Education section inv mat. In detail and classification

Inv mat.

1. DENTAL INVESTMENT MATERIALS

2. C ON T E NTS Introduction History Definition Requirements Material science Types of investments -composition -setting reaction -setting expansion -properties Conclusion

3. 3 Introduction Lost wax casting procedure is adopted for fabrication of metal alloy appliances like inlays, crowns etc outside the mouth. An accurate wax pattern prepared ,is invested in refractory mould materials. After wax burn out the mould is filled with alloy liquid, which solidifies & cools down. During this procedure the possible dimensional change taking place are to be suitably compensated.

4. 4 Total shrinkage = Wax Shrinkage + Alloy Shrinkage Methods of compensation : Compensation is done by carefully controlling the mould expansions, by availing large setting expansion & adjusting the thermal expansions of the investment materials. Setting Expansion + Thermal Expansion

5. Hi s t o ry Until the dawn of 20 th century the investing procedure was largely ignored by modern industries,but in ancient days this procedure was used to make statues, jewels, and artifacts. And it is the dental professionals who rediscovered it for preparing crowns and inlays. The first authenticated record of use of investing casting in dentistry was given by Dr.Philbrook in 1897.

6. But the true significance of this process was given after research by Dr.Williams.H.Taggart in 1907. Dr.Taggart not only developed and described the technique, he also reformulated a wax pattern component of excellent properties, he also developed an investment material and even invented an pressure casting machine.

7. 7 1933- BRODSKY –first refractory mold 70%-mullite,aluminium oxide and silicon dioxide and 30% plaster of paris (GERMAN BRILLAT NO-2) MC CABE-GYPSUM CONTRACTS ON HEATING Phillips-GBI should not be heated above 700 degrees

8. 8 1949-MOORE AND WATTS-phosphate bonded investment 1959-Ethyl silicate bonded investment 1964-HOBO-Evaluated a range of refractory and ceramic combinations 1985-CALAMIA-Suggested –PBI-fabrication of all ceramic laminate veneers 2000- has an ISO 11245 for phosphate bonded refractory die materials

9. DEFINITION Investment An investment can be described as a ceramic material that is suitable for forming a mold into which a metal or alloy is cast. Restorative dental materials-Robert . G.Craig

10. REQUIREMENTS OF INVESTMENT MATERIALS Easily manipulated. Provide sufficient strength. Capable of reproducing the shape, size and detail in the wax pattern.

11. Stability at higher temperatures: On being heated to higher temperatures the investment should not decompose to give off gases that would damage the surface of the alloy. Provide sufficient expansion.

12. Investment should be porous enough to permit the air or other gases in the mold cavity to escape easily during the casting Investment should produce a smooth surface and fine detail and margins on the casting. The material should be inexpensive.

13. MATERIAL SCIENCE In general an investment is a mixture of three distinct types of materials: Refractory material Binder material Modifiers to improve the properties

14. REFRACTORY MATERIAL Material capable of sustaining exposure to high temperature without significant degradation. SILICA (silicon dioxide) is used as refractory material & to regulate the thermal expansion . It is available in four allotropic forms such as Quartz Tridymite Cristobalite F us e d qu a r tz Quartz and Cristobalite are used extensively in dental investments

15. Each form of silica exists in two phases. 1.Low temperature phase or alpha phase 2.High temperature phase or Beta phase On heating the change between the two phases is rapid and readily reversible on cooling .this change is known as high – low inversion .

16. α quartz β quartz ▲ 575° C (high form) (low form) α cristobalite ▲ 200°- 270° C The beta allotropic forms are stable above the transition temperature and an inversion to the lower or alpha form occurs on cooling. β cristobalite

17. Thermal expansion of three forms of silica

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

19. BINDER MATERIAL Functions : - - Hold the ingredients together Provide rigidity The common binders used are: Gypsum -for low temperature casting investments. Phosphate&ethyl silicate -for high temperature casting investments.

