Casting procedures

COMPLETE PROCEDURE EXPLAINED IN NUTSHELL

1. DENTAL CASTING PROCEDURES Dr.BHUVANESH KUMAR.D.V

2. C ontents Introduction Definition Casting procedure Wax pattern Spruing Investing Burnout Casting retrieval Finishing & Polishing

3. Introduction Casting is one the most widely used methods for fabrication of metallic restorations . The lost wax casting technique was first described at the end of 19th century as a means of making dental castings

4. Definition Casting is defined as something that has been cast in a mold , an object formed by the solidification of fluid that has been cast into a refractory-- GPT 9

5. Casting procedure Steps Tooth preparation and Impression Wax pattern Spruing Attachment to crucible Investing Burnout Casting Recovery Finishing and polishing LAB PROCEDURES CLINICAL PROCEDURE

6. Tooth Preparation Impression and Pouring with Die Stone CLINICAL PROCEDURE

7. Conditioning of the Die 1 - DIE DITCHING 2 - MARKING THE MARGINS 3 -APPLYING DIE HARDENER 4 - APPLYING DIE SPACER AND DIE SEPERATOR 5 -WAX PATTERN FABRICATION

8. Wax pattern First procedure in the casting of an inlay or crown for the lost-wax process is the preparation of a dental wax pattern . It is contouring of wax pattern into desired shape and form. All aspects of final restoration are incorporated into the wax pattern. Shortest time should elapse between the time the pattern is removed from the die & the time it is invested.

9. Casting assembly

10. Spruing SPRUE:- The channel for elimination of wax after burnout and pushing of molten metal during casting .

11. Basic requirements of sprue Used as handle to remove wax pattern. To form a mount for the wax pattern & fix the pattern in space . Creates a channel for elimination of wax during burn out. Facilitate flow of molten metal from crucible to mold .

12. Types of sprue The sprue can be wax, plastic, or metal . Wax sprues are preferred for most castings because they melt at the same rate as the pattern and thus allow easy escape of the molten wax

13. Factors affecting sprue Diameter Position Attachment Direction Length

14. Sprue diameter The diameter of sprue should be equal to the thickest portion the wax pattern. Recommended sprue diameters – 2.5mm (10 gauge) for molar metal ceramic restorations. 2.0mm (12 gauge) for premolar partial veneer restoration.

15. Sprue position The sprue should be attached to the bulkiest part of the pattern, away from margins and occlusal contacts . Normally the largest noncentric cusp is used.

16. Sprue attachment The attachment of sprue former to the wax pattern should be smooth and do not posses pits or irregularities . Should be attached to the portion of the pattern with the l argest cross sectional area

17. Sprue Direction Should be directed away from any thin or delicate parts of pattern - molten metal may abrade or fracture investment in this area. It should be attached 45 degrees to the walls of mold, which decreases the turbulence of molten alloy Shouldn’t be sprued at 90 degrees to a broad flat surface as it causes turbulence within the mold cavity leading to severe porosity

18. Sprue Length Should be long enough to properly position the pattern in the casting ring within 6 mm of the end of the ring yet short enough so that the molten alloy doesn’t solidify before it fills the mold . ( 6mm – Gypsum bonded investments & 3-4mm – Phosphate bonded )

19. Reservoir Function Piece of wax attached to the sprue about 1mm away from the pattern , as a enlarged round mass or a connector bar between the wax pattern and sprue former.

20. V enting Small auxiliary sprues or vents have been recommended to improve casting of thin patterns and may help in : Escape of gases during casting. Compensate for the shrinkage during solidification. Solidification begins in critical areas by acting as a heat sink.

21. Crucible Former The sprue is attached to crucible former which constitutes the base of the casting ring during investing . The exact shape of the crucible former depends on the type of casting machine used.

22. Crucible formers are basically of 2 types: Steep-sided cone : used with metal when casted using centrifugal casting force. Shallow cone : used to cast metal using stream/air pressure.

23. They are available as- Rubber crucible former Metallic Crucible former Plastic crucible former

24. Casting ring The casting ring serves as a container for the investment while it sets & restricts setting expansion of the mold.

25. SHAPE R o und Oval COMPLE T E RING Rigid : Metal , plastic F l e x i b l e : Rubber S P LIT RING Metal Pl a stic Types of C asting R ing

26. Casting R ing L iners The most commonly used technique to provide room for I nvestment expansion . Triple fold function : - Freedom to expand which would otherwise be restricted by the ring . Helps to offset the contraction of the more rapidly cooling ring while the gold alloy is being melted . In a wet liner certain amount of hygroscopic expansion is afforded and a thicker liner provides even greater semi hygroscopic expansion .

