Dental Lab steps from mould preparation to finish/ dental education in india


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Dental Lab steps from mould preparation to finish/ dental education in india

  1. 1. GOOD MORNING INDIAN DENTAL ACADEMY Leader in continuing dental education
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  5. 5. Structural faults in dental castings can result from any combination of problems related to spruing, investing, wax elimination, alloy melting, casting & solidification of the casting. The ability to produce smooth and well fitting castings require strict adherence to certain fundamentals.....
  6. 6. SPRUING “Spruing is an art which is not well understood…..” Off all the procedures involving casting work, the spruing technique has a paramount importance in producing a complete & dense casting.
  7. 7. DEFINITIONS Sprue : the channel or hole through which plastic or metal is poured or cast into a gate or reservoir and then into a mould Sprue button : the material remaining in the reservoir of the mold after a dental casting Sprue former : a wax, plastic, or a metal pattern used to form the channel or channels allowing molten metal to flow into a mold to make a casting……. (GPT-8)
  8. 8. BASIC REQUIREMENTS To form a mount for the wax pattern To create a channel for elimination of wax during burn out To form a channel for entry of molten alloy during casting Provides a reservoir of molten metal which compensates for alloy shrinkage during solidification
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  10. 10. Taggart (1907), introduced the lost wax technique Asgar & Peyton (1959), stated that flaring should occur at the sprue/wax pattern junction Strickland et al (1959), stated the importance of the type, shape location & direction other than the size of the sprue Bruce (1964), tested the different methods of spruing patterns Rousseau (1964), described a casting method with a constricted sprue
  11. 11. MATERIALS  Wax  Plastic/Resin  Metal sprues
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  13. 13. FACTORS TO BE CONSIDERED  diameter  Shape  Number  Attachment  Location  Angulation
  14. 14. DIAMETER  Usually large-diameter sprue is recommended  The diameter of sprue should be equal to the thickest portion the wax pattern.  There are various gauges of sprue former  gauge 6, 8, 10, 12, 14, 16, 18 (0.4cm - 0.1cm)  Usually  for molar and metal ceramic restoration  10-gauge (2.5mm)   Premolars and partial coverage restoration  12-gauge (2.0mm)
  15. 15. SHAPE  The sprue former should be straight to reduce chances of creating turbulence in molten metal entering the mold.  High turbulence of alloy cause porosity.
  16. 16. NUMBER  Usually a single sprue is used for small castings.  When two thick sections of a pattern are connected by thin part of wax, 2 separate sprues should be attached to each thick portion
  17. 17. LOCATION OF ATTACHMENT  Attachment of sprue former and wax pattern: -
  18. 18. ANGLE OF ATTACHMENT  The sprue should be attached to pattern such that it makes 45 degrees to the walls of mold, which decreases the turbulence of molten alloy.  If the sprue is placed perpendicular to the mold wall, it induces high turbulence in molten alloy, leading to creation of a hot spot on mold wall. This results in localized shrinkage porosity.  It should be placed away from thin / delicate parts of pattern to prevent fracture of investment.
  19. 19. ATTACHMENT MORPHOLOGY  The transition should be smooth  Usually it is flared for high density gold alloys but restricted for low density alloys. Flaring acts as reservoir and facilitates the entry of molted alloy into the mold area.
  20. 20. SPRUING TECHNIQUE  Once spruing is done the wax pattern and sprue former assembly is carefully removed from the die.  During removal of pattern no pressure should be applied to prevent its distortion.
  21. 21.  Positioning the pattern on the crucible former  Application of surfactant  Ring liner increases the setting expansion  Pattern must be positioned sufficiently away from the end of the ring
  22. 22. INVESTING
  23. 23. GPT TERMINOLOGY…..  Investing : The process of covering, enveloping, wholly (or) in part an object such as denture tooth, wax form, crown, etc with a suitable material before processing, soldering, casting.  Dental casting Investment: Material consisting principally of an allotrope of silica and a bonding agent. The bonding substance may be gypsum (for use in lower casting temperature) (or) phosphates and silica (for use in higher casting temperatures).  Refractory : (adj) - Difficult to fuse/corrode, capable of enduring high temperatures  Refractory investment : An investment that can withstand high temperature using a soldering/casting.
