Burning-out Method of preparing a mould to receive casting metal Beginning of the elimination of wax*NOTE: burn out is different from wax elimination
3 Purposes of burnout: It drives off moisture in the mold. It vaporizes and thus eliminates the pattern, leaving a cavity in the mold. It expands the mold to compensate for contraction of the metal n cooling.
Materials Electric furnace with an adjustable rheostat is preferred over the gas furnace. Casting ring – which is placed into the oven with the sprue hole down. (this allows the melted wax to flow out of the main sprue channel.
According to Brumfield... The time required to drive off the water is mainly a function of: The amount of heat available. The closeness of the heating element to the investment when placed in the furnace.
According to Brumfield... The temperature of the mold is held down during early stages of burn-out by vaporization of the water. At the end of 60 mins, the water is practically all driven off and the temperature at the inside of the mold will rise fairly rapidly to near the temperature of the furnace.
According to Brumfield... Complete equalization however, requires another 70 minutes, giving a total burnout time of two to one and a quarter hours to eliminate the water and raising the temperature of the mold approx. 1300 F. SOAKING PERIOD- designed to allow time for the water to be eliminated from the mold and the mold temperature to rise that of the furnace.
According to Brumfield... During the time the water is being eliminated, the wax is being eliminated through vaporization and oxidation of carbon. Vaporization of wax does not require as much time as that for vaporization of the water and it will only require 1000 °F. The carbon residue may require more time for elimination.
According to Brumfield... It is important to eliminate all the moisture because: The casting would be porous because of the continued emission of steam by the investment. The venting of the mold cavity is largely accomplished through the interstices of the investment itself
According to Brumfield... Steam will then carry the heat into the investment during the early stages of the burnout (if the investment is not burned out on the same day it is poured, it should be soaked in water for few minutes before it is placed in the burnout furnace). The mold should be placed in the casting machine first to balance the weight against the weight of the mold.
Procedure1. Prepare and balance the casting machine before the casting ring is placed in the burnout oven. Ifthe machine is not an electric melt type, a fresh strip of asbestos should be adapted to the large ceramic crucible.a. Cut the strip slightly longer than the crucible.b. Taper one end.c. Wet the liner.
Procedured. Funnel is made at tapered end.e. Funnelled end is pushed through spout of crucible.f. Main portion of liner is adapted inside the crucible.g. Funneled end is adapted to inside of spout with instrument to form liner for spout.h. Completed liner in crucible.i. Lined crucible, metal and invested partial denture are placed in casting machine and then it is balanced.
Procedure2. Soak the investment in water for several minutes.3. Place the ring in a cool furnace sprue side down.4. Turn the burnout furnace on and adjust it to reach 1000°F in about 1 hour.5. Leave the furnace at 1000°F setting for 2 hours. Then increase the temperature so it will hold temperature Between 1250°F and 1300°F.
Procedure6. When the temperature of the furnace has reached 1250°F the mold must heat soak at this temperature for at least 1-1.2 hours (to completely remove the moisture or carbon residue from the interstices or space of the investment).
Procedure7. The mold is ready to cast. Invested pattern is removed from the burnout oven following complete elimination of the wax pattern. investment mold is placed into the casting machine.*The time it takes for the invested pattern to undergo complete burnout is 3 ½ to 4 hours.
Casting A manufacturing process wherein a liquid material is poured into a mold that contains a hollow cavity of a certain shape or figure and then allowed to set and solidify. The solidified part you obtain is called the casting which is removed from the mold once completely solidified.
Casting All methods use force to inject quickly the molten metal into the mold cavity. Centrifugal, steam, air pressure force. Too much or too little force is undesirable. If too little force is used, the mold is completely filled before the metal begins to freeze. If too much force is used, excessive turbulence may result in the entrapment of gases in the casting.
Casting Centrifugal casting machines is regulated by number of turns put on the actuating spring. The metal may be melted with a gas-oxygen blowtorch or by an electric muffle surrounding the metal. Induction method provides a rapid and accurate method of melting the metal.
Induction casting machineThe alloy (arrow) isplaced pinto a cleancrucible
Procedure1. Examine the liner in the crucible to see i it is clean and continuous to and through the spout.2. Wind the casting machine three or four turns depending on the tightness of the spring, and lock it in position.3. Place the metal in the crucible on the asbestos liner
Procedure4. Light the torch and adjust it to form a reducing flame.5. Heat the metal , keeping the tip of the blue cone on the surface of the metal. A large bulk of metal takes several minutes to melt. When the metal begins to ball up, add powdered flux.6. When the surface of the metal is clean, and a swirling action can be seen, position the casting ring in the casting machine. Do not move the flame from the metal.
