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Investment materials/ online orthodontic courses


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Indian Dental Academy: will be one of the most relevant and exciting training

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courses in Dental implants,Orthodontics,Endodontics,Cosmetic Dentistry,

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Investment materials/ online orthodontic courses

  1. 1. 11 INDIAN DENTAL ACADEMY Leader in continuing dental education
  2. 2. 22 INTRODUCTIONINTRODUCTION Following the production of wax pattern, the next stage in many dental procedures involves the investment of the pattern to form a mould. .
  3. 3. 33 A sprue former is attached to the pattern and assemblage is located in the casting ring. Investment material is poured around the wax pattern whilst in a fluid stage.
  4. 4. 44 When the investment sets hard the wax and the sprue former is removed by softening and/or burning out to leave a mould which can be filled with an alloy or ceramic using a casting technique.
  5. 5. 55 (GPT-7) Dental casting investment: A material consisting primarily of an allotrope of silica and a bonding agent. The bonding substance may be gypsum (for use in lower casting temperatures) or phosphates and silica (for use in higher casting temperatures) Refractory investment An investment material that can withstand
  6. 6. 66 (CRAIG) An investment may be described as a ceramic material that is suitable for forming a mold into which a metal or alloy is cast.
  7. 7. 77 Requirements of investmentsRequirements of investments for alloy casting proceduresfor alloy casting procedures 1.The investment material should be capable of reproducing the shape, size and detail recorded in the wax pattern. 2.The investment should be easily manipulated . Not only should it be possible to mix and manipulate the mass readily and it paint the wax pattern easily, but the investment also should harden in a relatively short
  8. 8. 88 3.The investment should be able to maintain the integrity at higher temperatures (as the casting is carried out in higher temperatures often as higher than 1000c) 4.On being heated to higher temperatures the investment should not decompose to give off gases that would damage the surface of the alloy. 5.The investment should have a sufficiently high value of compressive strength at the casting temperature so that it can withstand the stresses set up when the molten metal enters the mould
  9. 9. 99 6.The investment material should expand to compensate for the casting shrinkage 7. Casting temperature must not be critical. Preferably thermal expansion versus temperature curve should have a plateau of thermal expansion over a range of casting temperatures . 8. Investment should be porous enough to permit the air or other gases in the mold cavity to escape easily during the casting
  10. 10. 1010 9. Investment should produce a smooth surface and fine detail and margins on the casting. 10. After the casting is complete the investment should break away readily from the surface of the metal and should not have reacted chemically with it. 11.The investment material should be inexpensive.
  11. 11. 1111 Factors involved in selectionFactors involved in selection of investment materialof investment material 1. Type of alloy to be cast 2. Casting temperature to be used.
  12. 12. 1212 Types of investment materialsTypes of investment materials available for casting alloysavailable for casting alloys • GYPSUM-BONDED INVSTMENTS • PHOSPHATE BONDED INVESTMENTS • SILICA BONDED INVESTMENTS • NEWER INVESTMENT MATERIALS FOR TITANIUM AND TITANIUM BASED ALLOYS
  13. 13. 1313 COMPOSITIONCOMPOSITION Investment materials consist of a mixture of a 1. REFRACTORY MATERIAL 2. BINDER 3. MODIFIERS
  14. 14. 1414 REFRACTORY MATERIAL SILICA (silicon dioxide) is used as refractory material. It is available in four allotropic forms such as • Quartz • Tridymite • Cristobalite • Fused quartz Quartz and Cristobalite are used extensively in dental investments.
  15. 15. 1515 Quartz is a common mineral . Cristobalite occurs naturally as a rare mineral but is normally manufactured by prolonged heating of the quartz at high temperatures to induce the appropriate slow inversion. Each form of silica exists in two phases. 1. Low temperature phase or alpha phase 2. High temperature phase or Beta phase
  16. 16. 1616 High temperature phase is less dense than that of the Low temperature phase On heating the change between the two phases is rapid and readily reversible on cooling .this change is known as high – low inversion . • When heated a change in the crystalline form occurs at the transition temperature characteristic of the particular form of silica
  17. 17. 1717 Quartz when heated inverts from alpha phase to beta phase at a temperature of 575 c Cristobalite when heated inverts from alpha phase to beta phase at a temperature of 200 to 270 c The beta allotropic forms are stable above the transition temperature and an inversion to the lower or alpha form occurs on cooling. In powdered form the inversion occurs over a range of temperature instantaneously.
