CASTING PROCEDURE part 2
- 1. PRESENTED BY- DR. SNEHA BASU PG 1ST YEAR DEPARTMENT OF PROSTHODONTICS CASTING PROCEDURE PART-II
- 2. Contents Casting rings Casting ring liners Investing procedure Burnout Casting machines Cleaning the casting Casting defects
- 3. CASTING RINGS Casting rings are used to confine the fluid investment around the wax pattern while the investment sets They are available as:- 1. complete rings: a)rigid –metal --plastic b)flexible – rubber 2. split rings –metal --plastic 3. shape – round --oval
- 5. SELECTION OF CASTING RINGS The internal diameter of casting ring should be 5- 10 mm greater than the widest measurement of the pattern and about 6mm higher. For single crown/inlay – ring diameter-32mm For large FPD – ring diameter- 63mm, shape – round/oval
- 6. CASTING RING LINERS PURPOSE: to provide a buffer of pliable material against which the investment can expand to enlarge the mould. If there is no room for expansion outwards, the expansion forces will be exerted inwards towards the mould, resulting in distortion of the casting. Ring liners: 1. Asbestos liner 2.cellulose (paper) liner 3.aluminosilicate ceramic liner
- 7. Asbestos liner – not used anymore because of its carcinogenic potential. Ceramic liners- kaoliner, densply . . Ceramic material does not readily absorb water except under vaccume . It does not absorb water like cellulose liner, its network of fibers can retain water on the surface. {it has been pointed out, however,that ceramic material contains fibers in the size range likely to produce lung tumors in rats and therefore may be no less hazardous than asbestos}. Cellulose liners: absorbs water- becomes thicker and more compressible. It burns out before the casting is made and allowing unrestricted thermal expansion and easy escape of gases from the mould during casting.
- 8. PROCEDURE The liner is cut to fit the inside diameter of the casting ring with no overlap. The dry liner is tacked in position with stickey wax, and then it is used dry or wet. Wet liner technique: the lined ring is immersed in water for a time, and excess water is shaken away (squeezing the liner is avoided because this leads to variable amounts of water removal and nonuniform expansion). This absorbed water causes semi-hygroscopic expansion as it is drawn into the investment during setting (for gypsum bonded investments)
- 10. Using a thicker liner material or two layers of liner provides even greater semihygroscopic expansion and also affords a more unrestricted normal setting expansion of the investment. Thickness of the liner should not be less than approximately 1mm. The expansion of the investment is always greater in the unrestricted longitudinal direction than in the radical direction, that is, towards the ring. Therefore it is desirable to reduce the expansion in the longitudinal direction. Placing the liner somewhat short (3.25mm) of the ends of the ring tends to produce a more uniform expansion; thus there is less chance for distortion of
- 11. RINGLESS CASTING SYSTEM The high strength of the phosphate bonded investment material makes it possible to abandon the use of casting ring. Plastic ring with rubber crucible former are used. The ring is conical in shape with tapering walls. As the investment sets the investment is tapped out of ring. Then burnout is done without casting ring, this causes greater expansion. ADVANTAGES -allows unrestricted thermal expansion -smaller marginal discrepancy -easier technique -easy to divest -health hazard of liners are avoided
- 12. INVESTING PROCEDURE wax pattern should be cleaned of any debris, grease or oils using – a commercially wax pattern cleaner --a dilute synthetic detergent. Excess liquid is shaken off Pattern is left to air-dry while the investment is being prepared. The thin film of cleanser left on pattern - reduces the surface tension- allows wetting of the wax pattern by investment material to ensure complete coverage of the intricate portions of the pattern.
- 13. MIXING OF INVESTMENT Hand mixing Vacuum mixing Hand mixing:- appropriate amount of water(gypsum investment) or colloidal silica special liquid(phosphate investment)is dispensed in a clean dry mixing bowl. -powder is gradually added to the liquid with caution to avoid air entrapment until all the powder has been wet -hand mixing is an option but vacuum mixing is more common.
- 14. Vacuum mixing Mechanical mixing under vaccum removes air bubbles better. Once mixing is completed, the pattern may be- -hand invested -vacuum invested Hand investment: entire pattern is painted (inside out) with a thin layer of investment.the casting ring in positioned on the crucible former, the remainder of the investment is vibrated slowly into the ring. Vacuum investment: the same equipment used to
- 16. In one study it was found that 95% of vacuum- invested casting were free of nodules, whereas 17% of castings made in hand-invested moulds were entirely free of defects. PRECAUTIONS Air bubbles may can be entrapped on flat or concave surfaces (not oriented suitably for air evacuation) : tilting the ring slightly aids in releasing these bubbles so that they can rise to the surface. Exessive vibration should be avoided: it causes solids in investment to settle down and free water accumulation adjacent to the wax pattern, resulting in surface roughness. Exessive vibration may also dislodge small pattern from the sprue ,resulting in miscast.