  24. 24. ESSENTIALS…..  It should produce mould expansion to compensate for casting shrinkage.  Should have sufficient strength at room & high temperature.  Inner surface of the mold should not break at a high temperature.  Should exhibit sufficient strength, to withstand the force of molten alloy entering the mold.  Inner surface of mold should be smooth.
  25. 25. BASIC COMPOSITION….. Refractory Materials : The most commonly used refractory material is silicon dioxide in form quartz, crystoballite. Binder : commonly used binders are  - calcium sulfate hemihydrate Others are – Sodium silicate, ethyl silicate, ammonium sulfate, sodium phosphate. Other chemical modifiers Reducing agents – Ex : Charcoal Sodium chloride, boric acid – to  shrinkage, during dehydration of calcium sulfate dihydrate. Potassium sulfate (accelerator) Copper powder(reducing agent)/magnesium oxide
  26. 26.  When classifying investments by binder, three groups are recognized: gypsum-bonded, phosphate- bonded, and silica-bonded investments.  Each has specific applications.  The gypsum-bonded investments are used for castings made from ADA Type II, Type III, and Type IV gold alloys.  The phosphate-bonded materials are recommended for metal-ceramic frameworks.  The silica-bonded investments are for high-melting base metal alloys used in removable partial denture castings.
  27. 27. EXPANSION….. This property of investment is needed for compensation of casting shrinkage of alloy. The expansion occurs because of:- … Normal setting expansion … Hygroscopic setting expansion … Thermal expansion.
  28. 28. ARMAMENTARIUM  Vacuum mixer and bowl  Vibrator  Investment powder  Water or colloidal silica  Spatula  brush
  29. 29. CRUCIBLE FORMER  Helps to hold & place the sprue in the center of the casting ring  Helps to contour the investment surface  They may preformed or contoured with wax
  30. 30. CRUCIBLE  Carbon crucibles  Clay crucibles  Quartz /zirconia - alumina
  31. 31. Clay crucibles are used for crown and bridge alloys such as high noble and noble alloys .carbon crucibles are used for high noble crown and bridge alloys and for high fusing gold based metal-ceramic alloys Quartz and Zirconia-alumina crucibles are used for high fusing alloys or alloys that are sensitive to carbon contamination
  32. 32. CASTING RING LINER  A ring liner is placed on the inner surface of the casting ring  Should be placed a few mm short of the ring  It can be either asbestos liners (used earlier) non-asbestos liners (cellulose, ceramic-cellulose, alumino silicate)
  33. 33. CASTING RING Choice of the ring: Rubber ring Metal ring The diameter and the length of the ring should be selected rightly.
  34. 34. RINGLESS CASTING SYSTEM Versatile system for accurate & simple spruing, investing & casting with plastic or metal rings Produces consistent, accurate & predictable castings Designed to allow unrestricted expansion
  35. 35. ASSEMBLING THE RING This step should be coordinated with spruing & choosing the size of the ring Wetting agent is applied ( to reduce air bubbles) on the wax pattern
  36. 36. PREPARATION OF THE INVESTMENT MIX Mixing of the investment is done by: Hand mixing (brush technique) Vacuum mixing
  37. 37. BRUSH TECHNIQUE Add investment powder to the liquid in the mixing bowl and quickly incorporate with the hand. Attach the vacuum hose to the bowl, evacuate the bowl and mechanically spatulate Coat the wax pattern with the investment Carefully coat the internal surface & the margin of the pattern Fill the ring slowly, starting from the bottom
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  39. 39. VACUUM TECHNIQUE First hand spatulate the mix With the crucible former and pattern in place attach the ring to mixing bowl Attach the vacuum hose & mix accordingly to the manufacturers recommendations Invert the bowl & fill the ring under vibration Remove the vacuum hose before shutting of the mixer Immediately clean the bowl & mixing blade under running water
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  41. 41. SETTING OF INVESTMENT After the ring is filled to the rim allow the investment to set. If hygroscopic technique is used, the ring is placed in a 37 c water bath for 1 hr.