  18. 18. 1818 • The density decreases when the alpha form changes to beta form with a resulting increase in the new volume. The increase in the volume (or isothermal expansion) is probably due to straightening of the chemical bonds to form a less dense crystalline structure as illustrated in the figure •
  19. 19. 1919 • It is exhibited by a rapid increase in the linear expansion as indicated in the figure. • The graph shows that the over all thermal expansion and inversion expansion of materials containing cristobalite is greater than that of quartz.
  20. 20. 2020 The isothermal expansion for the Cristobalite is 1.3% at 250c Quartz is 0.6% at 573c Depending on type of silica used the investment materials are classified as 1. Quartz investments 2. Cristobalite investments
  21. 21. 2121 FUNCTIONS 1. It is added to provide refractory during heating which is capable of withstanding very high temperatures during casting without degradation . 2. It regulates thermal expansion. The wax pattern is eliminated from the mold by heat. During heating the Investment expands which is necessary to compensate partially or totally for the casting shrinkage of the alloy .
  22. 22. 2222 The expansion is accomplished by a combination of simple thermal expansion coupled with a crystalline inversion (isothermal expansion) which results in significant expansion.
  23. 23. 2323 It binds the refractory materials together. The nature of the binder characterizes the material Ex: • Alpha calcium hemi hydrate for casting gold alloys • Sodium silicate, ethyl silicate, ammonium sulphate , sodium phosphate for casting cobalt chromium alloys Binder
  24. 24. 2424 MODIFIERS • Usually a mixture of refractory materials and binder is not sufficient to produce all the properties of the investment materials • Other chemicals such as sodium chloride, boric acid, graphite, copper powder, are often added in small quantities to modify physical properties,.
  25. 25. 2525 Gypsum bonded investmentsGypsum bonded investments • They are the mold materials used in the casting of dental gold alloys with liquidus temperatures no more than 1080 c ADA SPECIFICATION NO2 for casting investments for dental gold alloys encompasses three types of investments .
  26. 26. 2626 They are TYPE1 : THERMAL EXPANSION TYPE employed in casting inlays and crowns TYPE 2 : HYGROSCOPIC EXPANSION type employed in casting inlays and crowns TYPE 3: for casting complete and partial denture bases
  28. 28. 2828 CompositionComposition • REFRACTORY MATERIAL – Silica –(60-65%) Increasing the proportion of silica in the investment powder increases the Manipulation time , Initial setting time ,
  29. 29. 2929 • setting expansion both in air and water and thermal expansion and reduces compressive strength
  30. 30. 3030 The increased manipulation and setting time and reduced compressive strength occur because the particles of the refractory filler interfere with the interlocking of growing gypsum crystals making this less effective in developing a solid structure.
  31. 31. 3131 • Setting expansion is increased when interlocking of growing gypsum crystals is inhibited by refractory particles because the crystal growth is directed outward • Thermal expansion is increased due to summing of Binder contraction + refractory expansion
  32. 32. 3232 • BINDER – Alpha hemi hydrate form of gypsum(30-35%) it is used as binder for investments used in casting gold containing alloys with melting ranges below 1000 c When this material is heated to the temperature required for complete dehydration and sufficiently high to ensure complete castings, it shrinks considerably and frequently fractures
  33. 33. 3333 All form s of gypsum shrink considerably after dehydration between 200- 400 c (due to loss of water of crystallization) a slight expansion occurs between 400c and approximately 700c, and then a larger contraction occurs (due to densification by sintering) This later shrinkage is most likely due to deposition and sulphur gases such as sulphur dioxide are emitted.
  34. 34. 3434 They not only cause shrinkage but also contaminates the castings with the sulphides of the non noble alloying metals such as silver and copper. Thus, it is imperative not to heat the gypsum products above 700 c for the gypsum products containing carbon the maximum temperature should be 650 c.
  35. 35. 3535 MODIFIER - (4-7%) Used are Reducing agents Modifying chemicals Coloring matter Reducing agents : they reduce any metal oxides formed on the metal by providing a non oxidizing atmosphere in the mold when the mold alloy enters • Ex– Copper
  36. 36. 3636 Modifying chemicals: They regulate setting expansion and thermal expansion and also prevent shrinkage of gypsum when heated above 300 c . • They act by reducing the two large contractions of gypsum binder on heating to temperatures above 300 c . • Ex– Boric acid Soluble salts of alkali or alkaline earth metals
  37. 37. 3737 • Boric acid: • when heated above 150 c forms a viscous liquid which impedes evaporation of last traces of water , delaying the gamma to beta transformation of calcium sulphate. • This viscous phase also reduces the high temperature contraction that results from sintering because it stabilizes the original contact formed between gypsum crystals and silica during setting
  38. 38. 3838 • Investments containing this boric acid when heated to 670-700 c shows increases its compressive strength ranging from 40-50%.
