Casting In Dentistry

Casting In Dentistry

Describe the steps in the casting procedure.
Answer:

Following are the steps in the casting procedure:

1. Tooth/teeth preparation: Teeth are prepared by the dentist to receive a cast restoration.

2. Impression: An accurate impression of tooth/teeth is placed with elastomers.

3. Dye Preparation:  A dye is prepared from dye stone or the impression is electroformed. A dye spacer is coated or painted over the dye which provides space for the luting cement.

4. Wax Pattern: A pattern of the final restoration is made with type II inlay wax or other casting waxes with all precautions to avoid distortion. Before making the pattrn, a dye lubricant is applied to help separate the wax pattern from the dye.

5. Sprue Former: A sprue former is made of wax, plastic or metal. Thickness is in proportion to the wax pattern. A reservoir is attached to the sprue or the attachment of the sprue to the wax pattern is flared. The length of the sprue is adjusted.

• The functions of sprue former or sprue are as follows:
  • To form a mount for the wax pattern
  • To create a channel for the elimination of wax during burnout
  • Forms a channel for entry of molten alloy during casting
  • Provides a reservoir of molten metal that compensates for alloy shrinkage during solidification.

6. Casting Ring Lining: A ring liner is placed inside of the casting ring. It should be short at one end. Earlier asbestos liners were used. Its use has been discontinued due to health hazards from breathing its dust.

• Types of non-asbestos ring liners used are as follows:
  • Fibrous ceramic aluminous silicate
  • Cellulose
  • Ceramic-cellulose combination.

The functions of the ring liner are as follows:

• • Allows for mold expansion (provides cushion effect).
  • When the ring is transferred from the furnace to the casting machine it reduces heat loss as it is a thermal insulator.
  • Permits easy removal of the investment after casting.

7. Investing:

• Apply a wetting agent (to reduce air bubbles) on the wax pattern.
• Seat the casting ring into the crucible former taking care that it is located near the center of the ring.
• Mix the investment and vibrate.
• Some investment is applied to the wax pattern with a brush to reduce trapping air bubbles.
• The ring is reseated on the crucible former placed on the vibrator and gradually filed with the remaining investment mix.
• Allow it to set for 1 hour.

8. Wax Elimination (burnout) and Thermal Expansion:

The purpose of burnout is:

• To eliminate the wax (pattern) from the mold
• To expand the mold (thermal expansion)
• The crucible is formed from the ring is separated. If a metallic sprue former is used, it should be removed before burnout.
• Burnout is started when the mold is wet.
• If burnout is delayed store the mold in a humidor.
• If burnout is delayed heating should be gradual.
• Rapid heating produces steam which causes the walls of the mold cavity to fake.
• In extreme cases, an explosion may occur.
• Rapid heating also causes cracks in investment due to uneven expansion. Follow the manufacturer’s instructions.
• The ring is placed in a burnout furnace and heated gradually to 400°C in 20 minutes.
• Maintain it for 30 minutes. In the next 30 minutes, raise the temperature to 700°C and maintain it for 30 minutes.

9. Casting: It is a process by which molten alloy is forced into the heated investment mold. Fusion in the case of noble metal alloy.

• Alloys are melted by the following ways:
  • Blow torch
  • Electrical resistance or induction.

10. Blow Torch:

• The fuel used is a combination of:
  • Natural or artificial gas and air
  • Oxygen and acetylene gas (high fusion alloys).
• The flame has four zones as follows:
  • Mixing zone: In this air and gas are mixed. No heat is present and it is dark in color.
  • Combustion zone: This surrounds the inner zone. It is green in color. It is a zone of partial combustion and has an oxidizing nature.
  • Reducing zone: It is a blue zone just beyond the green zone. It is the hottest part of the flame. This zone is used for the fusion of casting alloy.
  • Oxidizingzone: Outermost zone in which final combustion between the gas and surrounding air occurs. This zone is not used for fusion.

The air and gas mixture is adjusted to get a reducing flame which is used to melt the alloy because it does not contaminate the alloy and is the hottest part of the flame. The hot casting ring is shifted from the burnout furnace to the casting machine. The ring is placed in the casting cradle so that the sprue hole adjoins the crucible.

Slide the crucible against the ring to avoid spilling of molten metal. The alloy may be melted by a torch or by induction heating. In an induction casting machine, the molten metal is derived into the mold by centrifugal force.

One arm of the machine has a counterweight which balances the weight of the arm carrying the crucible and mould as it rotates. The red hot crucible and the casting ring are visible in the machine. The induction coil (copper colored) is half visible and is used to melt the metal.

Sprinkle flax powder over the molten metal to reduce the oxides and increase fluidity for casting. When the alloy is molten it has a mirror-like appearance like a ball of mercury. Release the arm and allow it to rotate till it comes to rest. This creates centrifugal force which forces the liquid metal into the mold cavity. The ring is allowed to cool for 10 minutes till the glow of the metal disappears.

11. Quenching (for Gold Alloys): The ring is then immersed in water. This leaves the casting metal in an annealed (softened) condition and also helps to fragment the investment. Base metal alloys should not be quenched.

12. Recovery of Casting: The investment is removed and the casting is recovered. A pneumatic (air-driven) chisel may be used to remove the investment. The final bits of investment are removed by sandblasting.

13. Sandblasting: Sandblasting is the process by which particles of an abrasive are projected at high velocity using compressed air in a continuous stream. The casting is held in a sandblasting machine to clean the remaining investment from its surface.

14. Pickling: The surface oxides from the casting are removed by pickling in 50% hydrochloric acid. HCl is heated, but not boiled with the casting in it. Pickling is not a routine procedure and is performed only when indicated.

15. Polishing: Minimum polishing is required if all the procedures from the wax pattern to casting are followed meticulously