- 17. HYGROSCOPIC TECHNIQUE: the filled casting ring is immediately placed in 37 degree water bath with crucible former side down. HIGH HEAT TECHNIQUE: the invested ring is allowed to bench set undisturbed for the manufacturer recommended time.
- 18. Once the investment is set for an appropriate amount of time- approximately 1 hr. for most gypsum and phosphate bonded investment – it is ready for burnout. Crucible former and any metal sprue former are carefylly removed. Any debris from the ingate area (funneled opening at the end of the ring) is cleaned with a brush. If burnout is not carried out immediately ,the investment is stored in a humidor at 100% humidity. Do not let the investment to dry out Rehydration of set investment that has been stored for an extended period may not replenish all of the lost water.
- 19. BURNOUT It prepares the mould for the molten alloy and allows thermal expansion to occur. It is advisible begin the burnout procedure while the mould is still wet. Water trapped in the pores of the investment reduces the absorption of wax, and as the water vaporizes, it flushes wax from the mould. This process is facilitated by placing the ring with the sprue hole down over a slot in a ceramic tray in the burnout furnace. When high heat technique is used, the mold temperature generates enough heat to convert carbon to either carbon monoxide or carbon dioxide. These gases can then escape through the pores in the heated investment. Gypsum bonded investment: 500˚C (hygroscopic technique) & 700˚ (thermal expansion technique) Phosphate bonded investment – 700˚ to 1030˚ C, depending on the type of alloy selected.
- 21. Wax elimination and heating techniques: Hygroscopic low heat technique High-heat thermal expansion technique Hygroscopic low heat technique: here the compensation expansion is from 3 sources: 1)37 degree Celsius water bath expands the wax pattern 2)Hygroscopic expansion 3)Thermal expansion at 500 degree Celsius. ADVANTAGES: -less investment degradation. -cooler surface for smoother castings -convenience of placing the mould directly in the 500 degree Celsius furnace.
- 22. The mold should remain in the furnace for at least 60 mis , they may be held up to 5 hrs. Even though the mold is usually held at this temperature for 60 to 90 mins, sufficient residual fine carbon may be retained to reduce the venting of the mold thereby causing backpressure porosity. High-heat thermal expansion technique: uses high heat burnout to obtain the required thermal expansion , while at the same time eliminating the wax pattern.
- 23. Time allowable for casting No more than 30 seconds should be allowed to elapse between the time the ring is removed from the oven and the molten alloy is centrifuged into the mold. Any undue delay will cause heat loss and resultant mold contraction.
- 24. Casting process and equipment Casting machines (based on the method of casting) 1. centrifugal type 2. air pressure type CENTRIFUGAL TYPE: May be spring driven or motor driven Can cast both large and small castings on the same machine
- 25. AIR PRESSURE TYPE : Either compressed air or gasses like carbon dioxideor nitrogen can be used to force the molten metal into the mould. Satisfactory to make small castings Some systems use argon gas to protect the alloy from oxidation (specially useful of melting titanium)
- 26. ATTACHED VACUUM SYSTEM: both centrifugal or air presure type are available with an attached vaccum system Vacuum creates a negative pressure within the mould, which helps to draw the alloy into the mold CASTING MACHINES CAN BE GROUPED BASED ON HEATING SYSTEM EMPLOYED 1. Torch melted 2. Induction melted 3. Arc melted
- 27. Torch melting of noble metal alloy Fuel used is a mixture of natural or artificial gas and air. Or oxygen and acetylene gas (high fusion alloys) Parts of the flame: 1. mixing zone- first long cone emanating directly from the nozzle. Here the air and gas are mixed before combustion. No heat is present in this zone. 2. Combustion zone-green color, immediately surrounding the inner cone. Here the gas and air are partially burned. This zone is oxidizing, should always be kept away from the molten alloy during fusion. 3. Reducing zone- dimly blue in color. Located just beyond the tip of the green combustion zone. Hottest part of the flame. This zone is used for melting alloy. 4. Oxidizing zone-the outer cone. Area in which combustion occurs with the oxygen in the air. Should not be used. Oxidises the alloy The air and gas mixture is adjusted to get a reducing flame,
- 29. Induction melting It is the process of heating an electrically conducting object (metal) by electromagnetic induction, where eddy currents are generated within the metal and resistance leads to Joule heating of the metal. An induction heater consists of an electromagnet, through which a high frequency alternating current is passed. Induction melting is useful for high fusing alloys like metal-ceramic and base metal alloys.
- 30. Arc melting Direct current is passed between two electrodes- a tungsten electrode and the alloy. Arc melting produces very high temperatures and is used to melt high fusion metals like titanium. Arc melting may be done under vacuum or an inert atmosphere like argon. Numerous combinations of these principles are employed in different machines.