Procedure7. Ifthe metal surface is still clean and the metal is swirling action can be seen, position the casting machine to drop the locking pin. Without moving the flame from the metal, release the casting machine.
8. When the casting machine stops, remove the ring and allow the mold to bench cool until the red sprue button has become somewhat blackish. Cooling of the mold depends on the flask Ney partial denture flask – a cooling period of 8-12 minutes is sufficient Kerr or Jelenko flask – it is suggested that 20 mins be allowed for bench cooling to avoid the possibility that the casting will be too soft
Recovering the castingAfter the casting is completed, allow the mold to cool until the sprue button has changed in color from red to black when viewed in shaded light. This will about 8 to 12 minute after completion of a large casting.At this time, quench the hot ring in water. With most gold castings this will produce a fairly soft and ductile condition in the metal and will make the finishing much easier.
Recovering the Casting The process of allowing the casting to cool slowly in the mold for a controlled period of time and then quenching is known as heat hardening the alloy. After the metal has been finished, but before it is polished, the properties of metal should be restored. This is known as heat hardening the metal. The process is done by simply most simply by placing the metal framework in an oven that has been heated for 845 degrees Fahrenheit (450 C) and permitting it to cool to 482 degrees(250 C) Fahrenheit over a period of 30 mins. After, it is removed from the furnace and again allowed to bench cool with the metal at its hardened state.
Sandblasting Process wherein a hard surface is smoothened, shaped, and etched. It can be done using a high speed sandblaster. It can occur: Naturally as a result of a particle blown by the wind causing erosion. Artificially using compressed air.
Sandblasting Most common medium for sandblasting is dentistry is aluminium oxide. The fine aluminium oxides are recycled within the sandblasting machine and replaced after 30- 40 minutes of total accumulated operation time. 60-80 psi of force is exerted by the machine.
Purpose of sandblasting Used to clean a surface of anything that may be clinging to it or remove impurities that are attached to the metal framework and prepare the surfaces for subsequent stoning and polishing. Provide mechanical retention between porcelain-metal. Removing the casting oxides as well as proper sandblasting of the alloy with 100-150 microns aluminum oxide results to mechanical bonding.
Purpose of sandblasting Micromechanical retention of denture base to RPD framework. Mechanical retention are provided by the lattice, mesh, beads, and posts. There is a low bond strength between the resin and metal that can lead to microleakage, discoloration, and breakage.
Uses of sandblasting Used to clean a surface of the metal framework. Removes stone particles around the teeth and from the resin-metal joints. Removes impurities that is attached to framework.
Artificial Sandblasting Patented by Benjamin Chew Tilghman on October 18, 1870. By the use of compressed air. Sand was sieved to a uniform size. The silica dust produced in the sandblasting process can yield to silicosis. A chronic lung disease due to continuous inhalation of silica dust particle. However, this may be prevented by using protective clothing and breathing air supply. Alternatives to sand are glass beads, metal pellets, dry ice, powdered abrasives of various fineness and ground coconut shells.
Alternate Abrasives Glass beads Metal pellets Dry ice Powder abrasives of various forms Ground coconut shells
Parts of Sandblasting machine Abrasives Air compressor Blaster gun
Types of Sandblaster machines Automatic Sandblaster – machines that eliminate the use of manual movement of either the shot stream of the workpiece, but rely on mechanical means to supply these features.
Types of Sandblaster machines Blast Cabinets – enclosures in which items are placed to be abraded. Useful in containing the blasting operation and preventing exposure of the blasting media to surrounding workers.
Cleaning of the Casting1. After casting has been made the ring is removed and allowed to bench cool for 10 minutes before it is plunged into the water, this will temper the casting. Then the casting is removed from the ring, freed of excess investment and cleaned carefully with a stiff brush under a steam of water.2. The casting should not be struck and pried with metallic instrument as to not dislodge the investment.3. Most of the investment, except for small particles difficult to reach can be removed with
Procedure of Sandblasting1. A stream of air filled with particles of walnut shell or particles of sand is blown from a nozzle under pressure.2. The casting rotated so that the stream of the air is directed towards those areas that contain the remaining particles of investment.3. The operator blasts the parts from the outside of the cabinet by placing his arms in gloves attached to glove holes on the cabinet viewing the part through a window, turning the blast on and off using a foot pedal.
Procedure of Sandblasting4. The sand is projected by compressed air through a fine nozzle onto the casting and these high speed silica particles abrade the surface of the casting and break away any debris.5. Sandblasters propel abrasive grits through a nozzle at high velocity in a steam of compressed air.6. Sandblasters clean off residual investment materials to prepare metal surfaces for further treatment.
