Casting procedures / education for a dentist


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Casting procedures / education for a dentist

  2. 2. CONTENTS Introduction The Sprue Casting ring liners Investing Casting Casting defects Conclusion References
  3. 3. Introduction Matthaeus gottfried purmann (1700) first mentioned wax models in connection with prosthetic work. Historical evidence suggest Philbrook first described lost wax method. Taggart is credited to introduction of this technique to profession in 1906.
  4. 4. Three steps after wax pattern are - Investing burn out Casting
  5. 5. Compensation for solidification shrinkage 1 .hygroscopic expansion of investments 2 .thermal expansion of investment 3 .setting expansion 4 .wax pattern expansion
  6. 6. Die preparation Die materials – type 1V , V dental stones , acrylic, polyester, epoxy resins & electroformed dies. Inelastic impression materials like compound amalgam can be condensed to form dies. Die stone investment combination - divestment - divestment phosphate .
  7. 7. The sprue Sprue is the channel or hole through which plastic or metal is poured or cast into a gate or reservoir & then into a mold. Wax, plastic or metal former used to form the channel or channels to allow molten metal to flow. Custom made Pre fabricated Selection of Sprue pin 1. Gauge selection –5 principles a. Select the gauge Sprue former with a diameter that is approximately same as the thickest area of wax pattern.
  8. 8. b. Sprue former should be attached to the pattern at the largest cross sectional areas. c. length of the Sprue . d.Type of the Sprue influences the burn out technique. e.Patterns may be sprued directly or indirectly.
  9. 9. 2 .Sprue former attachment 3 .Sprue former position 4 .Sprue former direction 5 .Sprue former length Reservoir
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  11. 11. Casting ring & liners Materials for rings & liners . Asbestos , aluminum silicate ceramic liner , cellulose liner. Placement – 3.25mm short of the ends . Effects . Ringless casting systems.
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  13. 13. Investing procedure Cleansing of the pattern Commercial wax pattern cleaner or dilute synthetic detergent.
  14. 14. Mixing of the investment Liquid is taken in a bowl and powder is added until all the powder has been wet. Hand mixing Vacuum mixing
  15. 15. Investment procedure Hand investing Vacuum investing
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  20. 20. Compensation for shrinkage Liners –no , Liquid ;powder ratio of investment. Hygroscopic expansion Time of immersion Temperature of water bath Burn out temperature Controlled water added technique
  21. 21. Casting procedures Casting –is act of forming an object in a mold. Wax elimination Gypsum bonded – 468degC –hygroscopic expansion 650 degC– thermal expansion Phosphate bonded 700 – 870 degC depending on type of alloy .
  22. 22. Hygroscopic low heat technique 37 deg water bath expands the wax pattern The warm water entering the investment mold from top adds some hygroscopic expansion . The thermal expansion at 500 deg provides the needed expansion .
  23. 23. Added expansion can be obtained by Increasing water bath temperature. Use 2 layers of liner Increasing the burn out temperatures to 600 to 650degC.
  24. 24. High heat thermal expansion technique Gypsum investments – mold is heated from room temp & slowly heated to 650 – 700 deg C in 60 minutes & held for 15 –30 minutes . Rate of heating rapid heating – cracking,flaking or spalling of mold walls, differential thermal expansion & radial cracks from the interior outwardly.
  25. 25. Temperature – at > 700 deg C CaSo4 + 4C--- CaS + 4Co 3CaSo4 +CaS-- 4CaO + 4So2 Time of casting – casting should be made immediately to prevent sulphur contamination& disintegration . Rapid burn out procedure Investments with crystobalite- mold is placed in a furnace at 350 degC.
  26. 26. Phosphate investments They expand by –expansion of wax pattern, the setting expansion, thermal expansion. The usual burn out temperature ranges from 750 – 900 degC. Because Higher temp ensures total elimination of wax residues The completion of physical & chemical changes . Prevention of premature solidification of high melting alloy .
  27. 27. Slow heating rate for 350 degC ,then hold at upper temp for 30 minutes. Some investments can be subjected to 2 stage heating more rapidly – placed directly in the furnace at the top temp for 30 min & cast . To save time – ring & ring liner are also eliminated – tapered plastic ring is used so set investment can be pushed out of the ring , when completely set placed directly into the hot furnace . Here expansion varies & can be adjusted by varying liquid concentration.
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  30. 30. Time allowable for casting High heat technique – 1 minute , as the mold contracts on cooling Low heat technique –casting should be done soon after removal from the oven.
  31. 31. Casting machines The alloy is melted in a separate crucible by a torch flame & cast into the mold by centrifugal force. The alloy is melted electrically by a resistance or induction furnace & cast into the mold by centrifugally by motor or spring action. The alloy is melted by first 2 ways ,but is cast by air pressure , a vacuum or both .
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  33. 33. Centrifugal casting machine The casting machine spring is first wound from 2 to 5 times.The metal is melted by torch flame in a glazed ceramic crusible attached to broken arm of the casting machine. The broken arm feature accelerate the initial rotational speed of the crusible and casting ring,increase in the linear speed of liquid casting alloy as it moves
  34. 34. Once metal reaches casting temperature and the heated casting ring is in position.The machine is released and the spring triggers rotation motion. As the metal fills the mould there is a hydrostatic pressure gradient along the length of the casting.This pressure form the tip of casting to the bottom surface is quite sharp and parabolic in form reaching zero at the button surface.
