casting laws & defects in prosthodontics.pptx

casting laws and defects in prosthodontics

1. Presented by : Dr. K. Lohitha I yr pg student Dept. of Prosthodontics Sibar Institute of Dental Sciences Guntur. What are the laws of casting ? Detail in various casting defects and describe in detail how to avoid them. Seminar on

2. PREVIOUS QUESTIONS What are the laws of casting? Detail in various casting defects and describe in detail how to avoid them. Casting machines and casting techniques. – RGUHS 2010. Casting defects – RGUHS 2006.

3. CONTENTS Introduction Review of literature Casting procedure Laws of casting Casting defects Classification Causes Solution Summary Conclusion Bibliography & references.

4. Introduction Casting is both an art and a science governed by numerous rules, or “laws”. Personal interpretation of these laws is demonstrated in a technician’s approach to the procedure, i.e.. The art of casting . Integrating the theoretical principles supporting this art enables you to blend science with art in dental technology.

5. CASTING :- It 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-8,2005). At times, casting problems can be attributed to poor casting technique and / or a failure to adhere to one or more of the basic principles of casting.

6. David B. Mahler and A. Bruce Ady (1963) They investigated the influence of factors on setting expansion and summarised as follows: Expansion of investment away from the wax pattern is relatively small than the expansion of the investment surrounded by the wax pattern. REVIEW OF LITERATURE

7. Greased and dry asbestos liners tend to decrease effective setting expansion whereas loose and double asbestos tend to increase it. Use of soft wax results in greater effective setting expansion and Over spatulation and thick mix increase setting expansion.

8. V.P.Delgado and F.A.Peyton (1953) They studied hygroscopic expansion of investment and summarised as follows: The use of mechanical spatulation or hand spatulation does not affect the amount of hygroscopic setting expansion, when a water bath at mouth temperature is used. Mechanical spatulation gives higher expansion values for thick mixes than hand spatulation when a water bath at room temperature is used.

9. CASTING PROCEDURE

10. 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 ” . These include : Laws of casting Introduction to Metal Ceramic Technology by W. Patrick Naylor

11. 1-7 laws – spruing and positioning of wax pattern. 8-12 laws – investing and wax elimination ( burn out) procedures. 13-17 laws – melting and casting.

12. 1 ST LAW Attach the pattern Sprue former to the thickest part of the wax pattern. Sprue former - occlusal / incisal surface.

13. As the molten metal alloy moves from the reservoir to the pattern margins it should flow from areas of greater volume to areas of lesser volume (i.e., margins). Molten metal flowing from thin area to a thicker region (full wax up) may solidify before the mold is completely filled.

14. Penalties for not obeying this law are: Cold shuts. Short margins. Incomplete castings.

15. 2 ND LAW Orient wax pattern so all the restoration margins will face the trailing edge when the ring is positioned in the casting machine. To identify that orientation, add a wax dot to the crucible former so you know how to place the ring in the casting cradle correctly after the pattern have been invested.

16. Penalties for not obeying this law are: Cold shuts. Short margins. 3 RD LAW Position the wax pattern in a “cold zone” of the investment mold and the reservoir in the “heat centre” of the casting ring.

17. Cold zones - end of the ring and along the ring periphery. The hottest portion - near the centre of the ring (heat centre).

18. Limit the amount of investment covering the patterns to no more than ¼ inch (6mm) & position the reservoir in the heat centre. Adherence to this law increases the likelihood that casting porosity will occur in the reservoir rather than in the restoration.

19. The penalty for not obeying this law is: Shrink-age porosity. 4 TH LAW A reservoir must have sufficient molten alloy to accommodate the shrinkage that occurs within the restorations.

20. Alloy filling the restoration solidify first Shrinkage of molten metal within the restoration & creation of vacuum Vacuum created will draw the molten metal from the adjacent source i.e., Reservoir Reservoir should be equal to or greater than the thickest cross-section area of the wax pattern.

21. Penalties for not obeying this law are: Shrink-age porosity. Suck-back porosity. 5 TH LAW Do not cast a button if a connector (runner) bar, or other internal reservoir, is used.

22. With indirect Spruing , the largest mass of metal should be the reservoir. A button is counterproductive because it can draw available molten alloy from the bar, shift the heat centre and reduce the feed of that metal to the restorations.

23. Likewise, the wax pattern should not be larger than the connector bar, if the bar is to act as a true reservoir. Weigh the sprued patterns and use the wax pattern-alloy conversion chart to determine the amount of alloy needed.

24. Penalties for not obeying this law are: Shrinkage porosity (and potential distortion during porcelain firing). Suck-back porosity. 6 TH LAW Turbulence must be minimized, if not totally eliminated.

25. Pathways for the flow of metal – smooth & gradual. Restrictions, or constrictions, can accelerate the metal's rate of flow and abrade the mold surface, mold wash.(Ingersoll & Wandling,1986). Eliminate sharp turns, restrictions, points, or impingements that might create turbulence and occlude air in the casting.

