casting procedures.pptx

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1. DENTAL CASTING PROCEDURES

2. CONTENTS Introduction Definitions Steps involved in Lost wax technique Fabrication of wax pattern Spruing the pattern Casting ring and liners Investment technique Burnout Casting technique Casting machines Removal and finishing of casting Casting defects Studies conducted Conclusion References

3. Introduction To get a good final metal restoration by taking into consideration of all the properties of metal being used; their manipulation; lab procedures and their applied aspects. W.H.TAGGART introduced lost wax technique by the process of casting, for fabrication of metal ceramic restorations.

4. SPRUE FORMER : The mold channel through which molten metal or ceramic flows into the mold cavity. CRUCIBLE FORMER : The base to which sprue former is attached while the wax pattern is invested in refractory investment, a convex rubber, plastic, or metal base that forms a concave depression or crucible in the refractory investment. INVESTING : The procedure for forming the mold. BURN OUT : Process of heating an investment mold to eliminate the embedded wax or plastic pattern. Definitions

5. LOST WAX TECHNIQUE : The technique is so named because wax pattern of a restoration is invested in a investment material, then the pattern is burned out (lost) to create a space into which molten metal is placed or cast. Definitions

6. STEPS INVOLVED

7. STEP1: Tooth preparation. STEP2 : Impression of the prepared tooth. STEP3 : Pouring the cast to form die. STEP4 : Fabrication of wax pattern on the die. STEP5 : Preparation of wax pattern for investment . Spruing the pattern. Attaching to crucible former. Placing in casting ring with liner.

8. STEP6: Investing the wax pattern. STEP7: Burn out /wax elimination technique. STEP8: Casting procedure. STEP9: Recovery of casting, finishing and polishing. Steps involved in lost wax technique

9. FABRICATION OF WAX PATTERN

10. Direct technique Indirect technique

11. SPRUING THE WAX PATTEN

12. The purpose of the sprue former ,or sprue pin, is to provide a channel through which molten alloy can reach the mold in an invested ring after the wax has been eliminated. Sprue former

13. Materials Wax Plastic Metal sprues

14. Factors to be considered Diameter Length Position Attachment Location Angulation

15. 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)

16. SPRUE LENGTH: Depends on the length of the casting ring. Top of the wax pattern from open end of the ring should be 6mm- gypsum bonded investment. 3mm-4mm - phosphate bonded investment.

17. Length The sprue should be long enough to place pattern 6mm from trailing end of casting Very short length- porosity at the junction of sprue and pattern Very long length- Incomplete casting

18. 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.

19. SPRUE POSITION Site of prefer placement are on the: Occlusal surface. Proximal surface. Just below a nonfunctional cusp. The ideal area is the point of greatest bulk in the pattern.

20. Sprue Attachment Should be attached to the largest cross-sectional areas of the pattern. The sprue former orientation should minimize the risk for metal flow onto flat areas of investment or onto small areas such as line angles.

21. SPRUE DIRECTION Should be directed away from any thin or delicate parts of the pattern. The sprue former should not be attached at a right angle to a broad flat surface. Ideally sprued at a 45-degree angle to the proximal area. 45

22. FUNCTION OF RESERVOIR: It should be at a distance of 1-2mm from the wax pattern. It prevent localized shrinkage porosity in the casting. The resulting solidification shrinkage occurs in the reservoir bar and not in the prosthesis. Reservoir

23. Vents: Vents are the additional sprues placed at thin or thick wax patterns to improve the quality of the casting.

24. Auxillary sprues : For large casting an additional auxillary sprue may be placed for filling the mold. Usually 14 to 16 gauge sprue are used

25. Crucible former: They are available as: Rubber, Metallic and Plastic They are of two types: Steep sided cone- to cast metal using centrifugal casting force Shallow cone- cast metal using stream or air pressure

26. CASTING RING AND LINERS

27. Casting ring used to invest the wax pattern in the investment medium .

28. With the use of solid metal rings or casting flasks, provision must be made to permit investment expansion . This effect can be overcome by using a split ring or flexible rubber ring that permits the setting expansion of the investment. Rubber rings Split rings

29. Ring liners line the walls of the ring to provide investment expansion. Types It can be either asbestos liners (used earlier) non-asbestos liners (cellulose, ceramic-cellulose, alumino silicate)

30. Ring liners are placed to ensure uniform expansion in form of Normal setting expansion. Semi hygroscopic expansion. The liner to fit the inside diameter of the casting ring with no overlap.

