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4. Dental Waxes and Dental Investments
- 1. DENTAL WAXES & DENTAL INVESTMENTS DR.AADIL THIMWALA PG DEPT. OF CONS. & ENDO. G.R.I.D.S.
- 2. Outline Definition History Classification Composition Properties of wax Processing waxes Impression waxes Pattern waxes Inlay wax RPD Casting wax
- 3. Invesments Gypsumbondedinvestment Phosphate bonded investment Ethyl silica bonded investment Brazing investment
- 4. Definition One ofseveral esters of fatty acids with higher alcohols, usually monohydric alcohol, dental waxes are combinations of various types of waxes compounded to provide desired physical properties. (G.P.T. 9th Edn. 2017)
- 5. History One ofthe oldest natural substances used by man as diet source obtained from insects. Oldest was used is beeswax. It was used for softening skin, binding together reeds used for flutes, coating and preserving valuable objects, candle production, and making sculptures Greek & Roman Literature – Used for sealing ships, as binder matrix, for protective coating of art objects, tablets etc. (S.Mahalaxmi: Materials used in Dentistry)
- 6. 3000 BC-Beeswax used by Egyptians in mummification process. 1700s- Mattheus Gottfried Purmann used wax models for prosthetic work. 1935- First synthetic liquid paraffin was produced by Fischer- Tropsch procedure. (Phillips science of dental materials 12th ed.)
- 7. Classification Type of waxExamples Sources Plants wax Carnauba wax Carnauba palm (leaves) • Copernica cerifera Candelilla wax Mexican shrubs • Euphorbia cerifera • Euphorbia antisyphilitica Japan wax Berries of • Rhus species • Toxicodendron species Insect wax Beeswax Honey bees Shellac wax Lac insect • Kerria lacca Animal wax Lanolin Sebaceous glands of sheep Spermaceti Head cavities and blubber of the sperm whale According to their source
- 8. Type of waxExamples Sources Petroleum waxes Paraffin wax Petroleum manufacturing processMicrocrystalline waxes Mineral wax Ozokerite Lignite beds Ceresin Ozokerite, a naturally occurring mineral wax Montan wax Lignite and brown coal
- 9. Based upon theiruses classified into three types Craig R. G. (1985 )Restorative dental materials 7th ed. Pattern waxes Inlay wax Resin wax Casting wax Baseplate wax Impression wax Corrective wax Biteplate wax Soft impression wax Disclosing wax Processing wax Boxing wax Beading wax Sticky wax Carding and blockout wax White and utility waxes
- 10. Type I (Inlaycasting wax) Class 1 : Soft Class 2 : Hard Type II (Baseplate wax) Class 1 : Soft Class 2 : Hard Class 3 : Extra hard ADA/ISO Classification(ADA SP NO 12 /ISO 15854:2005)
- 11. Composition Wax isa very complex mixture of many different types of compounds. Defined as a substance that is solid at ambient temperature and when subjected to moderate temperatures, becomes a low viscosity liquid. Waxes used in dentistry may be composed of several ingredients, including natural waxes, synthetics waxes, natural and synthetics resins, and other additions such as oil, fats, gums, fatty acids, and coloring agents of various types Zhang: Rheological Characterization of Dental Waxes
- 12. Mineral waxes : Madeup of hydrocarbons ranging from 17 to over 44 carbon atoms CH3—(CH2)—CH3 15 to 42
- 13. Plant & Animalwaxes : contain considerable concentrations of esters also contain acid, alcohols, and resins Japan wax Not true waxes but are chiefly fats Improves tackiness and emulsifying ability of paraffin
- 14. Carnauba wax : Contains85% alkyl esters Derived from leaves of the carnauba palm
- 15. Paraffin Waxes : Composed mainly of a complex mixture of chiefly straight saturated hydrocarbon chains possessing 26 to 30 carbon atoms of the methane series. Refining waxes and have less than 0.5% oil, which can lower the melting temperature. They crystallize in the form of plate, needles, or malcrystals but generally are of plate type
- 16. During solidificationand cooling, there is a volumetric contraction that varies from 11% to 15%. It likely to flake when it is trimmed, and it does not present a smooth, glossy surface, which is desirable requisite for an inlay wax. Other waxes and natural resins are added as modifying agents
- 17. Bees Waxes : Primary insect wax used in dentistry, obtained from honeycombs. Complex mixture of esters consisting mainly of mircyl palmitate plus saturated and unsaturated hydrocarbons. It is a brittle material at room temperature but becomes plastic at body temperature
- 18. Carnauba waxes : Composed of straight-chain esters, alcohols, acids, and hydrocarbons. They are characterized by quite high hardness, toughness, brittleness, and high melting points from 65°C to 9 0°C.