  39. 39. 3939 Salts of alkali and alkaline earth metals : ex- sodium chloride • Reduces first major shrinkage and eliminates second shrinkage of gypsum on heating • The effect of halide ion is nullified above 650 c and rapid contraction occurs (probably the result of accelerated sintering ) • A marked strength decreased on heating to 700 c ranging from -50 to -85 %
  40. 40. 4040 • The large high temperature shrinkage of the binder is not observed in gypsum bonded investments containing these modifiers because at a concentration of 50% or more of silica , the silica particles in the set investment form a continuous skeleton that resists over all shrinkage .
  41. 41. 4141 1.Particle size of the powder • Affects the smoothness of the mold cavity surface • Affects the inherent porosity of the mold • Only the particle size of the refractory filler is of practical importance as they remain unchanged in the said investment • The gypsum crystals formed during setting of the binder are much smaller than silica particles PropertiesProperties
  42. 42. 4242 • Refractory with the fine particle size– smooth mold surface and smooth casting • The venting of the mold cavity is normally provided by porosity inherent in set material (density) • So the refractory powder used in the investment should have uniform fine particle, size no more than 75 m
  43. 43. 4343 2.Manipulation time2.Manipulation time • Investing the wax pattern must be completed while the mix is still fluid • Loss of fluidity is indicated by disappearance of glossy surface from the mix.
  44. 44. 4444 3. Setting time3. Setting time • According to ADA sp no 2 the setting time for dental inlay casting investment should not be shorter than 5 min and not more than 25 min. • The modern inlay investments set initially in 9 – 18 min .
  45. 45. 4545 4.Expansion4.Expansion • Inlay investments have total expansion in the range of 1.5 – 2.5% . • Purpose of setting expansion is to aid in enlarging the mold to compensate partially for casting shrinkage. • Setting expansion of three types 1. normal setting expansion 2. hygroscopic expansion 3. thermal expansion
  46. 46. 4646 Normal setting expansionNormal setting expansion • Mixture of silica and gypsum hemi hydrate results in greater setting expansion than that of gypsum products when it is used alone. • The silica particles probably interfere with intermeshing and inter locking of the crystals as they form.
  47. 47. 4747 • Thus the thrust of crystals is outward during the growth and they increase expansion . • ADA sp no 2 for type 1 investment permits a maximum setting expansion in air of 0.6% setting expansion of modern investments is 0.4%which can be regulated by accelerators and retarders .
  48. 48. 4848 Hygroscopic setting expansionHygroscopic setting expansion • This is one of the methods for expanding the casting mold to compensate for casting shrinkage • When the gypsum product is allowed to set under or in contact with water and the amount of expansion exhibited is much greater than normal setting expansion
  49. 49. 4949 • The hygroscopic setting expansion may be 6 or more times greater than the normal setting expansion of a dental investment • The increased amount of expansion is because the water helps the outward growth of crystals • The investment should be immersed in water before the initial set is complete. • ADA sp no 2 for such type 2 investments require minimum setting expansion in water of 1.2% and maximum 2.2%.
  50. 50. 5050 Factors affecting hygroscopicFactors affecting hygroscopic setting expansionsetting expansion 1.Effect of composition: • Finer the particle size of silica greater the hygroscopic expansion. • Alpha hemi hydrate produces greater hygroscopic expansion than beta hemi hydrate in presence of silica. • Higher the silica content greater the expansion and at least 15%of binder is necessary to prevent drying shrinkage
  51. 51. 5151 2.Effect of water powder ratio The higher the water powder ratio of original investment water mixture the less the hygroscopic setting expansion 3.Effect of spatulation: • The shorter the mixing time the less is the hygroscopic expansion 4.Effect of shelf life: • The older the investment the lower is the hygroscopic expansion.
  52. 52. 5252 5.Effect of time of immersion: • The immersion before the initial set causes greater expansion . 6.Effect of confinement: • The confining effect of the opposing forces such as the walls of the container in which the investment is placed or the wall of the wax pattern is much more pronounced on the hygroscopic expansion than the normal setting expansion • The effective hygroscopic setting expansion is likely to be less in proportion than in the normal setting expansion.
  53. 53. 5353 7.Effect of amount of added water: • Magnitude of hygroscopic expansion is in direct proportion to the amount of water added during the setting period until a maximum expansion occurs • No further expansion is evident regardless of any amount of water added • Once the setting starts the later water is added to the investment the less is the hygroscopic setting expansion because part of crystallization has already started in normal fashion.
