1. Floating Topic
2. Floating Topic
3. casting
   1. a process which a detailed wax pattern of a dental restoration is converted into dental casting alloy/ceramic
   2. used to fabricate
      1. crowns
         1. inlays, onlays,MCC,FCC
      2. partial dentures
      3. some complete dentures
   3. Lost-wax technique
      1. 1. wax pattern
      2. 2. Spruing
      3. 3. investing
      4. 4. burnout
      5. 5. Casting
      6. 6. Removal of investment
      7. 7. Pickling
      8. 8. Finishing
4. Waxing & Spruing
   1. Wax pattern
      1. Detailed model of final restoration, includes all anatomy, contours, occlusal Fx, prox contact
      2. why use wax?
         1. easy to manipulate, inexpensive, well suited to making custom restoration
         2. easy to completely eliminate (via burnout) after investing
      3. Formed on working die of the tooth
      4. disadvantages
         1. tendency to flow, stress release, expand/contract with temp changes
            1. distort easily
            2. should be sprued and invest ASAP
   2. Sprue
      1. Forms a channel in the investment through which the molten alloy travels to from the restoration
      2. can be made from
         1. wax
         2. plastic
         3. metal
      3. can be single or multiple
         1. depends on wax pattern, type of alloy to be cast, experience of technician
         2. place on one of the cusps of wax pattern
         3. attached to a sprue base (made of rubber)
            1. the cone forms a depression in the investment
            2. guide molten alloy into sprue hole
      4. should be as big in diameter and as short as possible
         1. to prevent premature freezing of molten alloy in sprue channel
            1. must ensure molten alloy solidifies last in the sprue after successfully "feeding" the restoration
            2. Ball on a sprue
            3. as reservoir
5. Investing & Burnout
   1. Investments
      1. the stonelike material in which the wax pattern and sprue are embedded
         1. must be able to withstand high temp & forces of burnout & casting
      2. Comprised of
         1. binder
            1. holds the investment together
            2. gypsum or monoammonium-phosphate-magnesia
         2. refractory material
            1. helps resists the heat of burnout & casting
            2. Silica
      3. 2 types
         1. Gypsum-bonded investments
            1. Binder: Gypsum; Refractory: Silica
            2. used to cast
            3. inlays, full-metal crowns, some Ni-based RPD framework
            4. Melting temp of metal cannot exceed 1200*C
            5. casting not usable
         2. Phosphate-bonded investments
            1. Binder: monoammonium-phospate-magnesia; Refractory : Silica
            2. Used for higher melting alloys
            3. stronger than gypsum-bonded investments
      4. Investing
         1. casting ring placed onto sprue base
            1. to retain the investment while it sets
            2. To support investment during casting
            3. lined with ceramic-paper liner
            4. To aid in expansion of investment during setting & burnout
            5. Moistened beforehand
            6. To prevent it from absorbing water from investment
         2. wax pattern treted with Surfactant
            1. To aid flow of water-based(hydrophilic) investment mat over hydrophobic wax
         3. powder & water
            1. Premeasured packet of powder
            2. water MUST be measured
         4. mixed in power-driven vacuum mixer
            1. mixing occur at high speed
            2. make sure to precisely control mixing time
            3. vacuum limits incorporation of air into mixed investment
            4. reduce bubbles
         5. wax pattern is dimensionally unstable
            1. once removed from die, invest it ASAP
      5. Expansion of investment
         1. Because casting alloys shrink significantly when they cool from solid state(high temp) to room temp
            1. casting will be too small & won't fit the die/tooth
            2. therefore, investment are designed to expand & compensate the casting shrinkage
         2. caused by
            1. Hygroscopic expansion
            2. most common in gypsum-bonded investment
            3. excess intake of water from excess water during mixing or from air
            4. thermal expansion
            5. expansion of silica refractory when heated
   2. Burnout
      1. casting ring with set investment placed into an oven
         1. burn out wax pattern & sprue
            1. create space
      2. will cause thermal expansion of investment
      3. temp in oven commonly 500-600*C
         1. Varies, depending on
            1. type of investment
            2. temp of molten alloy
            3. amount of expansion needed
      4. Length of burnout time
         1. depends on
            1. size of the ring
            2. burnout temp
            3. number of casting ring in oven
            4. whether oven was hot at the beginning of burnout
         2. generally 1 hour
      5. once casting rings heated
         1. should be cast before it cools
            1. because
            2. thermal expansion of investment is not reversible
            3. if ring cools before it is cast]
            4. investment will crack / casting distorts
6. Casting & Pickling
   1. Casting
      1. Done in centrifugal casting machine
         1. operate on principle of centrifugal force
            1. molten alloy is accelerated outward by rapid spinning
         2. the process takes less than 1 second
      2. casting alloy is heated in crucible made of heat-resistance ceramic
      3. Flux is added
         1. to minimize formation of oxides
            1. these oxides impair heating& casting of the alloy & final quality of restoration
      4. casting alloys heated by blowtorch/ electric current
         1. if heated too little
            1. alloy will be too viscous to flow into all of finest details of casting ring
         2. if heated too much
            1. alloy will be damaged by oxidation elements
            2. investment may crack from thermal shock
      5. Cooling
         1. cool slowly on top of a bench
         2. rapid cooling in water
            1. A.K.A quenching
   2. Pickling
      1. a process to remove surface oxides on casting
      2. casting is grasped with special insulated tongs
         1. submerged in hot sulfuric acid (5-10sec)
      3. only done in high gold alloys
7. Finishing & Polishing
   1. remove sprue from restoration
      1. with thin carborundrum disc on handpiece
   2. recontour with carborundrum stones/green stones
   3. polished with progressively finer abbrasives
      1. should have high luster and smooth