- 19. Cocoa butter Containsglycerides of palmitic, stearic, oleic, lauric and lower fatty acids. Brittle at room temperature. Primarily used to protect the soft tissues against dehydration and also to protect the glass ionomer from moisture during setting
- 20. Microcrystalline waxes : Produced from a combination of heavy lube distillates and residual oils in the petroleum industry or from crude oil tank bottoms. Mixtures of solid, purified, mainly branched chain saturated microcrystalline hydrocarbons monocyclic and polycyclic compounds . They differ from paraffin waxes in that they have poorly defined, extremely smaller crystalline structure, darker color, and generally higher viscosity and melting range 65°C to 90°C.
- 21. Physical properties ofdental waxes I. General Physical Properties Melting Range Thermal expansion Dimensional stability Ductility II. Mechanical Properties Flow Residual stress
- 22. Melting range Waxeshave melting ranges rather than melting point. It varies depending upon their uses Impossible to obtain precise values for the top and bottom of the melting range because of the strong chemical similarities of components.
- 23. Thermal Expansion Waxesexpand when subjected to a rise in temperature and contract as the temperature is decreased. Dental waxes and their components have the largest coefficient of thermal expansion . Reiber and Hupfauf, S. tested three bite registration waxes (Kerr No.8- wax, Beauty Pink Hardwax and Aluwax) for their thermic dimensional behaviour under several heating and cooling conditions.
- 24. They shouldnot be heated more than necessary for achieving a sufficiently plastic quality for use It should be stored in ice water to be resistant enough to deformation during the model mounting. They Concluded..
- 25. Ductility Increases asthe temperature is increased Greatly influenced by the distribution of the melting temperature. Waxes with lower melting temperatures have a greater ductility at any temperature than those with higher melting temperatures have.
- 26. Residual stress Existsin the completed pattern, regardless of the method used to prepare a wax pattern. The warpage is due to the residual stresses which result from the heating of wax specimens formed under compression or tension. The longer the wax pattern is left before being invested, the greater the distortion that must be result. The effects are minimized by storing the shaped wax for the minimum possible period (eg. the wax pattern for an inlay) and at reduced temperature.
- 27. Flow Flow ofwaxes is clearly important, not only as part of the deliberate moulding process but also an undesirable aspect after the pattern. A direct relationship was found between the flow of the wax and the casting shrinkage Flow is dependent on: Temprature of the wax Force applied Length of time the force is applid.
- 28. Ito etal - investigated the relationship between flow charcteristics, bending strength, and softening temperature of paraffin and dental inlay waxes to casting shrinkage. They found that the casting shrinkage decreased as the flow of wax pattern increased. Ito, M., Yamagishi, T. , Oshida, Y. et al. (1996) Effect of selected physical properties of waxes on investments and casting shrinkage J Prosthet Dent. 75, 211-216
- 29. Processing wax Boxing &Beading wax Uses • Used to build up the vertical walls around the impressions, in the order to pour the stone and make a cast. • The procedure is known as boxing. Supplied as • Boxing wax as sheets, beading wax as strips.
- 30. Utility wax Consistsmainly of beeswax, petrolatum, and other soft waxes in varying proportions. Available in the form of sticks and sheets. Used to adjust the contour of perforated tray and for use with hydrocolloids.
- 31. Sticky wax consistsmainly of yellow beeswax, rosin, and natural resins such as gum dammar Properties: It is brittle at room temperature and assumes a thick liquid consistency when heated. It does not have more than 0.2% residue burnout and not more than 0.5% from 43°C to 28°C.
- 32. Uses It holdsbroken pieces of a denture together and assembles components of fixed partial dentures and wrought partial dentures in preparation for soldering
- 33. Impression wax Corrective Wax It is formulated from hydrocarbon waxes such as paraffin, ceresin, and beeswax and may contain metal particles. Properties and uses It has 100% flow at 37 C. It is indicated only for edentulous impressions, since it distorts on removal from the undercut areas. It is used as a wax veneer over an original impression to contact and register the detail of the soft tissue.
- 34. Bite registration wax Formulated from beeswax or hydrocarbon waxes such as paraffin or ceresin and may contain alumina or copper particle. Supplied as U-shaped rods or wafers
- 35. Steps inbite registration 1. The wax is softened in warm water. The soft wax is then placed between the teeth and the patient is asked to bite. 2. It is then taken out and placed in chilled water. 3. The casts of the patient is placed in the indentations formed by the teeth. 4. It is then mounted with plaster on the articulator.
- 37. Pattern wax Baseplate waxor Modeling wax ADA SPECIFICATION NO. 24 Used in the construction of dentures and other appliances made of acrylic and like materials.