20. CHEMICAL MODIFIERS Other chemicals such as sodium chloride, boric acid, graphite, copper powder, are often added in small quantities to modify physical properties - BORIC ACID&SODIUM CHLORIDE Regulate setting expansion and setting time and thus prevent most of the shrinkage of gypsum when heated above 300 ºC . -COPPER POWDER& GRAPHITE Act as reducing agents.

21. CLASSIFICATION Investments are classified According to type of binder used According to the type of silica used According to to use and melting range of alloy.

22. According to the type of binder used -Gypsum bonded investments -Phosphate bonded investments -Ethyl silicate bonded investments

23. According to type of silica used - Quartz investments -Cristobalite investments -

24. According to use - Gypsum bonded investments Used for conventional casting of gold alloy inlays,onlays ,crowns&FPD’s. ADA sp-2: divided them further into: TYPE I - for casting inlays/crowns. ; mode of expansion: Thermal TYPE II - for casting inlays/onlays/crowns mode of expansion: Hygroscopic TYPE III - for partial dentures with gold alloys.

25. Phosphate bonded investments For alloys used to produce copings or frameworks for metal-ceramic prosthesis,press able ceramics. ADA sp-42 divided them further into: TYPE 1: for inlays,crowns and other fixed restorations. TYPE 2 : for partial dentures and other cast,removable restorations.

26. Ethyl silicate bonded investments Used principally in casting of RPD with base metal alloys . Brazing investment or soldering investment Used for brazing parts of a restoration such as clasps on RPD. Acc to ADA sp-93 TYPE I: gypsum bonded dental brazing investment TYPE II: Phosphate bonded brazing investments.

27. GYPSUM BONDED INVESTMENTS They are used for casting gold alloys . They can withstand temp up to 700 o C C l a ssi f i c a ti on :- A cc o rd i n g A . D.A s p ec i f i c a ti o n n o . 2 f o r c a s t i n g investments for dental gold alloys, there are 3 types . Type – I Are those employed for the casting of inlays or crowns when the allow casting shrinkage compensation is accomplished principally by thermal expansion of the investment. Type – II compensation for alloy shrinkage is by the hygroscopic expansion Type III Investments are used in the construction of partial denture with gold alloys

28. C omposition Refractory Material Crystalline polymorphs of silica (quartz or cristobalite) 65-75% Silica is added to provide a refractory component during the heating of the investment and to regulate the thermal expansion .

29. When these silica forms are heated change in crystalline form occurs at transition temperature…inversion from a low form known as alpha to a high form beta occurs. :transition temperature is - quartz: 575ºc - cristoballite : 200-270 ºc Density decreases as alpha form changes to beta form with resulting increase in volume that is exhibited by a rapid increase in linear expansion.

30. B i nder Alpha hemi hydrate form of gypsum( 25-45%) It is used as binder for investments used in casting gold containing alloys with melting ranges below 1000 ºc

31. Dimensional changes of three forms of gypsum when heated..

32. MODIFIER - (4 - 7 %) Used are : Reducing agents Modifying chemicals Coloring matter

33. Reducing agents : they reduce any metal oxides formed on the metal by providing a non oxidizing atmosphere in the mold when the alloy enters mold. E g – Copper

34. Modifying chemicals : They regulate setting expansion and thermal expansion and also prevent shrinkage of gypsum when heated above 300 º c . They act by reducing the two large contractions of gypsum binder on heating to temperatures above 300 º c . Ex– Boric acid Soluble salts of alkali or alkaline earth metals

35. Setting reaction- same as gypsum The crystallization of calcium sulfate dihydrate occurs while most of the remaining hemihydrate particles dissolve.

36. Setting time According to ANSI/ADA Specification No. 2 for dental inlay casting investment, the setting time should not be shorter than 5 min or longer than 25 min . Usually, the modern inlay investments set initially in 9 to 18 min.

37. SETTING EXPANSION Setting expansion are of three type: Normal setting expansion Hygroscopic setting expansion Thermal expansion Purpose : To enlarge the mold to compensate for the casting shrinkage of the gold alloy

38. 1).Normal setting expansion Linear dimensional change occurs as the investment sets . Silica particles result in greater setting expansion S ilica particles interfere with the intermeshing and interlocking of crystals ; resulting in outward thrust of crystals resulting in expansion .

