casting defects and casting procedures in fixed p

Casting – Lost Wax Process
The lost wax casting process is widely used as it offers asymmetrical casting with
very fine details to be manufactured relatively inexpensively. The process involves
producing a metal casting using a refractory mould made from a wax replica pattern.
The steps involved in the process or the lost wax casting are:
• Create a wax pattern of the missing tooth / rim
• Sprue the wax pattern
• Invest the wax pattern
• Eliminate the wax pattern by burning it (inside the furnace or in hot water). This
will create a mould.
• Force molten metal into the mould - casting.
• Clean the cast.
• Remove sprue from the cast
• Finish and polish the casting on the die.
• Defects
Figure 1 - depiction of a dental casting mould
A general description of the process is as given above, however it might need to be
modified to account for different alloys being used. Cobalt alloys have melting
temperatures between 1250 and 1450oC which is higher than the investment made of
gypsum decomposition temperature. Because of that, the gypsum investment must be
replaced by phosphate or silica bonded type material.
The method of the alloy melting also depends on the alloy used since the temperatures
required are higher than the ones which can be achieved using a torch, or the alloy can
chemically react with gases.
Casting ring
Liner
Investment
Crucible
former
Sprue
Cavity formed
by wax

1. PRESENTATION BY SAYED SUMIYA CASTING DEFECTS GUIDED BY DR.GIRIJA.S.SAJJAN PROF AND HOD DEPARTMENT OF CONSERVATIVE DENTISTRY AND ENDODONTICS

2. CONTENTS INTRODUCTION DEFINITION OF THE CASTING AND CASTING DEFECTS LOST WAX TECHNIQUE CLASSIFICATION OF CASTING DEFECTS DIMENSIONAL INACCURACIES OR DIMENSIONAL ERRORS IN CASTING DISTORTION SURFACEROUGHNESS SURFACE IRREGULARITIES NODULES FINS RIDGES DISCOLORED CASTINGS PITS INCOMPLETE CASTING POROSITY

3. CONTENTS LITERATURE REVIEW CONCLUSION REFERENCES

4. DEFINITION C asting : An object formed by the solidification of a fluid that has been poured or injected into a mold. Casting defects are any impressions or irregularities that result in unsuccessful casting which interfere with the fit of the final restoration or its esthetic and mechanical properties. ROBERT.CRAIG-8THEDITION-PROPERTIES AND MANIPULATION,PAGE NO-256.

5. INTRODUCTION Lost-wax casting (also called by its French name, cire perdue , from the Latin cera perduta ) is the process by which a metal (such as silver , gold , brass or bronze ) sculpture is cast from an artist 's sculpture. Intricate works can be achieved by this method, primarily depending on the carver's skills. In industrial uses, the modern process is called investment casting . PROCESS IN WHICH A WAX –PATTERN ,PREPARED IN THE SHAPE OF A MISSING TOOTH STRUCTURE,IS EMBEDDED IN A CASTING INVESTMENT AND BURNED OUT TO PRODUCE A MOLD CAVITY INTO WHICH MOLTEN METAL IS CAST. PHILLIPS-11 TH EDITION ,PART-4,PAGE-564

6. HISTORY In the literature, credit is given to Dr. Swasey (1890 ),who introduced a technique where a solid gold inlay could be prepared . In his technique, the gold foil was first adapted to the shape and contour of the tooth,then removed, invested, and filled with 20-karat gold. Wax was used for making gold inlays for the first time by Martin (1891 ). According to Martin's technique, there was no need to use a foil liner . He filled the cavity with wax, removed it after hardening, invested it , burned it out, and then poured the molten gold into the mold formed in the investment .

7. HISTORY A few years later , Dr. Phil brook (1896 ) introduced a pressure-casting method of producing gold inlays. ( CASTING METALS IN DENTISTRY: PAST - PRESENT – FUTURE) K. ASGAR ,ADV DENT RES 2(L):33-43, AUGUST, 1988). The lost wax technique has been used for centuries but its use in dentistry was not common until 1907 when W.H. Taggart introduced this technique with the casting machine.