- 38. Composition composed mainlyof paraffin, ceresin, beeswax, carnauba wax, and coloring matter, which are mixed together, cast into blocks, and rolled into sheets. sheets are red or pink, 8.5-cm wide, 15-cm long, and 1.5-mm thick
- 39. Ideal requirements 1. Itmust be capable of holding porcelain or acrylic teeth in position both at normal room temperature and at mouth temperature. 2. Softened sheets must cohere readily. 3. It should not be flaky or adhering to fingers. 4. It should not cause irritation of oral tissues. 5. It must be easy to trim with a sharp instrument at 23°C. 6. It must have a smooth surface after gentle flaming. 7. It must leave no residue on porcelain or plastic teeth during processing. 8. The coloring should not separate or impregnate plaster during processing.
- 40. Types depending onthe melting range • Type 1: Soft wax used for building contour and veneers. • Type 2: Medium wax used for fabrication of patterns in mouth. This type of wax is preferred in temperate conditions. • Type 3: Hard wax used for fabrication of pattern in the mouth.
- 41. Uses 1. Making occlusalrims and registering the vertical dimension 2. Holding artificial teeth to base plates 3. Extension of the impression tray 4. Patterns for orthodontic appliances 5. Spacer in custom tray for complete denture 6. Checking the occlusal clearance after crown preparation
- 42. INLAY WAX ADASPECIFICATION NO. 4; ISO 15854 Inlay wax is a specialized dental wax that can be applied onto the prepared cavity or to the dies to form direct or indirect patterns Can be classified into two groups. type I inlay waxes(Medium wax): used in the mouth for direct waxing processes for pattern production type II inlay waxes(Soft wax): for making patterns on patients’ models using the indirect wax technique
- 43. Ideal requirements ofinlay wax 1. It must be able to closely adapt to the prepared portions of the tooth . 2. It should have low CTE. 3. It must have good cohesion, but should not adhere to the cavity walls. 4. It must permit detailed carving without flaking or chipping. 5. It must not leave any residue when the wax is eliminated from the mold.
- 44. 6. It shouldharden at oral temperature and remain plastic at temperatures slightly above oral temperature without injury to pulp or oral tissues. 7. At oral temperature, type I inlay wax must have essentially limited flow 8. Its color should contrast with the colors of teeth and oral tissues to facilitate carving.
- 45. 9. It mustalso be able to disintegrate, volatilize, and be eliminated completely from an investment mold during the burnout or wax elimination procedure. 10. If applied as a veneer in a sufficient thickness, its opacity must be sufficient to mask colored die stones. 11. The flow should be more than 70% at 45°C and less than 1% at 37°C.
- 46. Composition of inlaywax Composed of Paraffin wax, Gum dammar, Carnauba or candellila Ceresin. Coloring agent
- 47. Role of individualcomponent Paraffin wax (40-60%) Main ingredient Used to establish the melting point likely to flake when it is trimmed, and it does not produce a smooth, glossy surface, which is a desirable requisite for an inlay wax. Ceresin (10%) Partially replaces paraffin Increases toughness Easy to carve
- 48. Gum dammar (1%) It is a natural resin Derivative from pine tree improves the smoothness in moulding and makes it more resistant to cracking and flaking It also increases toughness of the wax and enhances the luster of the surface.
- 49. Carnauba wax (25%) Decreases the flow at mouth temperature It has an agreeable odor and gives glossiness to the wax surface Candellila wax Can be added to replace carnauba wax melting is lower and is not as hard as carnauba wax
- 50. Synthetic waxes Carnaubawax is often replaced partly by certain synthetic waxes (Montan). Their high melting point can imprpve general working qualities.
- 51. Properties of inlaywax Thermal properties Inlay waxes are softened with heat, forced into the prepared tooth cavity in either the tooth or the die, and cooled. The thermal conductivity of the waxes is low (e.g., kparaffin = 0.25 W/mK) High coefficient of thermal expansion. CTE for inlay wax is 350*10 per degree centigrade. Philips science of dental maerials: 12th ed.
- 52. Expand asmuch as 0.7% with an increase in temperature of 20 °C. Contract as much as 0.35% when it is cooled from 37 °C to 25 °C. Philips science of dental maerials: 12th ed.
- 53. glass transition temperature Abrupt change in the rate of expansion at certain point of temperature occurs. In the curve the change is at 35 degree. It is known is glass transition temp. Constituents of the wax probably change in their crystalline form. The wax is more plastic at higher temperatures Philips science of dental maerials: 12th ed.