26. Penalties for not obeying this law are: Voids in the casting ring. Surface pitting. Incomplete Casting. 7 TH LAW Select a casting ring of sufficient length and diameter to accommodate the patterns to be invested.

27. The casting ring should permit the patterns to be ¼ in. apart and ¼ in. from the top of the investment with a minimum 3/8 in. of investment between them and the ring liner.

28. Too little investment covering the wax pattern – break through the mold. Too much investment covering the wax pattern – impair the escape of gases. Penalties for not obeying this law are: Mold fracture. Casting fins. Shrinkage porosity.

29. 8 TH LAW Increase the wettability of the wax patterns A wetting agent should be brushed or sprayed on the patterns and dried thoroughly before investing. Penalty for not obeying this law is: Bubbles / fins.

30. 9 th law Weigh any bulk investment and measure the investment liquid for a precise powder-liquid ratio. The correct proportion of powder to liquid and any dilution of the (special) liquid with distilled water should be established for each alloy. Reduced liquid thick mix of investment Increases investment expansion Loose fitting castings

31. Increased liquid content less expansion and tight fitting castings. Using all special liquid provides more expansion but reduces working time compared to 50:50 dilution of the special liquid and distilled water. Penalty for not obeying this law is: Ill-fitting castings.

32. 10 th law Eliminate the incorporation of air in the casting investment and remove the ammonia gas by product of phosphate-bonded investments by mixing under vacuum. Vacuum mixing removes more air and gas than hand spatulation . Areas of the mold that contain dense, bubble-free investment will expand differently from sections that contain large voids (entrapped air).

33. Penalties for not obeying this law are: Small Nodules on the casting. Weak mold . Distortion of the casting. Allow the casting investment to set completely before initiating the burnout procedure 11 TH LAW

34. If setting is not complete at the time a ring is placed in the oven, the mold may be weak and unable to withstand steam expansion during burnout. Investment could fracture as a consequence. For best results, burnout should be initiated only after the recommended setting time. Penalties for not obeying this law are: Mold crackling / Blowout. Fins on the casting.

35. 12 th law 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. Plastic Sprues need to be heated slowly so they can soften gradually and not exert pressure on the mold , so use a two-stage burnout ( Tombasco & Reilly,1987).

36. Slow the rate of rise to permit the heat to move through the investment for uniform expansion. If burnout is incomplete, the spruing system channels may be blocked by wax or plastic residue (carbon) and, on casting, air cannot escape completely when metal enters the mold (Naylor,1990; Tombasco & Reilly,1987). Therefore, use at least a 30-minute heat-soaking at 800 F for the first burnout stage.

37. Penalties for not obeying this laws are: Cold Shuts. Short Margins. Cold Welds. Mold Cracks. Casting Fins.

38. 13 TH LAW Adequate heat must be available to properly melt and cast the alloy (Myers,1936) The selected heat source should be capable of melting the alloy to the point of sufficient fluidity. Prolonged heating, caused by an improperly adjusted torch, can prevent the alloy from attaining the fluidity. Too much heat, or too high temperature, can burn off minor alloying elements.

39. Penalties for not obeying this law are: Cold Shuts. Short Margins. Cold Welds (too little heat). Rough Castings. Investment Breakdown (too much heat).

40. 14 TH LAW When torch casting, use the “reducing zone” of the flame to melt the alloy and not the oxidizing zone. An improper adjusted torch can add carbon or oxygen to the alloy while heating. A melt achieved by the exclusive use of the Reducing zone minimizes the likelihood of metal oxidation and gas absorption and ensures a proper melt.

41. Penalties for not obeying this law are: Gas porosity Change in the alloy’s Coefficient of thermal expansion (due to alloy contamination)

42. 15 TH LAW Provide enough force to cause the liquid alloy to flow onto the heated mold . Adjust the casting machine to the requirements of each alloy. Lower-density metals generally need four winds of a centrifugal casting arm as compared to higher-density, gold based alloys. Don't over wind.

43. Penalties for not obeying this law are: Cold Shuts Short Margins Clod Welds (insufficient force) Mold Fracture & Fins (too much force)

44. 16 TH LAW Cast toward the margins of the wax patterns. Place the heated ring in the casting cradle using the orientation dot so the pattern margins face the trailing edge (the 2nd law). •In a centrifugal casting machine the metal will flow downward and to the right, taking advantage of the centrifugal, rotational, and gravitational forces of molten alloy (Ogura et al,1981; Ingresoll & Wandling,1986 ).

45. Penalties for not obeying this law are: Cold shuts Short margins Incomplete castings

46. 17th Law Do not quench the ring immediately after casting Allow the alloy and the investment to cool to room temperature. Uneven cooling and shrinkage between alloy and investment can apply tensile forces to the casting (Cascone,1976).

47. After casting, the alloy may not possess sufficient strength to resist these forces and the restoration could tear, if quenched. Penalty for not obeying this law is: Hot Tears in the restoration.

