Casting procedure & defects


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Casting procedure & defects

  1. 1. Page 1 Good morning Good morning
  2. 2. Page 2 Casting procedures Casting procedures AndAnd Casting defects Casting defects Presented by: Bhanuchandar. D Dept of Prosthodontics
  3. 3. Page 3 Contents • Introduction • History • Steps in casting procedure • Sprue formers • Crucible formers • Casting rings and ring liners • Investing procedure • Wax burnout • Casting of alloys into mold • Casting of titanium alloys • Cleaning of casting • Casting defects • Conclusion
  4. 4. Page 4 Introduction
  5. 5. Page 5 History • This met iculous procedure of cast ing was used t o produce j ewelry and ornament s. • 3500 B.C. – Egypt ians f irst manuf act ured ref ract ories which in t he f orm of glass vessels around a ref ract ory core of mud, sand, and animal dung. • 11t h Cent ury → Theophilus → Described lost wax t echnique, which was a common pract ice in j ewelry. • 1558 → B. Cellini → have at t empt ed use of wax and clay f or preparat ion of cast ings.
  6. 6. Page 6 • 1897 → Phillibrook described a met hod of cast ing met al f illing • 1907 – William. H .Taggart int roduced t he lost wax t echnique in dent ist ry. • 1949 – Moore and Walt developed phosphat e bonded invest ment . • 1959 -Asgar& Peyton st at ed t hat f laring should occur at t he sprue/ wax pat t ern j unct ion • 1959 - Strickland et alst at ed t he import ance of t he t ype, shape locat ion & direct ion ot her t han t he size of t he sprue • 1959 - Morrison and Warmick report ed t he f indings of et hyl silicat e ref ract ory mat erial f or dent al use.
  7. 7. Page 7 Steps involved in the casting
  8. 8. Page 8 Spruing Sprue former: A wax, plastic or metal used to form the channel or channels which allows molten metal to flow into a mold to make casting.
  9. 9. Page 9 • Purpose of the sprue former • Requirements of the sprue former
  10. 10. Page 10 Sprue materials:
  11. 11. Page 11
  12. 12. Page 12 Advantages of hollow sprue former: • It increases the contact area. • It holds less heat than the solid sprue former.  “Care must be taken to examine the orifice for small particles of investment that may break off while removing the metal sprue former.”
  13. 13. Page 13 Sprue former diameter: • The diameter and length of the sprue former depends on: - The type and size of the pattern. - The type of casting machine to be used. - The dimensions of the casting rings in which casting is made. • Pre fabricated sprue former are available in a wide variety of gauge from 6 to 18.
  14. 14. Page 14 • Usually → for molar and metal ceramic restoration - 10-gauge (2.5mm) → Premolars and partial coverage restoration - 12-gauge (2.0mm) • A narrow sprue may be useful in air pressure casting procedure where the metal is melted in conical depression formed by crucible former. so narrow sprue prevention premature metal flow into mold.
  15. 15. Page 15 Reservoir: • Reservoir is a small amount of additional wax added to the sprue former 1mm below the wax pattern.
  16. 16. Page 16 Sprue former length: • The length of the sprue former - it keeps the wax pattern 6mm from the
  17. 17. Page 17
  18. 18. Page 18 Sprue former position: • The ideal area- point of greatest bulk in the pattern. • The point of attachment should permit stream of metal to be directed to all parts of the mold without having to flow opposite the direction of casting force
  19. 19. Page 19 Sprue former direction: Attached 45 degrees to the walls of mold, which decreases the turbulence of molten alloy.
  20. 20. Page 20 Attachment morphology: • The attachment of sprue former to the wax pattern should be smooth and do not posses pits or irregularities.
  21. 21. Page 21 Spruing Direct Indirect
  22. 22. Page 22 Vents • Vents are the additional sprues placed at thin or thick wax patterns to improve the quality of the casting.
  23. 23. Page 23 Auxillary sprue • For large casting an additional auxillary sprue may be placed for filling the mold. • Usually 14 to 16 gauge sprue are used.
  24. 24. Page 24 Crucible former They are available as: Rubber, Metallic and Plastic They are of two types: 1) Steep sided cone- to cast metal using centrifugal casting force 2) Shallow cone- cast metal using stream or air pressure
  25. 25. Page 25 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:
  26. 26. Page 26 They are available as: 1) Shapes - Round - Oval 2) Complete rings – Rigid - Metal - Plastic Flexible - Rubber 3) Split rings - Metal - Plastic
  27. 27. Page 27 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
  28. 28. Page 28 Casting ring liner Materials used are: - Asbestos liner - Cellulose liner - Ceramic liner - Combination of ceramic and cellulose liner
  29. 29. Page 29 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.