31. Liner 3.25mm short of Casting ring.

32. 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

33. The system, called the power cast ring less system consists of 3 sizes of rings and formers with a release ring, preformed wax sprues and investment powder, and a special investment liquid.  

34. These rings are tapered to allow for removal of the investment mold after the material has set. This system is suited for casting of alloys that require greater mold expansion than traditional gold-based alloys

35. FORMING THE CRUCIBLE AND ATTACTHING THE PATTERN. The crucible part of the investment is funneled shaped component that is connected to the sprued wax pattern.

36. It comes in different Materials, Sizes, inclination, diameter. The deeper the crucible is and more the inclined its wall are, the more velocity will be imparted to the melt on its way to the mold .

37. Preparation of wax pattern before investing The wax pattern should be cleaned of any debris, grease, or oils. A commercial pattern cleaner or a diluted synthetic detergent is used. Synthetic detergent solution

38. A pattern should be invested as soon as possible, after it is removed from the die, and it should not be subjected to warm environment during this interval. In any case it should not stand more than 20-30 min before been invested.

39. INVESTING PROCEDURE

40. There are 3 types of investment materials available: Gypsum bonded investments. For conventional casting of gold alloy inlays, onlays , crowns & FPD’s. Phosphate bonded investments. For metal ceramic restorations, for pressable ceramics & for base metal alloys. Ethyl- silicate bonded investments. For casting of removable partial dentures with base metal alloy.

41. Wax pattern sprue Crucible former 6mm 3.25mm Investment material Casting ring Ring liner Cross sectional diagram of sprue base and metal casting ring 3.25mm

42. Investing is the process by which the sprued wax pattern is embedded in a material called an investment. The investment must be able to withstand the heat and forces of castings . Invested pattern

43. INVESTMENT TECNIQUES Vacuum investment Hand investment

44. VACCUM INVESTMENT With it, the powder and water are mixed under vacuum. The mix is permitted to flow into the ring and around the wax pattern with the vacuum present. Mechanical mixing under vacuum removes air bubbles created during mixing and evacuates any potentially harmful gases produced by the chemical reaction of the high-heat investment.

45. ADVANTAGES of vacuum investment . The amount of porosity in the investment is reduced by vacuum investing. The texture of the cast surface is somewhat smoother with better detail reproduction. The tensile strength of vacuum-mixed investment is also increased.

46. PRECAUTIONS During investing excessive vibration should be avoided because it can cause solids in the investment to settle and may lead to free water accumulation adjacent to the wax pattern, resulting in the surface roughness. Excessive vibrations may also dislodge small patterns from the sprue former, resulting in a miscast.

47. Setting of Investment Open air Hygroscopic technique Controlled water added technique

48. CASTING PROCEDURE

49. The invested pattern is ready for burnout procedures. The crucible former and any metal sprue former are carefully removed. Any debris from the ingate area are cleaned with a brush. Casting procedure Crucible former Removed.

50. BURNOUT PURPOSE Vaporize and thus eliminate wax pattern. Drives off moisture in the mould. Expand the mould to compensate for contraction of metal on cooling.

51. Wax Burnout Burnout is done in 2 stages: Initially furnace is raised to 310◦c for 30mins Second stage- temperature raise depends on type of thermal expansion required. Hygroscopic low heat expansion High heat thermal expansion

52. Wax Burn out For Gypsum investment - 650 – 700 Slowly heated for 60 min & held 15 – 30 min at upper temperature 700◦C for 30mins

53. For phosphate investments 2% or more expansion is required for Metal – Ceramic Prosthesis Burn Out temperature – 700 – 1030 C Slow – till 315 C & quite rapid thereafter

54. Advisable to begin the burnout procedure while the mold is still wet.

55. TIME ALLOWABLE FOR CASTING The investment contract thermally as it cools. Because of the liner and low thermal conductivity of investment, a short period can elapse before the temperature of the mold is appreciably affected. Under average condition of casting, approximately 1min can pass with out a noticeable loss in dimensions.