Procedure of Sandblasting7. Dust extraction systems keep the abrasive grit in the container; this can re-circulate back through blasting nozzle. Recirculation can degrade the grit rapidly and it can become much less effective; the best and strongest bonds are formed when fresh grit is used. Various grades of grit can be used. The finest grits will almost polish the surface of cast gold restorations and are very useful in finishing techniques.
Sandblasting VS Shellblasting Sandblasting Used for harder metal like cobalt— chromium. Uses zircon grit, silica sand, or aluminum oxide as an abrasive. Never be used on acrylic or porcelain because it will ruin them.
Sandblasting VS Shellblasting Shellblasting Used for softer metal like gold. Uses crushed walnut shells as abrasives. Walnut shells do not affect the teeth or denture base.
PICKLING/ELECTROPOLISHINGBATALLONES, Amery Rose
Pickling Pickling is the process in which a piece of metal (casting) is placed in an acid bath to remove surface oxides, investments particles and other contaminants from the surface of the metal framework. Goldalloy pickling Cobalt-chromium Electro polishing
Procedure1. Place the casting in a clean high-resistant porcelain dish then, pour the clean pickling solution to a depth just enough to cover the casting.2. Warm gently the pickling solution, until the surface of the metal brightens and show of development of a characteristic gold luster. The solution should not be boiled. Effective cleaning of the solution is accomplished before the boiling temperature is reached.3. Remove the casting from the solution with plastic tongs.4. Pickling solution is poured off (flushing generously with water or base solution) and the casting is washed with an abundance of water.
Disadvantage of boiling orprolonged heating of the acidsolution1. Acid fumes may be harmful and damaging.2. May corrode the laboratory equipment and instruments.
Precautions to be taken whilepickling1. It is highly recommended to use the pickling solution be acquired from the alloy supplier.2. Never heat the gold cast directly in a flame prior to immersing it in pickling solution.3. Casting should not be heated and dropped into the pickling solution.4. Metal or stainless steel tong should not be used to remove castings from the pickling solution.5. Since only small quantity is solution is needed (10ml), new pickling solution can be used each time to avoid excessive contamination. It should be clean and non-corrosive to laboratory equipments and instruments, relatively colorless rather than have the typical greenish blue color of contaminated acid.
Electropolishing Electro polishing is an electro-chemical process in which a metal is placed in an electrolytic solution and an electric current passes through it. Very minute particles on the surface of the metal framework are removed by electrolysis, in turn leaving a polished surface. It is used to polish, passivate, and deburr metal parts. Used for Cobalt Chromium Main electrolyte: Glycolic Acid (Glycerin) 12Volts Temperature: 18-20 degree Celsius
Procedure: Metal framework Electrolyte bath: 4- Electrolyte temp: attached to anode 5cm from the 18-20C of 12V rectifier cathode Rectifier: OFF 5 mins – leaving remove framework framerwork Rectifier: ON & wash under immersed in running water polishing bath Remove the 5 mins - Lightly dry Rinse under framework fro the & immerse again running water electrolyte bath
Advantages of electropolishing1. Improve corrosive resistance of the metal framework.2. Better surface finish.3. Ease of cleaning of the framework.4. Reduces the microscopic crevices in which bacteria can propagate and tiny particles can accumulate.
FINISHING AND POLISHINGFRAMEWORKBUSTO, Treblig Earl
Finishing and Polishing Smoothing the cast framework with progressively finer abrasive agents to remove scratches and to give a high luster to the completed framework. After grinding, the prosthesis must be in a: Completely smooth state. Required on areas contacting the teeth and soft tissue. Highly polished condition. Includes external surfaces such as clasp arms, rest and major connector.
Purpose Maintain cleanliness of appliance while in the oral cavity. Maintain health and cleanliness of the oral cavity. Roughed surfaces will rapidly collect food debris, plaque and calculus. To ensure the frameworks fit the working cast and natural dentition accurately.
Finishing of framework Cutting is done by the abrasive wheels or points and the speed of lathe or handpiece. Cobalt chromium alloy uses high speed lathes (24,000 rpm). The bur or stone must be locked in the chuck with no more than 2cms ticking out. Reinforced mandrel should be used to prevent shaft from bending or breaking. Heatless stone and disks should be held more than 1cm from the chuck. Avoid the use of heavy pressure not only for safety’s sake but because it will unnecessarily heat the casting, causing it to warp.
The bur or stone must be locked in the chuck withno more than 2cms ticking out.
This will happen if the bur ischucked too far.
Rules of finishing the casting High speeds are preferable to low speeds. In an experienced hand, there is less danger of the casting being caught and thrown out of the hands by the rotating instrument. The wheels or points and the speed of their rotation should do the cutting. Excessive pressure heats the work, crushes the abrasive particles, causes the wheels to clog and glaze, and slows the cutting. A definite sequence for finishing should be adopted and followed for every framework.