  42. 42. TIME OF PLACEMENT OF INVESTMENT FOR BURN OUT after 20 mins after 2 hrs After 1 hr Castings were undersized Castings were satisfactory Were found to be the best
  43. 43. WAX BURN OUT  For Gypsum investment - 6500 – Slowly heated for 60 min & held 15 – 30 min at upper temperature
  44. 44. FOR PHOSPHATE INVESTMENTS  2% or more expansion is required for Metal – Ceramic Prosthesis  Burn Out temperature – 7500 – 10300C  Slow – till 3150 C & quite rapid thereafter
  47. 47. CASTING  Casting machines requires a heat source to melt the alloy and a casting force  Heat source can be either the reducing flame of a torch or electricity  Present day casting machines still use either air pressure or centrifugal force to fill the mold
  49. 49.  Conventional alloys can be melted with gas air torch  Metal ceramic alloys in a higher melting range need a gas oxygen torch  Base metal alloys need a multiple orifice gas oxygen torch
  50. 50. TORCH MELTING/CENTRIFUGAL CASTING MACHINE  Most common  it rapidly spins the mold,crucible and molten alloy in a circle.  Casting machine spring is first wound from 2-4 turns  the rotating arm is composed of two parts connected by a pivot point and thus is called a broken arm casting machine.  The alloy is melted by a torch flame in ceramic crucible attached to the broken arm of the casting machine.
  51. 51. Once the metal has reached the casting temperature ,the machine is released and the spring triggers the rotational motion. The broken arm accelerates the initial rotational speed of the crucible and the casting ring, thus increasing the linear speed of the liquid casting alloy as it moves into the mold.
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  54. 54. QUENCHING & CLEANING THE CASTING Casting appears dark with oxides The process to remove this oxides is called as pickling 50% HCl is used to remove the oxides
  55. 55. RECOVERY OF THE CASTING  Trimming is done from the button end of the ring  Investment is being pushed out of the casting ring  The mold is broken open  Investment is removed from the casting
  57. 57. COMMON CAUSES OF CASTING FAILURE problem Possible causes 1.Rough casting 2.Large nodule Improper finishing of wax pattern Excess surfactant Improper w/p ratio Excessive burn out temperature Air trapped during investment procedure
  58. 58. problem possible causes Multiple nodules Nodules on occlusal surface Marginal discrepancy Suck back porosity Inclusion porosity Inadequate vacuum during investing Improper brush technique Lack of surfactant Excessive vibration Wax pattern distortion uneven expansion Improper pattern position narrow long sprue Particle of investment dislodged during casting
  59. 59. problem Possible causes fins Increased w/p ratio Pattern too near edge of the investmment Premature heating Too rapid heating
  60. 60. BACK PRESSURE POROSITY  entrapped air porosity seen on the inner surface of the casting produce large concave depressions  Inability of the air in the mold to escape through the pores in the investment  The entrapment is found on the cavity surface of the crown
  61. 61. INCOMPLETE CASTING  Molten metal is prevented in completely filling the mold  Insufficient venting  High viscosity of fused met  Mold is filled and alloy is solidified in 1 sec yet it is quite soft during early stages.
  62. 62.  Incomplete elimination of the wax residue from the mold  Pores in the investment gets filled and air cannot be vented completely.  Margins are rounded and quite shiny rather than dull ,this is due to the strong reducing atmosphere created by carbon monoxide left by the residual wax.