27. Other types of Ring Liners: Asbestos Cellulose Ceramic-ring Liner

28. Wax pattern- crucible former attachment Wax pattern is attached to the crucible former with a sprue ready for investing. A ring liner is in place.

29. Surface Treatment of Wax pattern Cleaning the wax pattern of debris, grease or oil by surfactants- Pattern cleanser. Synthetic detergent/ Acetylene. Debubblizer.

30. DEBUBBLIZER A debubblizer is a surface tension reducing agent that is used to reduce the prevalence of bubbles in industrial processes such as wax casting. It is also referred to as surfactant or a wetting agent that is sprayed on the set impression material. Composition – Citric acid Sodium 2-phenylpropane-2-sulfonate Propylene glycol

31. I nvesting The process of covering or enveloping an object such as a denture, tooth, wax form, crown, with a suitable investment material before processing, or casting Investment material – G ypsum bonded, Phosphate bonded , Ethyl silicate bonded

32. Mixing the investmnet While the wax pattern is air drying, the appropriate amount of distilled water (Gypsum Bonded investments)/ colloidal silica special liquid (Phosphate Bonded investments) is dispensed. Powder should be weighed before mixing it with liquid.

33. Mixing the investment The liquid is added to clean dry mixing bowl, and the powder is gradually added to the liquid using care and caution to minimize air entrapment.

34. Vacuum mixing of investment materials is highly recommended for consistent results in investing and casting with minimal surface defects, especially when phosphate- bonded investments are used. . Vacuum investing machines

35. DISPENSING Poured into the ring (thin consistency) from a height (20-30 cm), at a slight angle from bottom to the top of a ring.

36. PAINT ON TECHNIQUE Wet investment material is gently painted over a complex wax pattern by the use of fine hair brush, covering it completely. No. 6 or No. 8 brush is used to coat the pattern.

37. SETTING OF INVESTMENT (For thermal expansion technique) - Allow the investment ring to bench set undisturbed for the time recommended by manufacturer (approximately - 1 hour). (For hygroscopic technique) – Immediately place the filled casting ring in 37 degree water bath with crucible former-side down.

38. A - When the investment has set, the “skin” at the top of the ring is trimmed off. B - The rubber crucible former is removed, and any loose particles of investment are blown off. C - The ring is then placed in the furnace for the recommended burnout schedule.

39. BURNOUT Wax elimination or burnout consists of heating the investment in a thermostatically controlled furnace until all traces of the wax are vaporized . Once the investment has set for an appropriate period 1 hour it is ready for burnout . The crucible former is then carefully removed and the invested rings are placed in a room temperature furnace and heated to the prescribed maximum temperature.

40. The ring should be maintained long enough at the maximum temperature, i.e “heat-soak” to minimize a sudden drop in temperature upon removal from the oven . Such a drop could result in an incomplete casting because of excessively r a p i d solidification of the alloy as it enters the mold .

41. R i n g i s p l a c e d i n a r oo m temp e r a t u r e f u r n a c e a n d h e a te d to th e p r escr i b e d m a x . Temperature. For gypsum bonded investments – Hygroscopic technique – 500 C Thermal expansion technique – 700 C For Phosphate bonded investments – Maximum temperature (700 – 1030 C)

42. Manual S e miaut o - matic Fully Programm- able controls Burnout ovens

43. CASTING Casting procedure : It is a process of obtaining a metallic duplicate of a missing tooth structure by pouring molten metal into a mold of a required form & allowing it to solidify to obtain a metallic duplicate. 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 refractory mold.(GPT-9)

44. Casting of an alloy into the mold space uses 2 basic requirements: Heat source – to melt the alloy Casting force – to force molten alloy into mold

45. Torch Melting

46. Zones of Combustion Zones Color Zone 1 Non combustion zone Dark N o h ea t is present. Air and gas are mixed before combustion. Zone 2 Combustion zone Green Gas and air are partially burned here. Definitely oxidizing zone, should always be kept away from the molten alloy during fusion Zone 3 Reducing zone Dimly blue Hottest zone Should be kept constantly on the alloy during melting Zone 4 Oxidizing zone Area where combustion occurs with oxygen in the air. Shouldn’t be used to melt the alloy because it oxidizes the alloy and its temperature is lower than that of the Reducing zone.

47. Gas air torch Used to melt conventional noble metal alloys whose melting points less than 1000⁰ C (used for inlays, crown and bridge). Gas oxy torch Used to melt metal-ceramic alloys of higher temperature up to 1200⁰C . Oxygen pressure - 10 - 15 psi. The flame is directed onto metal with the nozzle of the torch about 1.5 cm away from the metal. Complete fluid should be obtained within 30 seconds at the point of which the metal is poured into the mold. Oxyacetylene torch One part acetylene + 2.5 parts oxygen Distance of 10 cm between the face of the nozzle and the base of crucible. If distance is reduced to : 7.5 mm - slight porosity. 5 mm - increased porosity due to occluded H2 gas.

