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Casting procedure


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Dental Material, Prosthodontics

Published in: Health & Medicine
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Casting procedure

  1. 1. CASTING PROCEDURE Dr. Deepak K. Gupta
  2. 2. Definition  Casting is defined as something that has been cast in a mold, an object formed by the solidification of a fluid that has been poured or injected into a mold.(GPT)  “Casting is the process by which a wax pattern of a prepared tooth is fabricated and converted to its metallic replica”(Rosenteil)
  3. 3. Steps in casting  Preparing the wax pattern  Spruing the wax pattern  Attaching the sprue to crucible former  Investing the pattern in the casting ring  Burnout of the wax  Casting  Recovery  Finishing and polishing
  4. 4. Steps involved in the casting
  5. 5. Preparing the wax pattern Prior to casting  margin of the wax pattern should be readapted  pattern checked for smoothness, finish and contour  Sprue should be attached to the thickest portion of the wax pattern  wax pattern can be removed from the die using sprue  surfactant should be applied on the wax to obtain wetting of the investment  invested immediately to prevent distortion
  6. 6.
  7. 7. SPRUING OF WAX PATTERN  Sprue is defined as “the channel or hole through which plastic or metal poured or cast into gate or reservoir and then into mold”
  8. 8. SPRUE DESIGN  it must allow the molten wax to escape from the mold  enable molten metal to flow into the mold with minimal turbulence  metal within the sprue must remain molten slightly longer than the alloy that has filled the mold this will act as a reservoir to compensate the shrinkage
  10. 10. SPRUE MATERIALS Wax: preferred for most casting because they melt at the same rate as the pattern and allow for easy escape for molten wax Plastic: resist distortion rigid,  may block the escape of wax,  hollow plastics are available Metal: non rusting metal should be used,  removed before casting
  11. 11. Diameter  should be larger than the thickest portion of the wax pattern  2.6 mm can be used for most patterns  2.0mm for premolar partial veneer restoration  narrow sprue are sufficient for casting to be done on centrifugal machine
  12. 12. POSITIONING OF THE SPRUE  Sprue should be attached to the point of greatest bulk  45 angulation near the bulk of the pattern  axial wall should have obtuse angle 135  This prevents air entrapment during investing and suck back porosity after casting  attachment should be flared to prevent turbulence during metal flow
  13. 13. VENTING  Small auxiliary sprues or vent should be placed to improve the casting  By allowing the gases to escape
  14. 14. CRUCIBLE FORMER  The sprue is attached to a crucible former, usually made of rubber, which constitutes the base of the casting ring during investing.  May be metal , plastic or rubber  The exact shape of the crucible former depends on the type of casting machine used.  With most modern machines, the crucible former is tall to allow use of a short sprue and allow the pattern to be positioned near the end of the casting ring.  also referred to as a sprue former
  15. 15. Diagrammatic representation of a dental casting mold A, Crucible former. B, Sprue. C, Cavity formed by wax pattern after burnout. D, Investment. E, Liner. F, Casting ring. G, Recommended maximum investment thickness of approximately 6 mm between the end of the mold cavity and the end of the invested ring to provide pathways for sufficient gas escape during casting.
  16. 16. Casting rings  Casting rings are used to confine the fluid investment around the wax pattern while the investment sets. And …….  Considerations in selection of casting rings:
  17. 17. They are available as: 1) Shapes - Round - Oval 2) Complete rings – Rigid - Metal - Plastic Flexible - Rubber 3) Split rings - Metal - Plastic
  18. 18. Ring less casting system: - plastic rings which is conical in shape with tapering walls are used. - Used for traditional gold-base alloys. Flexible rings Split casting rings
  19. 19. Casting ring liner Materials used are: - Asbestos liner - Cellulose liner - Ceramic liner - Combination of ceramic and cellulose liner
  20. 20. Function of casting ring liner:  Allow uniform expantion.  In case of wet liner technique- hygroscopic expansion.  Thickness of the liner should be less than 1mm.
  21. 21. RINGLESS CASTING TECHNIQUE  With the use of higher- strength, phosphate- bonded investments, the ringless technique has become quite popular.  The method uses a paper or plastic casting ring and is designed to allow unrestricted expansion.