48. According to Anusavice Distortion Surface roughness and irregularities Porosity Incomplete or missing details Based on location Internal external Classification CASTING DEFECTS

49. According to Rosensteil Roughness Nodules Fins Incompleteness Voids or porosity Marginal discrepancy Dimensional inaccuracies

50. DISTORTION Distortion of the casting is 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.

51. Mainly influenced by: a) configuration of the pattern. b) type of wax used. c) thickness of pattern. It can be minimised by: Immediate investment of wax pattern. Minimal pressure application. Should be stored in a refrigerator, if necessary.

52. 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

53. AIR BUBBLES (nodules) If there is a a) large nodule – air entrapped during investing procedure. b) multiple nodules – inadequate vacuum during investing. c) nodules on occlusal surface – due to excessive vibration. d) nodules on underside – prolonged vibration after pouring.

54. They can be minimised by: Using vacuum investing technique. Use of mechanical mixer with vibration before and after mixing if manual method is used. Wetting agent should be applied in thin layers and should properly be air-dried. Large or situated on a margin - remaking of the restoration. Small - removed with a no.1/4 or 1/2 round bur

55. 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)

56. Rapid Heating Rates Causes : Fins or spines Flaking of the investment Prevented by: Heat gradually for atleast 60min from room temperature to 700°c. Greater the bulk – more slowly heated.

57. Liquid/Powder Ratio The amount of water and powder measure should be accurate. Increased l/p ratio – rougher castings. Decreased l/p ratio – insufficient venting of air. Pattern position Should not place too close together. Should not place many patterns in same plane. Space between the pattern is atleast 3mm.

58. Under heating Incomplete elimination of wax residues. Voids due to gases formed when hot alloy comes into contact with carbon residues. Casting may be covered with carbon coating that is difficult to remove.

59. PROLONGED HEATING Results in disintegration of gypsum investment and thus roughening of mold walls. Furthermore, decomposition products contaminate the alloy and such contamination is resistant to pickling. COMPOSITION OF INVESTMENT Ratio of binder/quartz, influence surface texture. Coarse silica - surface roughness.

60. Temperature of the alloy Too high temperature – surface roughness. Other fuels if used – light orange color emitted by gold alloy. Casting pressure To high pressure – rough surface of the casting and fins. To low pressure – incomplete casting Average – 0.01 to 0.14 Mpa and 3 to 4 turns of the spring.

61. 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 .

62. Impact of molten metal alloy Cause: The direct impact of molten alloy may fracture or abrade the mold surface regardless of its bulk. Prevented by: proper spruing . Carbon inclusions Carbon from carbon crucible – absorbed by the alloys during casting results in formation of carbides or visible carbon inclusion .

63. 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 air (back pressure porosity)

64. Localized shrinkage porosity (shrink-spot porosity) Causes: Premature termination of the flow of molten metal during solidification. Usually occur near sprue - casting junction.

65. Prevented by Using sprue of correct thickness. Attaching sprue at thickest portion of wax pattern. Flaring sprue at the point of attachment. Placing reservoir close to the wax pattern.

66. Suck back porosity In interior of the crown near the area of the sprue . Occur often on occlusoaxial / incisoaxial line angles that are not well rounded. Can be eliminated by: Flaring the point of sprue attachment. Reducing the mold-melt temperature difference i.e., lowering the casting temperature by about 30 degrees.

67. Microporosity Occurs from solidification shrinkage but is generally present in fine grain alloy castings when solidification is too rapid for microvoids to seggregate liquid pool. Irregular in shape. Undetectable unless sectioned

68. 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

69. 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.

70. 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, such that solidification occurs before the escape of trapped air . Thickness of the investment Incomplete elimination of wax residues.

71. 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.

72. Factors that inhibit the mold filling is : In sufficient venting In sufficient casting pressure Incomplete elimination of wax Lower L/p ratio Viscosity of the fused metal Incomplete casting

73. conclusion Good technique pays off. By standardising the technique and paying strict attention to each step involved in spruing , investing and casting,it is often possible to control the location of defect and thus can minimise the number of actual miscasts.

74. Bibliography and references K. J. Anusavice ; Phillips Science of Dental Materials; 11 th edition. Saunders; 2003. W.P. Naylor; Introduction to metal ceramic technology; 2 nd edition. Quintessence Pub. Co.; 2009. Ronald L. Sakaguchi , John M. Powers; Craig’s Restorative Dental Materials; 13 th edition. Mosby; 2012. Stephen F. Rosenstiel etal ; Contemporary fixed Prosthodontics ; 3 rd edition. Mosby, Inc.; 2001. Kenneth D. Rudd etal ; Dental laboratory procedures - Removable partial denture; 2 nd edition. Mosby; 1986. Kenneth D. Rudd etal ; Dental laboratory procedures - Fixed partial denture; 2 nd edition. Mosby; 1985.

75.   Delgado V.P. Pytom F.A. - The hygroscopic Setting Expansion of dental casting Investment. J. Prosthet . Dent 1953; 3-423. David B.M., Bruceady - Influence of factors on setting expansion J.Prosthet.Dent 1963; 13:365.