- 54. Physical properties Contact angle It has low contact angle. Wet the surface of the material to which it is being added Flow It should exhibit a marked plasticity or flow at a temperature slightly above that of the mouth. The flow is measured by subjecting cylindrical specimens to a designated load at the stated temperature and measuring the percentage of reduction in length
- 55. Requirements for InlayWax Flow (%) Type of wax T = 30 °C Maximum T = 37 °C Maximum T = 40 °C Minimum T = 40 °C Maximum T = 45 °C Minimum T = 45 °C Maximum I - 1 - 20 70 90 II 1 - 50 - 70 90 ADA specification no. 4
- 56. Ductility Have moderateductility. Wax distortion The most serious problem one can experience in forming and the pattern from a tooth or die.
- 57. Causes of distortion Occluded air in the pattern Physical deformation (during molding, carving, or removal), release of stresses “trapped” during previous cooling. Excessive storage time. Extreme temperature changes during storage. If not held under uniform pressure during cooling.
- 58. Elastic memory Waxestend to return partially to their original shape after manipulation. To demonstrate this effect, a stick of inlay wax can be softened over a Bunsen burner, bent into a horseshoe shape, and chilled in this position. If it is then floated in room- temperature water for a number of hours, the horseshoe will open. Philips science of dental maerials: 11th ed.
- 59. To Avoid distortion Minimal carving and change in temperature. Minimal storage of pattern, invest immediately. Store it in a refrigerator if necessary
- 60. Manipulation of Inlaywax Direct technique Hold the stick of wax over the flame and rotate it rapidly until it becomes plastic. The soften wax shaped approximately to the prepared cavity.
- 61. Insert thewax into the cavity and held under finger pressure until it solidifies. The wax should be allowed to cool gradually to mouth tempreture. Cold carving instrument should be used. Withdraw the patern carefully in the long axis of the tooth The pattern shoud be touched as little as possible with hands to avoid temperature.
- 62. Indirect technique Patternis prepared on the lubricant die. i) Dipping method: The die can be dipped repeatedly in the hot liquid wax ii) Addition : The melted wax may be added layers with a spatula or a waxing
- 63. ACCORDING TO MARZOUK The 4 basic methods to produce wax replica of cast restorations are:- 1) To create a wax mass, then carve it to the restoration shape, using hot sharp instruments. 2) To incrementally built up the restoration, using wax cones, triangles, drops, etc. 3) Fabricate wax pattern directly intra orally 4) Depends on Pre-operative anatomic core. Marzuck’s modern operative dentistry
- 64. WAXING INSTRUMENTS CARVING WAX ADDING BURNISHING PKT instruments ( designed by Dr. Peter K. Thomas) are specifically used. PKT No 1 & 2 – wax addition instruments PKT No 3 - burnisher for refining occlusal anatomy PKT No 4 & 5 – wax carvers
- 65. PKT NO 1 PKTNO 2 PKT NO 4 PKT NO 3 PKT NO 5
- 66. RPD Casting wax They are available in the form of sheets. An ideal casting wax copies accurately the surface against which it is pressed. It should not be brittle on cooling. Uses To make pattern of metallic framework of RPD.
- 67. Properties They aretacky and ductile. Maximum flow at 35°C is 10% and the minimum flow at 38°C is 60%. It vaporizes at 500°C, leaving no residue
- 68.
- 69. Effect on generalhealth Inhalation of paraffin wax vapours may cause respiratory irritation. Molten wax causes burns and possible blindness in contact with eyes. Molten wax causes burns on contact with skin On swallowing the material is generally not toxic. Material Safety Data Sheet in Accordance with 91/155/EEC & ANSI 400.1.2003
- 70. EXPOSURE CONTROLS /PERSONAL PROTECTION Long term exposure limit (8hr ) – 2mg/m3 Short term exposure limit (10 min) – 6mg/m3 Respiratory protection: Adequate ventilation when heating wax above 150C Hand protection: Heavy duty butyl rubber gloves to be worn when handling molten wax. Eye protection: Wear safety goggles when handling molten wax. Skin protection: Wear protective clothing.
- 71. First-aid measures Afterinhalation: Remove individual to a well-ventilated area for fresh air, and call a physician if symptoms persist. After skin contact: Run exposed area under water for 15 minutes, then contact a physician. Do not attempt to remove material bonded to skin. After eye contact: Flush eyes with copious amounts of water for 15 minutes, administer first aid, and call a physician. After swallowing: Material is not generally toxic by ingestion.
- 72. DENTAL INVESTMENTS Definitions Investing: process of covering or enveloping, wholly or in part, an object such as a denture, tooth, wax form, crown, etc., with a suitable investment material before processing, soldering, or casting Investment : An investment can be described as a ceramic material that that is suitable for forming a mold into which a metal or alloy is cast GPT 9th ed. 2017
- 73. Introduction Investment materialis poured around the wax pattern whilst still in a fluid state. When the investment sets hard, the wax and sprue former are removed by softening or burning out to leave a mould. Needs to be constructed from a material which retains its integrity at the casting temperature Unmodified dental plasters or stones are not suitable McCabe- Applied dental materials
- 74. Gypsum, phosphate,and silicate are used for investing as they can withstand the higher temperatures of burnout and casting procedures. Investment material is available as a two component system 1. Powder 2. Liquid (usually water)
- 75. Requirements of andinvesting material 1. It should be capable of reproducing the shape, size, and details recorded in the wax pattern. 2. It should be easily manipulated and must readily wet the surface of the wax pattern. 3. It should have adequate setting time to allow the investing procedure 4. At the same time it should harden within a relatively short period of time.
