1. CASTING PROCEDURES AND CASTING DEFECTS Presented by: Dr. Shailja katiyar JR-1 Dept. of prosthodontics

2. Steps involved in the casting

3. CASTING:-  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)

4. 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. Advantages of hollow sprue former: It increases the contact area. It holds less heat than the solid sprue former.

5. Reservoir: Reservoir is a small amount of additional wax added to the sprue former 1mm below the wax pattern.

6. Sprue former length: The length of the sprue former - it keeps the wax pattern 6mm from the casting ring liner

7. 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

8. Attached 45 degrees to the walls of mold, which decreases the turbulence of molten alloy

9. Spruing Direct Indirect

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

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

12. 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

13. Casting rings Casting rings are used to confine the fluid investment around the wax pattern while the investment sets.

14. They are available as: 1) Shapes - Round - Oval 2) Complete rings – Rigid - Metal - Plastic Flexible – Rubber 3) Split rings - Metal - Plastic

15. Ring less casting system: - plastic rings which is conical in shape with tapering walls are used. - Used for traditional gold-base alloys. Flexible ringsSplit casting rings

16. Casting ring liner Materials used are: - Asbestos liner - Cellulose liner - Ceramic liner - Combination of ceramic and cellulose liner

17. Function of casting ring liner: Allow uniform expansion. In case of wet liner technique hygroscopic expansion. Thickness of the liner should be less than 1mm.

18. Investment mixing: 1. Hand mixing 2. Vacuum mixing Bubble free casting with different technique- 17% - open investment 95% - vacuum investment

20. 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

22. 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

23. 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.

24. 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.

25. 4. Induction melting machine: Metal is melted by an induction field that developed with in the crucible surrounded by water- cooled metal tubing.

26. Casting force: This can be done by use of following different types of forces Vacuum force Air or Gas Pressure Centrifugal force

27. 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

29. INTRODUCTION 2 CLASSIFICATION OF DEFECTS- A) Defects in the wax pattern B) Defects in the investing procedure C) Problems in burnout procedure D) Problems during casting procedure

30. Introduction In dentistry the lost-wax casting technic was not popular until 1907 when W.H. Taggart introduced his technic and casting machine. Today the lost-wax technic is common practice and is used for a variety of casting operations

31. Defects in Wax Pattern 1. Distortion 2. Surface roughness and irregularities 3. Air bubbles 4. Water film

32. Defects in Burn out Procedure: 1. Rapid Heating Rates 2. Under heating 3. Prolonged heating

33. Rough surface on casting Due to prolonged overheating of gypsum bonded investment. Prevention- use correct heating cycle for burnout procedure. Do not heat above 1290F (700C )

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

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

36. Distortion Any marked distortion of the casting is probably related to a distortion of the wax pattern. Distortion increases as the thickness of the pattern decreases. The less the setting expansion of the investment, the less the distortion

37. Surface Roughness, Irregularities, and Discoloration Surface roughness is defined as relatively finely spaced surface imperfections whose height, width, and direction establish the predominant surface pattern. Surface irregularities are isolated imperfections, such as nodules, that are not characteristic of the entire surface area.

38. Surface irregularities on an experimental casting caused by air bubbles (A), water film (B), inclusion of foreign bodies(C).

39. Air Bubbles Small nodules on a casting are caused by air bubbles that become attached to the pattern during or subsequent to the investing procedure. The best method to avoid air bubbles is to use the vacuum investing technique

40. If a manual method is used, various precautions can be observed from the investment mix before the investing. use of a mechanical mixer with vibration both before and after mixing should be practiced routinely. Wetting agent; A wetting agent may be helpful in preventing the collection of air bubbles on the surface of the pattern. The wetting agent be applied in a thin layer. Air-dry the wetting agent, because any excess liquid dilutes the investment, possibly producing surface irregularities on the casting

41. Water Films  Wax is repellent to water, and if the investment becomes separated from the wax pattern in some manner, a water film may form irregularly over the surface.  Appears as minute ridges or veins on the surface.  If the pattern is slightly moved, jarred, or vibrated after investing, or if the painting procedure does not result in an intimate contact  of the investment with the pattern, such a condition may result.  A wetting agent is of aid in the prevention of such irregularities. Too high a L/P ratio may also produce these surface irregularities

42. Rapid Heating  May result in fins or spines on the casting.  Also, a characteristic surface roughness may be evident because of flaking of the investment when the water or steam pours into the mold.  The mold should be heated gradually; atleast 60 min should elapse during the heating of the investment-filled ring from room temperature to 700° C.  The greater the bulk of the investment, the more slowly it should be heated.

43. Underheating Voids or porosity may occur in the casting from the gases formed when the hot alloy comes in contact with carbon residues. Occasionally, the casting may be covered with a tenacious carbon, coating that is virtually impossible to remove by pickling.