48. Gas air torch Gas oxy torch

49. DIFFERENCE BETWEEN TORCH AND ELECTRICAL MELTING OF ALLOYS A) TORCH MELTING For low temperature metals. Mixture of natural/artificial gas, oxygen/tank, gas–oxyacetylene. Slower than electric heating but more faster than resistance heating. B) ELECTRICAL MELTING For higher temperature metals. Electric resistance melting, induction melting. M e l t s all o y f as t e r a n d c a n b e easily over heated.

50. Air pressure casting machine . Centrifugal casting machine. Electrical resistance–heated casting machine. Induction melting casting machine. Vacuum or pressure assisted casting machine. Direct-current arc melting machine. CASTING MACHINES

51. Alloy is melted in the hollow left by the crucible former by torch flame and then air pressure is applied through a piston. Carbon dioxide, carbon monoxide or nitrogen gas can be used. Pressure - 10-15 psi. Air pressure casting machine

52. This machine makes use of centrifugal force to thrust the liquid metal into the mold. The method involves pouring molten metal into a cylindrical mold spinning about its axis of symmetry. The mold is kept rotating till the metal has solidified. Centrifugal casting machine.

53. Metal is melted by induction field that develops within the crucible surrounded by water cooled metal tubing. Molten metal is forced in to mold by air pressure or both. Induction melting casting machine

54. The casting crucible acts as a platform on which heat can be applied to the metal to facilitate the melting of the alloy. CERAMIC CRUCIBLE PLASTIC CRUCIBLE CASTING CRUCIBLES Clay - High and noble metal types. Carbon - High noble crown and bridge, alloys also for higher fusing gold-based metal ceramic alloys. Quartz - Higher fusing, gold based metal ceramic alloy & palladium alloys. Ceramic – alumina - High fusing alloys of any type : specially for alloys that have a high melting temperature or are sensitive to carbon contamination. TYPES

55. The crucibles used with noble metal alloys should not be used for melting base metal alloy. C ru c i b l e s h o u l d b e d i s c a rde d i f i t c o n t a i n s l a r g e a m o u n t o f o x i d e s a n d contaminants from previous metals. Sufficient mass of alloy must be present to sustain adequate casting pressure - 6 gm - adequate for premolar and anterior casting. 10 gm - adequate for molar casting. Casting force must be greater surface tension of alloy + resistance offered by gas in the mold. This can be done by use of following different type of forces - Vacuum force Air or Gas Pressure Centrifugal force

56. COURTESY : https://www.youtube.com/watch?v=L_s5IDKBJmc

57. DEVESTING The retrieval of casting or prosthesis from an investing medium A - Trimming is done from the bottom end of the ring. B - Investment is being pushed out of the casting ring. C - The mold is broken open. D - Investment is removed from the casting. Care must be taken to avoid damaging the margin

58. After the casting is recovered , it is quenched in water. Advantages of quenching - When water contacts the hot investment, a violent reaction ensues, resulting in a soft, granular investment that is easily removed. This leaves the cast metal in annealed condition for burnishing polishing. Quenching

59. Often the surface of casting appears dark with oxides and tarnish, such a surface film can be removed by process known as “PICKLING ”. Solutions used- 1. (50%) HCL 2. (50%) Sulphuric acid Others – Ultrasonic devices Abrasive devices Pickling When steel tongs contact the copper residues from previous casting, a small galvanic cell is created and copper is deposited on the casting at the point of contact of the tongs. The pickling sol. should be renewed frequently, because it is likely to become contaminated after reusing the sol. several times Caution while Pickling

60. The casting is held in a sandblasting machine to clean the remaining investment from its surface. Aluminum oxide particles of size 50 – 200 µ are fired. SANDBLASTING UNIT Sandblasting Cleaned casting

61. The casting is trimmed, shaped and smoothen with suitable burs or stones. The sprue is sectioned off with a cutting disc. White stone, rubber wheels, rubber disks, and fine-grits are included in the finishing and polishing agents. Trimming and polishing

62. Tiny air bubbles in the investment create very minute nodules on the inner surface, which interfere with the fitting of the casting. Inspection and Finishing

63. Inner surface should be carefully examined under higher magnification & illumination for any discrepancy. Tiny air bubble/ nodules should be removed by a small round bur. S l ig h t l y m o r e , r a th e r t h a n l es s , than the size of the nodule should be removed to ensure that the casting does not bind during seating