56. CASTING

57. Casting Heat source Torch flame - Gas air, Gas oxygen, Air acetylene, Oxygen acetylene. Electricity – electrical resistance or induction Casting force Gravitational force Vaccum Pneumatic pressure – Steam/ Gases Centrifugal force Piston-plunger force (for pressurable ceramics)

58. Methods of Melting the Alloys Generally two methods are employed for melting of metal alloy prior to casting: Flame / Blow pipe. Electrical .

59. FLAME The fuel employed in most cases is a mixture of natural or artificial gas and air, oxygen-air and acetylene. The temperature of gas-air flame is influenced by the nature of the gas and the proportion of gas and air in the mixture. Considerable care should be taken to obtain a non- luminous brush flame, with combustion zones clearly differentiated for melting the alloy .

60. ELECTRICAL There are two methods by which electricity can be employed to melt the alloy. Induction. Electric Arc (Resistance). Both the methods work on same principle i.e. heat energy is produced when electric current is passed through a conductor depending upon the voltage applied across it.

61. Induction The apparatus is simple in design consisting of high frequency induction coil across which high voltage is applied. This high frequency coil surrounds the crucible in which the alloy / metal pellets are melted. Types of crucibles: Ceramic crucible . Graphite crucible.

62. Graphite crucible Graphite is a good conductor of heat and electricity. It transmits the heat produced by the high frequency induction coil, to the metal / alloy pellets in the crucible. This process is the indirect heating of the pellets in the graphite crucible. Used for Noble metal alloys.

63. Ceramic crucible Ceramic is a bad conductor of heat and electricity. used for base metal alloys like nickel-chrome, chrome-cobalt.

64. Electric Arc / Resistance This is a crucible free technique where by the metal / alloy pellet is directly melted by the heat produced as a result of resistance offered by it to high voltage current. The apparatus consist of an electrode and a base plate on which the metal / alloy pellet is placed. The contact between the electrode terminal and the metal / alloy pellet is in form of an ‘electric arc’ thus the term ‘ electric arc furnace ’.

65. CASTING MACHINES

66. Centrifugal casting machine The alloy is melted in separate crucible by a torch flame and the metal is cast into the mold by centrifugal force. High frequency centrifugal casting machine The alloy is melted electrically by a resistance or induction furnace, then cast into the mold centrifugally by motor or spring action . Vacuum pressure casting machine The alloy is melted by a torch flame, or, electrically, and then cast by air pressure and / or by vacuum.

67. CENTRIFUGAL CASTING MACHINE This machine utilize the centrifugal force which is defined as a radial force radiating outward from the center of rotation of a body, for casting. casting machine

68. 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 .

69. The metal / alloy pellets are placed in the crucible. Flame/blowpipe is use to heat the alloy/metal in the crucible.

70. Once the alloy / metal is in a molten form. The flame is removed and the broken-arm is released.

71. Induction melting machine Alloy is melted by an induction field that develops within the crucible surrounded by water-cooled metal tubing. 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.

72. When the alloy reaches the casting temperature, it is forced into the mold by centrifugal force by air pressure/vacuum. Used for melting base metal alloys(to minimize risk of excessive oxidation) No difference in the accuracy of castings made with either of the three techniques

73. Direct-current arc melting machine Direct-current arc is produced between two electrodes, the alloy and the water cooled tungsten electrode. Temperature within the arc exceeds 4000°c and the alloy melts very quickly. Method has high risk of overheating the alloy

74. CASTING OF GLASS CERAMICS

75. Spruing : 8-10 guage Investing : A high quality casting ring that oxidises at a slow rate with one layer of kaolin ring liner Investment material : Phosphate bonded Burnout : Initially - 480 c for 30mins Final temp -1750 c for 2hrs

76. Casting : DICOR casting machine with platinum electrical resistance. Divesting : cut 1/4 th from the end, remaining is removed with 25µm aluminium oxide Sprue removal: double sided diamond disc Embedding: ceramic embodiment powder is mixed with 15cc distilled water is placed over the crown Ceramming : temp. upto 1075 c for 6hrs and is cooled to 392 F, then its removed.