  30. 30. Page 30 Investing . Investment materials are: 1. Gypsum bonded investment 2. Phosphate bonded investment 3. Ethyl silicate bonded investment 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
  31. 31. Page 31 -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
  32. 32. Page 32 • Investment mixing: 1. Hand mixing 2. Vacuum mixing Bubble free casting with different technique- 17% - open investment 95% - vacuum investment • Advantage of vacuum mixing:
  33. 33. Page 33 Setting of investment: • After mixing the investment is poured in to the casting ring up to its rim. • For hygroscopic expansion technique • For controlled water added technique • For Thermal expansion technique
  34. 34. Page 34 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.
  35. 35. Page 35 • Purpose of burn out: • Heat application: • Heating of ring should be done slowly. - Hygroscopic low-heat technique - High-heat thermal expansion technique
  36. 36. Page 36 Hygroscopic low-heat technique • The temperature used in this technique is 500°c for 60-90 mins. • Obtain compensation expansion from three sources: 1. Immersion of investment in 37°c water bath. 2. The warm water entering the investment mold from the top adds some of the expansion. 3. The thermal expansion at 500°c • This technique causes 0.55% of expansion.
  37. 37. Page 37 High-heat thermal expansion technique Gypsum bonded Investment : • 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
  38. 38. Page 38 Phosphate bonded investment : • They need higher 2nd stage temperature for - - total elimination of wax - and Prevent premature solidification of higher melting alloys. • After initial slow raise of temp to 315°c, the temperature is rapidly raised to 750- 900°c and maintained for 30 mins. • The technique cause 1.33-1.58 % of Thermal expansion
  39. 39. Page 39 Accelerated casting method (J Prost dent. 66: 155,1991) • To reduce the total time, Alternative Accelerated casting technique is proposed that 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
  40. 40. Page 40
  41. 41. Page 41 HEAT SOURCES: two basic modes- • Torch flame - Gas air torch - Gas oxygen torch - Oxy acetylene torch - hydrogen oxygen generator • Electricity 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
  42. 42. Page 42 Two type of torch tips: 1. Multi-orifice 2. Single-orifice Zones of the blow touch flame: • Zone 1 - colorless zone • Zone 2 – Combustion zone • Zone 3 - Reducing zone • Zone 4 - oxidizing zone
  43. 43. Page 43 Two types of flame can be obtained with a casting torch:
  44. 44. Page 44 The changes seen in metal during fusing are : • Initially appear spongy→ later small globules of fused metal appear → later 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.
  45. 45. Page 45 • 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. • Charcoal
  46. 46. Page 46 Gas air torch: - • To 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.
  47. 47. Page 47 They are: • Oxidizing flame - 6000°F • Carburizing flame- 5400°F • Neutral flame - 5600°F If distance is reduced to - 7.5 mm → slight porosity 5 mm → increased porosity due to occluded H2 gas.
  48. 48. Page 48 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.
  49. 49. Page 49 3. Electrical resistance - heated casting machine :- • It is used to melt ceramic alloys. Here the alloy is automatically melted in graphite crucible. • The crucible in the furnace is always against the casting ring. So the metal button remain molten slightly longer and ensures complete solidification.
  50. 50. Page 50 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.
  51. 51. Page 51 4. Induction melting machine: • Metal is melted by an induction field that developed with in the crucible surrounded by water- cooled metal tubing.
  52. 52. Page 52 • The electric induction furnace is a transformer in which an alternating current flows through the primary winding coil and generates a variable magnetic field in the location of the alloy to be melted in a crucible • It is more commonly used for melting base metal alloys not been used for noble alloy casting as much as other machines
  53. 53. Page 53 Casting force: • Casting force > 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
  54. 54. Page 54 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
  55. 55. Page 55 CASING OF TITANIUM ALLOYS • Titanium offers lot of advantages and can be used for casting. Problems associated are: • High melting point of titanium of 1671°C (when other dental casting alloys have liquidus temperature below 1500° C). • Tendency for the molten metal absorbs several gases in molten state.