44. Porosities in noble metal alloy castings may be classified as follows: I. Solidification defects A. Localized shrinkage porosity B. Microporosity II. Trapped gases A. Pinhole porosity H. Gas inclusions C. Subsurface porosity III. Residual air

45. Localized shrinkage  generally caused by premature termination of the flow of molten metal during solidification.  If the  sprue freezes in its cross-section before this feeding is completed to the casting proper, a localized shrinkage void will occur in the last portion of the casting that solidifies

46. Localized shrinkage porosity due to delayed solidification The porosity in the pontic area caused by the ability of the pontic to retain heat because of its bulk It was located in the heat center of the ring

47. This problem can be solved by attaching one or more small- gauge sprues These small chill set sprues ensure that solidification begins within the sprues, andthey act as cooling pins to carry heat away from the pontic

48. Suck-back porosity It often occurs at an occlusoaxial line angle or incisoaxial line angle that is not well rounded. The entering metal impinges onto the mold surface at this point and creates a higher localized mold temperature in this region, known as a hot spot

49. MICROPOROSITY AND PINHOLE POROSITY Microporosity occurs from solidification shrinkage but is generally present in fine-grain alloy castings when the solidification is too rapid for the microvoids to segregate to the liquid pool. Such phenomena can occur from rapid solidification if the mold or casting temperature is too low. This type of defect is not detectable unless the casting is sectioned

50. Pinhole and the gas inclusion porosities  These are related to the entrapment of gas during solidification. both are characterized by a spherical contour, but they are decidedly different in size.  The gas inclusion porosities are usually much larger than the pinhole porosity

51. Many metals dissolve or occlude gases while they are molten. For example, both copper and silver dissolve oxygen in large amounts in the liquid state. P latinum and palladium have a strong affinity for hydrogen as well as oxygen. On solidification, the absorbed gases are expelled and pinhole porosity results

52. A BLACK-COATED NOBLE METAL ALLOY CASTING FROM SULFUR CONTAMINATION OR OXIDATION

53. Subsurface porosity may be caused by the simultaneous nucleation of solid grains and gas bubbles at the first moment that the alloy freezes at the mold walls. can be diminished by controlling the rate at which the molten metal enters the mold Entrapped-air porosity or Back pressure porosity occurs on the inner surface of the casting, produce large concave depressions The entrapment is frequently found in a "pocket“ at the cavity surface of a crown or mesioocclusal-distal casting

56. CASTING DEFECTS BY RUDD AND MORROW Incomplete casting: -Inadequate spruing -Alloy not hot enough -Mold too cold -Ingate obstructed -Insufficient casting force

57. Rounded margins Incomplete burnout of wax pattern

58. Insufficient heating of alloybefore casting Margins melted while attaching pattern to sprue or former Improper length/diam of sprue restricts flow of alloy into mold metal freezes before margins are complete

59. Porosity Improper sprue former, size, length or placement

60. Rough surface on casting Excess moisture on pattern Water powder ratio too high in mixing investment Too much casting pressure causing investment breakdown from force of alloy

61. Pits in casting Debris in mould Dirty wax Loose debris in crucible Mold temperature too hot

62. Fins on casting Overvibration during investment or disturbing ring during set Heating ring too rapidly causes moisture in mold to form steam and rupture out mold

63. Bubbles or nodules on casting Investment not mixed under enough vacuum to remove entrapped air bubbles

64. Surface treatment Pickling consists of heating the discolored casting in an acid. For gypsum bonded investments the best pickling solution is 50% hydrochloric acid solution. It aids in the removal of any residual investment as well as of the oxide coating. Disadvantage is that acid fumes are likely to corrode laboratory metal furnishings. Never the casting be held with steel tongs so that both the casting and the tongs come in contact with the pickling solution, as this may contaminate the casting.

65. Gold based and palladium based metal ceramic alloys and base metal alloys are bench cooled to room temperature before the casting is removed from the investment. Castings from these alloys are generally not pickled, and when it isrecommended for certain metal –ceramic alloys, it is only to selectively remove specific surface oxides

66. An unsuccessful casting results in considerable trouble and loss of time, in almost all instances, defects in casting can be avoided by strict observance of procedures governed by certain fundamental rules and principles. Seldom is a defect in a casting attributable to factors others than the carelessness or ignorance of the operator. With present techniques, casting failures should be the exception, not the rule.. CONCLUSION

67.  SCIENCE OF DENTAL MATERIALS---ANUSAVICE—11TH EDITION  CONTEMPORARY FIXED PROSTHODONTICS ---STEPHEN F.  ROSENSTIEL---2ND EDITION  DENTAL LABORATORY PROCEDURES---RHOADS.RUDD.MORROW SCHILLINBURG.   FUNDEMENTALS OF FIXED PROTHODONTICS REFERENCES