurobond, Calsite, Cerafine, Deguvest, DVP, Eurocent, Nirobond, Roma exalet etc. Composition : 1. Binder : 20% It consists of 2 components. -
Acidic part Ammonium diacid phosphate (NH4H2PO4)
Basic part Magnesium oxide (mgo) Which react at room temperature to form a phosphate binder
Ammonium magnesium phosphate (NH4MgPO4.6H2O) which gives the investment green strength / room temperature strength. 2. Refractory : Silica. Either cristoballite / quartz or a mixture of two in a concentration of approximately 80%. 22
They function as refractory (i.e. to
provide high temperature thermal shock resistance) and provide thermal expansion at high temperature. 3. Modifiers : Carbon – Act as a reducing agent to produce clean casting and facilitate devesting of the casting from the mould. It is used for high temperature gold casting alloys. But with silver palladium or base metal alloy, carbon embrittles the alloy even though the investment is heated to temperatures that burnout the carbon. Palladium reacts with carbon at temperatures above 1504 oC. Thus if the casting temperature of a high palladium alloy exceeds this critical point, a phosphate investment without carbon should be used. s Mode of Supply : Powder in packets with special Liquid : 1. Powder contain NH4H2PO4, MgO, Silica, Traces of carbon. This powder may be mixed with water to form the investment. 2. Special liquid Contain colloidal silica. Colloidal silica suspensions are available for use with phosphate investments in place of water. This liquid shows increased setting expansion, produce significant amount of hygroscopic expansion (as with pure water the amount of hygroscopic expansion is less) and increases its strength. For base metal alloys a 33% dilution of colloidal silica is required. Setting Reaction : At room temperature ammonium diacid phosphate reacts with magnesium oxide to give the investment green strength or room temperature strength. 1. NH4H2PO4 + MgO + 5H2O NH4 MgPO4. 6H2O (Ammonium diacid phosphate)
(Magnesium ammonium phosphate)
The ammonium diacid phosphate is used in a greater amount than is necessary for this reaction, so that the additional amount can react with silica at 23
an elevated temperature. At higher temperature there is probably a superficial reaction between P2O5 and SiO2 to form silica phosphate, which increases strength of investment at higher temperature. Phosphates are quite complex and the reaction is not simple.
stoichometric (determination of the relative proportion of the compounds involved in a chemical reaction) quantities are equal molecules of magnesia and ammonium diacid phosphate, an excess of magnesia is usually present and some of it is never fully reacted.
The product formed is predominantly
colloidal multimolecular (NH4MgPO4.6H2O)n which aggregate around excess MgO and fillers. After the initial setting reaction, the set colloid undergoes various thermal reactions on heating. 2. MgO + NH4 H2PO4 + H2O â†“ (NH4MgPO4. 6H2O)n MgO
Colloidal type particles
NH4H2PO4 H2O Prolonged setting at room temperature or dehydration at 50oC (NH4MgPO4.6H2O)n H2O
Dehydrated at 160oC
(NH4 MgPO4.H2O)n Heated from 300 â€“ 650o C (Mg2P2O7)n
Non crystalline polymeric phase Above 690o C
Crystalline in nature Heated above 1040oC
The final products are crystalline Mg 2P2O7 and some excess MgO, along with essentially unchanged quartz, cristobalite or both. Some Mg 3(PO4)2 may be formed if the investment is grossly overheated or when the molten metal contacts the mold cavity surface. Manipulation : The powder is mixed with a measured amount of liquid using a bowl and spatula. Hand mixing or mechanical mixing under vacuum can be done. The mixed material is vibrated into the casting ring. Setting and Thermal Expansion : Theoretically, the setting reaction should show shrinkage but in practice there is slight expansion when colloidal silica solution is used instead of water. When phosphate investments are mixed with water, they exhibit a shrinkage at the same temperature range as gypsum bonded investments (200 oC to 400oC). This contraction is practically eliminated when a colloidal silica solution replaces the water. The shrinkage is due to decomposition of the binder, magnesium ammonium phosphate with evolution of ammonia, which is readily apparent by its odor, but this shrinkage is masked by expansion of cristoballite. According to ADA specification No.42 there are 2 types of phosphate bonded investments. Type I
: Inlays, crown and other fixed restorations.
: Partial dentures and other cast removable restorations
Working and Setting Time : Working time
: 2 minutes
: 1 hour
Factors Affecting Setting Time : 1. Increased Temperature Fast set The setting reaction itself is exothermic, and this further accelerates the rate of setting. 2. Increased mixing time Fast set Generally mechanical mixing under vacuum is preferred. 3. Increased L : P ratio Increased working time Miscellaneous Properties : Compressive Strengths
Should not be less than 2.5 MPa
0.6 – 0.8%
With water 0.8% With special liquid 1.2%
Advantages : 1. High green strength 2. High fired strength – Less mold cracking and few fins on casting. 3. They can withstand temperatures upto 1000oC for short periods of time. Disadvantages : 1. At temperatures greater than 1375oC Cause mold breakdown and roughen the surface of casting. 2. Due to high strength devesting is defect. 3. To increase expansion, with use of special liquid Cause less porous mold Incomplete casting 4. High tendency for reaction with non-precious alloy producing oxides which is difficult to remove from castings.