  56. 56. Page 56 • 1977 – Walter start :- casted Ti alloy as crown and bridges and R. P. D. frame work. • A new pressure / vacuum casting machine was developed. With argon – arc system for melting alloy. • Molten alloy drawn into the mold by gravity or vacuum & subjected to additional pressure to force the alloy into the mold
  57. 57. Page 57 Casting Crucible They are of 3 types: • Clay Crucibles • Carbon Crucibles • Quartz Crucibles (zircon-alumina)
  58. 58. Page 58 • Traditionally a wet lining of asbestos sheet was used on casting crucible. The moistened asbestos sheet provides a clean and good surface on which the alloy could be melted. • Advantages is, prevent alloy contamination with oxides and residuals that may be present in the crucible
  59. 59. Page 59 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.
  60. 60. Page 60 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.
  61. 61. Page 61  Hydrochloric acid  Sulfuric acid  Ultrasonic devices • Gold and palladium based metal ceramic alloys and base metals, these alloys are not generally pickled.
  62. 62. Page 62 • Casting is both an art and science governed by numerous rules, or “laws”. • Based on earlier work of Ingersoll & Wandling (1986), W. Patrick Naylor formulated an expanded set of 17 separate recommendations for Spruing, investing, burnout, and melting and casting procedures. • Collectively these guidelines are referred to as the laws of casting.
  63. 63. Page 63 Attach the pattern Sprue former to the thickest part of the wax pattern Orient the wax pattern so all the restoration margins will face the trailing edge when the ring is positioned in the casting machine Position the wax pattern in a “cold zone” of the investment mold and the reservoir in the “heat centre” of the casting ring
  64. 64. Page 64 A reservoir must have sufficient molten alloy to accommodate the shrinkage that occurs within the restorations Do not cast a button if a connector (runner) bar, or other internal reservoir, is used Turbulence must be minimized, if not totally eliminated
  65. 65. Page 65 Select a casting ring of sufficient length and diameter to accommodate the patterns to be invested Increase the wettability of the wax patterns Weigh any bulk investment and measure the investment liquid for a precise powder-liquid ratio
  66. 66. Page 66 Eliminate the incorporation of air in the casting investment and remove the ammonia gas by product of phosphate-bonded investments by mixing under vacuum Allow the casting investment to set completely before initiating the burnout procedure Use a wax elimination (burnout) technique that is specific for the type patterns involved (wax versus plastic) and recommended for the particular type of casting alloy selected
  67. 67. Page 67 When torch casting, use the “reducing zone” of the flame to melt the alloy and not the oxidizing zone Provide enough force to cause the liquid alloy to flow onto the heated mold Adequate heat must be available to properly melt and cast the alloy
  68. 68. Page 68 Cast toward the margins of the wax patterns Do not quench the ring immediately after casting
  69. 69. Page 69
  70. 70. Page 70 Classification According to Anusavice • Distortion • Surface roughness and irregularities • Porosity • Incomplete or missing details Based on location • Internal • external
  71. 71. Page 71 According to Rosensteil • Roughness • Nodules • Fins • Incompleteness • Voids or porosity • Marginal discrepancy • Dimensional inaccuracies
  72. 72. Page 72 Distortion • Distortion of the casting probably related to distortion of the wax pattern. Causes: • Can occur from the time of pattern preparation to the time of investing due to stress relaxation. • Distortion of the wax pattern occurs during the investment procedure. Minimized by:  Application of minimum pressure  Manipulation of wax at high temperature  Investing pattern immediately  If storage is necessary, store in refrigerator
  73. 73. Page 73 Surface Roughness and Irregularities surface roughness Defined as relatively finely spaced surface imperfections whose height, width and direction establish the predominant surface pattern. Surface irregularities Isolated imperfections such as nodules that are not characteristic of the entire surface area • The surface roughness of the casting is > wax pattern - the particle size of the investment and its ability to reproduce the pattern in microscopic detail
  74. 74. Page 74 Air bubbles: • Small nodules on the casting Prevented By: – Proper investment technique – Vibration of mix or by vacuum mixing – Application of wetting agent properly and correctly – important that it be applied in a thin layer.