8. A FAILED MOD AMALGAM RESTORATION IN THE SECOND PREMOLAR TOOTH. Indirect cast metal, porcelain, and composite intra-coronal restorations, Pickard’s Manual of Operative Dentistry, Eighth edition

9. DENTAL ALLOYS USED AS CASTING METALS FOR CAST METAL RESTORATIONS IN OPERATIVE DENTISTRY Type 1 Type 2 Type 3 Type 4 alloys

10. THE CASTING PROCESS-FLOW CHART ROBERT.CRAIG-8 th EDITION-PROPERTIES AND MANIPULATION,PAGE NO-257.

11. ALTERNATIVE METHODS FOR PRODUCING DIRECT AND INDIRECT TECHNIQUES

12. CLASSIFICATION OF DEFECTIVE CASTINGS PHILLIPS EDITION -11,CHAP-12,PAGENO:337.

13. CLASSIFICATION ROUGH CASTING LARGE AND MULTIPLE NODULES FINS INCOMPLETE CASTING INCOMPLETE CASTING WITH SHINY, ROUNDED DEFECT "SUCK-BACK" POROSITY MARGINAL DISCREPANCY INADEQUATE OR EXCESSIVE EXPANSION ROSENSTIEL -3 RD EDITION,INVESTING AND CASTING

14. CLASSIFICATION The faults which can occur in casting may be of four types. (1) Finning and bubbling. (2) Incomplete casting. (3) Porosity in casting. (4) Oversized or undersized casting.

15. REMELTING PREVIOUSLY CAST METAL When fusing an alloy to prepare a casting ,a dental laboratory will typically use new metal ingots ,and they may also add metal sprues that were removed from previous castings. The accepted guideline is that 50% new alloy should be used with the previously melted alloy. A study made on a type -3 alloy, found significant decrease in the yield strength, percentage elongation for specimens that consisted of entirely of metal that had been previously melted one time and 2 times . The prevalence of casting defects increased with the number of times that alloy was melted. Conclusion: There is no establishment limit on the number of times that previously cast metal can be reused along with 50% new alloy and still yield clinically acceptable castings of adequate strength and long term in-vivo performance.

16. CASTING SHRINKAGE Solidus temperature-at which an alloy becomes solid on cooling . Liquidus temperature-at which an alloy begins to freeze on cooling. It occurs in 3 stages PHILLIPS -11 TH EDITION –PAGE NO;577.

17. 2 methods to achieve compensation for casting shrinkage Low heat technique High heat technique

18. THE RELATIVE SOLIDIFICATION SHRINKAGE OF VARIOUS ALLOYS CAST AS SMOOTH CYLINDERS PHILLIPS,11 TH EDITION,PAGE NO:578

19. LOGICAL ASSUMPTIONS When the mold becomes filled with molten metal ,the metal starts to solidify at the walls of the mold ,because the temperature of the mold is less than that of the bulk molten metal During initial cooling the first layer of metal to solidify against the walls of the mold is weak,and tends to adhere to the mold until it gains sufficient strength as it cools to pull away. Prevention: Mechanical adhesion to the walls of the mold. when the metal is sufficiently strong to contract independently of the mold it shrinks thermally until it reaches room temperature.

20. DETAILED DESCRIPTION OF CASTING DEFECTS

21. DISTORTION Defect in casting is due to distortion in wax pattern. REASONS: A the investment around it hardens. The setting and hygroscopic expansions of the investment may produce a non uniform expansion of the walls of the pattern. It depends upon the configuration of the pattern, the type of wax, thickness of the pattern, setting expansion of the investment material. PREVENTION : PROPER MANIPULATION OF WAX AND HANDLING OF WAX PATTERN.

22. SURFACE ROUGNESS AND IRREGULARITIES AND DISCOLORATION AIR BUBBLES WATER FILMS RAPID HEATING RATES UNDERHEATING LIQUID/POWDER RATIO PROLONGED HEATING TEMPERATURE OF THE ALLOY CASTING PRESSURE COMPOSITION OF THE INVESTMENT FOREIGN BODIES IMPACT OF MOLTEN ALLOY PATTERN POSITION CARBON INCLUSIONS

23. 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 do not characterize the total surface area.