- 76. 5. It shouldbe able to withstand the high temperatures 6. It should have chemical stability at room temperature and at high temperatures during casting. 7. It should have suffcient compressive strength at the higher casting temperatures so that it can withstand the stresses created by molten metal as it enters the mold 8. It should have sufficient porosity to allow the gases to escape.
- 77. 8. It shouldhave a controlled expansion during setting 9. It should be easy to recover once the casting is done. 10. Smooth surface finish of the inner surface of the mold to have proper finish of the restoration.
- 78. Components Composed ofthree main materials 1. Refractory material, 2. Binder material, 3. Chemical modifiers Materials used in dentistry: S. Mahalaxmi
- 79. 1. Refractory material Usually it is a form of silica such as quartz, tridymite, or cristobalite or a combination of these Capable of sustaining exposure to a high temperature without significant degradation. 2. Binder material Material capable of binding the refractory material to form a coherent solid mass Common binders used are a-hemihydrates, ethyl silicates, or phosphates
- 80. 3. Modifiers Addedto the refractory and binder materials to enhance their physical properties. Includes nonoxidizing agents, retarders, accelerators, and coloring agents. Chemical modifiers like sodium chloride, boric acid, potassium sulfate, graphite, magnesium oxide, etc.
- 81. Classification 1. Gypsum-bonded investmentmaterials: conventional casting of inlays, onlays, crowns, and fixed partial dentures made of gold alloys. 2. Phosphate-bonded investment materials: designed primarily for alloys that have to be cast at higher temperatures than gold-based alloys 3. Ethyl silicate-bonded investment materials: used in casting of removable partial dentures with base metal alloys Based on the binders used.
- 82. Type Iinvestments: Expansion is mainly accomplished by thermal expansion of the investment Type II investments: Expansion is mainly through hygroscopic expansion of the investment Type III investments: Used for construction of partial dentures with gold alloys. Based upon Method of expansion Materials used in dentistry: S. Mahalaxmi
- 83. Setting expansion Itis essential to understand the concept of expansion of the investment materials. During casting, the molten metal is forced into the investment mold to occupy the space occupied by wax pattern before As the molten metal cools to room temperature, it shrinks dimensionally. This property of metal is called casting shrinkage or solidifi cation shrinkage.
- 84. Solidification shrinkageis compensated by expansion of the investment material. Wax shrinkage + casting shrinkage = Setting expansion + hygroscopic expansion + thermal expansion
- 85. Normal setting expansion The linear expansion that occurs during the normal setting of the investment material when it is surrounded by air. There is an initial contraction during the setting reaction as the crystals grow they impinge on each other resulting in a net expansion during setting. Philips dental materials 12th edi.
- 86. Hygroscopic expansion Expansionthat occurs when the investment is allowed to set in the presence of water is called hygroscopic expansion. It takes place by either of the following ways: 1. Placing a wet liner inside the casting ring 2. Placing the invested casting ring in a water bath 3. Adding water to the exposed areas of investment during hardening.
- 87. Thermal expansion Theinvested casting ring is placed in an oven to burnout the wax pattern to create a mold space. For gypsum-bonded investments, the temperature is gradually elevated till 700°C for no longer than 20–30 minutes. Some amount of expansion also occurs during this process The refractory material in the investment undergoes many changes leading to expansion.
- 88. Gypsum bonded invesment ADASPECIFICATION NO. 126 (OLD: 2) In 1929, Coleman and Weinstein invented a gypsum-bonded cristobalite investment Used for conventional casting gold alloy inlays, onlays, crowns, and fixed partial dentures. When this material is heated at temperatures sufficiently high to completely dehydrate and to ensure complete castings, it shrinks considerably and occasionally fractures. All the three common forms of gypsum products has different rates of thermal expansion. Introduction of dental materials: Rechard van Noort
- 89. A slightexpansion takes place between 400 °C and approximately 700 °C, and a large contraction then occurs. Above 700 °C the decomposition takes place and contaminates the casting. Not to be heated above 700 °C. Philips science of dental materials 12th edi.