  75. 75. Page 75 Water films: • If the Investment becomes separated from the wax pattern, a water film may form irregularly over the surface. • Appears as minute ridges or veins on the surface. Prevented By: 1.Use of wetting agent 2.Correct L/P ratio (Too high L/P ratio may produce these irregularities)
  76. 76. Page 76 Rapid Heating Rates Causes: • Fins or spines • Flaking of the investment Prevented by: • Heat gradually at least 60min from room temperature to 700°c. • Greater the bulk – more slowly heated. Under heating • Incomplete elimination of wax residues.
  77. 77. Page 77 Prolonged heating • Decomposition of the investment Liquid/Powder Ratio • The amount of water and powder measure should be accurate. Casting pressure • To high pressure – rough surface of the casting • To low pressure – incomplete casting • Average – 0.01 to 0.14 Mpa and 3 to 4 turns of the spring.
  78. 78. Page 78 Foreign bodies • Any casting that shows sharp, well- defined deficiencies indicates the presence of some foreign particles in the mold. They may be: - Pieces of the investment - Bits of the carbon from the flux - Sulfur components from – decomposition of the gypsum investment and high sulfur content torch flame. Pattern position • Should not place too close together • Should not place many patterns in same plane Space between the pattern is atleast 3mm
  79. 79. Page 79 Impact of metal alloy Cause: • The direct impact of molten alloy may fracture or abrade the mold surface regardless of its bulk. It results …….. Prevented by: • This type of surface roughness or irregularities can be avoided by proper spruing. Carbon inclusions • Carbon from- carbon crucible, carbon containing investment – absorbed by the alloys during casting results in formation of carbides or visible carbon inclusion. Other causes
  80. 80. Page 80 porosity Classified as follows: I. Solidification defects A. localized shrinkage porosity B. Micro porosity II. Trapped gases A. pin hole porosity B. gas inclusion porosity C. sub surface porosity III. Residual porosity
  81. 81. Page 81 Localized shrinkage porosity Cause: • By premature termination of the molten metal during solidification. • Porosity in the pontic is caused by- retain heat because of its bulk and located in the center of the rings.
  82. 82. Page 82 Suck back porosity – Interior of the crown near the area of the sprue create a hot spot on the mold wall
  83. 83. Page 83 Pin hole and Gas inclusion porosity • Characterized by spherical contour, but gas inclusion porosities are much larger than pin hole porosity. • Occur primarily because most metals dissolve gases when molten these gases expelled during solidification.. • Also be caused by gas occluded from a poorly adjusted torch flame or use of oxidizing zone rather than reducing zone. • Casting is usually black, do not clean easily on pickling
  84. 84. Page 84 Sub surface porosity • Caused by simultaneous nucleation of solid grains and gas bubbles at the first moment that the alloy freezes at mold walls • Prevented by controlling the rate at which the molten metal enters the mold.
  85. 85. Page 85 Back pressure porosity • Some times referred to as entrapped-air porosity. • The entrapment is frequently found in a “POCKET” at the cavity surface of a crown or MOD casting. • Also found on the outer surface of the casting when the casting or mold temperature is low, that solidification occurs before the trapped air can escape. • Thickness of the investment • Incomplete elimination of wax residues.
  86. 86. Page 86 Prevented by: • Proper burnout • Sufficiently high casting pressure • Investment of adequate porosity • Adequate L/P ratio • Adequate mold and casting temperature. • Thickness of investment between tip of pattern and end of ring is not greater than 6mm.
  87. 87. Page 87 In complete casting • Factors that inhibit the mold filling is: 1. In sufficient venting 2. In sufficient casting pressure 3. Incomplete elimination of wax 4. Lower L/p ratio 5. Viscosity of the fused metal
  88. 88. Page 88 Viscosity of the fused metal: • In complete casting resulting from too greater viscosity is due to insufficient heating of the alloy. • However, both the surface tension and the viscosity of the molten alloys are reduced by increased in temperature higher than its liquidus temperature
  89. 89. Page 89
  90. 90. Page 90 Reference • K. J. Anusavice, Phillips Science of Dental Materials, 11th edition • Introduction to metal ceramic technology, W.P. Naylor • Craig R.G, Restorative Dental Materials;10th edition • Rudd and morrow, Dental laboratory procedures- removable partial denture, 2nd edition • Rudd and morrow, Dental laboratory procedure- fixed partial denture, • John j. manappallil, basic dental materials, 2nd edition • Rosensteil, Contemporary fixed Prosthodontics • Shillinburg, fundamentals of fixed Prosthodontics • JPD- 1978; 3, 137-14 • internet
  91. 91. Page 91