  54. 54. 5454 • Some of the crystals have intermesh and inhibit further crystal growth after the water is added • On the same basis the less water that is added the lower is the expansion
  55. 55. 5555 8.Effect of casting ring liner: • Asbestos ring liner is used for lining the casting ring • This liner makes additional water available to the setting investment and causes an increased setting expansion. • Even when the mold sets in the air as in thermal expansion technique some hygroscopic setting expansion occurs.
  56. 56. 5656 • The investments used in the thermal expansion technique have relatively high silica content, so increase in setting expansion produced by exposure to water is high .As this high setting expansion is uncontrollable an isotopic a dry water proof asbestos ring liner is used • The combination of a low water powder ratio and a wet liner considerably increases the investment setting expansion
  57. 57. 5757 The hygroscopic setting expansion is a continuation of ordinary setting expansion because the immersion water replaces water of hydration and thus prevents the confinement of growing crystals by surface tension of the excess water. Because the diluent effect of the quartz particle, the hygroscopic setting expansion in these investments is greater than that of gypsum binder when used alone
  58. 58. 5858 • This phenomenon is purely physical . • The water is drawn between the refractory particles by the capillary action and thus causes the particles to separate creating an expansion • The effect is not permanent after the water is evaporated unless a binder is present
  59. 59. 5959 • The term hygroscopic is an mis inomer • This hygroscopic setting is as normal a phenomenon as that which occurs during normal setting expansion • The water is drawn into setting material by capillary action and not by hygroscopy
  60. 60. 6060 Thermal expansionThermal expansion • The thermal expansion is directly related to the amount and type of silica present. • Type 1 investments should have thermal expansion of not less than1% and not greater than 1.6%. • The desirable amount of thermal expansion depends on whether thermal expansion will compensate the casting shrinkage or it will be compensated by hygroscopic setting expansion
  61. 61. 6161 • If hygroscopic setting expansion is used the thermal expansion of 0.5- 0.6% is sufficient . • If only a thermal expansion is used with normal setting expansion then it should be 1 – 1.6%. • The maximum thermal expansion should be achieved at a temperature not greater than 700 c as the a breakdown of calcium sulphate binder occurs in presence of carbon ( present as graphite added to the investment as reducing agent or a residue from the burn out the wax pattern) liberating sulphur dioxide .
  62. 62. 6262 • The sulphur dioxide formed causes sulphide alloy formation and gold alloy casting resulting in discoloration and embrittlement of the alloy • Quartz has a low thermal expansion than cristobalite hence additives are added to investments containing quartz .
  63. 63. 6363 Factors affecting the thermal expansion : 1.Effect of water powder ratio: • More the amount of the water used for mixing less is the thermal expansion 2.Effect of chemical modifiers: • The addition of small amounts of sodium potassium or lithium chlorides to the investments eliminates the contraction caused by gypsum and increases the expansion without the presence of excessive amounts of silica . Silcas donot prevent gypsum shrinkage but counter balance it where as chlorides reduce gypsum shrinkage
  64. 64. 6464 5.Strength5.Strength • According to ADA sp no 2 the compressive strength should not be less than 2.5MPA • Alpha hemihydrate increases the compressive strength • Chemical modifiers increase the strength • More water during mixing less is the strength
  65. 65. 6565 • Heating the investments above to 700 c increase or decrease strength as much as 65% depending on composition • Greatest reduction in strength is found upon heating in investments containing sodium chloride • As the investment sets to room temperature strength decreases considerably because of fine cracks that formed during cooling
  66. 66. 6666 6.Fineness6.Fineness Fineness affects • Setting time • Surface roughness of the casting • Hygroscopic expansion Finer silica is preferrable
  67. 67. 6767 7.Porosity7.Porosity • During the casting process, the molten metal is forced into the mold under pressure . As the molten metal enters the air must be forced out ahead of it. If the air is not completely eliminated a back pressure builds up to prevent the gold alloy from completely filling the mold. • Common method for venting the mold is through the pores of the investment.
  68. 68. 6868 • More gypsum crystals ->less is the porosity • Lower the hemi hydrate -> greater the amount of water used to mix the investment ->more the porous is the investment • Uniform particle size ->greater is its porosity
  69. 69. 6969 StorageStorage • Should be stored in air tight and moisture proof containers • Should be purchased in small quantities • as the investment materials are composed of different ingredients each of which posses a different specific gravity , these components settle , under a normal vibration that occurs in dental laboratory.