- 90. Composition Refractory material Silica: If the proper forms of silica are employed in the investment, this contraction during heating can be eliminated and changed to an expansion. It exists in at least four allotropic forms: quartz, tridymite, cristobalite, and fused quartz. Quartz or cristobalite or a blend of the two in varying proportions— 65%–75% used particularly in gypsum bonded investments
- 91. When quartzis heated, it inverts reversibly from a “low” room- temperature crystal form, known as α quartz, to a “high” form, called β quartz, at a temperature of 573 °C. This α to β phase transformation is called an inversion, and it is accompanied by a linear expansion of 0.45%. In a similar manner, cristobalite undergoes an analogous transition between 200 °C and 270 °C from “low” (α cristobalite) to “high” (β cristobalite) Introduction of dental materials: Rechard van Noort
- 92. Binder Calcium a-hemihydrate is used as the binder On mixing with water it forms calcium sulfate dihydrate which binds the refractory material together. The temperature to which it is subjected should be not more than 700°C.
- 93. Reasons Gypsum isnot chemically stable at temperature exceeding 650°C. When heated to temperatures above 700°C, the calcium sulfate in the binder decomposes into sulfur dioxide and sulfur trioxides. Contamination causes shrinkage and contamination of the metal surface which tend to embrittle the casting metal.
- 94. Modifiers Reducing agentssuch as carbon or colloidal copper are used to produce nonoxidizing atmosphere within the mold when the metal is cast. Trace amounts of boric acid and sodium chloride are added to regulate setting time Coloring agents are added to distinguish between various investment materials. Oxalates added by deteriorating with heat forming carbon dioxide and carbon monoxide, thus preventing sulfur contamination of alloys
- 95. Setting time Accordingto ADA specification no. 2, investments should have setting time of not less than 5 minutes or not more than 25 minutes. Initial setting time of the modern gypsum-bonded investment is 9– 18 minutes.
- 96. Expansion The totalexpansion is the sum of the setting expansion + hygroscopic expansion + thermal expansion. Designed to compensate for the average contraction of the metal alloy being cast. The actual amounts of expansion involved can be balanced differently by different manufacturers
- 97. Normal setting expansion As water is mixed with investment powder, it surrounds the calcium sulfate crystals gradually converting them into calcium a -dihydrate. The effectiveness of setting expansion depends on the thermal expansion of the wax pattern The wax pattern expands due to the effect of exothermic heat that is released from the investment while setting
- 98. The amountof expansion of the wax is influenced by the softness and the thickness of the wax. The softer the wax and the thinner the wax pattern, the more is the expansion since the investment can move softer and thinner wax more easily. Amount of heat released depends on the amount of gypsum present. The setting expansion allowed for type I gypsum-bonded investment is 0.6%.
- 99. Hygroscopic expansion Thewater has to be incorporated before setting of the investment takes place. The water then replaces the water of hydration, thus preventing confinement of crystal growth. The crystals continue to grow outward in the presence of water, resulting in more expansion. Hygroscopic expansion is almost five to six times that of setting expansion. According to ANSI/ADA specification no. 2, the minimum and maximum setting expansion for type II investment is 1.2% and 2.2%
- 100. Thermal expansion Theamount of thermal expansion depends on the nature and amount of refractory material. Also on the type of gypsum binder, water/powder ratio, and temperature to which the mold is heated.
- 101. Factors influencing thesetting expansion 1. Particle size of silica : Acts as a diluent by delaying the interlocking of crystal and thus allowing them to grow further and expand. 2. Water/powder ratio: The greater the water content the lesser will be the binder crystals per unit volume which in turn decreases the expansion. 3. Silica/binder ratio: In order to increase the expansion, silica content can be increased but at the cost of decreasing the compressive strength of the material. The recommended ratio 65%–75% of silica, 25%–35% calcium sulfate hemihydrate, and 3% chemical modifiers.
- 102. 4. Spatulation time: More the spatulation time, more delayed will be the interlocking of crystals, thus increasing the expansion 5. Age of the investment: The older the investment the lower will be the hygroscopic expansion. 6. Confinement: The investment expansion is confined by the investment ring and wax pattern. 7. Flow of wax: The greater the flow of the wax, the greater will be the expansion
- 103. The expansion ofthe investment away from the wax pattern can be enhanced by the following: a. Placing a wet liner on the inner surface of the casting ring b. Placing the pattern toward the outer end of the ring c. Type of casting ring—rubber casting ring d. Type of technique—ringless technique
- 104. Thermal stability Thegypsum-bonded investment decomposes above 700°C as a result of the reaction of calcium sulfate with silica. CaSO4 + SiO2 → CaSiO3 + SO3 The sulfur trioxide causes porosity and weakening of the investment material. So the gypsum bonded investments should be restricted to gold alloys and lower melting base metal alloys which are cast below 700°C.