39. According to ADA sp no 2: FOR TYPE I investments:0.6% Value of setting expansion for modern investments is 0.4%,regulated by retarders and accelerators. Purpose : To aid in enlarging the mold to compensate partially for the casting shrinkage of gold.

40. 2). HYGROSCOPIC SETTING EXPANSION- E xpansion that occurs as the investment hardens while immersed in water This is one of the methods for expanding the casting mold to compensate for casting shrinkage. The hygroscopic setting expansion may be 6 or more times greater than the normal setting expansion of a dental investment

41. The water is drawn between the refractory particles by the capillary action and thus causes the particles to separate creating an expansion The investment should be immersed in water before the initial set is complete. ADA sp no 2 : type 2 investments require minimum setting expansion in water of 1.2% and maximum 2.2%.

42. HYGROSCOPIC SETTING EXPANSION & NORMAL SETTING EXPANSION

43. FACTORS AFFECTING HYGROSCOPIC SETTING EXPANSION 1 . Effect of composition T h e f i n e r t h e p arti c le s i z e of t h e s ili ca , t he greater is the hygroscopic expansion. H i g h e r t h e s ili c a c o n t en t g rea t e r is t he expansion. 2. Effect of w/p ratio The higher the W/p ratio of the original investment water mixture, the less is the hygroscopic setting expansion.

44. . 3.Effect of temperature Higher the temperature of immersion water, less is the surface tension and hence greater is the expansion 4. Effect of time of immersion initial se t causes I m m e r s ion b e fo r e t he greater expansion. 5. Effect of spatulation T h e sh o r t e r t h e mi x i n g t im e , t h e l es s is t he hygroscopic expansion.

45. 6. Effect of shelf life of the investment The older the investment, the less is the hygroscopic expansion. 7. Effect of confinement Both the normal and the hygroscopic setting expansions are confined by opposing forces, such as the walls of the container in which the investment is poured or the walls of a wax pattern.

46. Landgren , N., and Peyton, F. A.: Hygroscopic Expansion of Some Casting Investments,J . D. Res. 29:469-481, 1950. Landgren and Peyton” concluded : Hygroscopic expansion decreases with increased time between mixing and immersion when immersion is made after initial set. Greater expansion is obtained, and the results are most uniform when the investment is immersed before initial set. Higher temperature of the bath gives increased expansion when the investment is immersed after initial set.

47. Skinner E W.,Hygroscopic expansion of dental investments, JADA ,1957 The effects of various variables on the magnitude of the hygroscopic expansion in a dental investment have been studied. 1. The hygroscopic expansion is very sensitive to confinement of any type , whether it be pressure or friction Consequently, it is impossible to establish an absolute magnitude. 2. The greater the amount of Hydrocal in the investment in combination with quartz, the less is the hygroscopic expansion. 3. The hygroscopic expansion is less when the investment is allowed to set in an asbestos lined ring in air at room temperature than when it is immersed in water at 37°C. in the same asbestos-lined ring.

48. 3).Thermal expansion The thermal expansion of the gypsum-bonded investment is directly related to the amount of silica present and the type of silica employed. When the silica is subjected to high temperature a phase transformation occurs in the silica, producing a slightly different atomic configuration, which is accompanied by an expansion.

49. The amount of thermal expansion depends on the allotropic form of silica used, for example, the thermal expansion of Cristobalite is greater than that of quartz at most temperatures.

50. Type 1 investments should have thermal expansion of not less than1% and not greater than 1.6%. Type II investments should have a thermal expansion ranging from 0-0.6% Maximum thermal expansion should be attained at temperature not higher than 700degree centigrade.

51. Factors affecting thermal expansion Effect of water / powder ratio The magnitude of thermal expansion is related to the amount of solids present. T herefore more the water used in mixing the investment the less the thermal expansion achieved during subsequent heating.