  70. 70. 7070 • This separation influences on the setting time and other properties of the investment • For this reason and as well as to avoid accidental moisture contamination the investment should be purchased in small quantities
  71. 71. 7171 DivestmentDivestment • It is a gypsum bonded material mixed with colloidal silica • Setting expansion is 0.9% • Thermal expansion is 0.6% when it is heated to 677 c • As it is a gypsum bonded material it is not recommended for high fusing alloys. • Divestment phosphate is a phosphate bonded investment used as a divestment for fusing alloys.
  72. 72. 7272 Rapid heat investmentsRapid heat investments • Investments based on a cristobalite refractory require slow heating while the alpha to beta inversion is occurring some rapid heat investments have been introduced which are placed immediately after setting into a furnace pre heated to 700 c .
  73. 73. 7373 CompositionComposition • Cristobalite a form of silica is used as refractory material. Technique : • Place the mold 30 min after the pattern is invested into the preheated furnace for a n additional 30 min, the casting is then made .
  74. 74. 7474 Setting and thermal expansionSetting and thermal expansion • Setting expansion measured under ordinary conditions is still occurring rapidly at 30 min it is not complete until 2 hours after mixing and measures 1%. • The rapid rate of expansion at 30 min means the precise timing of placement of the mold in the furnace is critical if reproducible mold expansion is to occur
  75. 75. 7575 • This drastic heating program could be expected to cause severe thermal cracking in an ordinary cristobalite investment. • Measurements on a mold in a lined inlay ring showed that the periphery of the investment mass reached 250 c within 6 min of entering the hot furnace while the centre was at only 110 c until 10 min. • Both periphery and centre reached a maximum of 690 c within 30 min heating period.
  76. 76. 7676 • The expansion caused by the inversion of cristobalite shown on the graph beginning at 110 c and finishing at 170 c took place then enough of the outer parts of the specimen reached 250 c to produce a volume change.
  77. 77. 7777 AdvantagesAdvantages • They save the laboratory time as the furnace is maintained at 700 c instead of being repeatedly heated and cooled • The investment total expansion under these conditions was 1.95% , more than enough to compensate casting shrinkage of ordinary dental alloys
  78. 78. 7878 Gypsum bonded investments cannot withstand temperatures greater than 700c A large contraction occurs when gypsum bonded investments are heated above 700c. The later shrinkage is due to decomposition by interaction of silica with calcium sulphate to liberate sulphur trioxide gas. CaSO4+SiO2 -> CaSiO3+SO3
  79. 79. 7979 Another reaction which may take place is on heating is that of between calcium sulphate and carbon (May be derived from the residue left after burning out wax pattern or may be present as graphite in the investment): CaSO4+4C -> CaS+4CO further reaction can occur liberating sulphurdioxide 3CaSO4+CaS -> 4CaO +4SO2 This decomposition not only causes shrinkage but also contaminates the casting with the sulphides of the non noble alloying elements . So gypsum should not be heated above 700c
  80. 80. 8080 Most palladium and base metal alloys used for partial dentures and porcelain fused to metal restorations have high melting temperatures. They should be cast
  81. 81. 8181 As suggested by skinner (1963) “ the definitive advantage of this type of investment is that there is less chance for the contamination of the gold alloy during casting…. So far as is known at present such contamination is avoided with phosphate bonded investments . On this basis , I am inclined to predict that the dental investment of the future may be phosphate bonded not gypsum bonded”. As predicted the phosphate bonded investments are widely used. PhosphatePhosphate bondedbonded investmentsinvestments
  82. 82. 8282 APPLICATIONS • They are used in construction of high melting temperature dental alloys . • Soldering and porcelain veneering • To make soldering fixtures that hold prosthetic components in alignment while they are being joined with solders brazing alloys or welding alloys
  83. 83. 8383 ClassificationClassification Type 1 • For casting of inlays crowns and other restorations especially for alloys like gold, platinum ,palladium cobalt chromium and nickel chromium Type 2 • For casting of removable partial dentures
  84. 84. 8484 CompositionComposition Refractory materials – (concentration of approximately 80%)silica in quartz , cristobalite or a mixture of two . Purpose • To provide high temperature thermal shock resistance • High thermal expansion • To control thermal stresses related to thermal phase transformation of cristobalite and along with glasses and other metal oxides to provide bulk and help to control the surface finishing of
  85. 85. 8585 Binder (<20%) • Magnesium oxide (acid) and a phosphate (base) • Originally phosphoric acid was used but mono ammonium phosphate has replaced it (as it can be incorporated in powder form • Mono ammonium phosphate which in reaction with water in the presence of calcined magnesium oxide powder provides for binding of particles at ambient temperatures
  86. 86. 8686 Modifiers • Carbon is often added . • It produces clean casting. • Facilitates easy divesting of casting and mold . • Generally added when casting alloy is gold. • When silver palladium or base metal alloys are invested with the investment containing carbon ,it embrittles the alloys even though the investment is heated to the temperature that burn out the carbon.