- 105. The carbonpresent in the residue, left after burnout procedure, reacts with calcium sulphate to form calcium oxide and sulfur dioxide. This reaction occurs above 700°C This causes shrinkage and contamination of the casting with the sulfides of the non-noble alloys. Breakdown of gypsum above 700°C CaSO4 + 4C → CaS + 4CO 3CaSO4 + CaS → 4CaO + 4SO2
- 106. Factors influencing thermalexpansion 1. Type of refractory material: Cristobalite exhibits the maximum expansion of all polymorphs of silica. Quartz exhibits 1.4% expansion at 600°C, while cristobalite 1.6% at 250°C, and tridymite exhibits less than 1% at 600°C. 2. Temperature: Higher the temperature, higher will be the expansion. Rapid heating, prolonged heating, and overheating during wax burnout may fracture the investment and cause alloy liquid to enter into the cracks and solidify
- 107. 3. Percentage anduniformity of silica: The less uniform particle size results in greater expansion, as the packing ability of silica is better expansion will be less. 4. Water/powder ratio: The greater the ratio, lesser the silica content, leading to decrease in expansion. 5. Thickness of the mold wall: The thicker the walls of the mold, the more will be the expansion.
- 108. 6. Confinement: Thecasting ring is the outer confinement for the investment. The coefficient of thermal expansion of metal casting ring should coincide with the investment material for effective expansion to occur. The permissible minimum and maximum thermal expansion is 0– 0.5% at 500°C and 1%–1.6% at 700°C temperatures,
- 109. PHOPHATE BONDED INVESTMENTS ADASPECIFICATION NO. 126 (OLD: 42) Most commonly used investment in casting of palladium- based alloys and base metal alloys Metals which have high melting point are mainly cast by this investment material since it is more stable at high temperatures. This makes it most commonly used investment materials
- 110. Reasons include: Tremendousincrease in the use of metalceramic and all ceramic restorations which require higher melting alloys. Use of more economical base metal alloys Commercially pure titanium and titanium alloys which require specially formulated investments to minimize the interaction of the molten metal with the investment. Most base metals, palladium-based metal alloys, and titanium- based alloys are cast at temperatures of 850°C–1504°C
- 111. According to ANSI/ADAspecification no. 42 phosphate- bonded investments are classified into two types based on its application: • Type 1: For inlays, crowns, and other fixed restorations • Type 2: For partial dentures and other removable restorations
- 112. Composition Refractory material Silicais the principal refractory filler in the form of cristobalite or quartz or mixture of two components. Used in the concentration of 80% with a particle size ranging from submicron level to size of fi ne sand. The filler provides refractoriness (high temperature thermal shock resistance)and a high thermal expansion.
- 113. Binder Phosphate isthe binder present in this type of investment. It present as two components 1. water soluble phosphate ion (acid) 2. the magnesium oxide Initially phosphoric acid was used, which was later replaced by monoammonium phosphate. binding system undergoes an acid–base reaction between the acidic monoammonium phosphate and the basic magnesium oxide.
- 114. Modifiers same modifiersas those of gypsum-bonded investment Carbon can also be added to produce clean casting and enable divesting the casting. Carbon is not used for base metal alloys as the carbon residues affect the final alloy composition and embrittle the base metal alloys.
- 115. Liquid liquid componentconsists of either water or colloidal silica (silica sol) in water Colloidal silica has two purposes 1. hygroscopic expansion 2. strength of the investment For base metal alloys, 33% dilution of colloidal silica is necessary
- 116. Chemistry of settting The setting reaction is an exothermic type of reaction Ammonium phosphate and magnesium oxide react with water to form hydrated magnesium ammonium phosphate and water NH4H2PO4 + MgO + 5H2O Room temperature NH4MgPO4 .6H2O + MgO + NH4H2PO4 + H2O The set matrix consists of the multimolecular NH4MgPO. 6H2O aggregate around excess magnesium oxide and fi llers.
- 117. Thermal stability Athigher temperatures, some of the remaining phosphate reacts with silica forming complete silicophosphates This results in a significant increase in the strength of the materials. Thus this investment suits almost all types of alloys, namely, base metal alloys and palladium–silver alloys to be cast at higher temperature.
- 118. Setting time Settingtime and working time for phosphatebonded investments is relatively lesser than gypsum material. Temperature affects setting time adversely warmer the mix, the faster is the setting. The longer spatulation time accelerates the reaction. The addition of water to the silica increases the working time
- 119. Expansion In thecase of phosphate-bonded investments, the initial shrinkage occurs due to decomposition of magnesium ammonium phosphate, resulting in the liberation of ammonia But in practice, due to the outward growth of the crystals and inversion of silica, there is a slight net expansion. This can be greatly increased by the use of colloidal silica partly replacing water in the liquid.