  22. 22. INVESTING  The process of covering or enveloping an object such as a denture, tooth, wax form, crown, with a suitable investment material before processing, or casting  Investment materials are:  Gypsum bonded investment  Phosphate bonded investment  Ethyl silicate bonded investment
  23. 23. -Thin film of cleaner on pattern reduces surface tension of wax better “wetting” of wax pattern by the investment. -Some of the commercially available debubblizing agents can be used. •The wax pattern should not stand for more than 20-30 min before being invested. So, it is best to invest the wax pattern as soon as possible
  24. 24.  Investment mixing: 1. Hand mixing 2. Vacuum mixing  After mixing the investment is poured in to the casting ring up to its rim
  25. 25. Wax elimination or burn out  It is advisable to begin the burnout procedure while the mould is still wet.  Water trapped in the pores of the investment reduces the absorption of wax.  As the water vaporizes it flushes wax from the mold.  Purpose of burn out: to flush out wax pattern and create a hollow cavity  Heat application: Heating of ring should be done slowly.  Hygroscopic low-heat technique  High-heat thermal expansion technique
  27. 27. Hygroscopic low-heat technique  The temperature used in this technique is 500°c for 60-90 mins.  Obtain compensation expansion from three sources:  Immersion of investment in 37°c water bath.  The warm water entering the investment mold from the top adds some of the expansion.  The thermal expansion at 500°c  This technique causes 0.55% of expansion.
  28. 28. HIGH-HEAT THERMAL EXPANSION TECHNIQUE  The investment is slowly heated to 650°c - 700°c in 60mins. Then maintained for 15-30 mins at this temperature.  Above 700°c sulfur dioxide - Contaminates gold castings and makes them extremely brittle
  29. 29. ACCELERATED CASTING METHOD  To reduce the total time, Alternative Accelerated casting technique is proposed  Uses phosphate bonded investment which sets in 15 mins and then 15 min burn out is done at 815°c.  This method is used for preparing post and core restorations
  30. 30. HEAT SOURCES  Torch flame  Gas air torch: melt conventional noble metal alloys (used for inlays, crown and bridge) whose melting points less than 1000°c  Gas oxygen torch: Used to melt metal ceramic alloys of higher temperature up to 1200°c.  Oxy acetylene torch: One volume of acetylene and two and half volume of oxygen are needed.  Hydrogen oxygen generator  Electricity
  31. 31. Two type of torch tips: 1.Multi-orifice 2.Single-orifice Zones of the blow torch flame: Zone 1 - colorless zone /Non combustion zone Zone 2 – Combustion zone Zone 3 – Reducing zone Zone 4 - oxidizing zone
  32. 32. CHANGES SEEN IN METAL DURING FUSING  Initially appear spongy  later small globules of fused metal appear  molten metal flows assuming a spherical shape  At proper casting temperature the molten alloy is light orange and tend to spin or follow.  At this stage the temperature of molten alloy is 38°c above its liquidus temperature.  During melting of the gold alloys flux may be added  Minimizing porosity  To increase fusing of metal  Prevent oxidation  Commonly used fluxes are fused borax powder ground with boric acid power or Charcoal
  33. 33. Casting machines 1. Air pressure casting machines: Alloy is melted in situ in crucible hollow of the ring, followed by applied air pressure. 2. Centrifugal casting machine: Alloy is melted in a crucible, and forced in to mold by centrifugal force.
  34. 34. ELECTRICAL RESISTANCE - HEATED CASTING MACHINE  It is used to melt ceramic alloys.  Here the alloy is automatically melted in graphite crucible. Direct-current arc melting machine  Produce between two electrodes: the alloys and the water cooled tungsten electrode.  > 4000°C – alloy melts very quickly.  High risk of over heating of the alloy.
  35. 35. Induction melting machine  Metal is melted by an induction field that developed with in the crucible surrounded by water- cooled metal tubing.  It is more commonly used for melting base metal alloys  not been used for noble alloy casting as much as other machines
  36. 36. CASTING FORCE  Force required to overcome the surface tension of alloy + Resistance offered by gas in the mold.  This can be done by use of following different type of force  Vacuum force  Air or Gas Pressure  Centrifugal force  Sufficient mass of alloy must be present to sustain adequate casting pressure  6g is typically adequate for premolar and anterior casting  10g is adequate for molar casting  12 g is adequate for pontic
  37. 37. Casting Crucible They are of 3 types: • Clay Crucibles • Carbon Crucibles • Quartz Crucibles (zircon-alumina)
  38. 38. Cleaning of the casting  Consider the gold crown & bridge alloys.  After casting has been completed, ring is removed & quenched in water. Advantages: 1. Noble metal is left in an annealed condition for burnishing & Polishing. 2. When water contacts hot investment, violent reaction ensues. Investment becomes soft, granular & casting is more easily cleaned.