52. Effect of chemical modifiers As the weakening effect of silica is too great,therefore certain modifiers are added to eliminate the contraction caused by gypsum and increase the expansion without the presence of excessive amount of silica. Silicas do not prevent gypsum shrinkage but counterbalance it;whereas modifiers actually reduce gypsum shrinkage

53. Modifiers added are : Sodium, potassium, lithium chlorides, boric acid Although boric acid decreases contraction and hardens the investment, it disintegrates during heating of the investment and may result in roughened surface on the casting.

54. STRENGTH maximum stress required to fracture a structure. According to ADA sp no.2 : the compressive strength of gypsum bonded investment should not be less then 2.4Mpa. The strength of the investment is affected by the water/powder ratio . Ie ., more the water employed in mixing  the lower the compressive strength.

55. FINE N ESS Fineness affects the setting time,surface roughness of the casting. Although fine silica results in higher hygroscopic expansion than coarser silica , a fine particle is preferable because finer the investment smaller the surface irregularities .

56. POROSITY To a certain extend investment material should be porous. During casting process 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 to prevent gold alloy from completely filling the mold resulting in void formation. Thus the common method of venting the mold is through the pores of the investment .

57. Permeability of various gypsum- and phosphate-bonded investments was measured during conventional burn-out procedures. The porosity of the phosphate-bonded investment was approximately three-fourths that of the gypsum investments. These investments were modified by the addition of varying amounts of acrylic polymer for the purpose of altering permeability. The addition of acrylic polymer increased porosity and permeability of all of the materials included in this investigation. The acrylic additives, however, had no effect on the permeability of relatively Permeability and porosity of dental casting investments.Ballard , Leinfelder , and Taylor .J. Prosthet . Dent. Auqst , 1975

58. CLINICAL IMPLICATIONS Gypsum bonded investments cannot withstand temperatures greater than 700 ºc Heating above 700 ºc results in shrinkage with the liberation of: CaSO4+SiO2  CaSO4+4C  3CaSO4+CaS  C aS i O3 + S O 3 CaS+4CO 4CaO +4SO2

59. 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 700 º c

60. Hygroscopic thermal inlay casting investment Investment that can be used as hygroscopic or thermal type Available as Beauty cast- Whipmix Contains a blend of quartz and cristobalite as refractory For the Hygroscopic expansion technique, the investment is heated only upto 482degree celsius When the thermal casting technique is used the investment (is not immersed in water but) is heated to 649degree celsius to achieve expansion.

61. PHOSPHATE BONDED INVESTMENTS Rapid growth in use of metal ceramic & hot pressed ceramic prostheses has resulted in an increased use of phosphate bonded investments. Main use of this investment is for casting alloys that need to be heated to casting temperatures in excess of about 1200ºto 1300ºC.

62. APPLICATION TYPE I: for casting inlays,crowns and other restorations. ( according to ADA no.42) TYPE II :for removable partial dentures ( according to ADA no.42) 3. Make soldering fixtures. 4. Refractory dies for fabrication of custom veneers from dental porcelains .

63. COMPOSITION Phosphate bonded investments are available as two component system : 1) Powder 2). Liquid 1). Powder: consists of 2).Liquid: consists of colloidal silica. water - - - Refractory fillers Binders Carbon

64. R efractory fillers: Silica in the form of cristobalite,quartz-80 % Purpose: to provide high temperature thermal shock resistance and a high thermal expansion. Binder: consists of- Magnesium oxide:3-5% Mono ammonium phosphate:7-10% Originally phosphoric acid was used but now ammonium phosphate has replaced it as it can be incorporated into the investment powder.

65. Carb o n : Added to produce clean castings and facilitate the divesting of the casting from the mold. Used only when the casting alloy is gold . Not used with Ag-palladium alloys or base metal alloys as: carbon embrittles the alloys. Palladium reacts with carbon at temperatures above 1504ºC.