  87. 87. 8787 • The basic binding reactions is the same for all phosphate bonded investments, there are important differences in properties due to composition. Those used for: • Casting of high temperature alloys and • Making dies used in fabrication of porcelain veneers Contain quartz and cristobalite to achieve expansion to compensate shrinkage
  88. 88. 8888 Soldering investments do not require fine powders and are designed without high expansion fillers It is to keep parts that are to be joined from shifting while they and the surrounding investment is heated to the joining temperature Graphite is found in some of the investments to render them more permeable after burn out to provide a reducing atmosphere.
  89. 89. 8989 • It is available as two component systems • 1- It is a Powder which contains refractory materials and binders and modifiers • 2- Aqueous solution stabilized with colloidal silica Because the newer gold containing alloys and other alloys used for metal ceramic restorations have higher melting temperatures their contraction during solidification is also greater . Colloidal silica suspension facilitate greater expansion of the investment which can compensate the casting shrinkage.
  90. 90. 9090 Setting reactionsSetting reactions
  91. 91. 9191 Setting and thermal expansionSetting and thermal expansion • In practice there is a slight expansion and this can be increased by using colloidal silica solution instead of water .
  92. 92. 9292 • When phosphate investments were mixed with water they exhibited a shrinkage with in a range of 200 -400 c • This contraction is eliminated when colloidal silica solution replaces water • The early thermal shrinkage of phosphate investments is associated with a of binder magnesium ammonium phosphate and by evolution of ammonia .
  93. 93. 9393 • Expansion can be varied by the proportions of silica and water. 1.More silica and less water – more expansion . 2.Less silica and more water- less expansion. Liquid can be used as full strength or diluted with water to provide some degree of control over setting or thermal expansion.
  94. 94. 9494 PropertiesProperties • High temperature mold is achieved by formation of complex silicophosphates( from the reaction of some of the silica with the excess of dihydrogen phosphate)
  95. 95. 9595 PropertiesProperties • Casting investments 1 Compressive strength • Type 1- 2.5 mpa • Type 2- 3 mpa • Setting expansion within 15% of manufacture stated value • Full strength liquid .4% 2 Thermal expansion within 15% of manufacture stated value • 0.8% when 50:50 mixture of liquid and water 3 Modulus of rupture- .1to .5 mpa
  96. 96. 9696 • Refractory die stones 1. Compressive strength 13 Mpa 2.Setting expansion within 30%of manufacture stated value 3.Thermal expansion within 15% of manufacture stated value
  97. 97. 9797 Working and setting timeWorking and setting time Affected by 1.Temperature Warmer the mix faster it sets The setting reaction liberate the heat and accelerates rate of setting 2.Mixing time increased mixing time and mixing efficiency result in faster set and greater rise in temperature
  98. 98. 9898 • The more the efficient the the better the casting in smoothness and accuracy • Mechanical mixing under vaccum is preferred 3.Liquid:powder ratio • Increase in the liquid:powder ratio increases the setting time.