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  65. 65. TRIMMING PROCESS OF REMOVING EXCESS MATERIAL BY CUTTING OR GRINDING A MATERIAL WITH ROTARY INSTRUMENTS TO PROVIDE A DESIRED ANATOMIC FORM Finishing Process of removing surface defects or scratches created during the contouring process, through the use of cutting or grinding instruments or both
  66. 66.  The technique of finishing and polishing castings is a systematic reduction of surface irregularities.  The finished casting must maintain the fundamental requirements of fit ,form, function,and marginal integrity
  67. 67.  Ease in finishing a casting depends directly on the quality of wax pattern,the investing technique,and the casting technique  The smoother the wax pattern ,the smoother the casting;the denser the casting,the easier the finishing
  69. 69. Divested casting is sand blasted with 90-120µm alumina
  70. 70. Castings after sand blasting
  71. 71. Equipment
  72. 72. ARMAMENTARIUM Initial gross trimming instruments
  73. 73. Stones for smoothening the rough marks left by trimmers
  74. 74. Wire brushes, rubbers and bristle brushes for finishing
  76. 76. RECOMMENDED SEQUENCE FOR FINISHING OF A CAST RESTORATION  Zone 1. Internal margin  Zone 2. Internal surface(INTAGLIO)  Zone 3. The sprue  Zone 4. Proximal Contacts  Zone 5. Occlusal Surface  Zone 6. Axial Walls  Zone 7. External Margins
  77. 77. INTERNAL SURFACE (INTAGLIO) INSPECTION OF CASTING The best time to check for nodules is immediately after removing the investment from the casting and before attempting any trimming.
  78. 78. NEVER FORCE THE CASTING ONTO THE DIE, USE GREAT CAUTION WHEN FITTING THE CASTING When removing a nodule, remove slightly more than the defect to ensure complete seating of the restoration
  79. 79. THE SPRUE  To reestablish proper coronal morphology and function,the sprue must be sectioned,and the casting must be recontoured in the area of its attachment
  80. 80. Castings being seperated by gentle twisting
  81. 81. Thick carborandum disk followed by TC trimmers
  82. 82. PROXIMAL CONTACTS  Special care is needed to prevent the finishing procedures from producing an overreduced and consequently inadequate proximal contact.
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  84. 84. Articulating paper
  85. 85. Cross section showing properly finished connectors Twine impregnated with polishing compound is an efficient way to polish this hard to reach area.
  86. 86. OCCLUSAL SURFACE  Occlusal contacts are reestablished in static and dynamic relationships to the opposing arch.  Occlusal morphology must ensure positional stability and satisfy all funtional requirements
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  88. 88. Metal caliper is used to check for even thickness of coping
  89. 89. Check for contacts
  90. 90. AXIAL WALLS  The axial walls should be smoothly contoured and highly polished,enabling the patient to carry out optimum plaque control
  91. 91. Stones Rubber wheel Rubber point
  92. 92. EXTERNAL MARGINS  Margin finishing is critical to a restoration’s longevity and therefore deserves special attention.  The objective of all cast restoration finishing is a highly polished metal surface without ledges or steps as transition is made from restoration to unprepared tooth.
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  94. 94. SUMMARY AND CONCLUSION The accuracy of the fit of a cast restoration is essential for its longevity This allows for less plaque accumulation at the marginal area It also provides better mechanical properties like stability and resistance, less microleakage and good esthetics  Correct handling of the wax pattern with proper investing and casting technique should be used to get a accurate casting.
  95. 95. REFERENCES  Phillips’science of dental materials – Kenneth J.Anusavice,11th Edition.  Removable partial prosthodontics- McCracken,9th Edition.  Dental laboratory procedures – Fixed partial dentures -Robert M. Murrow, Kenneth D. Rudd, John E. Rhoads.  Notes on Dental materials – E.C.Combe,6th Edition.  Contemporary Fixed Prosthodontics – Stephen F. Rosenstiel, Martin F. Land, Junhei Fujimoto,3rd Edition.  Restorative dental materials – Robert G.Craig, John M. Powers.
  96. 96. Thank you For more details please visit