24. Air bubbles a) Air Bubbles on wax-pattern D ue to m anual investing method ,air might entrap during mixing of investment . Wetting agent should be applied in thin layer, it is best to air-dry the wetting agent ,any excess liquid dilutes the investment ,possibly producing surface irregularities on the casting. Avoided by : vacuum investing technique, vibrate before and after mixing. - Slightly tilt the casting ring, so that Surface entrapped air bubbles might rise to the surface. Use a wetting agent to reduce surface tension of wax.

25. WATER FILMS Wax is repellant to water, and if the investment becomes separated from wax pattern, a water film may form irregularly over the surface. - This may be seen if the, wax pattern is moved slightly or vibrated after investing. Too high L/P ratio may also produce such defects. The water films causes minute ridges or veins on the surface. Avoid by: Avoiding movement, vibration of pattern after investment. Use of wetting agent and painting investment properly on the pattern to ensure intimate contact .

26. TOO RAPID HEATING Features : Can cause fins / spines on casting. Due to rapid heating, flaking of investment occurs when steam forms. Also, the steam may carry some of the salts used as modifiers into the mold. Avoid by : Heat the mold gradually for 60 minutes from room temperature to 700"c .

27. UNDERHEATING If heating time is too short or if there is insufficient air in the furnace - can cause incomplete elimination of wax residues. Features : Seen as voids / porosity on casting. Hot alloy comes in contact with these carbon residues forming gases to form voids / porosity in the casting. Seen as a black coating because the casting is covered with carbon coating which cannot be eliminated by pickling. Avoid by : Heat the ring for adequate period of time so t hat the carbonaceous residue is removed.

28. INAPPROPRIATE WATER / POWDER RATIO Both increased water/ powder and decreased water / powder ratio causes rough castings . Increased water / powder ratio - rougher the casting. Decreased water/powder ratio - investment will be too thick. Therefore cannot be applied properly on the pattern. During vacuum investing, air might not be removed completely. Avoid by : Accurate water powder ratio.

29. CASTING PRESSURE Casting Pressure : Increase in casting pressure can cause rough surface on casting. Avoid by : Pressure should be adjusted to a guage pressure of 0.10 to 0.14 MPa in air pressure casting machine. 3-4 turns of spring in average type of centrifugal casting machine.

30. FOREIGN BODIES A rough crucible former with investment clinging to it may roughen the investment on its removal so that bits of investment are carried into the mold with molten alloy. Carelessness in the removal of the sprue former may also be a cause. Bits of carbon from a flux.

31. PATTERN POSITION If too many wax patterns placed in same ring, there should be a minimum distance of 3 mm between them . If it is less than 3 mm in distance (the expansion of wax is more than the expansion of the investment), the expansion of wax can cause breakdown of investment and cracking. Avoid by : Avoid placing too many wax pattern close to each other in same ring. Minimum distance of 3 mm among the wax pattern . Avoid the positioning of the patterns in the same plane in the mold .

32. POROSITY Porosity may occur both within the interior region of a casting and on the external surface . E xternal porosity can cause surface roughness. I nternal porosity weakens the casting and if it extends to the surface, it may be a cause for discoloration . If it is severe, it can produce leakage at the tooth - restoration interface and secondary caries may result . It cannot be prevented entirely but it can be minimized by the use of proper techniques.

33. CLASSIFICATION OF POROSITIES I)Those caused by solidification shrinkage. localized shrinkage porosity Micro porosity ll ) Porosities caused by trapped gases A. Pinhole porosity B . Gas inclusion porosity C. Subsurface porosity III. Due to residual air Back pressure porosity .

34. LOCALISED SHRINKAGE POROSITY cause: By premature termination of the flow of molten metal during solidification. It occurs near the sprue –casting junction. Suck-back porosity The void may occur externally ,usually in the interior of a crown near the area of a sprue ,if a hot spot has been created by the hot metal impinging from the sprue channel on a point of the mold wall . T his spot causes the local region to freeze last and results in what is called suck-back porosity . It occurs at an occluso -axial line angle ,when it is not rounded.

35. The entering metal impinges on to the mold surface at this point and creates a higher localized mold temperature in this region ,known as hot spot. Suck-back porosity is eliminated by flaring the point of sprue attachment and reducing the mold-melt temperature differential,that is ,lowering the casting temperature by about 30 degree.