- 31. Casting crucibles 4types:- 1. Clay 2. Carbon 3. Quartz 4. Zirconia-alumina Clay crucible is used for high noble and noble alloys. Carbon crucible for high noble crown and bridge alloys and higher fusing gold based metal ceramic alloys Quartz and zircinia-alunina crucible are recommended for high fusing alloys of any type. (specially those which are sensitive to carbon contamination)
- 33. CLEANING THE CASTING After the casting has solidified, the ring is removed and quenched in water as soon as the button exhibits a dull-red glow ADVANTAGES 1. the noble metal alloy is left in an annealed condition for burnishing, polishing and similar procedures 2. when the water contacts the hot investment , a violent reaction ensues, resulting in a soft, granular investment that is easily removed. Queching is done to achive the best grain structure. If it is quenced while it is too hot, the gold will be softer and weaker. If it is allowed to bech cool completely, the grain structure will be too large
- 34. The ring is removed from the water, and the investment and casting are pushed out of the ring. As much of the investment as possible is broken off by hand or with old instrument, then the casting and button are scrubbed with a stiff brush.the casting should appear smooth, with a dull, dark oxide layer. The oxide layer and any remaining particles of investment areremoved by lightly air abrading all surfaces with a 50µm abrasive, taking care not to abraid thin margins.
- 35. Often the surface of the casting appers dark with oxides and tarnish. This can be removed by a process known as pickling. Pickling consists of heating the discolored casting in an acid. For gypsum bonded investments 50% hydrochloric acid solution is used. Disadvantage is the acid fumes are a health hazard. Pickling process can be performed ultrasonically while the prosthesis is sealed in a teflon container. A solution of sulfuric acid may also be more advantageous in this respect. Only gold castings may be cleaned by pickling.
- 36. CASTING DEFECTS any irregularity in the metal casting process that is undesired is called Casting Defect. Casting defects can be classified under four headings: 1. Distortion 2. Surface roughness and irregularities 3. Porosity 4. Incomplete or missing details.
- 37. Distortion Related to distortion of the wax pattern Can be prevented by proper manipulation of the wax and handling the pattern Some distortion of the wax pattern occurs as the investment around it hardens. Distortion increases as the thickness of the pattern decreases. Generally, it is not a serious problem.
- 38. Surface roughness, irregularities and discoloration. Excessive roughness or irregularities on the outer surface of the casting necessitate additional finishing and polishing, whereas irregularities on the cavity surface prevent proper seating of an otherwise accurate casting. 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
- 39. Types of Surface Roughness 1. Nodules- Small nodules on the casting are caused by air bubbles that become attached to the pattern during or subsequent to the investing procedure.
- 40. Types a) Large nodules: Air trapped during investing procedure. b) Multiple nodules: - Inadequate vacuum during investing - Improper brush technique - Lack of surfactant
- 41. c) Nodules on occlusal surface: due to excessive vibration d)Nodules on underside: prolonged vibration after pouring.
- 42. Avoided by: The best method to avoid air bubbles is to use the vacuum investing technique. If manual method is used: 1. Use of mechanical mixer with vibration both before and after mixing 2. Surfactant may be used. Surfactant should be applied in thin layer and air – dried because any excess liquid dilutes the investment.
- 43. When nodules are large or situated on a margin, they usually necessitate remaking of the restoration. When small, they can often be removed with a No. ¼ or No. ½ round bur. A slight excess of metal should be removed to ensure that the nodule does not interfere with complete seating. Contemporary fixed prosthodontics – Rosensteil
- 44. 2. Ridges or Veins Wax is repellent to water, if the investment becomes separated from the wax pattern in some manner, a water film may form irregularly over the surface.
- 45. This condition occurs: -If the pattern is slightly moved, jarred or vibrated after investing. - If there is no intimate contact of the investment and the pattern - Too high a liquid /powder ratio -Use of surfactant help prevent such irregularities.
- 46. 3. Fins are caused by cracks in the investment that have been filled with molten metal.
- 47. Cause of Fins: - Weak mix of the investment i.e. high water/ powder ratio. - Improper positioning of the pattern in investment i.e. pattern placed too near the edge of the investment - Too rapid heating - Premature heating
- 48. -Excessive casting force - Rough handling of the ring after investing - Liner flushed with the end of the ring.
- 49. 4. Spines Rapid heating of investment results in spines. Sometime a characteristic surface roughness may also be evident because of flaking of the investment when the water/steam pours into the mold. Ideally, 60 min should elapse during the heating of the investment filled ring from room temperature to 700°C.
- 50. 5. Inclusions Carbon Inclusions: Carbon as from - A crucible - An improperly adjusted torch - Carbon containing investment.