77. CASTING OF POST CORE RESTORATION

78. Dowel core pattern can be made with direct or indirect technique. After the dowel core pattern is ready, it is sprued on the incisal / occlusal end. Add 15cc of extra water to 50g investment without a liner, so as to result in smaller dowel core to bind into the canal

79. CASTING OF TITANIUM ALLOY

80. The main problem with casting of titanium is its high reactivity to oxygen. Pressure/vacuum casting machine, with argon-arc system for melting of alloy Investment material should be non reactive and it should have adequate permeability for escape of gases Casting of titanium alloy is made in two chamber casting machine.

81. RECOVERY AND FINISHING OF CASTING

82. Recovery of the casting Trimming is done from the button end of the ring Investment is being pushed out of the casting ring

83. After the casting has solidified the ring is removed and quenched in water as soon as the button exhibits a dull yellow glow. Recovery of casting

84. When the water contacts hot investment, a violent reaction ensures , resulting in as soft, granular investment that is easily removed. Advantages of Quenching The ring in quenched in cold water.

85. The surface of casting appears dark with oxide and tarnish. Such a surface film can be removed by the process: Pickling. which consist of heating the discolored casting in an acid. Pickling solution: Gypsum bonded investment: 50% hydrochloric acid. Phosphate bonded investment: cold HF acid. PICKLING

86. Casting is placed in the warm pickling solution for few seconds. Acid should be heated, but not to be boiled. After pickling, acid is poured off and the casting is removed. Pickling solution should be renewed frequently. METHODS OF PICKLING

87. Another method is to heat the casting and then drop into the acid. The disadvantage of this method is that, a delicate margins may be melted in the flame, or the casting may be distorted by the sudden thermal shock when plunged into an acid . Gold and palladium based metal ceramic alloys and base metals , these alloys are not generally pickled.

88. Degassing Remove contamination or possibly to remove entrapped gas from the alloy . Base metal alloys heated to 1065 and held for 5-10mins in a 25-28 inch mercury vacuum & rapidly cooled to room temperature . A thin oxide layer is formed which is later sand blasted .

89. After recovery of casting the casting is held in a sandblasting machine to clean the investment from its surface SAND BLASTER & SAND BLASTING MACHINE

90. Remove sprue with double sided diamond disc. Removal of sprue. Polishing of gold casting is done with carbide burs , green stones, pink stone, tripoli or rouge. Polishing of base metal alloy casting is done with aluminum oxide,tin oxide.

91. DEFECTS IN CASTING

92. 1. Dimensional Inaccuracies &Distortion 2. Surface roughness and irregularities 3. Porosity Solidification defects Localized shrinkage porosity Microporosity Trapped gases Pin hole porosity Gas inclusion porosity Sub surface porosity Residual Air 4. Incomplete casting Classification of casting defects.

93. Dimensional inaccuracies and Distortion Problem Too small casting Likely causes Too little mold expansion Remedies Use correct temperature Too large casting Too much mold expansion Use correct temperature and investment material. Distorted Stress relief of wax pattern Warm wax thoroughly before creating pattern.

94. Surface roughness and irregularities Problem Likely causes Remedies Rough surface Breakdown of investment. Air bubbles on wax pattern. Weak investment Do not overheat mold or alloy. Use wetting agent and/or vacuum investing technique. Avoid using too much water when mixing investment or too much wetting agent on wax pattern.

95. Surface roughness and irregularities Problem Likely causes Remedies Fins Cracking of investment. Avoid heating investment too rapidly.

96. POROSITY Problem Likely causes Remedies SOLIDIFICATION DEFECTS Localized Shrinkage. Premature termination of flow of molten metal. Use of reservoir. Suck Back. Hot spot created by hot metal impinging on point on mold surface. Flare the point of attachment of sprue. Less temp.difference between mold and melt. Micro Porosity. Rapid solidification if mold or casting temp. is too low. Increase the mold or casting temp.

97. POROSITY Problem Likely causes Remedies TRAPPED GASES Pin hole Porosity. Absorbed gases are expelled on solidification. Prevent oxidation of alloys. Gas inclusion Porosity. Gas mechanically trapped by molten metal in mold. Gas incorporated during casting procedures. Correctly adjusting and positioning the torch during melting. Sub Surface Porosity. Simultaneous nucleation of solid grains and gas bubbles. Rate of entering of molten metal in mold.

98. POROSITY Problem Likely causes Remedies RESIDUAL AIR Back Pressure Porosity. Inability of escape of gases in the mold. Pressure gradient that displace air towards the end of investment. Tendency for mold to clog with residual carbon. Proper venting. Sufficient casting pressure. Proper burn out temperature.

