Casting procedures / education for a dentist

1,315 views
935 views

Published on



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

training center with best faculty and flexible training programs

for dental professionals who wish to advance in their dental

practice,Offers certified courses in Dental

implants,Orthodontics,Endodontics,Cosmetic Dentistry, Prosthetic

Dentistry, Periodontics and General Dentistry.

Published in: Education
0 Comments
9 Likes
Statistics
Notes
  • Be the first to comment

No Downloads
Views
Total views
1,315
On SlideShare
0
From Embeds
0
Number of Embeds
1
Actions
Shares
0
Downloads
1
Comments
0
Likes
9
Embeds 0
No embeds

No notes for slide

Casting procedures / education for a dentist

  1. 1. CASTING PROCEDURES www.indiandentalacademy.com
  2. 2. CONTENTS Introduction The Sprue Casting ring liners Investing Casting Casting defects Conclusion References www.indiandentalacademy.com
  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. www.indiandentalacademy.com
  4. 4. Three steps after wax pattern are - Investing burn out Casting www.indiandentalacademy.com
  5. 5. Compensation for solidification shrinkage 1 .hygroscopic expansion of investments 2 .thermal expansion of investment 3 .setting expansion 4 .wax pattern expansion www.indiandentalacademy.com
  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 . www.indiandentalacademy.com
  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. www.indiandentalacademy.com
  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. www.indiandentalacademy.com
  9. 9. 2 .Sprue former attachment 3 .Sprue former position 4 .Sprue former direction 5 .Sprue former length Reservoir www.indiandentalacademy.com
  10. 10. www.indiandentalacademy.com
  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. www.indiandentalacademy.com
  12. 12. www.indiandentalacademy.com
  13. 13. Investing procedure Cleansing of the pattern Commercial wax pattern cleaner or dilute synthetic detergent. www.indiandentalacademy.com
  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 www.indiandentalacademy.com
  15. 15. Investment procedure Hand investing Vacuum investing www.indiandentalacademy.com
  16. 16. www.indiandentalacademy.com
  17. 17. www.indiandentalacademy.com
  18. 18. www.indiandentalacademy.com
  19. 19. www.indiandentalacademy.com
  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 www.indiandentalacademy.com
  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 . www.indiandentalacademy.com
  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 . www.indiandentalacademy.com
  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. www.indiandentalacademy.com
  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. www.indiandentalacademy.com
  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. www.indiandentalacademy.com
  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 . www.indiandentalacademy.com
  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. www.indiandentalacademy.com
  28. 28. www.indiandentalacademy.com
  29. 29. www.indiandentalacademy.com
  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. www.indiandentalacademy.com
  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 . www.indiandentalacademy.com
  32. 32. www.indiandentalacademy.com
  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 www.indiandentalacademy.com
  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. www.indiandentalacademy.com
  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. www.indiandentalacademy.com
  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. www.indiandentalacademy.com
  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. www.indiandentalacademy.com
  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. www.indiandentalacademy.com
  39. 39. www.indiandentalacademy.com
  40. 40. www.indiandentalacademy.com
  41. 41. www.indiandentalacademy.com
  42. 42. www.indiandentalacademy.com
  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. www.indiandentalacademy.com
  44. 44. Pickling 50 % HCL – gypsum bonded 50%H2So4 Ultra sonic devices www.indiandentalacademy.com
  45. 45. www.indiandentalacademy.com
  46. 46. www.indiandentalacademy.com
  47. 47. CASTING DEFECTS www.indiandentalacademy.com
  48. 48. Casting defects Distortions Surface roughness ,irregularities & discolorations Porosity Incomplete or missing casting www.indiandentalacademy.com
  49. 49. Distortions Wax pattern Investing Expansion www.indiandentalacademy.com
  50. 50. rface roughness & irregularities Air bubbles www.indiandentalacademy.com
  51. 51. Water films www.indiandentalacademy.com
  52. 52. Rapid heating rates www.indiandentalacademy.com
  53. 53. Under heating www.indiandentalacademy.com
  54. 54. Liquid ;powder ratio www.indiandentalacademy.com
  55. 55. Prolonged heating www.indiandentalacademy.com
  56. 56. Temperature of the alloy www.indiandentalacademy.com
  57. 57. Casting pressure www.indiandentalacademy.com
  58. 58. Foreign bodies www.indiandentalacademy.com
  59. 59. Impact of molten alloy www.indiandentalacademy.com
  60. 60. Pattern position www.indiandentalacademy.com
  61. 61. Other causes www.indiandentalacademy.com
  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 www.indiandentalacademy.com
  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. www.indiandentalacademy.com
  64. 64. www.indiandentalacademy.com
  65. 65. www.indiandentalacademy.com
  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. www.indiandentalacademy.com
  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 . www.indiandentalacademy.com
  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. www.indiandentalacademy.com
  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. www.indiandentalacademy.com
  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 . www.indiandentalacademy.com
  71. 71. Incomplete casting Insufficient venting High viscosity of the fused metal Foreign bodies . www.indiandentalacademy.com
  72. 72. Review www.indiandentalacademy.com
  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. www.indiandentalacademy.com
  74. 74. References Phillips science of dental materials -Anusavice Text book of restorative materials - Robert .G. Craig www.indiandentalacademy.com
  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 www.indiandentalacademy.com
  76. 76. www.indiandentalacademy.com
  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 . www.indiandentalacademy.com
  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) www.indiandentalacademy.com
  79. 79. www.indiandentalacademy.com
  80. 80. Thank you For more details please visit www.indiandentalacademy.com www.indiandentalacademy.com

×