36. MICRO POROSITY It also occurs from solidification shrinkage but it is generally present in fine-grain alloy castings when the solidification is too rapid for the micro-voids to segregate the liquid pool. This premature solidification causes irregular voids. Such phenomena occurs when the rapid solidification occurs from the mold or casting temperature is too low.

37. PIN HOLE AND GAS INCLUSION POROSITIES Entrapment of gases during solidification process. Both are same but the sizes of porosities are different . These can be minimized by pre-melting the alloy on a graphite crucible if the alloy has been used before ,and by correctly adjusting and positioning the torch flame during melting.

38. BACK PRESSURE POROSITY Entrapped air porosity on the inner surface of the casting ,sometimes reffered to as back-pressure porosity. Produces large concave depressions. caused by inability of the air in the mold to escape through the pores in the investment or by the pressure gradient that displaces the air pocket towards the end of the investment via the molten metal enters the molten sprue and button.

39. Proper burn-out ,an adequate mold and casting temperature ,a sufficiently high casting pressure and proper L/P ratio can help to eliminate this porosity. Make sure that the thickness of the investment between the tip of the pattern and the end of the ring not be greater than 6 mm.

40. INCOMPLETE CASTING INSUFFICIENT VENTING INCOMPLETE ELIMINATION OF WAX RESIDUES

41. SPRUE DESIGN FIVE GENERAL PRINCIPLES SPRUE DIAMETER SPRUE POSITION SPRUE ATTACHMENT SPRUE DIRECTION

42. SPRUE DIAMETER Sprue former selection Diameter =thickest portion of the area on wax pattern. If the pattern is small…….. Attaching a large sprue former to a thin pattern could cause distortion. If the sprue diameter is too small ,this area will solidify before casting itself-localized shrinkage porosity.-reservoir serves to prevent such defects.

43. SPRUE POSITION It is often a matter of individual judgement based on the shape and form of the wax pattern. Occlusal surfaces Proximal wall Just below a non-functional cusp to minimize subsequent grinding of occlusal anatomy and contact areas. As indicated the ideal position is the point of greatest bulk in the pattern –avoids distorting thin areas of wax during attachment to the pattern and to permit complete flow of the alloy into the mold cavity.

44. SPRUE ATTACHMENT It is flared for high density gold alloys. The type of sprue former selected influences the burn out technique used. 2 types Wax sprue formers-commonly used Plastic sprue formers Spruing of the patterns Direct Indirect Reservoir It is added to the sprue network to prevent localized shrinkage porosity. The reservoir should remain molten to furnish liquid alloy into the mold as it solidifies.so the solidification shrinkage occurs in the reservoir bar ,not in the prosthesis.

45. SPRUE DIRECTION The sprue former should be directed away from the thin or delicate parts of the pattern,because the molten metal may abrade the investment- resulting in casting failure. It should not be attached at a right angle to a broad flat surface. It leads to turbulence and leads to severe porosity. When sprued at 45 degrees angulation to the proximal area ,a satisfactory casting is obtained.

46. SPRUE LENGTH The length of the sprue former depends on the length of the casting ring. If the sprue is too short…..the wax pattern may be so far removed from the end of the casting ring that gases cannot be adequately vented to permit the molten alooy to fill the ring completely.-leads to porosity….

47. LAWS OF CASTING Based on the earlier work of Ingersoll and Wandling (1986), Naylor formulated an expanded set of 17 separate recommendations for spruing , investing and burn-out and melting and casting procedure. collectively these guidelines are referred as LAWS OF CASTING’

48. 1st l aw Attach the pattern sprue former to the thickest part of the wax pattern. 2nd Law orient wax patterns so all the restoration margins will face the trailing edge when the ring is positioned in the casting machine. 3rd Law Position the wax patterns in a cold zone of the investment mould and the reservoir in the "heat center" of the casting ring. 4th Law A reservoir must have molten alloy to accommodate the shrinkage that occurs within the restorations.

49. 5th Law Do not cast a button if a connector bar or other internal reservoir is used. 6th Law Turbulence must minimized if not totally cannot be eliminated. 7th Law S elect a casting ring of sufficient length and diameter to accommodate the patterns to be invested. 8 th law Increase the wettability of the castings. 9 th law Weigh any bulk investment and measure precise power/liquid ratio. 10 th law Eliminate the incorporation of air in the casting investment and remove free Ammonia gas product of phosphate bonded investment by mixing under vacuum. 1lth Law Allow the casting investment to set completely before initiating the burn-out procedure.