99. INCOMPLETE CASTING Problem Likely causes Remedies Rounded margins. Back pressure of air due to low porosity of mold. Place pattern no more than 6-8mm from the end of the casting ring. Use porous investment or vents. Ensure there is no wax left. Cast with sufficient force.

100. INCOMPLETE CASTING Problem Likely causes Remedies Short castings Alloy deficiency. Mold too thin, too cold or had blocked sprue. Insufficient casting force. Use enough alloy that is completely molten. Use correct diameter sprue and heat to correct temp. Ensure casting machine is correctly balance and wound up.

101. Contamination Problem Likely causes Remedies Oxidation Overheating in air Do not use oxidizing flame or heat for too long. Use flux to protect molten alloy. Sulphur Breakdown of investment particularly gypsum bonded Do not overheat the investment

102. STUDIES CONDUCTED

103. Effect of Wet and Dry Cellulose ring liners on setting expansion and compressive strength of a gypsum bonded investment. J.of Prosth.Dent.1996;(76):519-21 OBJECTIVES: 1) Effect of water saturated or dry ring liner material on the fit of castings on steel die. 2) On compressive strength of investment core. RESULTS: 1) Use of dry cellulose ring liner resulted in significantly small occlusal and gingival spaces between casting and the die. 2) Use of dry cellulose ring liner resulted in significantly higher compressive strength of heated investment cores.

104. Three factors effecting investment setting expansion and casting size OBJECTIVE: The related effect of mixing rate, ring liner position and storage condition on setting expansion of investment and correlate casting size with measured expansion data. RESULTS: The rate and magnitude of setting expansion varied directly with rate of mixing. Setting expansion was least with no ring liner and greatest with full liner which allowed for no restriction of the investment in the ring. If mold is to be stored, max. dimensional stability is obtainted by placing in 100% humidor.

105. Inner surface roughess of complete cast crown made by centrifugal casting machine. OBJECTIVE: Six variables that could effect surface roughness were investigated. Type of alloy, mold temperature, metal casting temperature, casting machine, sand blasting and location of each section. RESULTS: 1)The trailing portion of the complete cast crown had rough surfaces than the leading portion. 2)Higher mold and casting temperature produce rough castings. 3)Sand blasting reduce the roughness.

106. Dimensional accuracy of castings produced with ringless and metal ring investment systems. J prosthet dent.july 2000;vol 84,no 1.Lombardas et al . Conducted a study to know the dimensional accuracy of castings produced with ringless and metal ring investment systems. The high strength of phosphate material makes it possible to abandon the use of casting ring. In ringless technique,the restriction of thermal expansion associated with the presence of metal ring is avoided

107. Investment without a metal casting ring produced smaller vertical discrepancies In terms of seating accuracy, the ringless and metal ring investing procedures can produce clinically acceptable castings.

108. References Phillips’science of dental materials – Kenneth J.Anusavice,11 th Edition . Dental laboratory procedures – Fixed partial dentures -Robert M. Murrow, Kenneth D. Rudd, John E. Rhoads . Contemporary Fixed Prosthodontics – Stephen F. Rosenstiel , Martin F. Land, Junhei Fujimoto,3 rd Edition. Restorative dental materials – Robert G.Craig , John M. Powers . Lombardas . Dimensional accuracy of castings produced with ringless and metal ring investment systems. J prosthet dent.july 2000;vol 84,no1

109.   The castability of pure titanium compared with Ni-Cr and Ni-Cr-Be alloys The Journal of Prosthetic Dentistry , Volume 98, Issue 6 , December 2007 , Pages 445-454 Silvana Maria Paulino Castability and resistance of ceramometal bonding in Ni-Cr and Ni-Cr-Be alloys The Journal of Prosthetic Dentistry , Volume 85, Issue 3 , March 2001 , Pages 299-304 Osvaldo L. Bezzon . New method for divesting cobalt-chromium alloy castings: Sandblasting with a mixed abrasive powder. Yoshiaki  Tagaa , Keiji  Kawai . (J Prosthet Dent 2001;85:357-62 .) Effect of Wet and Dry Cellulose ring liners on setting expansion and compressive strength of a gypsum bonded investment. J.of Prosth . Dent.1996;(76):519-21

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