  39. 39. PICKLING:  Surface of the casting appears dark with oxides and tarnish. Such a surface film can be removed by a process called Pickling.  Best method for pickling is to place a casting in a dish & pour acid over it.  Heat the acid but don't boil it.
  40. 40.  Hydrochloric acid  Sulfuric acid  Ultrasonic devices  Gold and palladium based metal ceramic alloys and base metals, these alloys are not generally pickled.
  41. 41. Casting Defect
  42. 42. Introduction  An unsuccessful casting results in considerable trouble and loss of time.  With present techniques, casting failures should be the exception, not the rule.  It can be classified under four headings  distortion;  surface roughness and irregularities;  Porosity  Incomplete or missing detail
  43. 43. DISTORTION  It is usually due to distortion of wax pattern  minimized or prevented by proper manipulation of the wax and handling of the pattern  Some distortion of the wax pattern occurs as the investment hardens around it.  Setting and hygroscopic expansions of the investment.  Produce a non-uniform expansion of the walls of the
  44. 44. SURFACE ROUGHNESS, IRREGULARITIES, AND DISCOLORATION  Accurate reproduction of the surface of the wax pattern from which it is made.  Surface roughness and irregularity should not confused, both are different terms  These depends on size of investment particles.  Various other causes  air voids  water films  liquid/powder ratio  composition of the investment  foreign bodies  impact of molten alloy on the mold wall  pattern position  carbon inclusions
  45. 45. Porosity  Porosity or void fraction is a measure of the void (i.e., "empty") spaces in a material, and is a fraction of the volume of voids over the total volume.  It can be either internal or external  What it does to the casting  Weakens  Discoloration  Surface roughness  Plaque accumlation
  46. 46. Classification  Solidification defects  Localized shrinkage porosity  Microporosity  Suck-back porosity  Trapped gases  Pinhole porosity  Gas inclusions  Subsurface porosity  Back pressure porosity
  47. 47. Localized shrinkage porosity  Caused by premature termination of the flow of molten metal during solidification.  Generally occurs near the sprue-casting junction, but it may occur anywhere
  48. 48. Microporosity  It also occurs due to solidification shrinkage.  generally present in fine-grain alloy castings.  solidification is too rapid for the microvoids to segregate to the liquid pool.  causes the formation of small, irregular voids.  mold or casting temperature is too low.  Not detectable unless the casting is sectioned and is not a serious defect
  49. 49. Suck-back porosity  localized shrinkage porosity,  interior of a crown near the area of the sprue  Occlusoaxial line angle or incisoaxial line angle that is not well rounded.  entering metal impinges onto the mold surface at this point – hot spot  This spot may retain a localized pool of molten metal after other areas of the casting have solidified.  Creates a shrinkage void or suck-back porosity.
  50. 50. Suck-back porosity
  51. 51. Pinhole and gas inclusion  Entrapment of gas during solidification  gas inclusion porosities are usually much larger than pinhole porosity,  Many metals dissolve or occlude gases while they are molten  Copper and silver: oxygen  Platinum and palladium: hydrogen as well as oxygen.
  52. 52. Subsurface porosity  simultaneous nucleation of solid grains and gas bubbles.  diminished by controlling the rate at which the molten metal enters the mold
  53. 53. Back-pressure Porosity  Entrapped air bubbles on the inner surface of the casting.  large concave depressions.  caused by the inability of the air in the mold to escape through the pores in the investment
  54. 54. INCOMPLETE CASTING  Occasionally only a partially complete casting or perhaps no casting at all is found.  The obvious cause is that the molten alloy has been prevented in some manner from completely filling the mold.  insufficient venting of the mold and high viscosity of the fused metal.  incomplete elimination of wax residues from the mold
  55. 55. INCOMPLETE CASTING  Incomplete venting  Magnitude of the casting pressure  pressure should be applied for at least 4 seconds.  lower L/P ratio is associated with less porosity of the investment.  Incomplete elimination of wax  Pores in the investment may become filled
  56. 56.
  57. 57. References  Phillips' Science of Dental Materials- Phillip Anusavice_12th  Basic Dental Materials -2nd.ed Mannapalli  Clinical Aspects of Dental Materials Theory, Practice, and Cases, 4th Edition  Craig's Restorative Dental Material 13th edition
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