Surfactant Containing Phosphate Bonded Investment : Addition of surfactants to phosphate bonded investment can increase the hygroscopic setting expansion. The surfactant also makes the unset investment more viscous and reduces the compressive strength. Ethyl-Silicate Bonded Investments : Another type of binding material for investments used with casting high melting alloys is silicate bonded investments. This investment material are being used since 1930. But now it is loosing popularity due to complicated and time consuming procedures involved. Common Brands : -
Nobilium rapid set (low iron and sodium investment)
Saddle lock (a ferruginous investment)
Howmet Vary rapid (intermediate iron and sodium)
Composition : A. Binder Silica gel that reverts to silica (cristobalite) on heating. B. Refractory Silica (cristobalite) C. Additive Magnesium oxide – Make it alkaline, strengthen the gel D. Wetting agent To reduce accumulation of air bubbles on surface of wax pattern. Various methods of producing silica or silicic acid gel binder. 1. pH of sodium silicate is lowered by the addition of an acid or acid salt Silicic acid gel forms. 2. Aqueous suspension of colloidal silica can be converted to gel by addition of an accelerator such as ammonium chloride Silicic acid gel.
Another system for binder formation is based on ethyl silicate. 3 Stages : Stage I : Hydrolysis : A colloidal silicic acid is first formed by hydrolyzing ethyl silicate in the presence of hydrochloric acid, ethyl alcohol and water. HCl, C2H5OH Si(OC2H5)4 + 4H2O Si(OH4) + 4C2H5OH (Ethyl alcohol) Ethyl silicate Sol of polysilicic acid Because of the use of polymerized form of ethyl silicate, a colloidal sol of polysilicic acid is expected instead of simpler silicic acid sol. Stage II : Gelation The sol is then mixed with quartz or cristoballite, to which is added a small amount of finally powdered magnesium oxide to render the mixture alkaline. nSi (OH)4 + Mgo Mgo [Si(OH)]n A coherent gel of polysilicic acid forms, accompanied by a setting shrinkage. Stage III : Drying (<168oC) This soft gel is dried at a temperature below 168 oC. During the drying process, the gel loses alcohol and water to form a concentrated hard gel. A volumetric contraction accompanies the drying, which reduces the size of the mold.
This contraction is known as “Green Shrinkage” and it occurs in
addition to the setting shrinkage. This gelatin process is slow and time consuming. An alternative and faster method for the production of silica gel is employed. Certain types of amines can be added to the solution of ethyl silicate so that hydrolysis and gelation occur simultaneously.
Simultaneous hydrolysis Ethyl silicate + Piperidine Silica gel (amine) Gelation With an investment of this type, the mold enlargement before casting must compensate not only for the casting shrinkage of the metal but also for the green shrinkage and setting shrinkage of the investment. Mode of Supply : 1. Powder
Refractory particles of silica and glass Calcined MgO Other refractory oxide
Single liquid of stabilized alcohol solution of silica gel. or
With 2 liquids -
One bottle contains a properly diluted water soluble silicate solution
Other bottle contains a properly diluted acid solution such as solution of hydrochloric acid. Before use equal volume of each bottle should be mixed and allowed to
stand for a prescribed time according to the manufacturers instruction. So that hydrolysis can take place and freshly prepared silicic acid formed. Manipulation : Powder is added to the hydrolyzed ethyl silicate liquid, mixed quickly and vibrated into a mold. This allows the heavier particles to settle quickly while the excess liquid and some of the fine particles rise to the top. In about 30 minutes The accelerator (NH4Cl) in the powder hardens the settled part, and the top excess is poured off. Thus the liquid: powder ratio in the settled part is greatly reduced and the setting shrinkage is reduced to 0.1%. It is little more complicated than phosphate type in that care must be exercised in handling and burnout, because flammable alcohol is given off. If 29
it is heated high enough, some silica converts to quartz and provides added expansion. This type of investment can be heated to 1090 oC to 1180oC and is compatible with the higher fusing alloys. Its low setting expansion minimizes distortion. Properties : Compressive strength
Not less than 1.5 MPa
0 – 0.4%
1.5 – 1.8% (This material has only thermal expansion and no other expansion)
Can withstand high temperature 1090oC to 1180oC Porosity The particles in the set material are very closely packed leading to low porosity. Air space / vents must be left in investment to permit escape of air from the mould. Advantages : 1. High permeability, yields sharply defined castings. 2. Low setting expansion 3. The investment has more refractory Form smooth castings 4. Low burnout strength results in easy removal of castings and cleaning of oxides from the castings. Disadvantages : 1. Limited shelf life of liquid 2. Must wait for substantial period of time, prior to using freshly mixed liquid. 3. Potential of cracking exits during burnout, owing to high thermal expansion. 4. Very expensive 5. Gives off flammable components during processing.
Uses : 1. They are mainly used for casting Co-Cr R.P.D. frame work. 2. Accurate casting of Nickel based alloys. Divestments (By Whipmix Corporation) : It is a combination of die stone and gypsum bounded investment material. The powder is mixed with colloidal silica. Properties: Setting expansion
: 0.6% (at 977oC)
Advantages : The wax pattern and die are invested simultaneously without removal of pattern. Useful with gold alloys. (Used for casting minute patterns) 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. Brazing Investment : ADA Sp. No.93 :
Type I â€“ Gypsum bonded dental brazing investment Type II â€“ Phosphate bonded
Steps : 1) Broken parts are stabilized by sticky wax. 2) The broken parts are then embedded in investment with portion to be solder is left exposed and free of investment. They should have low setting and thermal expansion.
Particle size is
usually not fine. They possess usually a compressive strength of 2-10 MPa. CONCLUSION : It should be emphasized that several investment techniques can produce comparable results. The dentist or technician should become familiar with different method and different investment materials. In any technique the fundamentals of that techniques should be applied. 31