  35. 35. Pressure gradient at the movement before solidification reaches about 0.21 to 0.28 Mpa [30 to 40 psi] at the tip of the casting. Because of this pressure gradient there is also gradient in the heat transfer,such that greatest transfer is at the high pressure end of the gradient. Because this end is also is the sharp edge margin of crown .It is assured that solidification progresses from thin margin to the button surface.
  36. 36. Electrical resistance- Heated casting machine. Here there is automatic melting of metal in a graphite crusible with in a furnace rather than a torch flame. Is advantageous for alloys like metal ceramic restorations, which are alloyed with base metal in trace amounts that tend to oxidize on over heating and crusible in the furnace is located flush against the casting ring so metal button remains molten slightly longer.
  37. 37. Induction melting machine: Metal is melted by an induction field that develops with in a crusible surrounded by an water cooled metal tubing. When metal reaches casting temperature it is forced into the mould by the air pressure,vaccume,or both at the other end of the ring. Mostly used for metal base metal alloys.
  38. 38. Casting crusibles: Clay crusibles: Crown bridge alloys , such as high noble and noble types. Carbon crusibles: Crown bridge alloys , and higher fusing ,gold based metal ceramic alloys. Quartz crusibles:for high fusing alloys of any type.
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  43. 43. Cleaning the casting For noble metals The ring is quenched in water Advantages are Noble metal is left in a annealed condition when water contacts the hot investment a violent reaction ensures . Investment becomes soft & granular & casting is more easily cleaned.
  44. 44. Pickling 50 % HCL – gypsum bonded 50%H2So4 Ultra sonic devices
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  48. 48. Casting defects Distortions Surface roughness ,irregularities & discolorations Porosity Incomplete or missing casting
  49. 49. Distortions Wax pattern Investing Expansion
  50. 50. rface roughness & irregularities Air bubbles
  51. 51. Water films
  52. 52. Rapid heating rates
  53. 53. Under heating
  54. 54. Liquid ;powder ratio
  55. 55. Prolonged heating
  56. 56. Temperature of the alloy
  57. 57. Casting pressure
  58. 58. Foreign bodies
  59. 59. Impact of molten alloy
  60. 60. Pattern position
  61. 61. Other causes
  62. 62. Porosity Solidification defects A. localized shrinkage porosity B. micro porosity Trapped gases A. pin hole porosity B. gas inclusions C. subsurface porosity Residual air
  63. 63. localized shrinkage porosity By incomplete feeding of molten metal. Common at Sprue casting junction. Or any way between dendrites . Suck back porosity – by hot spot . Can be eliminated by flaring , reducing casting temp by 30degC.
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  66. 66. Micro porosity Caused from solidification shrinkage In fine grain alloy casting when solidification is too rapid for micro voids to segregate to liquid pool. Controlled by increasing the mold melt temperature.
  67. 67. Pin hole porosity By entrapment of gas Molten metals dissolve or occlude gases, on solidification the gases are expelled and pin hole porosity results . Oxygen –copper, silver, platinum Hydrogen – platinum .
  68. 68. Gas inclusions Are due to inclusion of gases but are larger then pin hole porosities . Caused by occluded gases, contaminated castings , poorly adjusted torch flame or use of mixing or oxidizing zones of flame. Premelting of the metals & proper positioning of torch flame reduces this porosity.
  69. 69. Subsurface porosity Exact reason not known ,may be caused by the simultaneous nucleation of solid grains & gas bubbles at the first moment that the metal freezes at the mold walls . Controlling the rate at which the molten metal enters the mold reduces this type of porosity.
  70. 70. Entrapped air porosity or back pressure porosity –on inner surface of the castings can produce large concave depressions . Due to inability of air in the mold to escape through the pores of investment or pressure gradient that displaces the air towards the end of the investment via molten sprue & button . Increased density of investments, vacuum investing, & low heat technique which decreases venting of the mold .
  71. 71. Incomplete casting Insufficient venting High viscosity of the fused metal Foreign bodies .
  72. 72. Review
  73. 73. Conclusion Casting procedures are highly technique sensitive steps which converts wax pattern to a final restoration . Accurate & smooth restorations can be obtained if operator pays special attention to each step in the technique.
  74. 74. References Phillips science of dental materials -Anusavice Text book of restorative materials - Robert .G. Craig
  75. 75.  Fundamentals Of Fixed Prosthodontics - Herbert T Shillingburg  Contemporary Fixed Prosthodontics - Stephen F . Rosenstiel  Johnston’s Modern Practice In Fixed Prosthodontics - Roland W Dykema
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  77. 77. Dental casting alloys Classification of ADA (1984) High noble >_40wt %Au & _> 60wt%noble metals Noble metal _>25wt% of noble metals . Predominantly base metal <25wt% of noble metals .
  78. 78. According to National institutes of standards and technology gold casting alloys are classified as type 1 to IV based on dental function &hardness. Type I (soft) Type II (medium) Type III (hard) Type IV (extra hard)
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  80. 80. Thank you For more details please visit