  99. 99. 9999 AdvantagesAdvantages 1.They have high fired strength. This make them handle without breaking before they are placed in a furnace for the wax burn out process and strong enough to with stand the impact and the pressure of centrifugally cast molten alloy 2.They also provide high setting and thermal expansion enough to compensate cast metal prosthesis or porcelain veneers during cooling
  100. 100. 100100 3.They have ability to with stand the burn out process with temperatures that reach 900 c and also 1000 c for short period of time (for fabricating porcelain veneers or performing metal joining operations
  101. 101. 101101 DisadvantagesDisadvantages 1. When used with higher melting alloys those with casting temperatures higher than 1375 c they result in mold breakdown and rougher surfaces on casting 2. Their higher strength although an advantage make divesting a difficult and tedious task
  102. 102. 102102 3. When higher expansion is required more of silica liquid is used with the result that more dense and less porous mold is produced this results in incomplete casting if a release for trapped gases is not provided 4.When the powder is supplied in bulk form rather than in sealed pre measured packages it can react over time with moisture in air and result in lower expansion during setting or loss of ability to set to a strong mass
  103. 103. 103103 ETHYL SILICATE BONDEDETHYL SILICATE BONDED INVESTMENTSINVESTMENTS APPLICATIONS They are used in construction of high fusing base metal partial denture alloys
  104. 104. 104104 COMPOSITION Refractory material - Silica Binder –Silica gel or ethyl silicate Modifier – Magnesium oxide (strengthen the gel) Ammonium chloride - accelerator
  105. 105. 105105 It is supplied as a powder and liquid or two liquids If supplied as a powder and liquid • Powder consists of refractory particles of silicas and glasses along with the calcined magnesium oxide and some other refractory oxides in minor amounts • Liquid contains stabilized alcohol solution of silica gel
  106. 106. 106106 If supplied as 2 liquids • One is ethyl silicate Certain types of amines are added for hydrolysis and gelation to occur simultaneously • Other is acidified solution of denatured alcohol
  107. 107. 107107 SETTING REACTION When binder silica gel is used • Silicic aid gel is formed when sodium silicate is added to a acid or acid salt • MgO added strengthen the gel When ethyl silicate is used as a binder • Colloidal silicic is first formed by hydrolyzing ethyl silicate in presence of Hcl, ethyl alcohol, and water.
  108. 108. 108108 • Si(OC2H5)4 + 4H2O _ Si(OH)4+4C2H5OH • This silicic acid (sol) is mixed with the silica • to which MgO is added to render the mixture alkaline • A coherent gel of polysilicic acids is formed • The soft gel is dried at a temperature below 168 c
  109. 109. 109109 • During the drying process the gel loses alcohol and water to form a concentrated ,hard gel . • The volumetric contraction accompanies drying which reduces the size of the mold . • This contraction is known as “GREEN SHRINKAGE” occurs in addition to setting shrinkage
  110. 110. 110110 So the mold enlargement with this type of investment must compensate • Casting shrinkage • Setting shrinkage and • Green shrinkage
  111. 111. 111111 ManipulationManipulation These investments have a special particle size gradation and are handled in a different manner The powder is added to hydrolyzed ethyl silicate liquid, mixed quickly and vibrated into a mold , that has an extra collar to increase the height .
  112. 112. 112112 The mold is placed on a vibrator that has a tamping action This allows the heavier particles to settle while the excess liquid and some of the finer particles rise to the top . The top of the mold is prone to cracking due to greater drying shrinkage from evaporation of the ethyl alcohol.
  113. 113. 113113 The cracks must be removed before the firing process Otherwise ,when the mold is heated to burn out a pattern and achieve thermally induced expansion the cracks will grow and result in faulty casting. To overcome this problem a sufficient header of the investment is provided to allow for the removal of the cracked portion by grinding.(or)
  114. 114. 114114 In about 30 minutes the accelerator in the powder hardens the settled part, and the excess is poured off (to avoid crack formation). The liquid powder ratio in settled part is greatly reduced and the setting shrinkage is reduced to 0.1% Thus distortion is minimized and these investments are well suited for producing large ,precise castings. The expansion of the investment is all due to thermal expansion.
  115. 115. 115115 PropertiesProperties 1.Compressive strength -1.5Mpa 2.Thermal expansion( linear) – Within 15% of manufacturers value. About 1.5% to 1.8% can be attained between room temperature and 1000 c to 1177 c
  116. 116. 116116 AdvantagesAdvantages • High temperature cobalt chromium and nickel chromium alloys can be casted • Good surface finish is obtained • Low distortion • High thermal expansion • Thin sections with fine detail can be reproduced (as they are less dense) • Divesture is easier as they have low fired strength
  117. 117. 117117 DisadvantagesDisadvantages • Extra precaution needed in handling the low strength fired molds • Low strength and high thermal expansion require a more precise burn out process (flammable alcohol is released) and firing schedule to avoid cracking and hence destruction of
  118. 118. 118118 Newer investments for castingNewer investments for casting titanium based alloystitanium based alloys • Newer investments have been aimed at the casting of titanium or titanium based alloys . • Conventional phosphate bonded or ethyl silicate bonded investments are deficient for this purpose. • Molten titanium is highly reactive with the oxygen and is capable of reducing some of the oxides commonly found in the investment.
  119. 119. 119119 • Titanium can also dissolve residual oxygen, nitrogen, and carbon from the investments. • These elements can also harden and embrittle titanium in the solid state • As a result a modification of the existing refractory formulations and binders or new refractory formulations and binder systems are required.