Rules of finishing the casting Clean polishing wheels should be used. Ifcontaminated wheels are used, foreign particles may become embedded in the surface, which will lead to discoloration. Be sure that each finishing operation completely removes all scratches left by the preceding one. Remember that each successive finishing steps uses a finer abrasive and therefore cuts more slowly and requires more time to accomplish.
Procedure1. Cut the sprue from the casting with aseparating disk. Make the cut as close to the baras possible, but do not nick or damage theframework. When framework is made up of gold, use a jeweler’s saw to save gold and prevent accidental cutting.
Procedures 2. Grind off the sprue stumps with 7/8 inch stone(heatless stone). Restore the original shape of the part of the casting to which the sprue is attached.
Procedures3. Using a disk, barrel-shaped mountedstone, fine tapered mounted stone, or carbidebur, depending on the access to thearea, remove the flash from the clasparms, minor connector, rests, indirectretainers, and denture base retention areas. Mounted stones for grinding cannot be used interchangeably. For Gold- too soft, it may disintegrate rapidly when used on cobalt chormium. For Cobalt chromium- it is too hard for gold, may clod up with soft metal and will not cut.
Procedures4. Use larger wheels or stones to smooth thecontour of the major connector and to removesmall pits or defects. The stone should be keptmoving forward and backward along the majorconnector in strokes at least 15mm long to avoidcreating small depressions, waves or unevensurfaces of the contour. Cross grinding can weaken the bar when grinding on a thin spot or parts.
Procedures5. Smooth the rest of the framework except forthose areas which come in contact with the teethwith a fine tapered mounted stone. Use thisstone also to slightly round the sharp edges ofthe inferior border of the clasp arms.
Procedures6. Use an inverted cone stone or a separatingdisk to refine the undercut but joint of the finishline, both internally and externally. A wire brushis excellent for brightening nailhead.
Procedures 7. Optionally a series of sandpaper disks of decreasing grit can be used to smooth all clasp arms and occlusal rest. (Cannot be used on cobalt chromium castings). Rubber abrasive wheels, knife edge wheels, disks, and points are preferred for fine finishing on the castings. For gold- with fine pumice For Cobalt chromium- with carborandum
Procedures8. Finish the metal surfaces that contact the teeth next.a) Remove the nodules, small defects and irregularities with fine stones or burs.b) Shape a rubber abrasive point by running it against a truing stone or a heatless stone until it comes to a point.c) Using a cylindrical felt cone with Tripoli, produce a completely smooth surface. a) Hallow out the end of the cone using the tip of the knife to be used to polish the underside of occlusal rest b) After polishing with Tripoli, wash using soap and water, a mixture of 2/3 tincture of green soap 1/3 household ammonia, or a ready to use septisol solution.d) Use another flat cylinder with jewelers rouge to complete the finishing.
Use another flat cylinder withjewelers rouge to complete thefinishing.
Procedures9. Smooth the entire remaining framework,except the clasp arms, using the rubber wheelsand points. Run the wheel parallel to the length of the major connector. Use knife edge wheel on finish lines
Procedures10. Use small felt wheels the Tripoli next toremove traces of marks left by the rubber wheelsand points on some of the more difficult to reachareas.
Procedures11. The entire framework including the outersurfaces of the clasp is polished with a smallTripoli-impregnated cloth wheel.a) Polish the clasp arms with the cloth wheel running parallel to the length of the clasp from shoulder to the tip.b) Polish all other areas of the framework from several different directions to produce an even polished surface.
Tripoli impregnated cloth wheel Used in lathe Must be ―broken in‖ to make it softer, fluffier and to remove small strings Use a dull-edge knife on a revolving cloth Burn off strands Use a cloth for tripoli, and a cloth for the rouge
Procedures12. After the casting has been thoroughlypolished with tripoli, scrub the casting withsoap, water and a brush. Remove all traces oftripoli.
Procedures 13. Heat-treat the casting following the manufacturer’s directions because different alloys are heat-treated in various ways. 14. After heat treatment, it will be necessary to pickle and repolish the casting with tripoli.
15. Polish the framework with a cloth wheel and rouge.
Procedures16. Scrub the casting with soap and water ortincture of green soap and ammonia and ifnecessary, chloroform to remove all traces of therouge.
Procedures17. Dry the casting thoroughly with compressedair and examine it carefully under bright light andmagnification for any scratches or defects.
Procedures18. If the polish is satisfactory, the frameworkmay now be tried on the master cast or duplicatemaster cast.19. If the block – pout and relief wax is to beremoves it may be done by soaking the cast inwarm (120 F; 49 C) slurry water for 5 minutesthe pouring hot (200 F; 95 C) slurry water overthe cast to remove the wax.20. Check the fit of framework on the mastercast.