- 51. Foreign Body Inclusions Any casting that shows sharp, well defined deficiencies indicates the presence of some foreign particles in the mold, such as; I. Pieces of investment II. Bits of carbon from flux
- 52. A rough crucible former with bits of investment clinging to it may roughen the investment on its removal so that small particles of investment are carried into mold with molten alloy.
- 53. 6.Porosities Solidification porosity- Localized shrinkage porosity Shrink – Spot Porosity Suck Back Porosity Microporosity Trapped Gases- Pin hole porosity Gas Inclusions Sub surface porosities Residual Air Back pressure porosity
- 54. Shrink Spot Porosity Cause: Premature termination of flow of metal during solidification. Linear contraction of noble metal alloys in changing from liquid to solid is 1.25%. Therefore continual feeding of molten metal through the sprue must occur to compensate for casting shrinkage i. e. shrinkage during solidification. If the sprue solidifies before casting it usually results in localized shrinkage porosity.
- 57. It can be avoided by: - Using sprue of correct thickness -Attaching the sprue at the thickest portion of the wax pattern - Flaring the sprue at the point of attachment - Placing the reservoir close to the wax – pattern
- 58. Suck-Back Porosity Localized shrinkage may also occur in the interior of crown near the area of the sprue . The entering metal impinges on to the mold surface at occlusoaxial/ incisoaxial line angle and creates a higher localized mold temperature at this region known as “Hot Spot”. A hot spot may retain a localized pool of molten metal after other areas of the casting have solidified. This in turn creates shrinkage void or sucks back porosity.
- 59. These porosities can be eliminated: - By flaring the point of sprue attachment - Reducing the mold melt temperature differential i.e. lowering the casting temperature by about 30°
- 60. Left coping was casted at 1370 degree C and right coping at 1340 degree C
- 61. Microporosity Occurs from solidification shrinkage but is generally present in fine grain alloy casting when the solidification is too rapid for the microvoids to segregate to the liquid pool. Microporosity voids are irregular in shape. This defect is not detectable unless casting is sectioned.
- 62. Microporosity, pinhole, and gas inclusion porosity
- 63. Pinhole and Gas Inclusion Porosity Both these porosities are related to the entrapment of gas during solidification. Both these are characterized by spherical contour but size is varied i.e. gas inclusion is larger in size compared to pinhole. Many metals dissolve or occlude gases in their molten state. On solidification, absorbed gases are expelled resulting in these porosities. These can be minimized by Premelting the alloy, and by correctly adjusting the torch flame during melting.
- 64. Alloy after heat treatment at 725 degree C for 70 min. Pinhole Grain structure of the alloy
- 65. Subsurface Porosity Caused by simultaneous nucleation of solid grains and gas bubbles at the first moment that the alloy freezes at the mold walls. Can be minimized by controlling the rate at which the molten metal enters the mold.
- 67. Back Pressure Porosity Also known as entrapped air porosity. It can produce large concave depression. Caused by inability of air to escape through pores of investment or by pressure gradient that displaces air towards end of the investment via molten sprue and button. Occasionally found on the outer surface of casting when the casting or mold temperature is low that the solidification occurs before the entrapped air can escape
- 69. The incidence of entrapped air has increased by use of: - Dense modern investment - Increase in mold density - By tendency of mold to clog with residual carbon when low heat technique is used - Proper burnout, an adequate mold and casting temperature, a sufficiently high casting pressure and proper L/P ratio can help to eliminate entrapped air porosity - Thickness of investment between the tip of pattern and end of ring should be not more than
- 70. Incomplete Casting When due to some reason molten alloy is prevented from completely filling the mold. Causes: i. Insufficient venting of mold ii. Incomplete elimination of wax residue iii. High viscosity of fused metals iv. Inadequate metal v. Cool mold or melt vi. Wax pattern too thin vii. If there is marginal discrepancy due to wax pattern distortion and uneven expansion
- 71. If short rounded margins with lumpy/ rounded button- alloy not hot enough/ inadequate casting force. If short rounded margins with sharp button- pattern too far from the end of ring. If casting is shiny- incomplete burnout.
- 72. Incomplete casting resulting from incomplete wax elimination is characterized by rounded margins and shiny appearance.
- 73. References Anusavice KJ, Shen C, Rawls HR. Phillips' science of dental materials. Elsevier Health Sciences; 2013. Shillingburg HT, Sather DA, Wilson EL, Cain JR, Mitchell DL, Blanco LJ, Kessler JC. Fundamentals of fixed prosthodontics. Quintessence Publishing Company; 2012. Rosenstiel SF, Land MF. Contemporary fixed prosthodontics. Elsevier Health Sciences; 2015 Jul 28. Morrow RM, Rudd KD. Dental Laboratory Procedures: Fixed partial dentures. Mosby Incorporated; 1986.