  120. 120. 120120 CompositionComposition • According to the source of binder they can be classified as 1. Phosphate bonded 2. silicate bonded 3. Cemented Refractories that can be used are 1. Silica 2. Alumina 3. Magnesia 4. Zirconia
  121. 121.
  122. 122. 122122 PropertiesProperties • Standards do not exist for investments for the casting of titanium and its alloys so mechanical properties for the conventional phosphate and ethyl silicate based binders would apply here as well
  123. 123. 123123 Objectives for a titaniumObjectives for a titanium investment should beinvestment should be 1. To reduce breakdown of the investment 2. To reduce contamination of thetitanium.
  124. 124. 124124 To reduce breakdown of the investment • Reducing the reaction with investment is to employ molds that have been expanded by burn out process and then cooled back to near ambient temperature prior to casting process • This reduces the time that the alloy is in contact with mold at elevated temperatures and over all reactivity is reduced
  125. 125. 125125 • Lowering of the mold temperature requires that either non reversible expanders such as metals that expand by oxidation at elevated temperatures be used or that the temperature of the mold be kept just above the temperature where a reversal of expansion due to crystalline phase changes take place
  126. 126. 126126 • To avoid contamination of titanium by oxygen through the reduction of refractory oxides of the investment ,refractory materials that are less easily reduced by titanium should be used
  127. 127. 127127 • The GIBBS free energy of formation per mole of oxygen • Titanium oxide at 1727 c is -716 kJ/mol of oxygen • Titanium dioxide at 1727 c is -580 kJ/mol of oxygen • Cristobalite and quartz are -550 and -549kJ/mol respectively • From this it is clear that titanium may be expected to be oxidized by silicon dioxide which is reduced
  128. 128. 128128 • Some modifications of phosphate bonded investments have been explored for the purpose of rendering them more compatible with molten titanium alloys • One investment consisting of phosphate binder ,magnesia and quartz was developed under the hypothesis that quartz would not be as reactive as silica
  129. 129. 129129 • This investment was recommended for use of room temperature mold to reduce reaction with titanium but the contamination of castings by reaction with the investment was still encountered
  130. 130. 130130 • To make the use of setting expansion of phosphate binder , alumina and magnesia, both of which are good heat resistance, can be used as refractories: however the thermal expansion is low . • If either is of the powders are mixed with silica to raise expansion some contamination with silica again becomes as a problem
  131. 131. 131131 • To achieve expansion with out the use reactive powders a phosphate investments that contains both magnesia and alumina as refractories was developed . • This investments can attain large expansion by the spinel reaction of alumina and magnesia.
  132. 132. 132132 • Reaction of ethyl silicate bonded investments with liquid titanium have been reported to be some what less than that of phosphate bonded investments this is most likely due to use of highly refractory oxides in the powder. Regardless these investments require a more complex procedure for their use
  133. 133. 133133 • A more recent development is an investment using magnesia bonded by an aluminous cement which contains a mass fraction of 5% zirconium powder. • The aluminous cement serves as a binder for the magnesia as a refractory . • It sets by mixing with water.
  134. 134. 134134 • Oxidation of the zirconium powder to zirconia during the burn out process provides irreversible expansion to compensate for shrinkage of the casting during cooling from the solidification temperature. • The zirconia formed is highly stable as it has an FEFof -728kJ /mol of oxygen and it should not contaminate titanium • Titanium casting from this investments were reported to have smooth surfaces free of contamination from mold reaction
  135. 135. 135135 SUMMARYSUMMARY Of the various types of investments described the most commonly used investment used is phosphate bonded investment material. The increase in the use of the higher melting alloys resulted in the increase in the use of the phosphate bonded investments.
  136. 136. 136136 Gypsum bonded investments cannot withstand temperature higher than 700c and it can be used only with conventional gold alloys . The processing attention and extra care needed in burn out procedures limited the use of silicate bonded investments Newer investments are developed for titanium based alloys . Titanium is highly reactive with the oxygen and is capable of reducing some of the oxides commonly found in the investment.
  137. 137. 137137 Modification of the existing refractory formulations and binders is done or new refractory formulations and binder systems are developed The applications of the investments available are summarized below in the table:
  138. 138. 138138 GYPSUM BONDED INVESTMENTS (700c) PHOSPHATE BONDED INVESTMENTS (900-1000C) SILICA BONDED INVESTMENTS (1090 -1180c) NEWER INVESTMENTS FOR TITANIUM Mold for gold casting alloys Mold for base metal and gold casting alloys ; mold for cast ceramics and glasses Refractory die for ceramic build up Mould for base metal casting alloys Mould for titanium containing alloys
  139. 139. 139139 For more details please visit