66. SETTING REACTION Chemical reaction that causes the investment to set is: NH 4 H 2 PO 4 + MgO + 5H 2 O NH 4 MgPO 4 6H 2 O Thus predominantly colloidal multimolecular magnesium ammonium phosphate aggregate around excess of MgO and fillers is formed. The final products are crystalline Mg 2 P 2 O 7 and some excess MgO Along with unchanged quartz,cristoballite or both.

67. Setting and thermal expansion Depends on: Type of liquid used Concentration of liquid used.

68. Type of liquid used Phosphate bonded investments show slight expansion rather then shrinkage because of the presence of colloidal silica as liquid which results in increased conc . o f silica leading to expansion.

69. Thermal expansion of a phosphate bonded investment mixed with water as compared with the special liqui d

70. C oncentration of liquid The influence of the liquid concentration on the setting and thermal expansion o phosphate bonded investments.

71. Properties 1).Compressive strength Type 1: 2.5 mpa Type 2: 3 mpa 2). Thermal expansion 0.8 % when 50:50 mixture of liquid and water

72. W orking & setting tim e depends on, - Temperature Warmer the mix faster it sets. The setting reaction liberate the heat and further accelerates rate of setting - Mixing time Increased mixing time and mixing efficiency result in faster set.

73. Advantages Posses s high fired strength. Provide high setting and thermal expansion. They can withstand the burnout process (1-1.5hrs)with high temp eratures

74. Disadvantages When used with alloys having casting temperature greater than 1375ºC result in mold breakdown & rougher surfaces on castings. Although high strength of these investments is an advantage during casting,but can make divesting a difficult and tedious task.

75. ETHYL SILICATE-BONDED INVESTMENT These are used for high fusing base metal alloy. C omposition 1).Refractory material – Silica 2 ). Binder –Silica gel or ethyl silicate Several methods may be used to produce the silica or salicylic acid gel binders. -

76. FORMATION OF SILICA GEL When the Ph of sodium silicate is lowered by the addition of an acid or acid salt, a bonding silicylic acid gel forms. An aqueous suspension of colloidal silica can also be converted to a gel by the addition of an accelerator such as AMMONIUM CHLORIDE.

77. - Formation of binder by ethyl silicate Another method of binder formation is based on ethyl silicate. A colloidal silicic acid is first formed by hydrolyzing ethyl silicate in the presence of hydrochloric acid, ethyl alcohol and water The solution is then mixed with quartz or cristoballite, to which is added small amount of MgO -----to form polysilicic acid gel .

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

79. 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

80. 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

81. 7 6 Advantages Final set mass is cristobalite. Hence it can withstand temperature even beyond 1150 o C High strength.

82. 7 7 Disadvantages Non porous material. Complicated manipulation. Cannot be used for titanium and its alloys as silica can oxidize titanium or its alloys easily

83. 7 8 Other investment materials Soldering investments Divestment materials Investment material for titanium alloys Silica free investment s

84. 7 9 Soldering investments/Brazing investment RPD /FPD frameworks are cast sometimes in separate parts and then soldered/brazed. Parts are assembled on master casts , joined with sticky wax and invested in special investment material on a tile

85. 8 Dental brazing investments ADA specification no 93 Two types; 1: Type 1: Gypsum bonded dental brazing investments 2: Type 2 : Phosphate bonded dental brazing investments

86. 8 1 This method used to avoid distortion of the appliance during soldering Investment material should not undergo setting or thermal expansion which may distort the appliances

87. Gypsum mixed with colloidal silica liquid The die is made from this mix and the wax pattern is then constructed on it. The whole complex is then invested in a mixture of Divestment & water thereby eliminating the possibility of distortion of the pattern on removal from the die or during the setting of the investment.

88. 8 3 Properties Setting expansion : 0.9 % Thermal expansion : 0.6 % at 650 o C Thermal expansion : 1.2% at 850 o C

89. Divestment materials Die stone & investment combination. These mixed with colloidal silica liquid The die is made from this mix and the wax pattern is then constructed on it. The whole complex is then invested in a mixture of Divestment & water This combination used to compensate distortion of wax pattern of long span bridges or RPD frameworks during removal from die. S pecial GBI or PBI investment materials are used.