- 120. The expansionof the phosphate-bonded investments mainly depends on the thermal expansion of the material At higher temperatures, melting alloys have higher contraction during solidification, which necessitates a greater expansion of the investment. The replacement of liquid component from water to colloidal silica increases the hygroscopic expansion.
- 121. The expansionof the investment differs according to the ratio of colloidal silica to water. As the ratio of the special liquid to water increases, the expansion also increases. when the ratio of colloidal silica to water is 1. When the ratio 1:3, the expansion of phosphate-bonded investments at 700°C–900°C is 1.3%–1.5%. 2. When the ratio is 1:1, the expansion is 1.5%– 1.7% 3. When th ratio of 3:1 , th expansion is 1.7%–1.9%.
- 122. Phosphate-bonded investmentsoffer greater flexibility in controlling the amount of expansion as compared to gypsum-bonded investments. Increased expansion results from 1. Different particle sizes of silica produce higher expansion 2. A slight modification in the liquid/powder ratio 3. The setting reaction heat softens the wax allows free setting expansion. 4. Strength of the investment material at high temperatures reduces shrinkage of the alloy as it cools. 5. Use of colloidal silica reduces surface roughness and increases expansion
- 123. Ethyl Silicate bondedinvestment ADA SPECIFICATION NO. 126 Also known as silicate-bonded investment or silica-bonded investment. Rarely used in dentistry due to the time-consuming and complicated procedures of usage Nowadays, mainly used for casting high-fusing base metal partial denture alloys This investment is available in powder and liquid form.
- 124. Composition Refractory material Powderedquartz and cristobalite act as refractory materials. A small amount of magnesium oxide is added to the powder to reduce the pH of silica gel during manipulation. Binder Silica gel in the form of sodium silicate and colloidal silica is the commonly used binder that converts to silica at high temperatures.
- 125. Silica gelcan be produced by 1. By adding an acid or acid salt to sodium silicate, a bonding silicic acid gel is formed. 2. By mixing ethyl silicate with diluted hydrochloric acid to form silicic acid solution. Ethyl silicate is commonly used It is the liquid component of this investment. It is supplied as two bottles of special liquid with which the investment powder is mixed.
- 126. Hydrolysis takesplace between water present in the component and ethyl silicate in the presence of hydrochloric acid and ethyl alcohol. The formed product is called silicic acid solution. (C2H5O)4Si + 4H2O → Si(OH)4 + 4C2H5OH
- 127. Chemistry of setting On mixing silicic acid sol with silica refractory in an alkaline condition, a polysilicic acid or coherent gel of silica is formed. The soft silica gel is dried at a temperature below 168°C. During the drying, it loses alcohol and water to form a concentrated, hard, solid, nonporous mass of cristobalite This results in volumetric shrinkage called green shrinkage
- 128. Expansion Ethyl silicate-bondedinvestment does not show setting expansion. It undergoes only shrinkage during setting. When these investments are heated to suffciently high temperatures, considerable expansion takes place. This investment can be heated between 1090°C and 1180°C and this is compatible with high-fusing alloys. The total linear expansion is equal to the total linear thermal expansion. Total expansion of 1.6% is attained at a temperature of about 600°C.
- 129. The investmentprocedure for ethyl silicatebonded investment is little complicated and time consuming Ethyl alcohol vapor liberated during hydrolysis reaction can ignite at elevated temperatures
- 131. Brazing investment ADASPECIFICATION NO. 93 The process of assembling the parts of a restoration by soldering. Clasps on a removable partial denture, it is necessary to surround the parts with a suitable ceramic or investment The parts are temporarily held together with sticky wax until they are surrounded with the appropriate investment material Also called as soldering investment,
- 132. Composition The investmentfor soldering purpose is similar to casting investments containing quartz and a calcium sulfate hemihydrate binder.
- 133. Properties Soldering investmentsare designed to have lower setting and thermal expansions than casting investments Soldering investments do not have as fine a particle size as the casting investment, since the smoothness of the mass is less important Relatively little information is available in the dental literature on the properties of soldering investments.
- 134. References 1. Phillips scienceof dental materials, 12th ed 2. McCabe- Applied dental materials 3. Materials used in dentistry, S. Mahalaxmi. 4. Basic dental materials, Manapallil 5. Craig : Dental Materials : Properties & Manipulation 6. Introduction of dental materials: Rechard van Noort 7. S. Winkler : Essentials of Complete Denture Prosthodontics IInd Edn 8. Marzuck, operative dentistry, modern theory & practice. 9. Zhang: Rheological Characterization of Dental Waxes 10. Ito, M., Yamagishi, T. , Oshida, Y. et al. (1996) Effect of selected physical properties of waxes on investments and casting shrinkage J Prosthet Dent. 75, 211-216