50. 12 th law Use a elimination technique that is specific for the type of patterns involved and recommended for the particular type of casting alloys selected. 1 3th Law - Adequate heat must be available to properly melt and cast the alloy . 1 4th Law - when torch casting, use the reducing zone ,of the flame to melt the alloy donot use the oxidizing zone. I5th Law p rovide enough force to cause the liquid alloy to flow into the heated mould . 16 th law Cast towards the margins of the wax pattern. 17 th law Do not quench the ring immediately after casting.

51. Hygroscopic Low Heat Technique Obtains compensation expansion from 3 sources. 37°C water bath expands wax pattern. Warm water entering the investment mold from top adds some hygroscopic expansion. Thermal expansion at 500°C provides needed thermal expansion.

52. Advantages of Low Heat Technique Less investment degradation. Cooler surface for smoother castings. Convenience of placing molds directly in 500°C furnace. Care taken for sufficient burnout time. The molds should remain in furnace for atleast 60 min. Back pressure porosity great hazard in low heat technique.

53. Standardized hygroscopic technique was developed for alloys with high gold content; the newer noble alloy may require slightly more expansion. This added expansion may be obtained by making 1 or more of following changes. Increasing water bath temperature to 40°C. Using two layers of liners. Increasing burnout temperature to a range of 600°C to 650°C.

54. High-Heat Expansion Technique Depend almost entirely on high-heat burnout to obtain the required expansion, while at the same time eliminating the wax pattern. Additional expansion: Slight heating of gypsum investments on setting. Thus expanding the wax pattern. Water entering from wet liner adds a small amount of hygroscopic expansion to the normal setting expansion.

55. Gypsum Investments Fragile and require use of metal ring. Slowly heated to 600°C to 700°C in 60 mins. and held for 15 to 30 min. at the upper temperature. Rate of heating: Smoothness. Overall dimension of investment. Too rapid heating. Cracking of investment (outside layer expands >center section). Radial cracks interior to outward. Casting with fins or spines. Common with cristobalite investment.

56. Investment decomposition and alloy contamination is related to the chemical reaction between the residual carbon and sulfate binder. Calcium sulfate does not decompose unless heated above 1000°C. Reduction of calcium sulfate by carbon takes place rapidly above 700°C. CaSO 4 + 4C  CaS + 4CO 3CaSO 4 + CaS  4CaO + 4SO 2 This reaction occurs when gypsum investment are heated above 700°C in presence of carbon. SO2 as a product of this reaction contaminates gold castings and makes them extremely brittle.

57. Sulfur gases are generated when investment is heated above 700°C. After casting temperature reached  Casting should be made immediately. Sulfur contamination. Rough surface on casting. Improved investment formulations. Gypsum investment with considerable amount of cristobalite.

58. Few investments may be directly placed into furnace at final burnout temperature  held for 30 min. & cast. Factors affecting size and smoothness. Design of the furnace. Proximity of the mold to the heating element. Availability of air in the muffle.

59. SETTING TIME FORWAX ELIMINATION & HEATING For gypsum bonded investment 500°C for hygroscopic technique. 700°C for thermal expansion technique. Phosphate bonded 700°C to 1030°C.

60. TIME ALLOWABLE FOR CASTING Investment contracts thermally as it cools. When thermal expansion / high heat technique used. Investment loses heat after heated ring removed from furnace and mold contracts.

61. PHOSPHATE INVESTMENTS Obtain their expansion from following sources. Expansion of the wax pattern. Setting expansion (because of liquid used). Thermal expansion Phosphate investments much harder and stronger than gypsum investments. Burnout temperature range from 750°C to 1030°C. 315°C  rapid heating  held at the upper temperatures for 30 mins.

62. 2 stage heating: Placed directly in furnace at top temperature. Held for 20-30 min. then cast Elimination of metal ring and liner to save more time. Metal ring replaced with plastic ring i.e. tapered.

63. Low heat technique High heat technique Temperature gradient not great. Great temperature gradient. Thermal expansion no important to shrinkage compensation. Thermal expansion important. Burnout temperature on a fairly steep portion of thermal expansion curve. Burnout temperature on thermal expansion plateau.