90. 8 5 Properties Setting expansion : 0.9 % Thermal expansion : 0.6 % at 650 o C Thermal expansion : 1.2% at 850 o C

91. Investment Material For Ceramic/Silica free investment Casting of ceramic crowns using castable glasses is done in refractory moulds. Used with castable moldable ceramic are hot pressed into ceramic mould by pressure. where no compensation expansion is needed Special GBI of low thermal expansion are used, made from pure gypsum ,calcium sulfate h emihydrate , Same investment additives as GBI , prevent their contraction or deterioration on heating .

92. Investments For Titanium Alloys Molten titanium is highly reactive with oxygen and is capable of reducing some of the oxides commonly found in those investments. Titanium can also dissolve residual oxygen, nitrogen, and carbon from the investment; these elements can harden and embrittle titanium in the solid state. As a result, either modifications of existing refractory formulations and binders or new refractory formulations and binder systems are required. Dental Materials and Their Selection - 3rd Ed. (2002) by William J. O'Brien

93. Properties Reduce breakdown of the investment and the contamination of titanium : Reduction of refractory oxides of the investment. Refractory materials that are less easily reduced by titanium should be used Dental Materials and Their Selection - 3rd Ed. (2002) by William J. O'Brien

94. Modifications Of Investment Material Investment consisting of a phosphate binder, magnesia, and quartz was developed under the hypothesis that quartz would not be as reactive as silica. This investment was recommended for use as a room- temperature mold, to reduce reaction with titanium. However, contamination of castings by reaction with the investment was still encountered. Dental Materials and Their Selection - 3rd Ed. (2002) by William J. O'Brien

95. Modifications Of Investment Material A phosphate investment that contains both magnesia and alumina as refractories was developed. This investment can attain large expansion by the spinel reaction of alumina and magnesia (MgO + Al 2 O 3 ® MgO - Al 2 O 3 ) when it is burned out at 1,150°C to 1,200°C. Another approach to obtaining the needed expansion is through the use of Li 2 O - Al 2 O 3 - SiO 2 . It expands irreversibly upon heating through the temperature range of 900°C to 1100°C.

96. This paper presents a new phosphate-bonded ceramic mold material by modifying the composition of refractory aggregates and additive agents, which is promising to be used for rapid ceramic mold casting for medium-size super alloy components. The results indicate that the properties of ceramic mold slurry including initial setting time and fluidity are suitable for complete embedding medium-scale patterns and get accurate duplicate of external configurations when the amount of boric acid is 1.11wt% and sodium tri-poly phosphate (STP) is 0.92wt%. The strength of ceramic mold material can resist impact force when pouring molten metal and easy take casting out from ceramic mold . T. Z. Li et al., "A New Phosphate-Bonded Investment Material for Rapid Ceramic Molding of Medium-Size Castings", Advanced Materials Research, Vols. 79-82, pp. 1715-1718, 2009

97. Conclusion Thus ,the material of choice for investing should be one with the optimal properties as discussed, withstanding the varying degrees of high temperatures in the casting procedure

98. R efe r en ces Phillips science of dental materials Kenneth J.Annusavice -11 th edition Applied dental materials , John F.Mc.Cabe -8 TH edition • Dental materials and their selection , Willian J .O’Brien -2 nd edition Restorative dental materials , Robert .G.Craig -11 t t h EDITION

99. T . Z. Li et al., "A New Phosphate-Bonded Investment Material for Rapid Ceramic Molding of Medium-Size Castings", Advanced Materials Research, Vols. 79-82, pp. 1715-1718, 2009 Permeability and porosity of dental casting investments.Ballard , Leinfelder , and Taylor .J. Prosthet . Dent. Auqst , 1975 Landgren , N., and Peyton, F. A.: Hygroscopic Expansion of Some Casting Investments,J . D. Res. 29:469-481, 1950. Landgren , N., and Peyton, F. A.: Hygroscopic Expansion of Some Casting Investments,J . D. Res. 29:469-481, 1950. Taggart W H ,investment materials, Jou r. A .D .A . , July,1927