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Casting defects
- 1. CASTING DEFECTS BY AISHA HABEEB SVS INSTITUTE OF DENTAL SCIENCES
- 2. Introduction Classification of defect in casting/common causes of casting defects Dimensional Inaccuracies or Dimensional errors in casting Distortion Surface roughness Contents
- 3. Surface irregularities Nodules Fins Ridges/Veins on casting surface Discolored castings Pits (inclusion porosity) Incomplete casting Porosity Conclusion
- 9. Classification of defect in casting/common causes of casting defects.
- 11. Rough casting Large nodule Multiple nodules Nodules on occlusal surface
- 13. suck back porosity inadequate or excessive expansion.
- 15. General problems Problems with internal porosity Problems with external porosity accuracy distortion bubbles fins short rounded margins miscasting's pits localize shrinkage porosity subsurface porosity micro porosity back pressure porosity
- 18. Porosity Incomplete or missing detail
- 19. Dimensional Inaccuracies or Dimensional errors in casting The final fit of a casting depends on a balancing out of contraction and expansion which occurs during its construction.
- 20. Casting too large eliminated by correct type of investmentuse of correct temperature
- 21. Casting too small eliminated by Heating the mold sufficiently
- 22. The tolerance limit of dental castings are approximately one tenth the thickness of human hair.
- 23. Control of Dimensional accuracy Use mixing water of correct temperature (usually mouth temperature) use constant water powder ratio For hygroscopic expansion immerse the investment before it reaches initial set control time and temperature cast into the heated mould while it is still at the correct temperature.
- 25. Any marked distortion of the casting is probably related to wax pattern distortion
- 26. Unquestionably some distortion of the wax pattern occurs as the investment hardens around it
- 27. The distortion of the wax pattern after its removal from the die is a function of the temperature and time interval before investing
- 28. Distortion can occur during spruing the pattern because of the heat transferred to the pattern
- 30. Pattern should be removed carefully from the die, and should be invested immediately.
- 32. The surface of a dental casting should be an accurate reproduction of the surface of the wax pattern from which it is made
- 33. Definition: Relatively finely spaced surface imperfections whose height, width and direction establish the predominant surface pattern.
- 35. • Improper finishing of wax pattern • Improper water powder ratio • Excess surfactant • Direct wax pattern • Raid heating rates • Underheating • Too high a pressure during casting • Composition of investment • Foreign bodies • Impact of molten alloy • Pattern position
- 37. Control of surface roughness
- 38. Wetting agent be applied in thin layer
- 39. Before starting to invest a direct wax pattern taken from the mouth it should be washed in water
- 40. Mold should be heated gradually, at least 60 min should elapse during the heating of the investment filled ring from room temperature to 700ºC. A gauge pressure of 0.10 to 0.14 MPa in air pressure casting machine or 3 to 4 turns of the spring in an average type of centrifugal casting machine is sufficient for small casting.
- 42. Bubbles of gases trapped between the wax pattern and the investment produce nodules on the casting surface.
- 43. Large nodule- Air trapped during investing Multiple nodules – Inadequate vacum during mixing Improper brush technique Lack of surfactant. Nodules on occlusal surface – Prolonged vibration after pouring
- 44. Fins caused by cracks in the investment that have been filled with molten metal
- 45. Discolored castings With calcium sulphate bonded investments-color of the casting in black after removal from the investment
- 46. cause wax not completely eliminated
- 48. Mold remains in the oven too long
- 49. Oxidizing flame was used in melting the alloy
- 50. The investment did not contain deoxidizing agents.
- 55. The temperature of the alloy should be raised higher than its liquidus temperature Adequate venting the mold Sufficiently high casting pressure Accurate W/P ratio Use large sprue former Mold should soak heat approximately 1 hour at burnout temperature. Mold should be removed from burn out oven and cast immediately Ensure that no debris blocks the ingate Cast enough metal
- 56. Porosities
- 57. Localized shrinkage porosity caused by premature termination of the flow of the molten metal during solidification. The linear contraction of noble metal alloys - changing from a liquid to solid in at least 1.25%.
- 58. • Improper sprue design causes suck back porosity. • The entering metal impinges onto the mold surface and creates a higher localized mold temperature in this region known as Hot spot. • Hot spot may retain a localized pool of molten metal after area have solidified causing shrinkage void or suck back porosity. Suck back porosity
- 60. Simultaneous nucleation of solid grains and gas bubbles at the first movement that the alloy freezer at the mold walls. Subsurface porosity caused Can be diminished by controlling the rate at which the molten metal enters the mold.
- 62. Since back pressure porosity occurs more generally in full crown type castings, insertion of a wax rod into the core of the investment when investing the pattern will provide a good means of venting the hot gases quickly
- 63. Use of reservoir
- 64. The instance from the top of the pattern to the outside of the investment is kept to ¼ inch.
- 65. Furnace temperatures above 1100ºF help eliminate this back pressure porosity condition
- 66. Increasing the number of turns on a centrifugal machine and increasing the casting pressure.
- 67. Performing the casting operation in a vacuum.
- 68. Using more metal when casting so that a good size button is left us a important precaution.
- 69. Hygroscopic low burn out technique produces more of this porosity than high heat techniques.
- 70. CONCLUSION Thus, these are the various causes for the failure of the castings and methods by which these defects can be avoided, thereby producing a casting of good quality for clinical success.
- 71. REFERENCES Operative Dentistry Modern theory and practice-Marzouk Dental materials – Philiphs – Anusavice Full mouth rehabilitation – Kornfield Restorative Dental materials – Craig
- 72. Fixed prosthodontics – Rosenstiel Fundamentals of fixed prosthodontics – Schillinburg. Australian Dental Journal 1991 36 (5): 391-6 1992 37(1) 93-54 1991 36(4): 302-9 1992 37 (2): 91-7 Notes on Dental Materials – E.C. Coombe British Dental Journal 1972: 428-435.