64. CASTING MACHINES Alloys are melted in one of the 4 following ways: In a separate crucible by a torch flame, and cast centrifugally. Electrically by a resistance heating or induction furnace. Centrifugally by motor or spring action. By induction heating. Centrifugally by motor or spring action. Vacuum arc melted. Cast by pressure in argon atmosphere. Molten metal may be cast by air pressure, by vacuum, or both.

65. Torch melting centrifugal casting machine Casting machine spring wound from 2 to 5 turns Alloy is melted by a torch flame in a glazed ceramic crucible attached to the “broken-arm” of casting machine. Broken arm feature accelerates the initial rotational speed of the crucible and casting ring. Torch flame is generated from a gas mixture of propane and air. Machine is released and spring triggers the rotation motion. As the metal fills the mold, a hydrostatic pressure gradient develops along the length of the casting.

66. CENTRIFUGAL CASTING MACHINE

67. The pressure gradient from the tip of the casting to the bottom surface is quite, sharp and parabolic in form, reaching zero at the button surface. Pressure gradient before solidification reaches 0.21 to 0.28MPa (30 to 40 psi) at the tip of casting. Greatest rate of heat transfer to the mold is at the high pressure end of the gradient (tip of the casting).

68. ELECTRICAL RESISTANCE-HEATED CASTING MACHINE Current is passed through a resistance heating conductor, and automatic melting of the alloy occurs in a graphite or ceramic crucible. Advantages: For metal ceramic prosthesis. Base metals in trace amounts that tend to oxidize on overheating. Crucible located flush against casting ring. Carbon crucibles should not be used in melting of: High Pd Pd-Ag Ni-Cr Co-Cr

69. ELECTRIC RESISTANCE CASTING MACHINE

70. INDUCTION MELTING MACHINE Alloy is melted by induction field that develops within a crucible surrounded by water cooled metal tubing. The electrical 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 crucible. Once the alloy reaches casting temperature in air/vacuum it is forced into mold by centrifugal force by air pressure, or by vacuum. More commonly used for melting base metal alloys.

71. INDUCTION MELTING CASTING MACHINE

72. Direct Current Arc Melting Machine Direct current arc is produced between two electrodes- the alloy and water cooled tungsten electrode. The temperature between the arc exceeds 4000°C. Has high risk of overheating.

73. TORCH MELTING OF NOBLE ALLOY Melted by placing on inner sidewall of crucible. Fuel used - Natural or artificial gas and air. - Oxygen air and acetylene. Non-luminous brush flame with different combustion zones should be obtained. Mixing zone Combustion zone. Reducing zone. Oxidizing zone. At proper casting temperature the molten alloy is light orange and tend to spin or follow the flame. Alloy should be approx. 38°C to 66°C above liquidus temperature.

74. USE OF FLUX FOR GOLD ALLOYS Minimize porosity To increase the fluidity of the metal Film of flux formed over surface of the alloy prevents oxidation Eg: 1. powdered charcoal 2. Fused borax powder with boric acid powder

75. CLEANING THE CASTING ANNEALING PICKLING(50% HCl)

76. PURPOSE OF QUENCHING All intermediate phases are changed into a disordered solution and tensile strength, proportional limit and hardness are reduced and ductility is increased. When water contacts investment a violent reaction ensues, resulting in soft granular investment that is easily removed.

77. PICKLING ADVANTAGES OF HCl Aids in removal of residual investment Aids in removal of oxide coating DISADVANTAGES OF HCl Likely to corrode laboratory metal furnishings Fumes are health hazard, which are to be vented

78. FINISHING AND POLISHING BROWN OR WHITE ALUMINIUM OXIDE STONES WHEELS Cleaning can be done also by using hemostat, steam or clean framework

79. REFERENCES PHILLIPS –SCIENCE OF DENTAL MATERIALS-11 TH EDITION. CONTEMPORARY FIXED PROSTHODONTICS-ROSENSTIEL-4RTH EDITION. RESTORATIVE DENTAL MATERIALS-CRAIG. DENTAL MATERIALS;WILLIAM .J.OBRIEN-3 RD EDITION.